The concept of neuroglia.
neuroglia- These are cells that surround neurons and are part of the CNS and PNS together with them. The number of glial cells is an order of magnitude higher than the number of nerve cells.
Functions of neuroglia:
1. support - supports nerve cells
2. isolating - prevents the transition of nerve impulses from the body of one neuron to the body of another
3. regulatory - participates in the regulation of the central nervous system, in particular, ensuring the transmission of impulses in the right direction
4. trophic - participates in the metabolic processes of neurons
5. regulatory - regulates the excitability of nerve cells.
Membrane potential (or resting potential) is the potential difference between the outer and inner surface of the membrane in a state of relative physiological rest. resting potential arises due to two reasons:
1) uneven distribution of ions on both sides of the membrane;
2) selective permeability of the membrane for ions. At rest, the membrane is unequal permeable for various ions. The cell membrane is permeable to K ions, slightly permeable to Na ions, and impermeable to organic substances.
These two factors create conditions for the movement of ions. This movement is carried out without energy expenditure by passive transport - diffusion as a result of the difference in ion concentration. K ions leave the cell and increase the positive charge on the outer surface of the membrane, Cl ions passively pass into the cell, which leads to an increase in the positive charge on the outer surface of the cell. Na ions accumulate on the outer surface of the membrane and increase its positive charge. Organic compounds remain inside the cell. As a result of this movement, the outer surface of the membrane is charged positively, while the inner surface is negatively charged. The inner surface of the membrane may not be absolutely negatively charged, but it is always negatively charged with respect to the outer one. This state of the cell membrane is called the state of polarization. The movement of ions continues until the potential difference across the membrane is balanced, i.e., electrochemical equilibrium occurs. The moment of equilibrium depends on two forces:
1) diffusion forces;
2) strength electrostatic interaction. The value of electrochemical equilibrium:
1) maintenance of ionic asymmetry;
2) maintaining the value of the membrane potential at a constant level.
The diffusion force is involved in the appearance of the membrane potential (the concentration difference ions) and the strength of the electrostatic interaction, so the membrane potential is called concentration-electrochemical.
To maintain ionic asymmetry, electrochemical equilibrium is not enough. In a cage available another mechanism is the sodium-potassium pump. Sodium-potassium pump - mechanism ensuring active transport of ions. The cell membrane has system carriers, each of which binds three Na ions, which are inside the cell, and brings them out. From the outside, the carrier binds to two K ions located outside the cell and transfers them to the cytoplasm. Energy is taken from the breakdown of ATP.
2) (Resting potential mechanism)
An action potential is a shift in the membrane potential emerging in fabrics under the action of a threshold and suprathreshold stimulus, which is accompanied by a recharge of the cell membrane.
Under the action of a threshold or suprathreshold stimulus, the permeability cell membrane for ions to varying degrees. For Na ions, it increases and the gradient develops slowly. As a result, the movement of Na ions occurs inside the cell, ions To move out of the cage, what leads to recharge the cell membrane. The outer surface of the membrane is negatively charged, while the inner surface is positive.
Action potential components:
1) local response;
2) high-voltage peak potential (spike);
3) trace vibrations.
Na ions enter the cell by simple diffusion without energy expenditure. Reaching the threshold strength, the membrane potential decreases to a critical level of depolarization (about 50 mV). The critical level of depolarization is the number of millivolts that should the membrane potential decreases, so that an avalanche-like passage of Na ions into the cell occurs.
High voltage peak potential (spike).
The action potential peak is a constant component of the action potential. It consists of two phases:
1) ascending part - phases of depolarization;
2) descending part - phases of repolarization.
An avalanche-like flow of Na ions into the cell leads to a change in the potential on the cell membrane. The more Na ions enter the cell, the more the membrane depolarizes, the more activation gates open. The appearance of a charge with the opposite sign is called the inversion of the membrane potential. The movement of Na ions into the cell continues until the moment of electrochemical equilibrium for the Na ion. The amplitude of the action potential does not depend on the strength of the stimulus, it depends on the concentration of Na ions and on the degree of permeability of the membrane to Na ions. The descending phase (repolarization phase) returns the membrane charge to its original sign. When the electrochemical equilibrium for Na ions is reached, the activation gate is inactivated, the permeability to Na ions and the permeability to K ions increases. Full recovery of the membrane potential does not occur.
In the process of recovery reactions trace potentials are recorded on the cell membrane - positive and negative.
3) (Change in excitability during the passage of a wave of excitation)
With the development of the action potential, the excitability of the tissue changes, and this change proceeds in phases (Fig. 2). The state of the initial polarization of the membrane, which reflects the resting membrane potential, corresponds to the initial state of its excitability and, therefore, the cell is the normal level of excitability. During the prespike period, the excitability of the tissue is increased, this phase of excitability is called primary exaltation. During the development of the prespike, the resting membrane potential approaches the critical level of depolarization, and to achieve the latter, a stimulus strength less than the threshold (subthreshold) is sufficient.
During the development of the spike (peak potential), an avalanche-like flow of sodium ions into the cell occurs, as a result of which the membrane is recharged and it loses the ability to respond with excitation to stimuli even of a suprathreshold strength. This phase of excitability is called absolute refractoriness(absolute nonexcitability). It lasts until the end of the membrane recharge. Absolute refractoriness, i.e., complete non-excitability of the membrane, occurs due to the fact that sodium channels at the beginning are completely opened and then inactivated.
After the end of the membrane recharging phase, its excitability is gradually restored to its original level - phase of relative refractoriness. It continues until the membrane charge is restored to a value corresponding to the critical level of depolarization. Since during this period the resting membrane potential has not yet been restored, the excitability of the tissue is reduced and a new excitation can occur only under the action of a suprathreshold stimulus. The decrease in excitability in the phase of relative refractoriness is associated with partial inactivation of sodium channels and activation of potassium channels.
The period of negative trace potential corresponds to an increased level of excitability - phase of secondary exaltation. Since the membrane potential in this phase is closer to the critical level of depolarization, but compared to the state of rest (initial polarization), the threshold of irritation is reduced, i.e., excitability is increased. In this phase, a new excitation may arise under the action of stimuli of subthreshold strength. Sodium channels are not completely inactivated during this phase. During the period of development of a positive trace potential, the excitability of the tissue is reduced - phase of secondary refractoriness. In this phase, the membrane potential increases (the state of membrane hyperpolarization), moving away from the critical level of depolarization, the threshold of irritation rises and a new excitation can occur only under the action of stimuli of a superthreshold value. Membrane hyperpolarization develops due to three reasons: first, the continued release of potassium ions; secondly, the opening, possibly, of channels for chlorine and the entry of these ions into the cytoplasm of the cell; thirdly, the enhanced work of the sodium-potassium pump.
4) (Conduction of excitation along nerve fibers)
The mechanism of propagation of excitation in different nerve fibers is not the same. According to modern concepts, the propagation of excitation along nerve fibers is carried out on the basis of ionic mechanisms of action potential generation.
When excitation propagates along an unmyelinated nerve fiber, local electric currents that arise between its negatively charged and unexcited positively charged section cause membrane depolarization to a critical level, followed by generation of AP at the nearest point of the unexcited membrane section. This process is repeated many times. Throughout the entire length of the nerve fiber, a process of reproduction of a new AP occurs at each point of the fiber membrane. Such conduct of excitation is called and continuous.
The presence of a sheath in myelin fibers with a high electrical resistance, as well as sections of the fiber devoid of a sheath (interceptions of Ranvier) create conditions for a qualitatively new type of conduction of excitation along myelinated nerve fibers. Local electric currents arise between adjacent nodes of Ranvier, since the membrane of the excited node becomes negatively charged with respect to the surface of the adjacent unexcited node. These local currents depolarize the membrane of the unexcited interception to a critical level, and AP appears in it (Fig. 4). Consequently, the excitation, as it were, "jumps" over the sections of the nerve fiber covered with myelin, from one intercept to another. This propagation mechanism is called saltatory or spasmodic. The speed of this method of conducting excitation is much higher and it is more economical than continuous excitation, since not the entire membrane is involved in the active state, but only its small sections in the area of intercepts.
Rice. 4. Scheme of the propagation of excitation in unmyelinated (a) and myelinated (b) nerve fibers.
"Jumping" of the action potential through the area between interceptions is possible because the amplitude of AP is 5-6 times higher than the threshold value required to excite an adjacent interception. AP can "jump" not only through one, but also through two interceptive intervals. This phenomenon can be observed with a decrease in the excitability of the neighboring interception under the influence of any pharmacological substance, for example, novocaine, cocaine, etc.
