Departments of the central nervous system
The CNS has many functions. It collects and processes information about the environment coming from the PNS, forms reflexes and other behavioral reactions, plans (prepares) and performs arbitrary movements.
In addition, the central nervous system provides the so-called higher cognitive (cognitive) functions. In the central nervous system, processes associated with memory, learning and thinking take place. CNS includes spinal cord (medulla spinalis) and brain (encephalon) (Figure 5-1). The spinal cord is divided into successive sections (cervical, thoracic, lumbar, sacral and coccygeal), each of which consists of segments.
Based on information about the patterns of embryonic development, the brain is divided into five sections: myelencephalon (medulla), metencephalon (back brain) mesencephalon (mid brain) diencephalon (midbrain) and telencephalon (final brain). In the adult brain myelencephalon(medulla)
includes medulla oblongata (medulla oblongata, from medulla), metencephalon(hindbrain) - pons varolii (pons Varolii) and cerebellum (cerebellum); mesencephalon(midbrain) - midbrain; diencephalon(midbrain) - thalamus (thalamus) and hypothalamus (hypothalamus), telencephalon(final brain) - basal nuclei (nuclei bases) and cerebral cortex (cortex cerebri) (Fig. 5-1 B). In turn, the cortex of each hemisphere consists of lobes, which are named the same as the corresponding bones of the skull: frontal (lobus frontalis), parietal ( l. parietalis), temporal ( l. temporalis) and occipital ( l. occipitalis) shares. hemispheres connected corpus callosum (corpus callosum) - a massive bundle of axons crossing the midline between the hemispheres.
Several layers of connective tissue lie on the surface of the CNS. it meninges: soft(pia mater) gossamer (arachnoidea mater) and hard (dura mater). They protect the CNS. Subarachnoid (subarachnoid) the space between the pia mater and arachnoid is filled cerebrospinal (cerebrospinal) fluid (CSF)).
Rice. 5-1. The structure of the central nervous system.
A - brain and spinal cord with spinal nerves. Note the relative sizes of the components of the central nervous system. C1, Th1, L1 and S1 - the first vertebrae of the cervical, thoracic, lumbar and sacral regions, respectively. B - the main components of the central nervous system. The four major lobes of the cerebral cortex are also shown: occipital, parietal, frontal, and temporal.
Sections of the brain
The main structures of the brain are shown in Fig. 5-2 A. There are cavities in the brain tissue - ventricles, filled CSF (Fig. 5-2 B, C). CSF exerts a shock-absorbing effect and regulates the extracellular environment around neurons. CSF is formed mainly vascular plexus, lined with specialized ependyma cells. The choroid plexuses are located in the lateral, third and fourth ventricles. Lateral ventricles located one in each of the two cerebral hemispheres. They connect with third ventricle through interventricular holes (Monroy's holes). The third ventricle lies in the midline between the two halves of the diencephalon. It is connected to fourth ventricle through aqueduct of the brain (sylvian aqueduct), penetrating the midbrain. The “bottom” of the fourth ventricle is formed by the bridge and the medulla oblongata, and the “roof” is the cerebellum. The continuation of the fourth ventricle in the caudal direction is central channel spinal cord, usually closed in an adult.
CSF flows from the ventricles into the pons subarachnoid (subarachnoid) space through three holes in the roof of the fourth ventricle: median aperture(hole of Magendie) and two lateral apertures(holes of Lushka). Released from the ventricular system, CSF circulates in the subarachnoid space surrounding the brain and spinal cord. Extensions of this space are named subarachnoid (subarachnoid)
tanks. One of them - lumbar (lumbar) cistern, from which CSF samples are obtained by lumbar puncture for clinical analysis. Much of the CSF is absorbed through valved arachnoid villi into the venous sinuses of the dura mater.
The total volume of CSF in the cerebral ventricles is about 35 ml, while the subarachnoid space contains about 100 ml. Approximately 0.35 ml of CSF is formed every minute. At this rate, CSF renewal occurs approximately four times a day.
In a person in the supine position, the CSF pressure in the spinal subarachnoid space reaches 120-180 mm of water. The rate of CSF production is relatively independent of ventricular and subarachnoid pressures and systemic blood pressure. At the same time, the CSF reabsorption rate is directly related to CSF pressure.
The extracellular fluid in the CNS communicates directly with the CSF. Therefore, the composition of CSF influences the composition of the extracellular environment around neurons in the brain and spinal cord. The main components of CSF in the lumbar cistern are listed in Table. 5-1. For comparison, the concentrations of the corresponding substances in the blood are given. As shown in this table, the content of K+, glucose and proteins in the CSF is lower than in the blood, and the content of Na+ and Cl - is higher. In addition, there are practically no erythrocytes in the CSF. Due to the increased content of Na + and Cl - isotonicity of CSF and blood is ensured, despite the fact that there are relatively few proteins in CSF.
Table 5-1. Composition of cerebrospinal fluid and blood
Rice. 5-2. Brain.
A - midsagittal section of the brain. Note the relative positioning of the cerebral cortex, cerebellum, thalamus, and brainstem, as well as the various commissures. B and C - in situ cerebral ventricular system - side view (B) and front view (C)
Organization of the spinal cord
Spinal cord lies in the spinal canal and in adults it is a long (45 cm in men and 41-42 cm in women) cylindrical cord somewhat flattened from front to back, which at the top (cranially) directly passes into the medulla oblongata, and at the bottom (caudally) ends with a conical sharpening on level II of the lumbar vertebra. Knowledge of this fact is of practical importance (in order not to damage the spinal cord during a lumbar puncture for the purpose of taking cerebrospinal fluid or for the purpose of spinal anesthesia, it is necessary to insert a syringe needle between the spinous processes of III and IV lumbar vertebrae).
The spinal cord along its length has two thickenings corresponding to the nerve roots of the upper and lower extremities: the upper one is called the cervical thickening, and the lower one is called the lumbar. Of these thickenings, the lumbar one is more extensive, but the cervical one is more differentiated, which is associated with a more complex innervation of the hand as a labor organ.
In the intervertebral foramina near the junction of both roots, the posterior root has a thickening - the spinal ganglion (ganglion spinale) containing false-unipolar nerve cells (afferent neurons) with one process, which then divides into two branches. One of them, the central one, goes as part of the posterior root to the spinal cord, and the other, peripheral, continues into the spinal nerve. In this way,
there are no synapses in the spinal nodes, since only the cell bodies of afferent neurons lie here. In this way, these nodes differ from the vegetative nodes of the PNS, since in the latter intercalary and efferent neurons come into contact.
The spinal cord is made up of gray matter, which contains nerve cells, and white matter, which is made up of myelinated nerve fibers.
Gray matter forms two vertical columns placed in the right and left half of the spinal cord. In the middle of it is laid a narrow central canal containing cerebrospinal fluid. The central canal is a remnant of the cavity of the primary neural tube, so at the top it communicates with the IV ventricle of the brain.
The gray matter surrounding the central canal is called the intermediate substance. In each column of gray matter, two columns are distinguished: anterior and posterior. On transverse sections, these pillars look like horns: anterior, expanded, and posterior, pointed.
The gray matter consists of nerve cells grouped into nuclei, the location of which basically corresponds to the segmental structure of the spinal cord and its primary three-membered reflex arc. The first sensitive neuron of this arc lies in the spinal nodes, its peripheral process goes as part of nerves to organs and tissues and contacts receptors there, and the central one penetrates the spinal cord as part of the posterior sensory roots.
Rice. 5-3. Spinal cord.
A - nerve pathways of the spinal cord; B - transverse section of the spinal cord. Conducting paths
The structure of a neuron
Functional unit of the nervous system - neuron. A typical neuron has a receptive surface in the form cell body (soma) and several shoots - dendrites, on which are synapses, those. interneuronal contacts. The axon of a nerve cell forms synaptic connections with other neurons or with effector cells. The communication networks of the nervous system are made up of neural circuits formed by synaptically interconnected neurons.
catfish
In the soma of neurons are nucleus and nucleolus(Fig. 5-4), as well as a well-developed biosynthetic apparatus that produces membrane components, synthesizes enzymes and other chemical compounds necessary for the specialized functions of nerve cells. The apparatus for biosynthesis in neurons includes Nissl bodies- flattened cisterns of the granular endoplasmic reticulum, tightly adjacent to each other, as well as a well-defined golgi apparatus. In addition, soma contains numerous mitochondria and elements of the cytoskeleton, including neurofilaments and microtubules. As a result of incomplete degradation of membrane components, a pigment is formed lipofuscin, accumulating with age in a number of neurons. In some groups of neurons in the brainstem (for example, in the neurons of the substantia nigra and the blue spot), the melatonin pigment is noticeable.
Dendrites
Dendrites, outgrowths of the cell body, in some neurons reach a length of more than 1 mm, and they account for more than 90% of the surface area of the neuron. In the proximal parts of the dendrites (closer to the cell body)
contains Nissl bodies and sections of the Golgi apparatus. However, the main components of the dendritic cytoplasm are microtubules and neurofilaments. Dendrites were considered to be electrically non-excitable. However, it is now known that the dendrites of many neurons have voltage-controlled conduction. This is often due to the presence of calcium channels, which, when activated, generate calcium action potentials.
axon
A specialized section of the cell body (usually the soma, but sometimes the dendrite), from which the axon departs, is called axon hillock. The axon and axon hillock differ from the soma and proximal portions of dendrites in that they lack the granular endoplasmic reticulum, free ribosomes, and the Golgi apparatus. The axon contains a smooth endoplasmic reticulum and a pronounced cytoskeleton.
Neurons can be classified according to the length of their axons. At type 1 neurons according to Golgi axons short, ending, like dendrites, close to the soma. Neurons of the 2nd type according to Golgi characterized by long axons, sometimes more than 1 m.
Neurons communicate with each other using action potentials, propagating in neuronal circuits along axons. Action potentials are transmitted from one neuron to the next as a result synaptic transmission. In the process of transmission, reached presynaptic ending An action potential usually triggers the release of a neurotransmitter, which is either excites the postsynaptic cell so that a discharge from one or more action potentials occurs in it, or slows down her activity. Axons not only transmit information in neural circuits, but also deliver chemicals through axonal transport to synaptic endings.
Rice. 5-4. Diagram of an "ideal" neuron and its main components.
Most afferent inputs coming along the axons of other cells terminate in synapses on dendrites (D), but some terminate in synapses on the soma. Excitatory nerve endings are more often located distally on the dendrites, and inhibitory nerve endings are more often located on the soma.
Neuron organelles
Figure 5-5 shows the soma of neurons. The soma of neurons shows the nucleus and nucleolus, the biosynthetic apparatus that produces membrane components, synthesizes enzymes and other chemical compounds necessary for the specialized functions of nerve cells. It includes Nissl bodies - flattened cisterns of granular
endoplasmic reticulum, as well as a well-defined Golgi apparatus. The soma contains mitochondria and cytoskeletal elements, including neurofilaments and microtubules. As a result of incomplete degradation of membrane components, the pigment lipofuscin is formed, which accumulates with age in a number of neurons. In some groups of neurons in the brainstem (for example, in the neurons of the substantia nigra and the blue spot), the melatonin pigment is noticeable.
Rice. 5-5. Neuron.
A - organelles of the neuron. In the diagram, typical organelles of a neuron are shown as they are seen under a light microscope. The left half of the scheme reflects the structures of the neuron after Nissl staining: nucleus and nucleolus, Nissl bodies in the cytoplasm of the soma and proximal dendrites, and the Golgi apparatus (unstained). Note the absence of Nissl bodies in the axon colliculus and axon. Part of a neuron after staining with salts of heavy metals: neurofibrils are visible. With appropriate staining with salts of heavy metals, the Golgi apparatus can be observed (not shown in this case). On the surface of the neuron are several synaptic endings (stained with salts of heavy metals). B - The diagram corresponds to the electron microscopic picture. The nucleus, nucleolus, chromatin, nuclear pores are visible. Mitochondria, rough endoplasmic reticulum, Golgi apparatus, neurofilaments and microtubules are visible in the cytoplasm. On the outer side of the plasma membrane - synaptic endings and processes of astrocytes
Types of neurons
Neurons are very diverse. Neurons of different types perform specific communication functions, which is reflected in their structure. So, dorsal root ganglion neurons (spinal ganglia) receive information not by synaptic transmission, but from sensory nerve endings in organs. The cell bodies of these neurons are devoid of dendrites (Fig. 5-6 A5) and do not receive synaptic endings. After leaving the cell body, the axon of such a neuron is divided into two branches, one of which (peripheral process)
is sent as part of the peripheral nerve to the sensory receptor, and the other branch (central branch) enters the spinal cord back spine) or in the brain stem (as part of cranial nerve).
Neurons of a different type, such as pyramidal cells cerebral cortex and Purkinje cells cerebellar cortex, are busy processing information (Fig. 5-6 A1, A2). Their dendrites are covered with dendritic spines and are characterized by an extensive surface. They have a huge number of synaptic inputs.