Nerve fibers have lability- the ability to reproduce a certain number of excitation cycles per unit of time in accordance with the rhythm of the acting stimuli. The measure of lability is the maximum number of excitation cycles that a nerve fiber can reproduce per unit time without transformation of the stimulation rhythm. Lability is determined by the duration of the peak of the action potential, i.e., the phase of absolute refractoriness. Since the duration of the absolute refractoriness of the spike potential of the nerve fiber is the shortest, its lability is the highest. The nerve fiber is capable of reproducing up to 1000 impulses per second.
N. E. Vvedensky discovered that if a section of a nerve is subjected to alterations(i.e., exposure to a damaging agent) through, for example, poisoning or damage, then the lability of such a site is sharply reduced. Restoration of the initial state of the nerve fiber after each action potential in the damaged area is slow. When this area is exposed to frequent stimuli, it is not able to reproduce the given rhythm of stimulation, and therefore the conduction of impulses is blocked. This state of reduced lability was called by N. E. Vvedensky parabiosis. In the development of the state of parabiosis, three successively replacing each other phases can be noted: equalizing, paradoxical, inhibitory.
AT equalization phase there is an equalization of the magnitude of the response to frequent and rare stimuli. Under normal conditions of functioning of the nerve fiber, the magnitude of the response of the muscle fibers innervated by it obeys the law of force: for rare stimuli, the response is less, and for frequent stimuli - more. Under the action of a parabiotic agent and with a rare stimulation rhythm (for example, 25 Hz), all excitation impulses are conducted through the parabiotic site, since the excitability after the previous impulse has time to recover. With a high stimulation rhythm (100 Hz), subsequent impulses can arrive at a time when the nerve fiber is still in a state of relative refractoriness caused by the previous action potential. Therefore, part of the impulses is not carried out. If only every fourth excitation is carried out (i.e., 25 impulses out of 100), then the amplitude of the response becomes the same as for rare stimuli (25 Hz) - the response is equalized.
AT paradoxical-phase there is a further decrease in lability. At the same time, a response occurs to rare and frequent stimuli, but to frequent stimuli it is much less, because frequent stimuli further reduce lability, lengthening the phase of absolute refractoriness. Therefore, a paradox is observed - the response to rare stimuli is greater than to frequent ones.
AT braking phase lability is reduced to such an extent that both rare and frequent stimuli do not cause a response. In this case, the nerve fiber membrane is depolarized and does not go into the stage of repolarization, i.e., its original state is not restored.
The phenomenon of parabiosis underlies medical local anesthesia. The influence of anesthetic substances is also associated with a decrease in lability and a violation of the mechanism for conducting excitation along nerve fibers.
Parabiosis is a reversible phenomenon. If the parabiotic substance does not act for long, then after the termination of its action, the nerve exits the state of parabiosis through the same phases, but in reverse order.
The mechanism of development of the parabiotic state is as follows. When a nerve fiber is exposed to a parabiotic factor, the ability of the membrane to increase sodium permeability in response to irritation is disrupted. In the site of alteration, the inactivation of sodium channels caused by the damaging agent is added to the inactivation caused by the nerve impulse, and the excitability decreases to such an extent that the conduction of the next impulse is blocked.
5) (Synapses, their types, structural features)
Physiology of synapses.
In the CNS, nerve cells are connected to each other through synapses. Synapse - This is a structurally functional formation that provides the transfer of excitation or inhibition from the nerve fiber to the innervated cell.
synapses by localization They are divided into central (located within the CNS, as well as in the ganglia of the autonomic nervous system) and peripheral (located outside the CNS, provide communication with the cells of the innervated tissue).
Functionally synapses are divided into exciting, in which, as a result of depolarization of the postsynaptic membrane, an excitatory postsynaptic potential is generated, and brake, in the presynaptic endings of which a mediator is released, hyperpolarizing the postsynaptic membrane and causing the appearance of an inhibitory postsynaptic potential.
According to the transmission mechanism synapses are divided into chemical and electrical. Chemical synapses transmit excitation or inhibition due to special substances - mediators. depending on the type of mediator chemical synapses are divided into:
1. cholinergic (mediator - acetylcholine)
2. adrenergic (mediators - adrenaline, norepinephrine)
According to anatomical classification synapses are divided into neurosecretory, neuromuscular and interneuronal.
Synapse consists of three main components:
1. presynaptic membrane
2. postsynaptic membrane
3. synaptic cleft
The presynaptic membrane is the end of the process of the nerve cell. Inside the process, in the immediate vicinity of the membrane, there is an accumulation of vesicles (granules) containing one or another mediator. Bubbles are in constant motion.
The postsynaptic membrane is part of the cell membrane of the innervated tissue. The postsynaptic membrane, unlike the presynaptic membrane, has protein chemoreceptors to biologically active (mediators, hormones), medicinal and toxic substances. An important feature of the postsynaptic membrane receptors is their chemical specificity, i.e. the ability to enter into biochemical interaction only with a certain type of mediator.
The synaptic cleft is a space between pre- and postsynaptic membranes filled with a fluid similar in composition to blood plasma. Through it, the neurotransmitter slowly diffuses from the presynaptic membrane to the postsynaptic one.
Structural features of the neuromuscular synapse determine its physiological properties.
1. Unilateral conduction of excitation (from the pre- to the postsynaptic membrane), due to the presence of receptors sensitive to the mediator only in the postsynaptic membrane.
2. Synaptic delay in the conduction of excitation (the time between the arrival of an impulse at the presynaptic ending and the onset of the postsynaptic response), associated with the low rate of diffusion of the mediator into the synaptic cleft compared with the rate of passage of the impulse along the nerve fiber.
3. Low lability and high fatigue of the synapse, due to the propagation time of the previous impulse and the presence of a period of absolute refractoriness in it.
4. High selective sensitivity of the synapse to chemicals, due to the specificity of the chemoreceptors of the postsynaptic membrane.
Stages of synaptic transmission.
1. Synthesis of mediator. In the cytoplasm of neurons and nerve endings, chemical mediators are synthesized - biologically active substances. They are synthesized constantly and deposited in synaptic vesicles of nerve endings.
2. Secretion of the neurotransmitter. The release of the mediator from synaptic vesicles has a quantum character. At rest, it is insignificant, and under the influence of a nerve impulse it increases sharply.
3. Interaction of the mediator with the receptors of the postsynaptic membrane. This interaction consists in a selective change in the permeability of ion-selective channels of the effector cell in the area of active mediator-binding sites. The interaction of a mediator with its receptors can cause excitation or inhibition of a neuron, contraction of a muscle cell, the formation and release of hormones by secretory cells. In the case of an increase in the permeability of sodium and calcium channels, the flow of Na and Ca into the cell increases, followed by membrane depolarization, the occurrence of AP and further transmission of the nerve impulse. Such synapses are called excitatory. If the permeability of potassium channels and channels for chlorine increases, there is an excess release of K from the cell with simultaneous diffusion of Cl into it, which leads to hyperpolarization of the membrane, a decrease in its excitability and the development of inhibitory postsynaptic potentials. The transmission of nerve impulses becomes difficult or completely stops. Such synapses are called inhibitory.
Receptors that interact with ACh are called cholinergic receptors. Functionally, they are divided into two groups: M- and H-cholinergic receptors. In the synapses of skeletal muscles, only H-cholinergic receptors are present, while in the muscles of internal organs, predominantly M-cholinergic receptors are present.
Receptors that interact with NA are called adrenoreceptors. Functionally, they are divided into alpha and beta adrenoreceptors. In the postsynaptic membrane of the smooth muscle cells of the internal organs and blood vessels, both types of adrenoreceptors often coexist. The action of NA is depolarizing if it interacts with alpha-adrenergic receptors (contraction of the muscular membrane of the walls of blood vessels or intestines), or inhibitory - when interacting with beta-adrenergic receptors (their relaxation).
4. Mediator inactivation. Inactivation (complete loss of activity) of the neurotransmitter is necessary for repolarization of the postsynaptic membrane and restoration of the initial MP level. The most important route of inactivation is hydrolytic cleavage with inhibitors. For ACh, the inhibitor is cholinesterase, for NA and adrenaline, monoamine oxidase and catecholoxymethyltransferase.
Another way to remove the mediator from the synaptic cleft is “reuptake” by its presynaptic endings (pinocytosis) and reverse axon transport, which is especially pronounced for catecholamines.
The coordination activity of the central nervous system is based on the interaction of the processes of excitation and inhibition.
Excitation- This is an active process, which is a tissue response to irritation and is characterized by an increase in tissue functions.
Braking- This is an active process, which is a tissue response to irritation and is characterized by a decrease in tissue functions.
Primary inhibition in the CNS occurs due to inhibitory neurons. This is a special type of intercalary neurons, which, when transmitting an impulse, release an inhibitory mediator. There are two types of primary inhibition: postsynaptic and presynaptic.
Postsynaptic inhibition occurs if the axon of an inhibitory neuron forms a synapse with the body of the neuron and, releasing a mediator, causes hyperpolarization of the cell membrane, inhibiting cell activity.
presynaptic inhibition occurs when an axon of an inhibitory neuron synapses with an axon of an excitatory neuron, preventing impulse conduction.
6) (The spinal cord, its functions, participation in the regulation of muscle tone)
The spinal cord performs reflex and conduction functions. The first is provided by its nerve centers, the second by pathways.
It has a segmental structure. Moreover, the division into segments is functional. Each segment forms anterior and posterior roots. The rear ones are sensitive, i.e. afferent, anterior motor, efferent. This pattern is called the Bell-Magendie law. The roots of each segment innervate 3 metameres of the body, but as a result of overlap, each metamere is innervated by three segments. Therefore, when the anterior roots of one segment are affected, the motor activity of the corresponding metamere is only weakened.
Morphologically, the neuron bodies of the spinal cord form its gray matter. Functionally, all its neurons are divided into motor neurons, intercalary, neurons of the sympathetic and parasympathetic divisions of the autonomic nervous system. Motor neurons, depending on their functional significance, are divided into alpha and gamma motor neurons. To a-motoneurons there are fibers of afferent pathways that start from intrafusal, i.e. receptor muscle cells. The bodies of a-motoneurons are located in the anterior horns of the spinal cord, and their axons innervate skeletal muscles. Gamma motor neurons regulate the tension of muscle spindles i.e. intrafusal fibers. Thus, they are involved in the regulation of skeletal muscle contractions. Therefore, when transection of the anterior roots, muscle tone disappears.
Interneurons provide communication between the centers of the spinal cord and the overlying sections of the central nervous system.
The neurons of the sympathetic division of the autonomic nervous system are located in the lateral horns of the thoracic segments, and the parasympathetic in the sacral division.
The conductor function is to ensure the connection of peripheral receptors, centers of the spinal cord with the overlying parts of the central nervous system, as well as its nerve centers among themselves. It is carried out by conducting paths. All pathways of the spinal cord are divided into proper or propriospinal, ascending and descending. Propriospinal tracts connect the nerve centers of different segments of the spinal cord. Their function is to coordinate muscle tone, movements of various body metameres.
The ascending paths include several tracts. The Gaulle and Burdach bundles conduct nerve impulses from the proprioreceptors of the muscles and tendons to the corresponding nuclei of the medulla oblongata, and then to the thalamus and somatosensory cortical zones. Thanks to these pathways, the body posture is assessed and corrected. The Gowers and Flexig bundles transmit excitation from proprioreceptors, mechanoreceptors of the skin to the cerebellum. Due to this, the perception and unconscious coordination of the posture is ensured. The spinothalamic tracts carry signals from pain, temperature, and tactile skin receptors to the thalamus, and then to the somatosensory cortex. They provide the perception of the corresponding signals and the formation of sensitivity.
Descending paths are also formed by several tracts. Corticospinal pathways run from the pyramidal and extrapyramidal cortical neurons to the a-motoneurons of the spinal cord. Due to them, the regulation of voluntary movements is carried out. The rubrospinal pathway conducts signals from the red nucleus of the midbrain to the a-motoneurons of the flexor muscles. The vestibulospinal pathway transmits signals from the vestibular nuclei of the medulla oblongata, primarily the nucleus of Deiters, to the a-motoneurons of the extensor muscles. Due to these two ways, the tone of the corresponding muscles is regulated with changes in body position.
All reflexes of the spinal cord are divided into somatic, i.e. motor and vegetative. Somatic reflexes are divided into tendon or myotatic and cutaneous. Tendon reflexes occur with mechanical stimulation of the muscles and tendons. Their slight stretching leads to excitation of tendon receptors and a-motor neurons of the spinal cord. As a result, there is a contraction of muscles, primarily extensor muscles. Tendon reflexes include knee, achilles, ulnar, carpal, etc., arising from mechanical irritation of the corresponding tendons. For example, the knee is the simplest monosynaptic, since there is only one synapse in its central part. Skin reflexes are caused by irritation of skin receptors, but are manifested by motor reactions. They are plantar and abdominal (explanation). The spinal nerve centers are under the control of the overlying ones. Therefore, after transection between the medulla oblongata and the spinal cord, spinal shock occurs and the tone of all muscles will decrease significantly.
Vegetative reflexes of the spinal cord are divided into sympathetic and parasympathetic. Both are manifested by the reaction of internal organs to irritation of receptors in the skin, internal organs, and muscles. Vegetative neurons of the spinal cord form the lower centers of regulation of vascular tone, cardiac activity, bronchial lumen, sweating, urination, defecation, erection, ejaculation, etc.
7) (The medulla oblongata and the bridge, their functions, participation in the regulation of muscle tone)
Medulla
Features of the functional organization. The human medulla oblongata is about 25 mm long. It is a continuation of the spinal cord. Structurally, in terms of the variety and structure of the nuclei, the medulla oblongata is more complex than the spinal cord. Unlike the spinal cord, it does not have a metameric, repeatable structure; the gray matter in it is located not in the center, but with nuclei to the periphery.
In the medulla oblongata there are olives associated with the spinal cord, the extrapyramidal system and the cerebellum - this is a thin and wedge-shaped nucleus of proprioceptive sensitivity (the nucleus of Gaulle and Burdach). Here are the intersections of the descending pyramidal paths and the ascending paths formed by the thin and wedge-shaped bundles (Gaulle and Burdakh), the reticular formation.
The medulla oblongata, due to its nuclear formations and the reticular formation, is involved in the implementation of autonomic, somatic, gustatory, auditory, and vestibular reflexes. A feature of the medulla oblongata is that its nuclei, being excited sequentially, ensure the implementation of complex reflexes that require the sequential inclusion of different muscle groups, which is observed, for example, when swallowing.
The nuclei of the following cranial nerves are located in the medulla oblongata:
a pair of VIII cranial nerves - the vestibulocochlear nerve consists of the cochlear and vestibular parts. The cochlear nucleus lies in the medulla oblongata;
pair IX - glossopharyngeal nerve (p. glossopharyngeus); its core is formed by 3 parts - motor, sensory and vegetative. The motor part is involved in the innervation of the muscles of the pharynx and oral cavity, the sensitive part receives information from the taste receptors of the posterior third of the tongue; autonomic innervates the salivary glands;
pair X - the vagus nerve (n.vagus) has 3 nuclei: the autonomic innervates the larynx, esophagus, heart, stomach, intestines, digestive glands; sensitive receives information from the receptors of the alveoli of the lungs and other internal organs, and motor (the so-called mutual) provides a sequence of contraction of the muscles of the pharynx, larynx when swallowing;
pair XI - accessory nerve (n.accessorius); its nucleus is partially located in the medulla oblongata;
pair XII - hypoglossal nerve (n.hypoglossus) is the motor nerve of the tongue, its core is mostly located in the medulla oblongata.
Touch functions. The medulla oblongata regulates a number of sensory functions: the reception of skin sensitivity of the face - in the sensory nucleus of the trigeminal nerve; primary analysis of taste reception - in the nucleus of the glossopharyngeal nerve; reception of auditory stimuli - in the nucleus of the cochlear nerve; reception of vestibular stimuli - in the upper vestibular nucleus. In the posterior superior sections of the medulla oblongata, there are paths of skin, deep, visceral sensitivity, some of which switch here to the second neuron (thin and sphenoid nuclei). At the level of the medulla oblongata, the enumerated sensory functions implement the primary analysis of the strength and quality of stimulation, then the processed information is transmitted to the subcortical structures to determine the biological significance of this stimulation.
conductor functions. All ascending and descending pathways of the spinal cord pass through the medulla oblongata: spinal-thalamic, corticospinal, rubrospinal. The vestibulospinal, olivospinal and reticulospinal tracts originate in it, providing tone and coordination of muscle reactions. In the medulla, the paths from the cerebral cortex end - the corticoreticular paths. Here ends the ascending pathways of proprioceptive sensitivity from the spinal cord: thin and wedge-shaped. Brain formations such as the pons, midbrain, cerebellum, thalamus, hypothalamus, and cerebral cortex have bilateral connections with the medulla oblongata. The presence of these connections indicates the participation of the medulla oblongata in the regulation of skeletal muscle tone, autonomic and higher integrative functions, and the analysis of sensory stimuli.
reflex functions. Numerous reflexes of the medulla oblongata are divided into vital and non-vital, but such a representation is rather arbitrary. The respiratory and vasomotor centers of the medulla oblongata can be attributed to vital centers, since a number of cardiac and respiratory reflexes are closed in them.