Rice. 5-6. Types of neurons
A - neurons of various shapes: 1 - a neuron resembling a pyramid. Neurons of this type, called pyramidal cells, are characteristic of the cerebral cortex. Note the spine-like processes dotting the surface of the dendrites; 2 - Purkinje cells, named after the Czech neuroanatomist Jan Purkinje who first described them. They are located in the cerebellar cortex. The cell has a pear-shaped body; on one side of the soma is an abundant plexus of dendrites, on the other - an axon. Thin branches of dendrites are covered with spines (not shown in the diagram); 3 - postganglionic sympathetic motor neuron; 4 - alpha motor neuron of the spinal cord. It, like the postganglionic sympathetic motor neuron (3), is multipolar, with radial dendrites; 5 - sensory cell of the spinal ganglion; does not have dendrites. Its process is divided into two branches: central and peripheral. Since in the process of embryonic development the axon is formed as a result of the fusion of two processes, these neurons are considered not unipolar, but pseudo-unipolar. B - types of neurons
Types of non-neuronal cells
Another group of cellular elements of the nervous system - neuroglia(Fig. 5-7 A), or supporting cells. In the human CNS, the number of neuroglial cells is an order of magnitude greater than the number of neurons: 10 13 and 10 12, respectively. Neuroglia is not directly involved in short-term communication processes in the nervous system, but contributes to the implementation of this function by neurons. So, neuroglial cells of a certain type form around many axons myelin sheath, significantly increases the speed of conduction of action potentials. This allows axons to quickly transmit information to distant cells.
Types of neuroglia
Glial cells support the activity of neurons (Fig. 5-7 B). In the CNS, neuroglia are astrocytes and oligodendrocytes, and in the PNS - Schwann cells and satellite cells. In addition, cells are considered to be central glial cells. microglia and cells ependyma.
Astrocytes(named for their stellate shape) regulate the microenvironment around CNS neurons, although they are in contact with only part of the surface of the central neurons (Fig. 5-7 A). However, their processes surround groups of synaptic endings, which as a result are isolated from neighboring synapses. Special branches - "legs" astrocytes form contacts with capillaries and with connective tissue on the surface of the CNS - with pia mater(Fig. 5-7 A). Legs limit the free diffusion of substances in the CNS. Astrocytes can actively absorb K + and neurotransmitter substances, then metabolizing them. Thus, astrocytes play a buffer role, blocking direct access for ions and neurotransmitters to the extracellular environment around neurons. The cytoplasm of astrocytes contains glial cells.
filaments that perform a mechanical support function in the CNS tissue. In case of damage, the processes of astrocytes containing glial filaments undergo hypertrophy and form a glial "scar".
Other elements of neuroglia provide electrical insulation to neuronal axons. Many axons are covered with insulating myelin sheath. It is a multi-layered wrapping spirally wound over the plasma membrane of axons. In the CNS, the myelin sheath is created by cell membranes oligodendroglia(Fig. 5-7 B3). In the PNS, the myelin sheath is made up of membranes Schwann cells(Fig. 5-7 B2). Unmyelinated (non-myelinated) axons of the CNS do not have an insulating coating.
Myelin increases the speed of conduction of action potentials due to the fact that ion currents during an action potential enter and exit only in interceptions of Ranvier(areas of interruption between adjacent myelinating cells). Thus, the action potential "jumps" from intercept to intercept - the so-called saltatory conduction.
In addition, neuroglia contain satellite cells, encapsulating ganglion neurons of spinal and cranial nerves, regulating the microenvironment around these neurons in the same way that astrocytes do. Another type of cell microglia, or latent phagocytes. In case of damage to CNS cells, microglia contributes to the removal of cellular decay products. This process involves other neuroglial cells, as well as phagocytes penetrating the CNS from the bloodstream. The CNS tissue is separated from the CSF, which fills the ventricles of the brain, by an epithelium formed ependymal cells(Fig. 5-7 A). The ependyma mediates the diffusion of many substances between the extracellular space of the brain and the CSF. Specialized ependymal cells of the choroid plexuses in the ventricular system secrete a significant
share of CSF.
Rice. 5-7. non-neuronal cells.
A is a schematic representation of non-neuronal elements of the central nervous system. Two astrocytes are depicted, the legs of the processes of which end on the soma and dendrites of the neuron, and also contact the pia mater and/or capillaries. The oligodendrocyte forms the myelin sheath of axons. Microglial cells and ependymal cells are also shown. B - different types of neuroglial cells in the central nervous system: 1 - fibrillar astrocyte; 2 - protoplasmic astrocyte. Note the astrocytic stalk in contact with the capillaries (see 5-7 A); 3 - oligodendrocyte. Each of its processes ensures the formation of one or more intergap myelin sheaths around the axons of the central nervous system; 4 - microglial cells; 5 - ependyma cells
Scheme of distribution of information on a neuron
In the synapse zone, a locally formed EPSP propagates passively electrotonically throughout the entire postsynaptic membrane of the cell. This distribution is not subject to the all-or-nothing law. If a large number of excitatory synapses are excited simultaneously or almost simultaneously, then a phenomenon occurs summation, manifested in the form of the appearance of an EPSP of a significantly larger amplitude, which can depolarize the membrane of the entire postsynaptic cell. If the magnitude of this depolarization reaches a certain threshold value (10 mV or more) in the area of the postsynaptic membrane, then voltage-controlled Na+ channels open at lightning speed on the axon hillock of the nerve cell, and the cell generates an action potential that is conducted along its axon. With abundant release of the transmitter, the postsynaptic potential may appear as early as 0.5-0.6 ms after the action potential that has arrived in the presynaptic region. From the beginning of the EPSP to the formation of the action potential, another 0.3 ms passes.
threshold stimulus is the weakest stimulus reliably distinguished by the sensory receptor. To do this, the stimulus must cause a receptor potential of such an amplitude that is sufficient to activate at least one primary afferent fiber. Weaker stimuli may elicit a subthreshold receptor potential, but they will not result in firing of the central sensory neurons and hence will not be perceived. In addition, the number
excited primary afferent neurons required for sensory perception depends on spatial and temporary summation in sensory pathways (Fig. 5-8 B, D).
Interacting with the receptor, ACh molecules open nonspecific ion channels in the postsynaptic cell membrane so that their ability to conduct monovalent cations increases. The operation of the channels leads to a basic inward current of positive ions, and therefore to a depolarization of the postsynaptic membrane, which, in relation to synapses, is called excitatory postsynaptic potential.
The ionic currents involved in EPSPs behave differently than sodium and potassium currents during action potential generation. The reason is that other ion channels with different properties (ligand-gated rather than voltage-gated) are involved in the EPSP generation mechanism. At an action potential, voltage-gated ion channels are activated, and with increasing depolarization, the following channels open, so that the depolarization process reinforces itself. At the same time, the conductivity of transmitter-gated (ligand-gated) channels depends only on the number of transmitter molecules bound to receptor molecules (resulting in the opening of transmitter-gated ion channels) and, consequently, on the number of open ion channels. The amplitude of the EPSP lies in the range from 100 μV up to 10 mV in some cases. Depending on the type of synapse, the total duration of EPSP in some synapses ranges from 5 to 100 ms.
Rice. 5-8. Information flows from the dendrites to the soma, to the axon, to the synapse.
The figure shows the types of potentials in different places of the neuron, depending on the spatial and temporal summation
Reflex- This is a response to a specific stimulus, carried out with the mandatory participation of the nervous system. The neural circuit that provides a specific reflex is called reflex arc.
In its simplest form reflex arc of the somatic nervous system(Fig. 5-9 A), as a rule, consists of sensory receptors of a certain modality (the first link of the reflex arc), information from which enters the central nervous system along the axon of a sensitive cell located in the spinal ganglion outside the central nervous system (the second link reflex arc). As part of the posterior root of the spinal cord, the axon of the sensory cell enters the posterior horns of the spinal cord where it forms a synapse on the intercalary neuron. The axon of the intercalary neuron goes without interruption to the anterior horns, where it forms a synapse on the α-motor neuron (the interneuron and α-motor neuron, as structures located in the central nervous system, are the third link of the reflex arc). The axon of the α-motor neuron emerges from the anterior horns as part of the anterior root of the spinal cord (fourth link of the reflex arc) and goes to the skeletal muscle (fifth link of the reflex arc), forming myoneural synapses on each muscle fiber.
The simplest scheme reflex arc of the autonomic sympathetic nervous system
(Fig. 5-9 B), usually consists of sensory receptors (the first link of the reflex arc), information from which enters the central nervous system along the axon of a sensitive cell located in the spinal or other sensitive ganglion outside the central nervous system (the second link of the reflex arcs). The axon of the sensory cell as part of the posterior root enters the posterior horns of the spinal cord, where it forms a synapse on the intercalary neuron. The axon of the intercalary neuron goes to the lateral horns, where it forms a synapse on the preganglionic sympathetic neuron (in the thoracic and lumbar regions). (Intercalary neuron and preganglionic sympathetic
the neuron is the third link in the reflex arc). The axon of the preganglionic sympathetic neuron exits the spinal cord as part of the anterior roots (fourth link of the reflex arc). The next three options for the paths of this type of neuron are combined in the diagram. In the first case, the axon of the preganglionic sympathetic neuron goes to the paravertebral ganglion, where it forms a synapse on the neuron, the axon of which goes to the effector (the fifth link of the reflex arc), for example, to the smooth muscles of the internal organs, to secretory cells, etc. In the second case, the axon of the preganglionic sympathetic neuron goes to the prevertebral ganglion, where it forms a synapse on a neuron, the axon of which goes to the internal organ (the fifth link of the reflex arc). In the third case, the axon of the preganglionic sympathetic neuron goes to the adrenal medulla, where it forms a synapse on a special cell that releases adrenaline into the blood (all this is the fourth link of the reflex arc). In this case, adrenaline through the blood enters all target structures that have pharmacological receptors for it (the fifth link of the reflex arc).
In its simplest form reflex arc of the autonomic parasympathetic nervous system(Fig. 5-9 C) consists of sensory receptors - the first link of the reflex arc (located, for example, in the stomach), which send information to the central nervous system along the axon of a sensitive cell located in the ganglion located along the vagus nerve (second link reflex arc). The axon of the sensory cell transmits information directly to the medulla oblongata, where a synapse is formed on the neuron, the axon of which (also within the medulla oblongata) forms a synapse on the parasympathetic preganglionic neuron (the third link of the reflex arc). From it, the axon, for example, as part of the vagus nerve, returns to the stomach and forms a synapse on the efferent cell (fourth link of the reflex arc), the axon of which branches through the stomach tissue (fifth link of the reflex arc), forming nerve endings.
Rice. 5-9. Schemes of the main reflex arcs.
A - Reflex arc of the somatic nervous system. B - Reflex arc of the autonomic sympathetic nervous system. B - Reflex arc of the autonomic parasympathetic nervous system
taste buds
familiar to all of us taste sensations are actually mixtures of the four elemental tastes: salty, sweet, sour, and bitter. Four substances are especially effective in causing the corresponding taste sensations: sodium chloride (NaCl), sucrose, hydrochloric acid (HC1) and quinine.
Spatial distribution and innervation of taste buds
Taste buds are contained in taste buds of various types on the surface of the tongue, palate, pharynx and larynx (Fig. 5-10 A). On the front and side of the tongue are located mushroom-shaped and foliate
papillae, and on the surface of the root of the tongue - grooved. The composition of the latter may include several hundred taste buds, the total number of which in humans reaches several thousand.
Specific taste sensitivity is not the same in different areas of the surface of the tongue (Fig. 5-10 B, C). Sweet taste is best perceived by the tip of the tongue, salty and sour - by the side zones, and bitter - by the base (root) of the tongue.
Taste buds are innervated by three cranial nerves, two of which are shown in Fig. 5-10 G. drum string(chorda tympani- branch of the facial nerve) supplies the taste buds of the anterior two-thirds of the tongue, glossopharyngeal nerve- rear third (Fig. 5-10 D). Nervus vagus innervates some taste buds of the larynx and upper esophagus.
Rice. 5-10 Chemical sensitivity - taste and its basics.
A is a taste bud. Organization of taste buds in papillae of three types. A taste bud is shown with a taste opening at the top and nerves extending from below, as well as two types of chemoreceptor cells, supporting (supporting) and taste cells. B - three types of papillae are presented on the surface of the tongue. B - distribution of zones of four elementary taste qualities on the surface of the tongue. D - innervation of the two anterior thirds and the posterior third of the surface of the tongue by the facial and glossopharyngeal nerves
taste bud
Taste sensations arise from the activation of chemoreceptors in the taste buds (taste buds). Each taste bud(calicilus gustatorius) contains from 50 to 150 sensory (chemoreceptive, gustatory) cells, and also includes supporting (supporting) and basal cells (Fig. 5-11 A). The basal part of the sensory cell forms a synapse at the end of the primary afferent axon. There are two types of chemoreceptive cells containing different synaptic vesicles: with an electron-dense center or round transparent vesicles. The apical surface of the cells is covered with microvilli directed towards the taste pore.
Chemoreceptor molecules microvilli interact with stimulating molecules that enter the taste pore(gustatory opening) from the fluid that bathes the taste buds. This fluid is partly produced by glands between the taste buds. As a result of a shift in membrane conductance, a receptor potential arises in the sensory cell, and an excitatory neurotransmitter is released, under the influence of which a generator potential develops in the primary afferent fiber and a pulsed discharge begins, which is transmitted to the CNS.
The coding of the four primary taste qualities is not based on the complete selectivity of sensory cells. Each cell responds to more than one gustatory stimuli, but most actively, as a rule, only one. Distinguishing taste quality depends on spatially ordered input from a population of sensory cells. The intensity of the stimulus is encoded by the quantitative characteristics of the activity caused by it (the frequency of impulses and the number of excited nerve fibers).
On fig. 5-11 shows the mechanism of work of taste buds, which is turned on for substances of different taste.