The medulla oblongata organizes and implements a number of protective reflexes: vomiting, sneezing, coughing, tearing, closing of the eyelids. These reflexes are realized due to the fact that information about irritation of the receptors of the mucous membrane of the eye, oral cavity, larynx, nasopharynx through the sensitive branches of the trigeminal and glossopharyngeal nerves enters the nuclei of the medulla oblongata, from here comes the command to the motor nuclei of the trigeminal, vagus, facial, glossopharyngeal, accessory or hypoglossal nerves, as a result, one or another protective reflex is realized. In the same way, due to the sequential inclusion of muscle groups of the head, neck, chest and diaphragm, reflexes of eating behavior are organized: sucking, chewing, swallowing.
In addition, the medulla oblongata organizes postural reflexes. These reflexes are formed by afferentation from the receptors of the vestibule of the cochlea and the semicircular canals to the superior vestibular nucleus; from here, the processed information for assessing the need for a change in posture is sent to the lateral and medial vestibular nuclei. These nuclei are involved in determining which muscle systems, segments of the spinal cord should take part in a change in posture, therefore, from the neurons of the medial and lateral nuclei, along the vestibulospinal pathway, the signal arrives at the anterior horns of the corresponding segments of the spinal cord, innervating the muscles, whose participation in changing the posture in necessary at the moment.
Posture change is carried out due to static and statokinetic reflexes. Static reflexes regulate skeletal muscle tone in order to maintain a certain body position. The statokinetic reflexes of the medulla oblongata provide a redistribution of the tonus of the trunk muscles to organize a posture corresponding to the moment of rectilinear or rotational movement.
Most of the autonomic reflexes of the medulla oblongata are realized through the nuclei of the vagus nerve located in it, which receive information about the state of activity of the heart, blood vessels, digestive tract, lungs, digestive glands, etc. In response to this information, the nuclei organize motor and secretory reactions of these organs.
Excitation of the nuclei of the vagus nerve causes an increase in the contraction of the smooth muscles of the stomach, intestines, gallbladder and, at the same time, relaxation of the sphincters of these organs. At the same time, the work of the heart slows down and weakens, the lumen of the bronchi narrows.
The activity of the nuclei of the vagus nerve is also manifested in increased secretion of the bronchial, gastric, intestinal glands, in the excitation of the pancreas, secretory cells of the liver.
The center of salivation is localized in the medulla oblongata, the parasympathetic part of which provides an increase in general secretion, and the sympathetic part - protein secretion of the salivary glands.
The respiratory and vasomotor centers are located in the structure of the reticular formation of the medulla oblongata. The peculiarity of these centers is that their neurons are able to be excited reflexively and under the influence of chemical stimuli.
The respiratory center is localized in the medial part of the reticular formation of each symmetrical half of the medulla oblongata and is divided into two parts, inhalation and exhalation.
In the reticular formation of the medulla oblongata, another vital center is represented - the vasomotor center (regulation of vascular tone). It functions in conjunction with the overlying structures of the brain and, above all, with the hypothalamus. Excitation of the vasomotor center always changes the rhythm of breathing, the tone of the bronchi, intestinal muscles, bladder, ciliary muscle, etc. This is due to the fact that the reticular formation of the medulla oblongata has synaptic connections with the hypothalamus and other centers.
In the middle sections of the reticular formation there are neurons that form the reticulospinal pathway, which has an inhibitory effect on the motor neurons of the spinal cord. At the bottom of the IV ventricle, the neurons of the "blue spot" are located. Their mediator is norepinephrine. These neurons cause activation of the reticulospinal pathway during REM sleep, which leads to inhibition of spinal reflexes and a decrease in muscle tone.
Damage symptoms. Damage to the left or right half of the medulla oblongata above the intersection of the ascending pathways of proprioceptive sensitivity causes disturbances in the sensitivity and work of the muscles of the face and head on the side of the injury. At the same time, on the opposite side relative to the side of the injury, there are violations of skin sensitivity and motor paralysis of the trunk and limbs. This is explained by the fact that the ascending and descending pathways from the spinal cord and into the spinal cord intersect, and the nuclei of the cranial nerves innervate their half of the head, i.e., the cranial nerves do not intersect.
Bridge
The bridge (ponscerebri, ponsVarolii) is located above the medulla oblongata and performs sensory, conductive, motor, integrative reflex functions.
The structure of the bridge includes the nuclei of the facial, trigeminal, abducens, vestibulocochlear nerve (vestibular and cochlear nuclei), the nuclei of the vestibular part of the vestibulocochlear nerve (vestibular nerve): lateral (Deiters) and superior (Bekhterev). The reticular formation of the bridge is closely related to the reticular formation of the middle and medulla oblongata.
An important structure of the bridge is the middle cerebellar peduncle. It is she who provides functional compensatory and morphological connections of the cerebral cortex with the cerebellar hemispheres.
The sensory functions of the bridge are provided by the nuclei of the vestibulocochlear, trigeminal nerves. The cochlear part of the vestibulocochlear nerve ends in the brain in the cochlear nuclei; the vestibular part of the vestibulocochlear nerve - in the triangular nucleus, Deiters' nucleus, Bekhterev's nucleus. Here is the primary analysis of vestibular stimuli of their strength and direction.
The sensory nucleus of the trigeminal nerve receives signals from receptors in the skin of the face, the anterior scalp, mucous membranes of the nose and mouth, teeth, and the conjunctiva of the eyeball. The facial nerve (p. Facialis) innervates all facial muscles of the face. The abducens nerve (n. abducens) innervates the rectus lateral muscle, which abducts the eyeball outwards.
The motor portion of the trigeminal nucleus (n. trigeminus) innervates the masticatory muscles, the muscle that stretches the eardrum, and the muscle that pulls the palatine curtain.
The conductive function of the bridge. Provided with longitudinal and transverse fibers. Transversely located fibers form the upper and lower layers, and between them pass the pyramidal paths coming from the cerebral cortex. Between the transverse fibers are neuronal clusters - the nuclei of the bridge. From their neurons, transverse fibers begin, which go to the opposite side of the bridge, forming the middle cerebellar peduncle and ending in its cortex.
In the tire of the bridge there are longitudinally running bundles of fibers of the medial loop (lemniscus medialis). They are crossed by transversely running fibers of the trapezoid body (corpustrapezoideum), which are axons of the cochlear part of the vestibulocochlear nerve of the opposite side, which end in the nucleus of the superior olive (olivasuperior). From this nucleus, the paths of the lateral loop (lemniscus lateralis) go to the posterior quadrigemina of the midbrain and to the medial geniculate bodies of the diencephalon.
The anterior and posterior nuclei of the trapezoid body and the lateral loop are localized in the tegmentum of the brain. These nuclei, together with the superior olive, provide the primary analysis of information from the organ of hearing and then transmit information to the posterior colliculus of the quadrigemina.
The tegmentum also contains a long medial and tectospinal tract.
The intrinsic neurons of the pons structure form its reticular formation, the nuclei of the facial and abducens nerves, the motor portion of the nucleus, and the middle sensory nucleus of the trigeminal nerve.
The reticular formation of the bridge is a continuation of the reticular formation of the medulla oblongata and the beginning of the same midbrain system. The axons of the neurons of the reticular formation of the bridge go to the cerebellum, to the spinal cord (reticulospinal pathway). The latter activate the neurons of the spinal cord.
The pontine reticular formation affects the cerebral cortex, causing it to awaken or sleep. In the reticular formation of the bridge there are two groups of nuclei that belong to a common respiratory center. One center activates the inhalation center of the medulla oblongata, the other activates the exhalation center. The neurons of the respiratory center, located in the pons, adapt the work of the respiratory cells of the medulla oblongata in accordance with the changing state of the body.
8) (The midbrain, its functions, participation in the regulation of muscle tone)
Morphofunctional organization. The midbrain (mesencephalon) is represented by the quadrigemina and the legs of the brain. The largest nuclei of the midbrain are the red nucleus, the substantia nigra and the nuclei of the cranial (oculomotor and trochlear) nerves, as well as the nuclei of the reticular formation.
Touch functions. They are realized due to the receipt of visual, auditory information.
conductor function. It consists in the fact that all ascending paths pass through it to the overlying thalamus (medial loop, spinothalamic path), cerebrum and cerebellum. Descending paths go through the midbrain to the medulla oblongata and spinal cord. This is the pyramidal path, cortical-bridge fibers, rubroreticulospinal path.
motor function. It is realized due to the nucleus of the trochlear nerve (n. trochlearis), the nuclei of the oculomotor nerve (n. oculomotorius), the red nucleus (nucleusruber), the black substance (substantianigra).
Red nuclei are located in the upper part of the legs of the brain. They are connected with the cerebral cortex (paths descending from the cortex), the subcortical nuclei, the cerebellum, and the spinal cord (the red nuclear-spinal path). The basal ganglia of the brain, the cerebellum have their endings in the red nuclei. Violation of the connections of the red nuclei with the reticular formation of the medulla oblongata leads to decerebrate rigidity. This condition is characterized by a strong tension in the extensor muscles of the limbs, neck, and back. The main cause of decerebrate rigidity is the pronounced activating effect of the lateral vestibular nucleus (Deiters' nucleus) on the extensor motor neurons. This influence is maximal in the absence of inhibitory influences of the red nucleus and overlying structures, as well as the cerebellum. When the brain is transected below the nucleus of the lateral vestibular nerve, the decerebrate rigidity disappears.
Red nuclei, receiving information from the motor zone of the cerebral cortex, subcortical nuclei and the cerebellum about the upcoming movement and the state of the musculoskeletal system, send corrective impulses to the motor neurons of the spinal cord along the rubrospinal tract and thereby regulate muscle tone, preparing its level for the emerging voluntary movement .
Another functionally important core of the midbrain - the substantia nigra - is located in the legs of the brain, regulates the acts of chewing, swallowing (their sequence), provides precise movements of the fingers of the hand, for example, when writing. The neurons of this nucleus are able to synthesize the mediator dopamine, which is supplied by axonal transport to the basal ganglia of the brain. The defeat of the substantia nigra leads to a violation of the plastic tone of the muscles. Fine regulation of plastic tone when playing the violin, writing, performing graphic works is provided by the black substance. At the same time, when a certain posture is held for a long time, plastic changes occur in the muscles due to a change in their colloidal properties, which ensures the lowest energy costs. The regulation of this process is carried out by the cells of the substantia nigra.
The neurons of the nuclei of the oculomotor and trochlear nerves regulate the movement of the eye up, down, out, towards the nose and down to the corner of the nose. The neurons of the accessory nucleus of the oculomotor nerve (Yakubovich's nucleus) regulate the lumen of the pupil and the curvature of the lens.
reflex functions. Functionally independent structures of the midbrain are the tubercles of the quadrigemina. The upper ones are the primary subcortical centers of the visual analyzer (together with the lateral geniculate bodies of the diencephalon), the lower ones are auditory (together with the medial geniculate bodies of the diencephalon). In them, the primary switching of visual and auditory information occurs. From the tubercles of the quadrigemina, the axons of their neurons go to the reticular formation of the trunk, the motor neurons of the spinal cord. Neurons of the quadrigemina can be polymodal and detector. In the latter case, they react only to one sign of irritation, for example, a change of light and darkness, the direction of movement of a light source, etc. The main function of the colliculus of the quadrigemina is to organize the reaction of alertness and the so-called start reflexes to sudden, not yet recognized, visual or sound signals. Activation of the midbrain in these cases through the hypothalamus leads to an increase in muscle tone, increased heart rate; there is a preparation for avoidance, for a defensive reaction.
The quadrigemina organizes orienting visual and auditory reflexes.
In humans, the quadrigeminal reflex is a watchdog. In cases of increased excitability of the quadrigemina, with a sudden sound or light irritation, a person experiences a shudder, sometimes jumping to his feet, screaming, the fastest possible removal from the stimulus, sometimes an unrestrained flight.
In violation of the quadrigeminal reflex, a person cannot quickly switch from one type of movement to another. Therefore, the quadrigemina take part in the organization of voluntary movements.
Reticular formation of the brain stem
The reticular formation (formatioreticularis; RF) of the brain is represented by a network of neurons with numerous diffuse connections between themselves and with almost all structures of the central nervous system. The RF is located in the thickness of the gray matter of the medulla oblongata, middle, diencephalon and is initially associated with the RF of the spinal cord. In this regard, it is advisable to consider it as a single system. The network connections of RF neurons with each other allowed Deiters to call it the reticular formation of the brain.
RF has direct and feedback connections with the cerebral cortex, basal ganglia, diencephalon, cerebellum, middle, medulla and spinal cord.
The main function of the RF is to regulate the level of activity of the cerebral cortex, cerebellum, thalamus, and spinal cord.
On the one hand, the generalized nature of RF influence on many brain structures gave grounds to consider it a nonspecific system. However, studies with brainstem RF stimulation have shown that it can selectively exert an activating or inhibitory effect on various forms of behavior, on sensory, motor, and visceral systems of the brain. The network structure provides high reliability of RF functioning, resistance to damaging effects, since local damage is always compensated by the remaining network elements. On the other hand, the high reliability of RF functioning is ensured by the fact that irritation of any of its parts is reflected in the activity of the entire RF of the given structure due to the diffuseness of connections.
Most RF neurons have long dendrites and a short axon. There are giant neurons with long axons that form pathways from the RF to other areas of the brain, such as downstream, reticulospinal, and rubrospinal. The axons of RF neurons form a large number of collaterals and synapses, which terminate on neurons in various parts of the brain. The axons of RF neurons, going to the cerebral cortex, end here on the dendrites of layers I and II.
The activity of RF neurons is different and, in principle, similar to the activity of neurons in other brain structures, but among RF neurons there are those that have a stable rhythmic activity that does not depend on incoming signals.
At the same time, in the RF of the midbrain and pons, there are neurons that are “silent” at rest, i.e., do not generate impulses, but are excited when visual or auditory receptors are stimulated. These are the so-called specific neurons, which provide a quick response to sudden, unidentified signals. A significant number of RF neurons are polysensory.
In the RF of the medulla oblongata, midbrain and pons converge signals of different sensory. The neurons of the bridge receive signals mainly from somatosensory systems. Signals from the visual and auditory sensory systems mainly come to RF neurons in the midbrain.
The RF controls the transmission of sensory information passing through the nuclei of the thalamus, due to the fact that with intense external stimulation, the neurons of the nonspecific nuclei of the thalamus are inhibited, thereby removing their inhibitory effect from the relay nuclei of the same thalamus and facilitating the transmission of sensory information to the cerebral cortex.
In the RF of the bridge, medulla oblongata, midbrain, there are neurons that respond to pain stimuli coming from muscles or internal organs, which creates a general diffuse uncomfortable, not always clearly localized, pain sensation of "dull pain".
Repetition of any type of stimulation leads to a decrease in the impulse activity of RF neurons, i.e., the processes of adaptation (addiction) are also inherent in RF neurons of the brainstem.
The RF of the brainstem is directly related to the regulation of muscle tone, since the RF of the brainstem receives signals from the visual and vestibular analyzers and the cerebellum. From the RF to the motor neurons of the spinal cord and nuclei of the cranial nerves, signals are received that organize the position of the head, torso, etc.
Reticular pathways, which facilitate the activity of the motor systems of the spinal cord, originate from all departments of the Russian Federation. Pathways from the pons inhibit the activity of the motor neurons of the spinal cord that innervate the flexor muscles and activate the motor neurons of the extensor muscles. Pathways coming from the RF of the medulla oblongata cause opposite effects. Irritation of the RF leads to tremor, increased muscle tone. After the cessation of stimulation, the effect caused by it persists for a long time, apparently due to the circulation of excitation in the network of neurons.
RF of the brainstem is involved in the transmission of information from the cerebral cortex, spinal cord to the cerebellum and, conversely, from the cerebellum to the same systems. The function of these connections is to prepare and implement motor skills associated with addiction, orienting reactions, pain reactions, organization of walking, eye movements.
The regulation of the vegetative activity of the RF organism is described in section 4.3; here we note that this regulation is most clearly manifested in the functioning of the respiratory and cardiovascular centers. In the regulation of autonomic functions, the so-called starting RF neurons are of great importance. They give rise to the circulation of excitation within a group of neurons, providing the tone of regulated autonomic systems.
RF influences can be broadly divided into downward and upward. In turn, each of these influences has an inhibitory and exciting effect.
The ascending influences of RF on the cerebral cortex increase its tone, regulate the excitability of its neurons without changing the specificity of responses to adequate stimuli. RF affects the functional state of all sensory areas of the brain, therefore, it is important in the integration of sensory information from different analyzers.