The cellular mechanisms of taste perception are reduced to various ways of depolarization of the cell membrane and further opening of potential-gated calcium channels. Entered calcium makes possible the release of the mediator, which leads to the appearance of a generator potential at the end of the sensory nerve. Each stimulus depolarizes the membrane in a different way. Salt stimulus interacts with epithelial sodium channels (ENaC), opening them to sodium. An acidic stimulus can open ENaC on its own or close potassium channels due to a decrease in pH, which will also lead to depolarization of the taste cell membrane. Sweet taste arises due to the interaction of a sweet stimulus with a G-protein-coupled receptor that is sensitive to it. Activated G-protein stimulates adenylate cyclase, which increases the content of cAMP and further activates the dependent protein kinase, which, in turn, closes them by phosphorylation of potassium channels. All this also leads to membrane depolarization. A bitter stimulus can depolarize the membrane in three ways: (1) by closing potassium channels, (2) by interacting with G-protein (gastducin) to activate phosphodiesterase (PDE), thereby reducing cAMP levels. This (for reasons not entirely understood) causes the membrane to depolarize. (3) The bitter stimulus binds to a G-protein capable of activating phospholipase C (PLC), resulting in an increase in inositol 1,4,5 triphosphate (IP 3), which leads to the release of calcium from the depot.
Glutamate binds to glutamate-regulated non-selective ion channels and opens them. This is accompanied by depolarization and opening of potential-gated calcium channels.
(PIP 2) - phosphatidyl inositol 4,5 biphosphate (DAG) - diacylglycerol
Rice. 5-11. Cellular mechanisms of taste perception
Central taste pathways
The cell bodies to which the taste fibers of the VII, IX and X cranial nerves belong are located in the geniculate, stony and nodular ganglia, respectively (Fig. 5-12 B). The central processes of their afferent fibers enter the medulla oblongata, are included in the solitary tract and terminate in synapses in the nucleus of the solitary tract (nucleus solitarius)(Fig. 5-12 A). In a number of animals, including some rodent species, secondary gustatory neurons in the nucleus of the solitary tract project rostral to the ipsilateral parabrachial nucleus.
In turn, the parabrachial nucleus sends projections to the small cell (right cellular) part ventral posteromedial (VZM MK) nucleus (MK - small cell part of VZM) thalamus (Fig. 5-12 B). In monkeys, the projections of the nucleus of the solitary tract to the VZM MK-nucleus are direct. VZM MK-nucleus is associated with two different taste areas of the cerebral cortex. One of them is part of the facial representation (SI), the other is in the insula (insula- island) (Fig. 5-12 D). The central taste pathway is unusual in that its fibers do not cross over to the other side of the brain (unlike the somatosensory, visual, and auditory pathways).
Rice. 5-12. Pathways that conduct taste sensation.
A - the end of gustatory afferent fibers in the nucleus of the solitary tract and ascending paths to the parabrachial nucleus, ventrobasal thalamus and cerebral cortex. B - peripheral distribution of gustatory afferent fibers. C and D - taste areas of the thalamus and cerebral cortex of monkeys
Smell
In primates and humans (microsmats) olfactory sensitivity developed much worse than in most animals (macrosmats). Truly legendary is the ability of dogs to find a trail by smell, as well as the attraction of insects of the opposite sex with the help of pheromones. As for a person, his sense of smell plays a role in the emotional sphere; odors effectively contribute to the extraction of information from memory.
Olfactory receptors
The olfactory chemoreceptor (sensory cell) is a bipolar neuron (Fig. 5-13B). Its apical surface bears immobile cilia that react to odorous substances dissolved in the mucus layer covering them. An unmyelinated axon emerges from the deeper edge of the cell. Axons unite into olfactory bundles (fila olfactoria), penetrating the skull through holes in the cribriform plate (lamina cribrosa) ethmoid bone (os ethmoidale). The olfactory nerve fibers terminate in synapses in the olfactory bulb, and the central olfactory structures are at the base of the skull just below the frontal lobe. Olfactory receptor cells are part of the mucous membrane of the specialized olfactory zone of the nasopharynx, the total surface of which on both sides is approximately 10 cm 2 (Fig. 5-13 A). Humans have about 10 7 olfactory receptors. Like taste buds, olfactory receptors have a short lifespan (about 60 days) and are constantly being replaced.
Molecules of odorous substances enter the olfactory zone through the nostrils when inhaling or from the oral cavity while eating. Smelling movements increase the flow of these substances, which temporarily combine with the olfactory binding protein of mucus secreted by the glands of the nasal mucosa.
There are more primary olfactory sensations than gustatory ones. There are at least six classes of odors: floral, ethereal(fruit), musky, camphorous, putrid and caustic. Examples of their natural sources are rose, pear, musk, eucalyptus, rotten eggs and vinegar, respectively. The olfactory mucosa also contains trigeminal receptors. When clinically testing the sense of smell, pain or temperature stimulation of these somatosensory receptors should be avoided.
Several molecules of an odorous substance cause a depolarizing receptor potential in the sensory cell, which triggers the discharge of impulses in the afferent nerve fiber. However, activation of a certain number of olfactory receptors is necessary for a behavioral response. The receptor potential, apparently, arises as a result of an increase in the conductivity for Na + . At the same time, the G-protein is activated. Therefore, a cascade of second messengers is involved in the olfactory transformation (transduction).
Olfactory coding has much in common with gustatory coding. Each olfactory chemoreceptor responds to more than one class of odors. The encoding of a specific quality of smell is provided by the responses of many olfactory receptors, and the intensity of sensation is determined by the quantitative characteristics of impulse activity.
Rice. 5-13. Chemical sensitivity - the sense of smell and its basics.
A&B - layout of the olfactory zone of the mucous membrane in the nasopharynx. At the top is the cribriform plate, and above it is the olfactory bulb. The olfactory mucosa also extends to the sides of the nasopharynx. C and D - olfactory chemoreceptors and supporting cells. G - olfactory epithelium. D - scheme of processes in olfactory receptors
Central olfactory pathways
The olfactory pathway first switches in the olfactory bulb, which is related to the cerebral cortex. This structure contains three types of cells: mitral cells, fascicular cells and interneurons (granule cells, periglomerular cells)(Figure 5-14). The long branching dendrites of the mitral and fascicular cells form the postsynaptic components of the olfactory glomeruli (glomeruli). Olfactory afferent fibers (running from the olfactory mucosa to the olfactory bulb) branch near the olfactory glomeruli and terminate in synapses on the dendrites of the mitral and fascicular cells. In this case, there is a significant convergence of olfactory axons on the dendrites of mitral cells: on the dendrite of each mitral cell there are up to 1000 synapses of afferent fibers. Granule cells (granular cells) and periglomerular cells are inhibitory interneurons. They form reciprocal dendrodendritic synapses with mitral cells. When mitral cells are activated, depolarization of the interneurons in contact with it occurs, as a result of which an inhibitory neurotransmitter is released in their synapses on mitral cells. The olfactory bulb receives inputs not only through the ipsilateral olfactory nerves, but also through the contralateral olfactory tract running in the anterior commissure (commissure).
The axons of the mitral and fascicular cells leave the olfactory bulb and enter the olfactory tract (Fig. 5-14). Starting from this site, olfactory connections are very complicated. The olfactory tract goes through anterior olfactory nucleus. The neurons of this nucleus receive synaptic connections from the neurons of the olfactory
bulbs and project through the anterior commissure to the contralateral olfactory bulb. Approaching the anterior perforated substance at the base of the brain, the olfactory tract is divided into the lateral and medial olfactory strips. The axons of the lateral olfactory stria terminate in synapses in the primary olfactory region, including the pre-piriform (prepyriform) cortex, and in animals, the piriform (pyriform) lobe. The medial olfactory strip projects to the amygdala and to the basal forebrain cortex.
It should be noted that the olfactory pathway is the only sensory system without the obligatory synaptic switching in the thalamus. Probably, the absence of such a switch reflects the phylogenetic antiquity and the relative primitiveness of the olfactory system. However, olfactory information still enters the posteromedial nucleus of the thalamus and from there is sent to the prefrontal and orbitofrontal cortex.
In a standard neurological examination, an olfaction test is usually not performed. However, the perception of odors can be tested by asking the subject to smell and identify the odorous substance. At the same time, one nostril is examined, the other must be closed. In this case, strong stimuli such as ammonia should not be used, since they also activate the endings of the trigeminal nerve. Olfactory disturbance (anosmia) observed when the base of the skull is damaged or one or both olfactory bulbs are compressed by a tumor (for example, when olfactory fossa meningioma). An aura of unpleasant odor, often the smell of burnt rubber, occurs with epileptic seizures generated in the area of the uncus.
Rice. 5-14. Diagram of a sagittal section through the olfactory bulb showing the olfactory chemoreceptor cell endings on the olfactory glomeruli and on the olfactory bulb neurons.
Axons of mitral and fascicular cells exit as part of the olfactory tract (to the right)
The structure of the eye
The wall of the eye consists of three concentric layers (shells) (Fig. 5-15 A). The outer support layer, or fibrous sheath, includes a transparent cornea with its epithelium, conjunctiva and opaque sclera. In the middle layer, or choroid, are the iris (iris) and the choroid itself (choroidea). AT iris there are radial and annular smooth muscle fibers that form the dilator and sphincter of the pupil (Fig. 5-15 B). choroid(choroid) is richly supplied with blood vessels that feed the outer layers of the retina, and also contains pigment. The inner nerve layer of the eye wall, or retina, contains rods and cones and lines the entire inner surface of the eye, with the exception of the "blind spot" - optic disc(Fig. 5-15 A). Axons of retinal ganglion cells converge to the disc, forming the optic nerve. The highest visual acuity is in the central part of the retina, the so-called yellow spot(macula lutea). The middle of the macula is depressed in the form fossa(fovea centralis)- zones of focusing visual images. The inner part of the retina is nourished by the branches of its central vessels (arteries and veins), which enter together with the optic nerve, then branch in the disk area and diverge along the inner surface of the retina (Fig. 5-15 C), without touching the yellow spot.
In addition to the retina, there are other formations in the eye: lens- a lens that focuses light on the retina; pigment layer, limiting light scattering; aqueous humor and vitreous body. Aqueous moisture is a fluid that makes up the environment of the anterior and posterior chambers of the eye, and the vitreous fills the interior of the eye behind the lens. Both substances contribute to maintaining the shape of the eye. Aqueous moisture is secreted by the ciliary epithelium of the posterior chamber, then circulates through the pupil to the anterior chamber, and from there
gets through Schlemm's channel into the venous circulation (Fig. 5-15 B). The intraocular pressure depends on the pressure of aqueous humor (normally it is below 22 mm Hg), which should not exceed 22 mm Hg. The vitreous body is a gel composed of extracellular fluid with collagen and hyaluronic acid; unlike aqueous humor, it is replaced very slowly.
If the absorption of aqueous humor is impaired, intraocular pressure increases and glaucoma develops. With an increase in intraocular pressure, the blood supply to the retina becomes difficult and the eye can become blind.
A number of functions of the eye depend on the activity of the muscles. The external eye muscles, attached outside the eye, direct the movements of the eyeballs towards the visual target. These muscles are innervated oculomotor(nervus oculomotorius),bloc(n. trochlearis) and diverting(n. abducens)nerves. There are also internal eye muscles. Due to the muscle that dilates the pupil (pupil dilator), and the muscle that constricts the pupil (pupil sphincter) the iris acts like an aperture and regulates the diameter of the pupil in a manner similar to a camera aperture device that controls the amount of incoming light. The pupillary dilator is activated by the sympathetic nervous system, and the sphincter is activated by the parasympathetic nervous system (via the oculomotor nerve system).
The shape of the lens is also determined by the work of the muscles. The lens is suspended and held in place behind the iris by fibers. ciliary(ciliary, or cinnamon) belt, attached to the pupil capsule and to the ciliary body. The lens is surrounded by fibers ciliary muscle, acting like a sphincter. When these fibers are relaxed, the tension in the girdle fibers stretches the lens, flattening it. By contracting, the ciliary muscle counteracts the tension of the girdle fibers, which allows the elastic lens to take on a more convex shape. The ciliary muscle is activated by the parasympathetic nervous system (via the oculomotor nerve system).
Rice. 5-15. Vision.
A - diagram of the horizontal section of the right eye. B - the structure of the anterior part of the eye in the area of the limbus (connection of the cornea and sclera), the ciliary body and the lens. B - back surface (bottom) of the human eye; view through an ophthalmoscope. Branches of the central artery and vein leave the region of the optic disc. Not far from the optic nerve head on its temporal side is the fovea centralis (fovea). Note the distribution of ganglion cell axons (thin lines) converging at the optic disc.
In the following figures, the details of the structure of the eye and the mechanisms of operation of its structures are given (explanations in the figures)
Rice. 5-15.2.
Rice. 5-15.3.
Rice. 5-15.4.
Rice. 5-15.5.
Optical system of the eye
Light enters the eye through the cornea and passes through successive transparent fluids and structures: the cornea, aqueous humor, lens, and vitreous body. Their collection is called diopter apparatus. Under normal conditions, there refraction(refraction) of light rays from a visual target by the cornea and lens so that the rays are focused on the retina. The refractive power of the cornea (the main refractive element of the eye) is equal to 43 diopters * [“D”, diopter, is a unit of refractive (optical) power, equal to the reciprocal of the focal length of the lens (lens), given in meters]. The convexity of the lens can vary, and its refractive power varies between 13 and 26 D. Due to this, the lens provides accommodation of the eyeball to objects that are close or far away. When, for example, rays of light from a distant object enter a normal eye (with a relaxed ciliary muscle), the target is brought into focus on the retina. If the eye is directed to a near object, the light rays are first focused behind the retina (i.e., the image on the retina blurs) until accommodation occurs. The ciliary muscle contracts, loosening the tension of the girdle fibers, the curvature of the lens increases, and as a result, the image is focused on the retina.