RF is directly related to the regulation of the wake-sleep cycle. Stimulation of some structures of the RF leads to the development of sleep, stimulation of others causes awakening. G. Magun and D. Moruzzi put forward the concept that all types of signals coming from peripheral receptors reach the medulla oblongata and the pons through the RF collaterals, where they switch to neurons that give ascending paths to the thalamus and then to the cerebral cortex.
Excitation of the RF of the medulla oblongata or pons causes synchronization of the activity of the cerebral cortex, the appearance of slow rhythms in its electrical parameters, and sleep inhibition.
Excitation of the midbrain RF causes the opposite effect of awakening: desynchronization of the electrical activity of the cortex, the appearance of fast low-amplitude β-like rhythms in the electroencephalogram.
G. Bremer (1935) showed that if the brain is cut between the anterior and posterior tubercles of the quadrigemina, then the animal stops responding to all types of signals; if the transection is made between the medulla oblongata and the midbrain (while the RF retains its connection with the forebrain), then the animal reacts to light, sound, and other signals. Therefore, maintaining an active analyzing state of the brain is possible while maintaining communication with the forebrain.
The reaction of activation of the cerebral cortex is observed with RF stimulation of the medulla oblongata, midbrain, diencephalon. At the same time, irritation of some nuclei of the thalamus leads to the appearance of limited local areas of excitation, and not to its general excitation, as happens with stimulation of other parts of the RF.
RF of the brainstem can have not only an excitatory, but also an inhibitory effect on the activity of the cerebral cortex.
The descending influences of the RF of the brainstem on the regulatory activity of the spinal cord were established by I.M. Sechenov (1862). He showed that when the midbrain is irritated by salt crystals in a frog, paw withdrawal reflexes arise slowly, require stronger stimulation, or do not appear at all, i.e., they are inhibited.
G. Megun (1945-1950), applying local irritations to the RF of the medulla oblongata, found that when some points are stimulated, the forepaw flexion reflexes, knee reflexes, and corneal reflexes become sluggish. When stimulated by the RF at other points of the medulla oblongata, these same reflexes were evoked more easily, were stronger, i.e., their implementation was facilitated. According to Magun, inhibitory influences on the reflexes of the spinal cord can only be exerted by the RF of the medulla oblongata, while facilitating influences are regulated by the entire RF of the stem and spinal cord.
9) (The cerebellum, its participation in the regulation of motor and autonomic functions)
The cerebellum (cerebellum, small brain) is one of the integrative structures of the brain, which is involved in the coordination and regulation of voluntary, involuntary movements, in the regulation of autonomic and behavioral functions.
Features of the morphofunctional organization and connection of the cerebellum. The implementation of these functions is provided by the following morphological features of the cerebellum:
1) the cerebellar cortex is built quite uniformly, has stereotyped connections, which creates conditions for fast information processing;
2) the main neural element of the cortex, the Purkinje cell, has a large number of inputs and forms the only axon output from the cerebellum, the collaterals of which end at its nuclear structures;
3) almost all types of sensory stimuli are projected onto Purkinje cells: proprioceptive, skin, visual, auditory, vestibular, etc.;
4) exits from the cerebellum provide its connections with the cerebral cortex, with stem formations and the spinal cord.
The cerebellum is anatomically and functionally divided into old, ancient and new parts.
The old part of the cerebellum (archicerebellum) - the vestibular cerebellum - includes the flocculo-floccular lobe. This part has the most pronounced connections with the vestibular analyzer, which explains the importance of the cerebellum in the regulation of balance.
The ancient part of the cerebellum (paleocerebellum) - the spinal cerebellum - consists of sections of the vermis and the pyramid of the cerebellum, uvula, parietal division and receives information mainly from the proprioceptive systems of muscles, tendons, periosteum, and joint membranes.
The new cerebellum (neocerebellum) includes the cortex of the cerebellar hemispheres and sections of the worm; it receives information from the cortex, mainly via the fronto-cerebellopontine pathway, from visual and auditory receptor systems, which indicates its participation in the analysis of visual and auditory signals and the organization of the reaction to them.
The cerebellar cortex has a specific structure that is not repeated anywhere in the central nervous system. The upper (I) layer of the cerebellar cortex is a molecular layer, consists of parallel fibers, branchings of dendrites and axons of layers II and III. In the lower part of the molecular layer, basket and stellate cells are found, which provide interaction between Purkinje cells.
The middle (II) layer of the cortex is formed by Purkinje cells lined up in one row and having the most powerful dendritic system in the CNS. On the dendritic field of one Purkinje cell, there can be up to 60,000 synapses. Therefore, these cells perform the task of collecting, processing and transmitting information. The axons of Purkinje cells are the only way in which the cerebellar cortex transmits information to its nuclei and the nuclei of the brain structure.
Under the II layer of the cortex (under the Purkinje cells), there is a granular (III) layer, consisting of granule cells, the number of which reaches 10 billion. The axons of these cells rise up, divide in a T-shape on the surface of the cortex, forming contact paths with Purkinje cells. Here are the Golgi cells.
Information leaves the cerebellum through the upper and lower legs. Through the upper legs, the signals go to the thalamus, the pons, the red nucleus, the nuclei of the brain stem, and the reticular formation of the midbrain. Through the lower legs of the cerebellum, the signals go to the medulla oblongata to its vestibular nuclei, olives, and the reticular formation. The middle cerebellar peduncle connects the new cerebellum with the frontal lobe of the brain.
The impulse activity of neurons is recorded in the layer of Purkinje cells and the granular layer, and the frequency of generation of impulses of these cells ranges from 20 to 200 per second. The cells of the cerebellar nuclei generate impulses much less frequently - 1-3 impulses per second.
Stimulation of the upper layer of the cerebellar cortex leads to prolonged (up to 200 ms) inhibition of Purkinje cell activity. Their same inhibition occurs with light and sound signals. The total changes in the electrical activity of the cerebellar cortex on irritation of the sensory nerve of any muscle look like a positive oscillation (inhibition of cortical activity, hyperpolarization of Purkinje cells), which occurs after 15-20 ms and lasts 20-30 ms, after which a wave of excitation occurs, lasting up to 500 ms (depolarization of Purkinje cells).
Signals from skin receptors, muscles, articular membranes, and periosteum enter the cerebellar cortex through the so-called spinal cerebellar tracts: along the posterior (dorsal) and anterior (ventral) tracts. These paths to the cerebellum pass through the inferior olive of the medulla oblongata. From the olive cells come the so-called climbing fibers that branch on the dendrites of the Purkinje cells.
The nuclei of the bridge send afferent pathways to the cerebellum, forming mossy fibers that terminate on the granule cells of layer III of the cerebellar cortex. Between the cerebellum and the bluish part of the midbrain there is an afferent connection with the help of adrenergic fibers. These fibers are capable of diffusely ejecting norepinephrine into the intercellular space of the cerebellar cortex, thereby humorally changing the state of excitability of its cells.
Axons of cells of the third layer of the cerebellar cortex cause inhibition of Purkinje cells and granule cells of their own layer.
Purkinje cells, in turn, inhibit the activity of neurons in the cerebellar nuclei. The nuclei of the cerebellum have a high tonic activity and regulate the tone of a number of motor centers of the intermediate, middle, medulla oblongata, and spinal cord.
The subcortical system of the cerebellum consists of three functionally different nuclear formations: the tent nucleus, the corky, spherical, and dentate nuclei.
The tent nucleus receives input from the medial cerebellar cortex and is connected to the Deiters nucleus and RF of the medulla and midbrain. From here, the signals travel along the reticulospinal pathway to the motor neurons of the spinal cord.
The intermediate cortex of the cerebellum projects to the cork and globular nuclei. From them, connections go to the midbrain to the red nucleus, then to the spinal cord along the rubrospinal path. The second path from the intermediate nucleus goes to the thalamus and further to the motor cortex.
The dentate nucleus, receiving information from the lateral zone of the cerebellar cortex, is connected with the thalamus, and through it - with the motor zone of the cerebral cortex.
Cerebellar control of motor activity. Efferent signals from the cerebellum to the spinal cord regulate the strength of muscle contractions, provide the ability for prolonged tonic muscle contraction, the ability to maintain optimal muscle tone at rest or during movements, to balance voluntary movements with the purpose of this movement, to quickly switch from flexion to extension and vice versa.
The cerebellum provides synergy of contractions of different muscles during complex movements. For example, when taking a step while walking, a person brings his leg forward, at the same time the center of gravity of the body is transferred forward with the participation of the back muscles. In cases where the cerebellum does not fulfill its regulatory function, a person has disorders of motor functions, which is expressed by the following symptoms.