The cornea and lens together form a convex lens. Rays of light from an object pass through the nodal point of the lens and form an inverted image on the retina, as in a camera. The retina processes a continuous sequence of images, and also sends messages to the brain about the movements of visual objects, threatening signs, periodic changes in light and dark, and other visual data about the external environment.
Although the optical axis of the human eye passes through the nodal point of the lens and through the point of the retina between the fovea and the optic disc, the oculomotor system orients the eyeball to the area of the object called fixation point. From this point, a beam of light passes through the nodal point and is focused in the fovea. Thus, the beam passes along the visual axis. The rays from the rest of the object are focused in the retinal area around the fovea (Fig. 5-16 A).
The focusing of rays on the retina depends not only on the lens, but also on the iris. The iris acts as the diaphragm of a camera and regulates not only the amount of light entering the eye, but, more importantly, the depth of the visual field and the spherical aberration of the lens. As the pupil diameter decreases, the depth of the visual field increases, and the light rays are directed through the central part of the pupil, where spherical aberration is minimal. Changes in pupil diameter occur automatically, i.e. reflexively, when adjusting (accommodating) the eye to the examination of close objects. Therefore, during reading or other eye activities associated with the discrimination of small objects, the image quality is improved by the optical system of the eye. Image quality is affected by another factor - light scattering. It is minimized by limiting the beam of light, as well as its absorption by the pigment of the choroid and the pigment layer of the retina. In this respect, the eye again resembles a camera. There, too, light scattering is prevented by confining the beam of rays and absorbing it by the black paint covering the inner surface of the chamber.
Image focusing is disturbed if the size of the eye does not match the refractive power of the diopter apparatus. At myopia(myopia) images of distant objects are focused in front of the retina, not reaching it (Fig. 5-16 B). The defect is corrected with concave lenses. And vice versa, when hyperopia(farsightedness) images of distant objects are focused behind the retina. Convex lenses are needed to fix the problem (Figure 5-16 B). True, the image can be temporarily focused due to accommodation, but the ciliary muscles get tired and the eyes get tired. At astigmatism there is an asymmetry between the radii of curvature of the surfaces of the cornea or lens (and sometimes the retina) in different planes. For correction, lenses with specially selected radii of curvature are used.
The elasticity of the lens gradually decreases with age. As a result, the efficiency of its accommodation decreases when viewing close objects. (presbyopia). At a young age, the refractive power of the lens can vary over a wide range, up to 14 D. By the age of 40, this range is halved, and after 50 years it drops to 2 D and below. Presbyopia is corrected with convex lenses.
Rice. 5-16. Optical system of the eye.
A - the similarity between the optical systems of the eye and the camera. B - accommodation and its violations: 1 - emmetropia - normal accommodation of the eye. Rays of light from a distant visual object are focused on the retina (upper diagram), and focusing of rays from a close object occurs as a result of accommodation (lower diagram); 2 - myopia; the image of a distant visual object is focused in front of the retina, concave lenses are needed for correction; 3 - hypermetropia; the image is focused behind the retina (upper diagram), convex lenses are required for correction (lower diagram)
hearing organ
Peripheral hearing aid, ear, subdivided into outer, middle and inner ear
(Fig. 5-17 A). outer ear
The outer ear consists of the auricle, external auditory canal and auditory canal. Ceruminous glands in the walls of the auditory canal secrete earwax- waxy protective substance. The auricle (at least in animals) directs sound into the auditory canal. Sound is transmitted through the auditory canal to the eardrum. In humans, the auditory canal has a resonant frequency of approximately 3500 Hz and limits the frequency of sounds reaching the eardrum.
Middle ear
The outer ear is separated from the middle tympanic membrane(Fig. 5-17 B). The middle ear is filled with air. A chain of bones connects the tympanic membrane to the oval window that opens into the inner ear. Not far from the oval window is a round window, which also connects the middle ear with the inner ear (Fig. 5-17 C). Both holes are sealed with a membrane. The ossicular chain includes hammer(malleus),anvil(incus) and stirrup(stapes). The base of the stirrup in the form of a plate fits tightly into the oval window. Behind the oval window is a fluid-filled vestibule(vestibulum)- part snails(cochlea) inner ear. The vestibule is integral with the tubular structure - vestibule stairs(scala vestibuli- vestibular ladder). The vibrations of the tympanic membrane, caused by sound pressure waves, are transmitted along the ossicular chain and push the stirrup plate into the oval window (Fig. 5-17 C). The movements of the stirrup plate are accompanied by fluctuations of the fluid in the vestibule ladder. Pressure waves propagate through the liquid and are transmitted through main (basilar) membrane snails to
drum stairs(scala tympani)(see below), causing the membrane of the round window to bulge towards the middle ear.
The tympanic membrane and the ossicular chain perform impedance matching. The fact is that the ear must distinguish between sound waves propagating in the air, while the mechanism of the neural transformation of sound depends on the movements of the fluid column in the cochlea. Therefore, a transition is needed from air vibrations to liquid vibrations. The acoustic impedance of water is much higher than that of air, so without a special impedance matching device, most of the sound entering the ear would be reflected. Impedance matching in the ear depends on:
the ratio of the surface areas of the tympanic membrane and the oval window;
mechanical advantage of the lever design in the form of a chain of movably articulated bones.
The efficiency of the impedance matching mechanism corresponds to a 10-20 dB improvement in audibility.
The middle ear also performs other functions. It contains two muscles: tympanic membrane muscle(m. tensor tympani- innervated by the trigeminal nerve) stirrup muscle
(m. stapedius- innervated by facial nerve The first is attached to the malleus, the second to the stirrup. Contracting, they reduce the movement of the auditory ossicles and reduce the sensitivity of the acoustic apparatus. This helps to protect hearing from damaging sounds, but only if the body expects them. A sudden explosion can damage the acoustic apparatus because the reflex contraction of the muscles of the middle ear is delayed. The middle ear cavity is connected to the pharynx by Eustachian tube. This passage equalizes the pressure in the outer and middle ear. If fluid accumulates in the middle ear during inflammation, the lumen of the Eustachian tube may close. The resulting pressure difference between the outer and middle ear causes pain due to the tension of the tympanic membrane, even rupture of the latter is possible. Pressure differences can occur in an airplane and while diving.
Rice. 5-17. Hearing.
A - General scheme of the outer, middle and inner ear. B - diagram of the tympanic membrane and the chain of auditory ossicles. B - the diagram explains how, when the oval plate of the stirrup is displaced, the fluid moves in the cochlea and the round window bends
inner ear
The inner ear consists of the bony and membranous labyrinths. They form the cochlea and the vestibular apparatus.
A snail is a tube twisted in the form of a spiral. In humans, the spiral has 2 1/2 turns; the tube begins with a wide base and ends with a narrowed apex. The cochlea is formed by the rostral end of the bony and membranous labyrinths. In humans, the apex of the cochlea is located in the lateral plane (Fig. 5-18 A).
Bone labyrinth (labyrinthus osseus) The snail includes several chambers. The space near the oval window is called the vestibule (Fig. 5-18 B). The vestibule passes into the staircase of the vestibule - a spiral tube that continues to the top of the cochlea. There, the staircase of the vestibule joins through the opening of the cochlea (helicotrema) with a drum ladder; this is another spiral tube that descends backwards along the cochlea and ends at a round window (Fig. 5-18 B). The central bone rod, around which spiral staircases are twisted, is called snail stem(modiolus cochleae).
Rice. 5-18. The structure of the snail.
A - the relative location of the cochlea and the vestibular apparatus of the middle and outer ear of a person. B - the relationship between the spaces of the cochlea
Organ of Corti
membranous labyrinth (labyrinthus membranaceus) snails are also called middle staircase(scala media) or cochlear duct(ductus cochlearis). It is a membranous flattened spiral tube 35 mm long between the scala vestibuli and the scala tympani. One wall of the middle staircase is formed by the basilar membrane, the other - Reisner membrane, third - vascular strip(stria vascularis)(Fig. 5-19 A).
The snail is filled with liquid. In the scala vestibule and the scala tympani is perilymph, close in composition to CSF. The middle staircase contains endolymph, which differs significantly from CSF. This fluid contains a lot of K+ (about 145 mM) and little Na+ (about 2 mM), so that it is similar to the intracellular environment. Since the endolymph is positively charged (about +80 mV), the hair cells inside the cochlea have a high transmembrane potential gradient (about 140 mV). Endolymph is secreted by the vascular streak, and drainage occurs through the endolymphatic duct into the venous sinuses of the dura mater.
The nervous apparatus for converting sound is called "organ of Corti"(Fig. 5-19 B). It lies at the bottom of the cochlear duct on the basilar membrane and consists of several components: three rows of outer hair cells, one row of inner hair cells, a jelly-like tectorial (integumentary) membrane, and supporting (supporting) cells of several types. The human organ of Corti contains 15,000 outer and 3,500 inner hair cells. The supporting structure of the organ of Corti is made up of columnar cells and the reticular plate (mesh membrane). From the tops of the hair cells protrude bundles of stereocilia - cilia immersed in the tectorial membrane.
The organ of Corti is innervated by nerve fibers of the cochlear part of the eighth cranial nerve. These fibers (humans have 32,000 auditory afferent axons) belong to the sensory cells of the spiral ganglion enclosed in the central bone shaft. Afferent fibers enter the organ of Corti and terminate at the bases of the hair cells (Fig. 5-19 B). The fibers supplying the outer hair cells enter through the tunnel of Corti, an opening under the columnar cells.
Rice. 5-19. Snail.
A - diagram of a transverse section through the cochlea in the foreshortening shown in the inset in Fig. 5-20 B. B - the structure of the organ of Corti
Sound transformation (transduction)
The organ of Corti transforms sound in the following way. Reaching the tympanic membrane, sound waves cause its vibrations, which are transmitted to the fluid that fills the scala vestibuli and scala tympani (Fig. 5-20 A). Hydraulic energy leads to displacement of the basilar membrane, and with it the organ of Corti (Fig. 5-20 B). The shear force developed as a result of the displacement of the basilar membrane relative to the tectorial membrane causes the stereocilia of the hair cells to bend. When the stereocilia bend towards the longest of them, the hair cell depolarizes, when they bend in the opposite direction, it hyperpolarizes.
Such changes in the membrane potential of hair cells are due to shifts in the cationic conductivity of the membrane of their apex. The potential gradient that determines the entry of ions into the hair cell is the sum of the resting potential of the cell and the positive charge of the endolymph. As noted above, the total transmembrane potential difference is approximately 140 mV. The shift in the conductivity of the membrane of the upper part of the hair cell is accompanied by a significant ion current, which creates the receptor potential of these cells. An indicator of ion current is extracellularly recorded the microphonic potential of the cochlea- oscillatory process, the frequency of which corresponds to the characteristics of the acoustic stimulus. This potential is the sum of the receptor potentials of a certain number of hair cells.
Like retinal photoreceptors, hair cells release an excitatory neurotransmitter (glutamate or aspartate) upon depolarization. Under the action of a neurotransmitter, a generator potential arises at the ends of the cochlear afferent fibers, on which the hair cells form synapses. So, the sound transformation ends with the fact that the vibrations of the basilar
membranes lead to periodic discharges of impulses in the afferent fibers of the auditory nerve. The electrical activity of many afferent fibers can be recorded extracellularly as a composite action potential.
It turned out that only a small number of cochlear afferents responded to a sound of a certain frequency. The occurrence of a response depends on the location of the afferent nerve endings along the organ of Corti, since at the same sound frequency the amplitude of the displacements of the basilar membrane is not the same in its different parts. This is partly due to differences in the width of the membrane and its tension along the organ of Corti. Previously, it was believed that the difference in resonant frequency in different parts of the basilar membrane is due to differences in the width and tension of these areas. For example, at the base of the cochlea, the width of the basilar membrane is 100 μm, and at the apex it is 500 μm. In addition, at the base of the cochlea, the membrane tension is greater than at the apex. Therefore, the area of the membrane near the base must vibrate at a higher frequency than the area at the top, like the short strings of musical instruments. However, experiments have shown that the basilar membrane oscillates as a whole and is followed by traveling waves. At high-frequency tones, the amplitude of wave-like oscillations of the basilar membrane is maximum closer to the base of the cochlea, and at low-frequency ones, at the apex. In reality, the basilar membrane acts as a frequency analyzer; the stimulus is distributed along it along the organ of Corti in such a way that hair cells of different localization respond to sounds of different frequencies. This conclusion forms the basis place theory. In addition, hair cells located along the organ of Corti are tuned to different sound frequencies due to their biophysical properties and the characteristics of stereocilia. Thanks to these factors, the so-called tonotopic map of the basilar membrane and the organ of Corti is obtained.
Rice. 5-20. Organ of Corti
Peripheral vestibular system
The vestibular system perceives the angular and linear accelerations of the head. Signals from this system trigger head and eye movements that provide a stable visual image on the retina, as well as correct body posture to maintain balance.