1) asthenia (asthenia - weakness) - a decrease in the strength of muscle contraction, rapid muscle fatigue;
2) astasia (astasia, from Greek a - not, stasia - standing) - loss of the ability to prolonged muscle contraction, which makes it difficult to stand, sit, etc .;
3) dystonia (distonia - violation of tone) - an involuntary increase or decrease in muscle tone;
4) tremor (tremor - trembling) - trembling of the fingers, hands, head at rest; this tremor is aggravated by movement;
5) dysmetria (dismetria - violation of the measure) - a disorder of the uniformity of movements, expressed either in excessive or insufficient movement. The patient tries to take an object from the table and brings his hand behind the object (hypermetry) or does not bring it to the object (hypometry);
6) ataxia (ataksia, from Greek a - negation, taksia - order) - impaired coordination of movements. Here, the impossibility of performing movements in the right order, in a certain sequence, is most clearly manifested. Manifestations of ataxia are also adiadochokinesis, asynergy, drunk-shaky gait. With adiadochokinesis, a person is not able to quickly rotate his palms up and down. With muscle asynergy, he is unable to sit up from a prone position without the help of hands. The drunken gait is characterized by the fact that a person walks with his legs wide apart, staggering from side to side from the line of walking. There are not so many innate motor acts in a person (for example, sucking), but he learns most of the movements during his life and they become automatic.
Reticular formation - a set of various located throughout the brain stem, which have an activating or inhibitory effect on various structures of the central nervous system, thereby controlling their reflex activity.
The reticular formation of the brain stem has an activating effect on the cells and an inhibitory effect on the motor neurons of the spinal cord. By sending inhibitory and excitatory impulses to the spinal cord to its motor neurons, the reticular formation is involved in the regulation of skeletal muscle tone.
The reticular formation maintains the tone of the vegetative centers, integrates sympathetic and parasympathetic influences, and transmits the modulating influence from the hypothalamus and cerebellum to the internal organs.
Functions of the reticular formation
Somatomotor control(activation of skeletal muscles), can be direct through tr. reticulospinalis and indirect through, olives, tubercles of the quadrigemina, red nucleus, black substance, striatum, nuclei of the thalamus and even somatomotor cortical zones.
Somatosensory control, i.e. decrease in levels of somatosensory information - "slow pain", modification of the perception of various types of sensory sensitivity (hearing, vision, vestibulation, smell).
Visceromotor control the state of the cardiovascular, respiratory systems, the activity of smooth muscles of various internal organs.
Neuroendocrine transduction through the influence on neurotransmitters, centers of the hypothalamus and further the pituitary gland.
Biorhythms through connections with the hypothalamus and pineal gland.
Various functional states of the body(sleep, awakening, state of consciousness, behavior) are carried out through numerous connections of the nuclei of the reticular formation with all parts of the central nervous system.
Coordination works of different brain stem centers providing complex visceral reflex responses (sneezing, coughing, vomiting, yawning, chewing, sucking, swallowing, etc.).
The structure of the reticular formation
Reticular formation made up of many neurons lying separately or grouped into nuclei (see Fig. 1 and 2). Its structures are localized in the central regions of the trunk, starting from the upper segments of the cervical spinal cord to the upper level of the brainstem, where they gradually merge with nuclear groups. The reticular formation occupies the spaces between the nuclei of the cranial nerves, other nuclei and tracts passing through the brainstem.
Neurons of the reticular formation are characterized by a wide variety of shapes and sizes, but their common feature is that they form numerous synaptic contacts both among themselves and with neurons of other brain nuclei with long dendrites and widely branching axons. These branches form a kind of network ( reticulum), where the name came from - the reticular formation. The neurons that form the nuclei of the reticular formation have long axons, forming pathways to the spinal cord, brainstem nuclei, and other areas of the brain.
Rice. 1. The most important structural formations of the midbrain (cross section)
The neurons of the reticular formation receive numerous afferent signals from various structures of the CNS. There are several groups of neurons to which these signals are received. This is group of neurons in the lateral nucleus reticular formation located in the medulla oblongata. The neurons of the nucleus receive afferent signals from the intercalary neurons of the spinal cord and are part of one of the indirect spinocerebellar pathways. In addition, they receive signals from the vestibular nuclei and can integrate information about the state of activity of the interneurons associated with the motor neurons of the spinal cord, and about the position of the body and head in space.
The next group is neurons of the reticulotegmental nucleus located on the border of the dorsal edge of the bridge. They receive afferent synaptic inputs from neurons in the pretectal nuclei and superior colliculi and send their axons to cerebellar structures involved in eye movement control.
Neurons of the reticular formation receive a variety of signals through pathways that connect them to the cerebral cortex (corticoreticulospinal pathways), substantia nigra, and.
Rice. 2. The location of some nuclei in the brain stem and hypothalamus: 1 - paraventricular; 2 - dorsomedial: 3 - preoptic; 4 - supraoptic; 5 - back
In addition to the described afferent pathways, the reticular formation receives signals via axon collaterals pathways of sensory systems. At the same time, signals from different receptors (tactile, visual, auditory, vestibular, pain, temperature, proprioreceptors, receptors of internal organs) can converge to the same one.
From the above list of the main afferent connections of the reticular formation with other areas of the CNS, it can be seen that the state of its tonic neuronal activity is determined by the influx of almost all types of sensory signals from sensitive neurons, as well as signals from most CNS structures.
Classification of the reticular formation depending on the direction of the fibers|
Departments |
Characteristic |
|
Descending department |
Vegetative centers:
Motor centers:
|
|
ascending division |
Reticulothalamic ReticulologyPothalamic Reticulocerebellar Reticulocortical: activating; hypnogenic |
The nuclei of the reticular formation and their functions
For a long time it was believed that the reticular formation, whose structure is characterized by wide interneuronal connections, integrates signals of various modalities without highlighting specific information. However, it is becoming more and more obvious that the reticular formation is not only morphologically, but also functionally heterogeneous, although the differences between the functions of its individual parts are not as obvious as is typical for other areas of the brain.
Indeed, many neuronal groups of the reticular formation form its nuclei (centers), which perform specific functions. These are the neuronal groups that form vasomotor center medulla oblongata (giant cell, paramedian, lateral, ventral, caudal nuclei of the medulla oblongata), respiratory center(giant cell, small cell nuclei of the medulla oblongata, oral and caudal nuclei of the bridge), chewing centers and swallowing(lateral, paramedian nucleus of the medulla oblongata), eye movement centers(paramedian part of the bridge, rostral part of the midbrain), muscle tone control centers(the rostral nucleus of the bridge and the caudal nucleus of the medulla oblongata), etc.
One of the most important nonspecific functions of the reticular formation is regulation of general neuronal activity of the cortex and other structures of the CNS. In the reticular formation, the biological significance of incoming sensory signals is assessed, and depending on the results of this assessment, it can activate or inhibit neuronal processes through nonspecific or specific neural groups of the thalamus in the entire cerebral cortex or in all individual zones. Therefore, the stem reticular formation is also called trunk activating system brain. Due to these properties, the reticular formation can influence the level of general activity of the cortex, the maintenance of which is the most important condition for maintaining consciousness, the state of wakefulness, and the formation of the focus of attention.
An increase in the activity of the reticular formation (against a generally high background) in individual sensory, associative areas of the cortex makes it possible to isolate and process specific, most important information for the body at a given moment in time and organize adequate response behavioral reactions. Usually these reactions, organized with the participation of the reticular formation of the brain stem, are preceded by orientational movements of the eyes, head and body in the direction of the signal source, changes in respiration and blood circulation.
The activating effect of the reticular formation on the cortex and other structures of the central nervous system is carried out along ascending pathways coming from the giant cell, lateral and ventral reticular nuclei of the medulla oblongata, as well as from the nuclei of the pons and midbrain. Along these pathways, the flows of nerve impulses are conducted to the neurons of the nonspecific nuclei of the thalamus and, after their processing, are switched in the thalamic nuclei for subsequent transmission to the cortex. In addition, signal flows are conducted from the listed reticular nuclei to the neurons of the posterior hypothalamus and basal ganglia.
In addition to regulating the neural activity of the higher parts of the brain, the reticular formation can regulate sensory functions. This is done by influencing the conduction of afferent signals to the nerve centers, the excitability of the neurons of the nerve centers, and the sensitivity of the receptors. An increase in the activity of the reticular formation is accompanied by an increase in the activity of the neurons of the sympathetic nervous system, which innervates the sense organs. As a result, visual acuity, hearing, tactile sensitivity may increase.