The structure of the vestibular labyrinth
Like the cochlea, the vestibular apparatus is a membranous labyrinth located in the bony labyrinth (Fig. 5-21 A). On each side of the head, the vestibular apparatus is formed by three semicircular canals [horizontal, vertical anterior (upper) and vertical rear] and two otolith organs. All these structures are immersed in the perilymph and filled with endolymph. The otolith organ contains utriculus(utriculus- elliptical pouch, uterus) and sacculus(sacculus- spherical bag). One end of each semicircular canal is dilated ampoules. All semicircular canals enter the utriculus. Utriculus and sacculus communicate with each other through connecting duct(ductus reuniens). It originates from endolymphatic duct(ductus endolymphaticus), ending with an endolymphatic sac that forms a connection with the cochlea. Through this connection, the endolymph secreted by the vascular stria of the cochlea enters the vestibular apparatus.
Each of the semicircular canals on one side of the head is located in the same plane as the corresponding canal on the other side. Due to this, the corresponding areas of the sensory epithelium of the two paired canals perceive head movements in any plane. Figure 5-21B shows the orientation of the semicircular canals on either side of the head; note that the cochlea is rostral to the vestibular apparatus and that the apex of the cochlea lies laterally. The two horizontal canals on either side of the head form a pair, as do the two vertical anterior and two vertical posterior canals. Horizontal channels have an interesting feature: they
are in the plane of the horizon when the head is tilted 30°. The utriculus is oriented almost horizontally, while the sacculus is oriented vertically.
The ampulla of each semicircular canal contains sensory epithelium in the form of the so-called ampullary scallop(crista ampullaris) with vestibular hair cells (a diagram of the cut through the ampullar comb is shown in Fig. 5-21 C). They are innervated by the primary afferent fibers of the vestibular nerve, which is part of the VIII cranial nerve. Each hair cell of the vestibular apparatus, like similar cells in the cochlea, carries a bundle of stereocilia (cilia) at its apex. However, unlike cochlear cells, vestibular hair cells still have a single kinocilium. All cilia of ampullar cells are immersed in a jelly-like structure - kupula, which is located across the ampoule, completely blocking its lumen. With angular (rotational) acceleration of the head, the cupula deviates; accordingly, the cilia of the hair cells are bent. The cupula has the same specific gravity (density) as the endolymph, so it is not affected by the linear acceleration created by gravity (gravitational acceleration). Figure 5-21 D, E shows the position of the cupula before turning the head (D) and during the turn (D).
The sensory epithelium of the otolith organs is elliptical pouch spot(macula utriculi) and spot of spherical pouch(macula sacculi)(Fig. 5-21 E). Each macula (spot) is lined with vestibular hair cells. Their stereocilia and kinocilium, as well as the cilia of the hair cells of the ampulla, are immersed in a jelly-like mass. The difference between the jelly-like mass of otolithic organs is that it contains numerous otoliths (the smallest "stony" inclusions) - crystals of calcium carbonate (calcite). The jelly-like mass together with its otoliths is called otolithic membrane. Due to the presence of calcite crystals, the specific gravity (density) of the otolithic membrane is about two times higher than that of the endolymph, so the otolithic membrane is easily shifted under the action of linear acceleration created by gravity. Angular acceleration of the head does not lead to such an effect, since the otolithic membrane almost does not protrude into the lumen of the membranous labyrinth.
Rice. 5-21. vestibular system.
A - the structure of the vestibular apparatus. B - top view of the base of the skull. The orientation of the structures of the inner ear is noticeable. Pay attention to the pairs of contralateral semicircular canals that are in the same plane (two horizontal, upper - anterior and lower - rear canals). B - scheme of the incision through the ampullar comb. The stereocilia and kinocilium of each hair cell are immersed in the cupula. The position of the cupula before turning the head (D) and during the turn (D). E - the structure of the otolith organs
Innervation of the sensory epithelium of the vestibular apparatus
The cell bodies of the primary afferent fibers of the vestibular nerve are located in ganglia Scarpae. Like spiral ganglion neurons, they are bipolar cells; their bodies and axons are myelinated. The vestibular nerve sends a separate branch to each macula of the sensory epithelium (Fig. 5-22A). The vestibular nerve runs along with the cochlear and facial nerves in the internal auditory canal (meatus acusticus internus) skulls.
vestibular hair cells divided into two types (Fig. 5-22 B). Type I cells are flask-shaped and form synaptic connections with the goblet endings of primary affinities.
vestibular nerve rents. Type II cells are cylindrical, their synaptic contacts are on the same primary afferents. The synapses of the vestibular efferent fibers are located at the ends of the primary afferents of type I cells. With type II cells, vestibular efferent fibers form direct synaptic contacts. Such an organization is similar to that discussed above when describing the contacts of the afferent and efferent fibers of the cochlear nerve with the internal and external hair cells of the organ of Corti. The presence of efferent nerve endings on type II cells may explain the irregular discharges in the afferents of these cells.
Rice. 5-22.
A - innervation of the membranous labyrinth. B - vestibular hair cells of types I and II. Right inset: dorsal view of stereocilia and kinocilia. Pay attention to where the contacts of the afferent and efferent fibers are located.
Transformation (transduction) of vestibular signals
Similar to cochlear hair cells, the membrane of vestibular hair cells is functionally polarized. When the stereocilia bend towards the longest cilium (kinocilia), the cationic conductivity of the cell apex membrane increases and the vestibular hair cell depolarizes (Fig. 5-23B). Conversely, when stereocilia are tilted in the opposite direction, hyperpolarization of the cell occurs. An excitatory neurotransmitter (glutamate or aspartate) is tonically (constantly) released from the hair cell, so that the afferent fiber on which this cell forms a synapse generates impulse activity spontaneously, in the absence of signals. When the cell depolarizes, the release of the neurotransmitter increases, and the frequency of discharge in the afferent fiber increases. In the case of hyperpolarization, on the contrary, a smaller amount of the neurotransmitter is released, and the discharge frequency decreases until the impulse stops completely.
Semicircular canals
As already mentioned, when turning the head, the hair cells of the ampulla receive sensory information, which they send to
brain. The mechanism of this phenomenon is that angular accelerations (turns of the head) are accompanied by flexion of the cilia on the hair cells of the ampullar comb and, as a consequence, a shift in the membrane potential and a change in the amount of the released neurotransmitter. With angular accelerations, the endolymph, due to its inertia, is displaced relative to the wall of the membranous labyrinth and presses on the cupula. The shear force causes the cilia to bend. All cilia of the cells of each ampullar comb are oriented in the same direction. In the horizontal semicircular canal, the cilia face the utriculus; in the ampullae of the other two semicircular canals, they face away from the utriculus.
Changes in the discharge of vestibular nerve afferents under the action of angular acceleration can be discussed using the example of the horizontal semicircular canal. The kinocilia of all hair cells usually face the utriculus. Consequently, when the cilia are bent towards the utriculus, the frequency of the afferent discharge increases, and when they are bent away from the utriculus, it decreases. When the head is turned to the left, the endolymph in the horizontal semicircular canals shifts to the right. As a result, the cilia of the hair cells of the left canal are bent towards the utriculus, and in the right canal - away from the utriculus. Accordingly, the discharge frequency in the afferents of the left horizontal channel increases, and in the afferents of the right it decreases.
Rice. 5-23. Mechanical transformations in hair cells.
A - hair cell;
B - Positive mechanical deformation; B - Negative mechanical deformation; D - Mechanical sensitivity of the hair cell;
D - functional polarization of vestibular hair cells. When the stereocilia are bent towards the kinocilium, the hair cell depolarizes and excitation occurs in the afferent fiber. When the stereocilia are bent away from the kinocilium, the hair cell hyperpolarizes and the afferent discharge weakens or stops.
Several important spinal reflexes are activated by muscle stretch receptors, the muscle spindles and the Golgi tendon apparatus. it muscle stretch reflex (myotatic reflex) and reverse myotatic reflex needed to maintain the posture.
Another significant reflex is the flexion reflex, which is caused by signals from various sensory receptors in the skin, muscles, joints, and internal organs. The afferent fibers that cause this reflex are often called flexion reflex afferents.
The structure and function of the muscle spindle
The structure and function of muscle spindles are very complex. They are present in most skeletal muscles, but they are especially abundant in muscles that require fine regulation of movement (for example, in the small muscles of the hand). As for large muscles, muscle spindles are most numerous in muscles containing many slow phasic fibers (type I fibers; slow twitch fibers).
The spindle consists of a bundle of modified muscle fibers innervated by both sensory and motor axons (Fig. 5-24A). The diameter of the muscle spindle is approximately 100 cm, the length is up to 10 mm. The innervated part of the muscle spindle is enclosed in a connective tissue capsule. The so-called lymphatic space of the capsule is filled with fluid. The muscle spindle is loosely located between normal muscle fibers. Its distal end is attached to endomysium- connective tissue network inside the muscle. Muscle spindles lie parallel to normal striated muscle fibers.
The muscle spindle contains modified muscle fibers called intrafusal muscle fibers unlike the usual extrafusal muscle fibers. The intrafusal fibers are much thinner than the extrafusal fibers and are too weak to participate in muscle contraction. There are two types of intrafusal muscle fibers: with a nuclear bag and with a nuclear chain (Fig. 5-24 B). Their names are associated with the organization of cell nuclei. Fibers with a nuclear bag larger than fibers
nuclear chain, and their nuclei are densely packed in the middle part of the fiber like a bag of oranges. AT nuclear chain fibers all nuclei are in one row.
Muscle spindles receive complex innervation. Sensory innervation consists of one afferent axon of group Ia and several group II afferents(Fig. 5-24 B). Group Ia afferents belong to the class of sensory axons of the largest diameter with a conduction velocity of 72 to 120 m/s; group II axons have an intermediate diameter and conduct impulses at a speed of 36 to 72 m/s. Group Ia afferent axon forms primary end, spirally wrapped around each intrafusal fiber. There are primary endings on intrafusal fibers of both types, which is important for the activity of these receptors. Group II afferents form secondary endings on fibers with a nuclear chain.
The motor innervation of muscle spindles is provided by two types of γ-efferent axons (Fig. 5-24 B). dynamicγ -efferents terminate on each fiber with a nuclear bag, staticγ -efferents- on fibers with a nuclear chain. γ-efferent axons are thinner than α-efferents of extrafusal muscle fibers, so they conduct excitation at a slower rate.
The muscle spindle responds to muscle stretch. Figure 5-24B shows the change in afferent axon activity as the muscle spindle moves from a shortened state during extrafusal contraction to a lengthened state during muscle stretch. Contraction of the extrafusal muscle fibers causes the muscle spindle to shorten as it lies parallel to the extrafusal fibers (see above).
The activity of the afferents of the muscle spindles depends on the mechanical stretching of the afferent endings on the intrafusal fibers. When the extrafusal fibers contract, the muscle fiber shortens, the distance between the coils of the afferent nerve ending decreases, and the discharge frequency in the afferent axon decreases. Conversely, when the entire muscle is stretched, the muscle spindle also lengthens (because its ends are attached to the connective tissue network inside the muscle), and stretching the afferent end increases the frequency of its impulse discharge.
Rice. 5-24. Sensory receptors responsible for inducing spinal reflexes.
A - diagram of the muscle spindle. B - intrafusal fibers with a nuclear bag and a nuclear chain; their sensory and motor innervation. C - changes in the frequency of the pulsed discharge of the afferent axon of the muscle spindle during muscle shortening (during its contraction) (a) and during muscle lengthening (during its stretching) (b). B1 - during muscle contraction, the load on the muscle spindle decreases, since it is located parallel to normal muscle fibers. B2 - when the muscle is stretched, the muscle spindle lengthens. R - recording system
Muscle stretch receptors
A known way of influencing afferents on reflex activity is through their interaction with intrafusal fibers with a nuclear bag and fibers with a nuclear chain. As mentioned above, there are two types of γ motor neurons: dynamic and static. Dynamic motor γ-axons terminate on intrafusal fibers with a nuclear bag, and static - on fibers with a nuclear chain. When the dynamic γ-motor neuron is activated, the dynamic response of the afferents of group Ia increases (Fig. 5-25 A4), and when the static γ-motor neuron is activated, the static responses of the afferents of both groups - Ia and II (Fig. 5-25 A3) increase (Fig. 5-25 A3), and at the same time can decrease dynamic response. Different descending pathways have a preferential effect on dynamic or static γ-motoneurons, thus changing the nature of the reflex activity of the spinal cord.
Golgi tendon apparatus
In skeletal muscle, there is another type of stretch receptor - golgi tendon apparatus(Fig. 5-25 B). The receptor with a diameter of about 100 μm and a length of about 1 mm is formed by the endings of group Ib afferents - thick axons with the same impulse conduction velocity as those of group Ia afferents. These endings wrap around bundles of collagen filaments in the tendon of the muscle (or in tendon inclusions within the muscle). The sensitive ending of the tendon apparatus is organized sequentially with respect to the muscle, in contrast to the muscle spindles, which lie parallel to the extrafusal fibers.
Due to its sequential arrangement, the Golgi tendon apparatus is activated either by contraction or stretching of the muscle (Fig. 5-25B). However, muscle contraction is a more effective stimulus than stretching, since the stimulus for the tendon apparatus is the force developed by the tendon in which the receptor is located. Thus, the Golgi tendon apparatus is a force sensor, in contrast to the muscle spindle, which gives signals about the length of the muscle and the rate of its change.
Rice. 5-25. Muscular stretch receptors.
A - the influence of static and dynamic γ-motor neurons on the responses of the primary ending during muscle stretching. A1 - time course of muscle stretching. A2 - group Ia axon discharge in the absence of γ-motoneuron activity. A3 - response during stimulation of a static γ-efferent axon. A4 - response during stimulation of the dynamic γ-efferent axon. B - layout of the Golgi tendon apparatus. B - activation of the Golgi tendon apparatus during muscle stretch (left) or muscle contraction (right)
The function of muscle spindles
The discharge frequency in group Ia and group II afferents is proportional to the length of the muscle spindle; this is noticeable both during linear stretching (Fig. 5-26A, left) and during muscle relaxation after stretching (Fig. 5-26A, right). Such a reaction is called static response afferents of the muscle spindle. However, primary and secondary afferent endings respond to stretch differently. Primary endings are sensitive to both the degree of stretch and its speed, while secondary endings respond primarily to the amount of stretch (Fig. 5-26A). These differences determine the nature of the activity of the endings of the two types. The frequency of the discharge of the primary ending reaches a maximum during muscle stretching, and when the stretched muscle relaxes, the discharge stops. This type of reaction is called dynamic response afferent axons of group Ia. The responses in the center of the figure (Figure 5-26A) are examples of dynamic primary ending responses. Tapping a muscle (or its tendon) or sinusoidal stretching more effectively induces a discharge in the primary afferent ending than in the secondary.
Judging by the nature of the responses, the primary afferent endings signal both the muscle length and the rate of its change, while the secondary endings transmit information only about the muscle length. These differences in the behavior of primary and secondary endings depend mainly on the difference in the mechanical properties of intrafusal fibers with a nuclear bag and with a nuclear chain. As mentioned above, primary and secondary endings are found on both types of fibers, while secondary endings are located predominantly on nuclear chain fibers. The middle (equatorial) part of the fiber with the nuclear bag is devoid of contractile proteins due to the accumulation of cell nuclei, so this part of the fiber is easily stretched. However, immediately after stretching, the middle part of the fiber with the nuclear bag tends to return to its original length, although the end parts of the fiber are elongated. The phenomenon that
called "slide" due to the viscoelastic properties of this intrafusal fiber. As a result, a burst of activity of the primary ending is observed, followed by a decrease in activity to a new static level of impulse frequency.
In contrast to nuclear bag fibers, nuclear chain fibers change in length more closely with changes in the length of extrafusal muscle fibers because the middle portion of nuclear chain fibers contains contractile proteins. Therefore, the viscoelastic characteristics of the nuclear chain fiber are more uniform, it is not prone to shedding, and its secondary afferent endings generate only static responses.
So far, we have considered the behavior of muscle spindles only in the absence of γ-motoneuron activity. At the same time, the efferent innervation of muscle spindles is extremely significant, since it determines the sensitivity of muscle spindles to stretch. For example, in fig. 5-26 B1 shows the activity of the muscle spindle afferent during continuous stretch. As already mentioned, with the contraction of the extrafusal fibers (Fig. 5-26 B2), the muscle spindles cease to experience stress, and the discharge of their afferents stops. However, the effect of muscle spindle unloading is counteracted by the effect of stimulation of γ-motoneurons. This stimulation causes the muscle spindle to shorten along with the extrafusal fibers (Figure 5-26 B3). More precisely, only two ends of the muscle spindle are shortened; its middle (equatorial) part, where the cell nuclei are located, does not contract due to the lack of contractile proteins. As a result, the middle part of the spindle lengthens, so that the afferent endings are stretched and excited. This mechanism is very important for the normal activity of muscle spindles, since as a result of descending motor commands from the brain, as a rule, simultaneous activation of α- and γ-motor neurons occurs and, consequently, conjugated contraction of extrafusal and intrafusal muscle fibers.
Rice. 5-26. Muscle spindles and their work.
A - responses of the primary and secondary endings to various types of changes in muscle length; differences between dynamic and static responses are demonstrated. The upper curves show the nature of changes in muscle length. The middle and bottom row of records are impulse discharges of primary and secondary nerve endings. B - activation of the γ-efferent axon counteracts the effect of muscle spindle unloading. B1 - pulsed discharge of the afferent of the muscle spindle with constant stretching of the spindle. B2 - the afferent discharge stopped during the contraction of the extrafusal muscle fibers, since the load was removed from the spindle. B3 - activation of the γ-motor neuron causes shortening of the muscle spindle, counteracting the effect of unloading
Myotatic reflex, or stretch reflex
The stretch reflex plays a key role in maintaining posture. In addition, its changes are involved in the implementation of motor commands from the brain. Pathological disturbances of this reflex serve as signs of neurological diseases. The reflex manifests itself in two forms: phasic stretch reflex, triggered by the primary endings of muscle spindles, and tonic stretch reflex depends on both primary and secondary endings.
phasic stretch reflex
The corresponding reflex arc is shown in Fig. 5-27. The group Ia afferent axon from the muscle spindle of the rectus femoris muscle enters the spinal cord and branches. Its branches enter the gray matter of the spinal cord. Some of them terminate directly (monosynaptically) on α-motor neurons, which send motor axons to the rectus femoris (and its synergists, such as the vastus intermedius), which extends the leg at the knee. Group Ia axons provide monosynaptic excitation of the α-motor neuron. With a sufficient level of excitation, the motor neuron generates a discharge that causes muscle contraction.
Other branches of the group Ia axon form endings on inhibitory interneurons of group Ia (such an interneuron is shown in black in Figure 5-27). These inhibitory interneurons terminate in α-motor neurons that innervate the muscles that are connected to the hamstring (including the semitendinosus), the antagonistic knee flexor muscles. When inhibitory interneurons Ia are excited, the activity of motoneurons of antagonist muscles is suppressed. Thus, the discharge (stimulatory activity) of group Ia afferents from the muscle spindles of the rectus femoris muscle causes a rapid contraction of the same muscle and
conjugate relaxation of the muscles connected to the hamstring.
The reflex arc is organized in such a way that activation of a certain group of α-motor neurons and simultaneous inhibition of an antagonistic group of neurons is ensured. It is called reciprocal innervation. It is characteristic of many reflexes, but not the only one possible in systems of regulation of movements. In some cases, motor commands cause conjugate contraction of synergists and antagonists. For example, when the hand is clenched into a fist, the extensor and flexor muscles of the hand contract, fixing the position of the hand.
Group Ia afferent impulse discharge occurs when the physician applies a light blow to the tendon of a muscle, usually the quadriceps femoris, with a neurological hammer. The normal reaction is a short-term muscle contraction.
Tonic stretch reflex
This type of reflex is activated by passive flexion of the joint. The reflex arc is the same as that of the phasic stretch reflex (Fig. 5-27), with the difference that the afferents of both groups - Ia and II - are involved. Many group II axons form monosynaptic excitatory connections with α motor neurons. Hence, the tonic stretch reflexes are mostly monosynaptic, as are the phasic stretch reflexes. Tonic stretch reflexes contribute to muscle tone.
γ - Motor neurons and stretch reflexes
γ-Motoneurons regulate the sensitivity of stretch reflexes. Muscle spindle afferents do not have a direct effect on γ-motoneurons, which are activated polysynaptically only by flexor reflex afferents at the spinal level, as well as by descending commands from the brain.
Rice. 5-27. myotatic reflex.
Arc of the stretch reflex. The interneuron (shown in black) is an inhibitory group Ia interneuron.
Reverse myotatic reflex
Activation of the Golgi tendon apparatus is accompanied by a reflex reaction, which at first glance is the opposite of the stretch reflex (in fact, this reaction complements the stretch reflex). The reaction is called reverse myotatic reflex; the corresponding reflex arc is shown in fig. 5-28. The sensory receptors for this reflex are the Golgi tendon apparatus in the rectus femoris muscle. Afferent axons enter the spinal cord, branch out, and form synaptic endings on interneurons. The path from the Golgi tendon apparatus does not have a monosynaptic connection with α-motor neurons, but includes inhibitory interneurons that suppress the activity of α-motor neurons of the rectus femoris muscle, and excitatory interneurons that cause the activity of α-motoneurons of antagonist muscles. Thus, in its organization, the reverse myotatic reflex is opposite to the stretch reflex, hence the name. However, in reality, the reverse myotatic reflex functionally complements the stretch reflex. The Golgi tendon apparatus serves as a sensor of force developed by the tendon to which it is connected. When while maintaining a stable
posture (for example, a person is standing at attention), the rectus femoris muscle begins to tire, the force applied to the knee tendon decreases and, consequently, the activity of the corresponding Golgi tendon receptors decreases. Since these receptors usually suppress the activity of α-motor neurons of the rectus femoris, the weakening of impulse discharges from them leads to an increase in the excitability of α-motor neurons, and the force developed by the muscle increases. As a result, a coordinated change in reflex reactions occurs with the participation of both muscle spindles and afferent axons of the Golgi tendon apparatus, contraction of the rectus muscle increases, and the posture is maintained.
With excessive activation of reflexes, a "jackknife" reflex can be observed. When a joint passively flexes, the resistance to such flexion initially increases. However, as the flexion continues, the resistance suddenly drops, and the joint abruptly moves into its final position. The reason for this is reflex inhibition. Previously, the jackknife reflex was explained by the activation of the Golgi tendon receptors, since it was believed that they had a high threshold for responding to muscle stretch. However, the reflex is now associated with the activation of other high-threshold muscle receptors located in the muscle fascia.
Rice. 5-28. Reverse myotatic reflex.
The arc of the reverse myotatic reflex. Both excitatory and inhibitory interneurons are involved.
Flexion reflexes
The afferent link of flexion reflexes starts from several types of receptors. During flexion reflexes, afferent discharges lead to the fact that, firstly, excitatory interneurons cause the activation of α-motor neurons supplying the flexor muscles of the ipsilateral limb, and, secondly, inhibitory neurons do not allow activation of α-motor neurons of antagonistic extensor muscles (Fig. 5-29). As a result, one or more joints are bent. In addition, commissural interneurons cause functionally opposite activity of motoneurons on the contralateral side of the spinal cord, so that the muscle is extended - a cross-extension reflex. This contralateral effect helps maintain body balance.
There are several types of flexion reflexes, although the nature of the muscle contractions corresponding to them is close. An important stage of locomotion is the flexion phase, which can be considered as a flexion reflex. It is provided mainly by the neural network of the spinal
brain called locomotor generator
cycle. However, under the influence of afferent input, the locomotor cycle can adapt to momentary changes in limb support.
The most powerful flexion reflex is flexion withdrawal reflex. It predominates over other reflexes, including locomotor reflexes, apparently for the reason that it prevents further damage to the limb. This reflex can be observed when a walking dog draws up an injured paw. The afferent link of the reflex is formed by nociceptors.
In this reflex, a strong painful stimulus causes the limb to withdraw. Figure 5-29 shows the neural network for a specific knee flexion reflex. However, in reality, during the flexion reflex, there is a significant divergence of the signals of the primary afferents and interneuronal pathways, due to which all the main joints of the limb (femoral, knee, ankle) can be involved in the withdrawal reflex. Features of the flexion withdrawal reflex in each specific case depend on the nature and localization of the stimulus.
Rice. 5-29. Flexion reflex
Sympathetic division of the autonomic nervous system
The bodies of preganglionic sympathetic neurons are concentrated in the intermediate and lateral gray matter. (intermediolateral column) thoracic and lumbar segments of the spinal cord (Fig. 5-30). Some neurons are found in C8 segments. Along with localization in the intermediolateral column, localization of preganglionic sympathetic neurons was also found in the lateral funiculus, intermediate region, and plate X (dorsal to the central canal).
Most preganglionic sympathetic neurons have thin myelinated axons - B-fibers. However, some axons are unmyelinated C-fibers. Preganglionic axons leave the spinal cord as part of the anterior root and enter the paravertebral ganglion at the level of the same segment through the white connecting branches. White connecting branches are present only at the levels T1-L2. Preganglionic axons terminate in synapses in this ganglion or, having passed through it, enter the sympathetic trunk (sympathetic chain) of the paravertebral ganglia or into the splanchnic nerve.
As part of the sympathetic chain, preganglionic axons are sent rostral or caudal to the nearest or remote prevertebral ganglion and form synapses there. After leaving the ganglion, the postganglionic axons go to the spinal nerve, usually through the gray connecting branch that each of the 31 pairs of spinal nerves has. As part of the peripheral nerves, postganglionic axons enter the effectors of the skin (piloerector muscles, blood vessels, sweat glands), muscles, and joints. Typically, postganglionic axons are unmyelinated. (FROM fibers), although there are exceptions. Differences between white and gray connecting branches depend on the relative content
they have myelinated and unmyelinated axons.
As part of the splanchnic nerve, preganglionic axons often go to the prevertebral ganglion, where they form synapses, or they can pass through the ganglion, ending in a more distant ganglion. Some preganglionic axons that run as part of the splanchnic nerve terminate directly on the cells of the adrenal medulla.
The sympathetic chain stretches from the cervical to the coccygeal level of the spinal cord. It functions as a distribution system, allowing preganglionic neurons located only in the thoracic and upper lumbar segments to activate postganglionic neurons supplying all segments of the body. However, there are fewer paravertebral ganglia than spinal segments, since some ganglia fuse during ontogenesis. For example, the superior cervical sympathetic ganglion is made up of fused C1-C4 ganglia, the middle cervical sympathetic ganglion is made up of C5-C6 ganglia, and the inferior cervical sympathetic ganglion is made up of C7-C8 ganglia. The stellate ganglion is formed by the fusion of the inferior cervical sympathetic ganglion with the T1 ganglion. The superior cervical ganglion provides postganglionic innervation to the head and neck, while the middle cervical and stellate ganglia supply the heart, lungs, and bronchi.
Normally, the axons of the preganglionic sympathetic neurons distribute to the ipsilateral ganglia and therefore regulate autonomic functions on the same side of the body. An important exception is the bilateral sympathetic innervation of the intestines and pelvic organs. As well as the motor nerves of skeletal muscles, the axons of preganglionic sympathetic neurons, related to certain organs, innervate several segments. So, preganglionic sympathetic neurons, which provide sympathetic functions of the head and neck regions, are located in the C8-T5 segments, and those related to the adrenal glands are in T4-T12.
Rice. 5-30. Autonomic sympathetic nervous system.
A are the basic principles. See the reflex arc in fig. 5-9 B
Parasympathetic division of the autonomic nervous system
Preganglionic parasympathetic neurons lie in the brainstem in several nuclei of the cranial nerves - in the oculomotor Westphal-Edinger nucleus(III cranial nerve), top(VII cranial nerve) and lower(IX cranial nerve) salivary nuclei, as well as dorsal nucleus of the vagus nerve(nucleus dorsalis nervi vagi) and double core(nucleus ambiguus) X cranial nerve. In addition, there are such neurons in the intermediate region of the sacral segments S3-S4 of the spinal cord. Postganglionic parasympathetic neurons are located in the cranial nerve ganglia: in the ciliary ganglion (ganglion ciliare), receiving preganglionic input from the Westphal-Edinger nucleus; in the pterygoid node (ganglion pterygopalatinum) and submandibular node (ganglion submandibulare) with inputs from superior salivary nucleus (nucleus salivatorius superior); in the ear (ganglion oticum) with input from inferior salivary nucleus (nucleus salivatorius inferior). The ciliary ganglion innervates the pupillary sphincter muscle and the ciliary muscles of the eye. From the pterygopalatine ganglion axons go to the lacrimal glands, as well as to the glands of the nasal and oral parts of the pharynx. The neurons of the submandibular ganglion project to the submandibular and sublingual salivary glands and glands of the oral cavity. The ear ganglion supplies the parotid salivary gland and oral glands.
(Fig. 5-31 A).
Other postganglionic parasympathetic neurons are located near the internal organs of the chest, abdominal and pelvic cavity or in the walls of these organs. Some cells of the enteric plexus can also be considered
as postganglionic parasympathetic neurons. They receive inputs from the vagus or pelvic nerves. The vagus nerve innervates the heart, lungs, bronchi, liver, pancreas, and the entire gastrointestinal tract from the esophagus to the splenic flexure of the colon. The rest of the colon, rectum, bladder, and genitals are supplied with axons from the sacral preganglionic parasympathetic neurons; these axons are distributed via the pelvic nerves to the postganglionic neurons of the pelvic ganglia.
Preganglionic parasympathetic neurons, which project to the internal organs of the chest cavity and part of the abdominal cavity, are located in the dorsal motor nucleus of the vagus nerve and in the double nucleus. The dorsal motor nucleus performs mainly secretomotor function(activates the glands), while the double core - visceromotor function(regulates the activity of the heart muscle). The dorsal motor nucleus supplies the visceral organs of the neck (pharynx, larynx), chest cavity (trachea, bronchi, lungs, heart, esophagus) and abdominal cavity (a significant part of the gastrointestinal tract, liver, pancreas). Electrical stimulation of the dorsal motor nucleus causes the secretion of acid in the stomach, as well as the secretion of insulin and glucagon in the pancreas. Although projections to the heart are anatomically traced, their function is not clear. In the double nucleus, two groups of neurons are distinguished:
Dorsal group, activates the striated muscles of the soft palate, pharynx, larynx and esophagus;
The ventrolateral group innervates the heart, slowing down its rhythm.
Rice. 5-31. Autonomic parasympathetic nervous system.
A - basic principles
autonomic nervous system
autonomic nervous system can be considered as part of the motor (efferent) system. Only instead of skeletal muscles, smooth muscles, myocardium and glands serve as effectors of the autonomic nervous system. Since the autonomic nervous system provides efferent control of visceral organs, it is often called the visceral or autonomic nervous system in foreign literature.
An important aspect of the activity of the autonomic nervous system is assistance in maintaining the constancy of the internal environment of the body. (homeostasis). When signals are received from the visceral organs about the need to adjust the internal environment, the CNS and its vegetative effector site send the appropriate commands. For example, with a sudden increase in systemic blood pressure, baroreceptors are activated, as a result of which the autonomic nervous system starts compensatory processes and normal pressure is restored.
The autonomic nervous system is also involved in adequate coordinated responses to external stimuli. So, it helps to adjust the size of the pupil in accordance with the illumination. An extreme case of autonomic regulation is the fight-or-flight response that occurs when the sympathetic nervous system is activated by a threatening stimulus. This includes a variety of reactions: the release of hormones from the adrenal glands, an increase in heart rate and blood pressure, bronchial dilatation, inhibition of intestinal motility and secretion, increased glucose metabolism, dilated pupils, piloerection, narrowing of the skin and visceral blood vessels, vasodilatation of skeletal muscles. It should be noted that the “fight or flight” response cannot be considered ordinary; it goes beyond the normal activity of the sympathetic nervous system during the normal existence of the organism.
In peripheral nerves, along with autonomic efferent fibers, afferent fibers from sensory receptors of visceral organs follow. Signals from many of these receptors trigger reflexes, but activation of some receptors causes
sensations - pain, hunger, thirst, nausea, a feeling of filling the internal organs. Visceral sensitivity can also be attributed to chemical sensitivity.
The autonomic nervous system is usually divided into sympathetic and parasympathetic.
Functional unit of the sympathetic and parasympathetic nervous system- a two-neuron efferent pathway, consisting of a preganglionic neuron with a cell body in the CNS and a postganglionic neuron with a cell body in the autonomous ganglion. The enteric nervous system includes neurons and nerve fibers of the myoenteric and submucosal plexuses in the wall of the gastrointestinal tract.
Sympathetic preganglionic neurons are located in the thoracic and upper lumbar segments of the spinal cord, so the sympathetic nervous system is sometimes referred to as the thoracolumbar division of the autonomic nervous system. The parasympathetic nervous system is arranged differently: its preganglionic neurons lie in the brainstem and in the sacral spinal cord, so it is sometimes called the craniosacral section. Sympathetic postganglionic neurons are usually located in the paravertebral or prevertebral ganglia at a distance from the target organ. As for the parasympathetic postganglionic neurons, they are located in the parasympathetic ganglia near the executive organ or directly in its wall.
The regulatory influence of the sympathetic and parasympathetic nervous systems in many organisms is often described as mutually antagonistic, but this is not entirely true. It would be more accurate to consider these two departments of the system of autonomous regulation of visceral functions as acting in a coordinated manner: sometimes reciprocally, and sometimes synergistically. In addition, not all visceral structures receive innervation from both systems. Thus, smooth muscles and skin glands, as well as most blood vessels, are innervated only by the sympathetic system; Few vessels are supplied with parasympathetic nerves. The parasympathetic system does not innervate the vessels of the skin and skeletal muscles, but supplies only the structures of the head, chest and abdominal cavity, as well as the small pelvis.
Rice. 5-32. Autonomic (autonomous) nervous system (Table 5-2)
Table 5-2.Responses of effector organs to signals from autonomic nerves *
The end of the table. 5-2.
1 A dash means that the functional innervation of the organ was not detected.
2 “+” signs (from one to three) indicate how important the activity of adrenergic and cholinergic nerves is in the regulation of specific organs and functions.
3 in situ expansion due to metabolic autoregulation predominates.
4 The physiological role of cholinergic vasodilation in these organs is controversial.
5 In the range of physiological concentrations of adrenaline circulating in the blood, skeletal muscle and liver vessels are dominated by the expansion reaction mediated by β receptors, while the vessels of other abdominal organs are dominated by the constriction reaction mediated by α receptors. In the vessels of the kidneys and mesentery there are, in addition, specific dopamine receptors that mediate expansion, which, however, does not play a big role in many physiological reactions.
6 The cholinergic sympathetic system causes vasodilation in skeletal muscle, but this effect is not involved in most physiological responses.
7 It has been hypothesized that adrenergic nerves supply inhibitory β-receptors in smooth muscle
and inhibitory α-receptors on parasympathetic cholinergic (excitatory) ganglion neurons of Auerbach's plexus.
8 Depending on the phase of the menstrual cycle, on the concentration of estrogen and progesterone in the blood, as well as on other factors.
9 Sweat glands of the palms and some other areas of the body ("adrenergic sweating").
10 The types of receptors that mediate certain metabolic responses vary significantly among animals of different species.
Neuron(neurocyte), neuronum(neurocytus), has a body, corpus, a long process-axon, axon and short branching processes-dendrites, dendrite.
Neurons form circuits that transmit a signal - a nerve impulse - from the dendrites to the body and then to the axon, which, branching out, contacts the bodies of other neurons, their dendrites or axons. Neurons are connected through the contact zone - synapse, providing transmission of a nerve impulse.
Chemical mediators usually take part in this transmission. When the pulse is transmitted, there is a slight delay in the passage of the pulse. Throughout a person's life, synapses can be destroyed and new synapses can form. With the formation of new contacts between neurons, in particular, the mechanisms of memory are associated.
Chains of neurons, including an afferent neuron, the dendrites of which have sensitive endings in various organs, and an efferent neuron, whose axon ends in the working organ (muscle, gland), are designated as the simplest reflex arcs. Usually, in a reflex arc, an impulse from a sensitive neuron is transmitted to an intercalary (associative neuron), and from the latter to an efferent (effector neuron).
Numerous connections of an associative neuron include a reflex arc in the most complex neural complexes.
The nervous system develops from the outer germ layer, the ectoderm. The anlage of the nervous system has the appearance of a neural plate, which is a thickening of the ectoderm along the dorsal surface of the body. In the future, the edges of the neural plate, becoming thinner, approach each other, while the plate itself, deepening, forms a neural groove. The edges of the plate, which have taken the form of neural folds, are connected and form a neural tube, which, plunging into the depth, is laced from the ectoderm.
At the same time, nodal (ganglion) plates are formed from the cells that make up the neural folds. Subsequently, they split: part of them, located in the form of rollers on the sides of the neural tube, closer to its dorsal surface, forms spinal nodes; the other part of the nerve cells migrates to the periphery, forming nodes of the autonomic nervous system.
Various differentiation and uneven growth of the neural tube significantly change its internal structure, appearance and shape of the cavity.
An enlarged cranial neural tube develops into brain, and the rest of it in the spinal cord.
Neural tube cells differentiate into neuroblasts, which form neurons with their processes, and into spongioblasts, which give elements of neuroglia.
Neurons develop as highly specialized cells. Through their processes, some neurons establish connections between different parts of the brain - this is intercalary (associative) neurons, others carry out the connection of the nervous system with other organs - are afferent (receptor) and efferent (effector) neurons.
Axons of afferent and efferent neurons are part of the nerves extending from the brain and spinal cord.
Neuron(from the Greek neuron - nerve) is a structural and functional unit of the nervous system. This cell has a complex structure, is highly specialized and contains a nucleus, a cell body and processes in structure. There are over 100 billion neurons in the human body.
Functions of neurons Like other cells, neurons must maintain their own structure and functions, adapt to changing conditions, and exert a regulatory influence on neighboring cells. However, the main function of neurons is the processing of information: receiving, conducting and transmitting to other cells. Information is received through synapses with receptors of sensory organs or other neurons, or directly from the external environment using specialized dendrites. Information is carried along axons, transmission - through synapses.
The structure of a neuron
cell body The body of a nerve cell consists of protoplasm (cytoplasm and nucleus), externally bounded by a membrane of a double layer of lipids (bilipid layer). Lipids consist of hydrophilic heads and hydrophobic tails, arranged in hydrophobic tails to each other, forming a hydrophobic layer that allows only fat-soluble substances (eg oxygen and carbon dioxide) to pass through. There are proteins on the membrane: on the surface (in the form of globules), on which outgrowths of polysaccharides (glycocalix) can be observed, due to which the cell perceives external irritation, and integral proteins penetrating the membrane through, they contain ion channels.
The neuron consists of a body with a diameter of 3 to 100 microns, containing a nucleus (with a large number of nuclear pores) and organelles (including a highly developed rough ER with active ribosomes, the Golgi apparatus), as well as processes. There are two types of processes: dendrites and axons. The neuron has a developed cytoskeleton that penetrates into its processes. The cytoskeleton maintains the shape of the cell, its threads serve as "rails" for the transport of organelles and substances packed in membrane vesicles (for example, neurotransmitters). In the body of the neuron, a developed synthetic apparatus is revealed, the granular ER of the neuron stains basophilically and is known as the "tigroid". The tigroid penetrates into the initial sections of the dendrites, but is located at a noticeable distance from the beginning of the axon, which serves as a histological sign of the axon. A distinction is made between anterograde (away from the body) and retrograde (towards the body) axon transport.
Dendrites and axon
Axon - usually a long process adapted to conduct excitation from the body of a neuron. Dendrites are, as a rule, short and highly branched processes that serve as the main site for the formation of excitatory and inhibitory synapses that affect the neuron (different neurons have a different ratio of the length of the axon and dendrites). A neuron may have several dendrites and usually only one axon. One neuron can have connections with many (up to 20 thousand) other neurons. Dendrites divide dichotomously, while axons give rise to collaterals. The branch nodes usually contain mitochondria. Dendrites do not have a myelin sheath, but axons can. The place of generation of excitation in most neurons is the axon hillock - a formation at the place where the axon leaves the body. In all neurons, this zone is called the trigger zone.
Synapse A synapse is a point of contact between two neurons or between a neuron and a receiving effector cell. It serves to transmit a nerve impulse between two cells, and during synaptic transmission, the amplitude and frequency of the signal can be regulated. Some synapses cause neuron depolarization, others hyperpolarization; the former are excitatory, the latter are inhibitory. Usually, to excite a neuron, stimulation from several excitatory synapses is necessary.
Structural classification of neurons
Based on the number and arrangement of dendrites and axons, neurons are divided into non-axonal, unipolar neurons, pseudo-unipolar neurons, bipolar neurons, and multipolar (many dendritic trunks, usually efferent) neurons.
Axonless neurons- small cells, grouped near the spinal cord in the intervertebral ganglia, which do not have anatomical signs of separation of processes into dendrites and axons. All processes in a cell are very similar. The functional purpose of axonless neurons is poorly understood.
Unipolar neurons- neurons with one process, are present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain.
bipolar neurons- neurons with one axon and one dendrite, located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia;
Multipolar neurons- Neurons with one axon and several dendrites. This type of nerve cells predominates in the central nervous system.
Pseudo-unipolar neurons- are unique in their kind. One process departs from the body, which immediately divides in a T-shape. This entire single tract is covered with a myelin sheath and structurally represents an axon, although along one of the branches, excitation goes not from, but to the body of the neuron. Structurally, dendrites are ramifications at the end of this (peripheral) process. The trigger zone is the beginning of this branching (that is, it is located outside the cell body). Such neurons are found in the spinal ganglia.
Functional classification of neurons By position in the reflex arc, afferent neurons (sensitive neurons), efferent neurons (some of them are called motor neurons, sometimes this is not a very accurate name applies to the entire group of efferents) and interneurons (intercalary neurons) are distinguished.
Afferent neurons(sensitive, sensory or receptor). Neurons of this type include primary cells of the sense organs and pseudo-unipolar cells, in which dendrites have free endings.
Efferent neurons(effector, motor or motor). Neurons of this type include final neurons - ultimatum and penultimate - non-ultimatum.
Associative neurons(intercalary or interneurons) - this group of neurons communicates between efferent and afferent, they are divided into commissural and projection (brain).
Morphological classification of neurons The morphological structure of neurons is diverse. In this regard, when classifying neurons, several principles are used:
take into account the size and shape of the body of the neuron,
the number and nature of branching processes,
the length of the neuron and the presence of specialized shells.
According to the shape of the cell, neurons can be spherical, granular, stellate, pyramidal, pear-shaped, fusiform, irregular, etc. The size of the neuron body varies from 5 microns in small granular cells to 120-150 microns in giant pyramidal neurons. The length of a neuron in humans ranges from 150 microns to 120 cm. The following morphological types of neurons are distinguished by the number of processes: - unipolar (with one process) neurocytes, present, for example, in the sensory nucleus of the trigeminal nerve in the midbrain; - pseudo-unipolar cells grouped near the spinal cord in the intervertebral ganglia; - bipolar neurons (have one axon and one dendrite) located in specialized sensory organs - the retina, olfactory epithelium and bulb, auditory and vestibular ganglia; - multipolar neurons (have one axon and several dendrites), predominant in the central nervous system.
Development and growth of a neuron A neuron develops from a small precursor cell that stops dividing even before it releases its processes. (However, the issue of neuronal division is currently debatable.) As a rule, the axon begins to grow first, and dendrites form later. At the end of the developing process of the nerve cell, an irregularly shaped thickening appears, which, apparently, paves the way through the surrounding tissue. This thickening is called the growth cone of the nerve cell. It consists of a flattened part of the process of the nerve cell with many thin spines. The microspinules are 0.1 to 0.2 µm thick and can be up to 50 µm in length; the wide and flat area of the growth cone is about 5 µm wide and long, although its shape may vary. The spaces between the microspines of the growth cone are covered with a folded membrane. Microspines are in constant motion - some are drawn into the growth cone, others elongate, deviate in different directions, touch the substrate and can stick to it. The growth cone is filled with small, sometimes interconnected, irregularly shaped membranous vesicles. Directly under the folded areas of the membrane and in the spines is a dense mass of entangled actin filaments. The growth cone also contains mitochondria, microtubules, and neurofilaments found in the body of the neuron. Probably, microtubules and neurofilaments are elongated mainly due to the addition of newly synthesized subunits at the base of the neuron process. They move at a speed of about a millimeter per day, which corresponds to the speed of slow axon transport in a mature neuron.
Since the average rate of advance of the growth cone is approximately the same, it is possible that neither assembly nor destruction of microtubules and neurofilaments occurs at the far end of the neuron process during the growth of the neuron process. New membrane material is added, apparently, at the end. The growth cone is an area of rapid exocytosis and endocytosis, as evidenced by the many vesicles present here. Small membrane vesicles are transported along the process of the neuron from the cell body to the growth cone with a stream of fast axon transport. Membrane material, apparently, is synthesized in the body of the neuron, transferred to the growth cone in the form of vesicles, and is included here in the plasma membrane by exocytosis, thus lengthening the process of the nerve cell. The growth of axons and dendrites is usually preceded by a phase of neuronal migration, when immature neurons settle and find a permanent place for themselves.
The functioning of the body as a whole, the interaction of its individual parts, the preservation of the constancy of the internal environment (homeostasis) are carried out by two regulatory systems: nervous and humoral.
Significance of the nervous system. The main functions of the nervous system are: 1) fast and accurate transmission of information about the state of the external and internal environment of the body; 2) analysis and integration of all information; 3) organization of adaptive response to external signals; 4) regulation and coordination of the activity of all organs and systems in accordance with the specific conditions of activity and changing factors of the external and internal environment of the body. The activity of the higher parts of the nervous system is associated with the implementation of mental processes and the organization of purposeful behavior.
The nervous system, being a single and highly integrated, on the basis of structural and functional features, is divided into two main parts - central and peripheral.
Central nervous system (CNS) includes the brain and spinal cord, where clusters of nerve cells are located - nerve centers that receive and analyze information, integrate it, regulate the integral activity of the body, and organize an adaptive response to external and internal influences.
Peripheral nervous system consists of nerve fibers located outside the central nervous system. It is represented by bundles of processes of neurons (nerve trunks) lying in the central nervous system or in ganglia (nodes) outside it (vegetative nervous system). Some of them - afferent (sensory) fibers - transmit signals from receptors located in different parts of the body to the central nervous system, others - effector (motor) fibers - from the central nervous system to the periphery. Depending on the object of innervation, peripheral nerves are divided into somatic (cranial and spinal) and autonomic (sympathetic and parasympathetic).
Neuron (neurocyte) - the main structural and functional unit of the nervous system. Neurons are highly specialized cells adapted for receiving, encoding, processing, integrating, storing and transmitting information. The neuron consists of a body and processes of two types: short branching dendrites and a long process - an axon.
Body The nerve cell has a diameter of 5 to 150 microns. It is the biosynthetic center of the neuron, where complex metabolic processes take place. The body contains a nucleus and cytoplasm, which contains many organelles involved in the synthesis of cellular proteins (proteins). A long filamentous process of axon departs from the cell body, which performs the function of transmitting information. The axon is covered with a special myelin sheath that creates optimal conditions for signal transmission. The end of the axon strongly branches, its terminal branches form contacts with many other cells (nerve, muscle, etc.). Clusters of axons form a nerve fiber. Dendrites- strongly branching processes that extend in many from the cell body. Up to 1000 dendrites can depart from one neuron. The body and dendrites are covered with a single membrane and form the receptive (receptive) surface of the cell. It contains most of the contacts from other nerve cells - synapses. cell membrane - membrane- is a good electrical insulator. On both sides of the membrane there is an electrical potential difference - the membrane potential, the level of which changes when synaptic contacts are activated.
Synapse has a complex structure. It is formed by two membranes: presynaptic and postsynaptic. The presynaptic membrane is located at the end of the axon that transmits the signal; postsynaptic - on the body or dendrites to which the signal is transmitted. In synapses, when a signal arrives, two types of chemicals are released from the synaptic vesicles - excitatory (acetylcholine, adrenaline, norepinephrine) and inhibitory (serotonin, gamma-aminobutyric acid). These substances are mediators, acting on the postsynaptic membrane, change its properties in the area of contacts. When excitatory mediators are released, an excitatory postsynaptic potential (EPSP) arises in the contact area, and under the action of inhibitory mediators, an inhibitory postsynaptic potential (IPSP) arises, respectively. Their summation leads to a change in the intracellular potential towards depolarization or hyperpolarization. When depolarized, the cell generates impulses that are transmitted along the axon to other cells or a working organ. During hyperpolarization, the neuron enters an inhibitory state and does not generate impulse activity. The multiplicity and diversity of synapses provides the possibility of wide interneuronal connections and the participation of the same neuron in different functional associations.
Classification of neurons . Having a fundamentally common structure, neurons differ greatly in size, shape, number, branching and arrangement of dendrites, length and branching of the axon, which indicates their high specialization. The following two main types of neurons are distinguished.
pyramidal cells- large neurons of different sizes ("collectors"), on which impulses from different sources converge (converge).
Dendrites pyramidal neurons are spatially organized. One process - the apical dendrite - emerges from the top of the pyramid, is oriented vertically and has terminal horizontal branches. Others - basal dendrites - branch out at the base of the pyramid. Dendrites are densely dotted with special outgrowths (spines) that increase the efficiency of synaptic transmission. Along the axons of pyramidal neurons, impulses are transmitted to other parts of the central nervous system. Pyramidal neurons are divided into two types according to their function: afferent and efferent. Afferent transmit and receive a signal from sensory receptors, muscles, internal organs to the central nervous system. Nerve cells that transmit signals from the central nervous system to the periphery are called efferent.
Insertion (contact) cells or interneurons. They are smaller in size, diverse in the spatial arrangement of processes (fusiform, star-shaped, basket-shaped). Common to them is a wide branching of dendrites and a short axon with varying degrees of branching. Interneurons provide interaction between different cells and therefore are sometimes called associative.
The representation of different types of neurons and the nature of their relationship differ significantly in different brain structures.
Age-related changes in the structure of the neuron and nerve fiber . In the early stages of embryonic development, a neuron, as a rule, consists of a body that has two undifferentiated and unbranched processes. The body contains a large nucleus surrounded by a small layer of cytoplasm. The process of maturation of neurons is characterized by a rapid increase in the cytoplasm, an increase in the number of ribosomes in it and the formation of the Golgi apparatus, the intensive growth of axons and dendrites. Different types of nerve cells mature heterochronously in ontogeny. The earliest (in the embryonic period) large afferent and efferent neurons mature. The maturation of small cells (interneurons) occurs after birth (in postnatal ontogenesis) under the influence of environmental factors, which creates the prerequisites for plastic rearrangements in the central nervous system. Separate parts of the neuron also mature unevenly. The dendritic spiny apparatus is formed the latest, the development of which in the postnatal period is largely provided by the influx of external information. The myelin sheath covering axons grows rapidly in the postnatal period, its growth leads to an increase in the speed of impulse conduction along the nerve fiber. Myelination proceeds in this order: first - the peripheral nerves, then the fibers of the spinal cord, the brain stem, the cerebellum, and later - the fibers of the cerebral hemispheres. Motor nerve fibers are covered with a myelin sheath already by the time of birth, sensitive (for example, visual) fibers - during the first months of the child's postnatal life.
The human body is a complex system in which many individual blocks and components take part. Outwardly, the structure of the body is seen as elementary and even primitive. However, if you look deeper and try to identify the schemes according to which the interaction between different organs occurs, then the nervous system will come to the fore. The neuron, which is the basic functional unit of this structure, acts as a transmitter of chemical and electrical impulses. Despite the outward resemblance to other cells, it performs more complex and responsible tasks, the support of which is important for the psychophysical activity of a person. To understand the features of this receptor, it is worth understanding its device, principles of operation and tasks.
What are neurons?
A neuron is a specialized cell that is able to receive and process information in the process of interaction with other structural and functional units of the nervous system. The number of these receptors in the brain is 10 11 (one hundred billion). At the same time, one neuron can contain more than 10 thousand synapses - sensitive endings, through which they occur. Taking into account the fact that these elements can be considered as blocks capable of storing information, it can be concluded that they contain huge amounts of information. A neuron is also called a structural unit of the nervous system, which ensures the functioning of the sense organs. That is, this cell should be considered as a multifunctional element designed to solve various problems.
Features of a neuron cell
Types of neurons
The main classification involves the division of neurons on a structural basis. In particular, scientists distinguish axon-free, pseudo-unipolar, unipolar, multipolar and bipolar neurons. It must be said that some of these species are still little studied. This refers to axon-free cells that are grouped in the region of the spinal cord. There is also controversy regarding unipolar neurons. There are opinions that such cells are not present at all in the human body. If we talk about which neurons predominate in the body of higher beings, then multipolar receptors will come to the fore. These are cells with a network of dendrites and one axon. We can say that this is a classic neuron, the most common in the nervous system.
Conclusion
Neuronal cells are an integral part of the human body. It is thanks to these receptors that the daily functioning of hundreds and thousands of chemical transmitters in the human body is ensured. At the present stage of development, science provides an answer to the question of what neurons are, but at the same time leaves room for future discoveries. For example, today there are different opinions regarding some of the nuances of the work, growth and development of cells of this type. But in any case, the study of neurons is one of the most important tasks of neurophysiology. Suffice it to say that new discoveries in this area may shed light on more effective treatments for many mental illnesses. In addition, a deep understanding of the principles of how neurons work will allow the development of tools that stimulate mental activity and improve memory in the new generation.