Along with ascending activating and inhibitory influences on the higher parts of the brain, the reticular formation takes part in regulation of movements, providing activating and inhibitory effects on the spinal cord. Its nuclei switch both ascending pathways from proprioreceptors and the spinal cord to the brain, and descending motor pathways from the cerebral cortex, basal ganglia, cerebellum, and red nucleus. Although the ascending neural pathways from the reticular formation to the thalamus and cortex play a role primarily in maintaining the general level of activity of the cerebral cortex, it is this function that is important for the waking cortex to plan, launch, execute movements and control their execution. There are a large number of collateral connections between the ascending and descending pathways through the reticular formation, through which they can exert mutual influence. The existence of such close interaction creates conditions for the mutual influence of the area of the reticular formation, which influences the activity of the cortex through the thalamus, planning and initiating movements, and the area of the reticular formation, which affects the executive neural mechanisms of the spinal cord. In the reticular formation there are groups of neurons that send most of the axons to the cerebellum, which is involved in the regulation and coordination of complex movements.
Through the descending reticulospinal pathways, the reticular formation directly affects the functions of the spinal cord. Direct influence on its motor centers is carried out by medial reticulospinal tract coming from the nuclei of the bridge and activating mainly the inter- and y-motor neurons of the extensor and inhibiting the motor neurons of the flexor muscles of the trunk and limbs. By lateral reticulospinal tract, starting from the giant cell nucleus of the medulla oblongata, the reticular formation has an activating effect on the inter- and y-motor neurons of the limb flexor muscles and an inhibitory effect on the neurons of the extensor muscles.
From experimental observations on animals, it is known that stimulation of more rostrally located neurons of the reticular formation at the level of the medulla oblongata and midbrain has a diffuse facilitating effect on spinal reflexes, and stimulation of neurons in the caudal part of the medulla oblongata is accompanied by inhibition of syinal reflexes.
The activating and inhibitory influence of the reticular formation on the motor centers of the spinal cord can be carried out through y-motoneurons. At the same time, the reticular neurons of the rostral region of the reticular formation activate y-motoneurons, which innervate the intrafusal muscle fibers with their axons, cause their contraction, and activate the muscle spindle receptors. The flow of signals from these receptors activates a-motoneurons and causes the contraction of the corresponding muscle. The neurons of the caudal part of the reticular formation inhibit the activity of the y-motor neurons of the spinal cord and cause muscle relaxation. The distribution of tone in large muscle groups depends on the balance of neuronal activity in these areas of the reticular formation. Since this balance depends on descending influences on the reticular formation of the cerebral cortex, basal ganglia, hypothalamus, cerebellum, these brain structures can also influence the distribution of muscle tone and body posture through the reticular formation and other nuclei of the brain stem.
The wide branching of the axons of the reticulospinal tracts in the spinal cord creates conditions for the influence of the reticular formation on almost all motor neurons and, accordingly, on the state of the muscles of various parts of the body. This feature provides an effective effect of the reticular formation on the reflex distribution of muscle tone, posture, orientation of the head and body in the direction of action of external stimuli and the participation of the reticular formation in the implementation of voluntary movements of the muscles of the proximal parts of the body.
In the central part of the reticular giant cell nucleus there is a site, the irritation of which inhibits all motor reflexes of the spinal cord. The presence of such inhibition of brain structures on the spinal cord was discovered by I.M. Sechenov in experiments on frogs. The essence of the experiments was to study the state of spinal cord reflexes after crossing the brain stem at the level of the diencephalon and stimulating the caudal section of the cut with a salt crystal. It turned out that motor spinal reflexes did not appear during stimulation or became weakened and recovered after elimination of stimulation. Thus, it was revealed for the first time that one nerve center can inhibit the activity of another. This phenomenon has been called central braking.
The reticular formation plays an important role in the regulation of not only somatic, but also vegetative functions (the reticular nuclei of the brain stem are part of the structure of the vital parts of the respiratory center and the centers for regulating blood circulation). The lateral group of the reticular nuclei of the pons and the dorsolateral nucleus of the tegmentum form urinary center of the bridge. The axons of the neurons of the nuclei of this center reach the preganglionic neurons of the sacral spinal cord. Stimulation of the neurons of these nuclei in the bridge is accompanied by contraction of the muscles of the bladder wall and urination.
The parabrachial nucleus is located in the dorsolateral bridge, on the neurons of which the fibers of taste sensory neurons end. The neurons of the nucleus, like the neurons of the bluish macula and substantia nigra, contain neuromelanin. The number of such neurons in the parabrachial nucleus decreases in Parkinson's disease. The neurons of the parabrachial nucleus have connections with the neurons of the hypothalamus, amygdala, nuclei of the raphe, solitary tract, and other nuclei of the brainstem. It is assumed that parabrachial nuclei are related to the regulation of vegetative functions and a decrease in their number in parkinsonism explains the occurrence of autonomic disorders in this disease.
In experiments on animals, it was shown that stimulation of certain local areas of the reticular structures of the medulla oblongata and the pons can cause inhibition of cortical activity and sleep. At the same time, low-frequency (1-4 Hz) waves appear on the EEG. Based on the facts described, it is believed that the most important functions of the ascending influences of the reticular formation are the regulation of the sleep-wake cycle and the level of consciousness. It turned out that a number of nuclei of the reticular formation of the brain stem are directly related to the formation of these states.
So, on each side of the central suture of the bridge are paramedian reticular nuclei, or seam core containing serotonergic neurons. In the caudal part of the bridge, they include the lower central nucleus, which is a continuation of the nucleus of the raphe of the medulla oblongata, and in the rostral part of the bridge, the nucleus of the raphe bridge includes the upper central nucleus, called Bekhterev's nucleus, or the median nucleus of the raphe.
In the rostral part of the pons, on the dorsal side of the tegmentum, there is a group of nuclei bluish spot. They have about 16,000-18,000 melanin-containing noradrenergic neurons, the axons of which are widely represented in various parts of the central nervous system - the hypothalamus, hippocampus, cerebral cortex, cerebellum and spinal cord. The bluish macula extends into the midbrain, and its neurons can be traced in the ssry substance of the periaqueductal space. The number of neurons in the nuclei of the bluish spot decreases in parkinsonism, Alzheimer's disease and Down's syndrome.
Both serotonergic and norepinephrine neurons of the reticular formation play a role in the control of the sleep-wake cycle. Suppression of serotonin synthesis in the raphe nuclei leads to the development of insomnia. Serotonergic neurons are thought to be part of the neural network that regulates slow-wave sleep. Under the action of serotonin on the neurons of the bluish spot, paradoxical sleep occurs. The destruction of the nuclei of the bluish spot in experimental animals does not lead to the development of insomnia, but causes the disappearance of the paradoxical sleep phase for several weeks.
Reticular formation (from lat. reticulum - mesh, formatio - education)
reticular formation, a set of nerve structures located in the central parts of the brain stem (medulla oblongata and midbrain, visual tubercles). Neuron s ,
components of R. f., are diverse in size, structure and length of axons ;
their fibers are densely intertwined. The term R. f.", introduced by the German scientist O. Deiters, reflects only its morphological features. R. f. morphologically and functionally connected with the spinal cord, cerebellum (See Cerebellum), limbic system (See Limbic system) and the cerebral cortex. In the area of R. f. the interaction of both ascending - afferent, and descending - efferent impulses entering it is carried out. Circulation of impulses through closed neural circuits is also possible. Thus, there is a constant level of excitation of R. f. neurons, as a result of which the tone and a certain degree of readiness for the activity of various parts of the central nervous system are provided. Degree of excitation R. f. regulated by the cerebral cortex (See. Cerebral cortex).
Downward Influences. In R. f. distinguish areas that have inhibitory and facilitating effects on the motor responses of the spinal cord (See Spinal cord) ( rice. one
). The relationship between stimulation of various regions of the brainstem and spinal reflexes was first noted in 1862 by IM Sechenov. In 1944-46, the American neurophysiologist H. Magone and his co-workers showed that stimulation of various areas of R. f. medulla oblongata has a facilitating or inhibitory effect on the motor reactions of the spinal cord. Electrical stimulation of the medial part of R. t. medulla oblongata in anesthetized and decerebrated cats and monkeys is accompanied by a complete cessation of movements caused both reflexively and by stimulation of the motor areas of the cerebral cortex. All inhibitory effects are bilateral, but on the side of irritation, such an effect is often observed at a lower threshold of irritation. Some manifestations of inhibitory influences R. t. medulla oblongata correspond to the picture of central inhibition described by Sechenov (see Sechenov's inhibition). Irritation of the lateral region R. t. medulla oblongata along the periphery of the area that has inhibitory effects, is accompanied by a facilitating effect on the motor activity of the spinal cord. The area of R. f., which has facilitating effects on the spinal cord, is not limited to the medulla oblongata, but extends anteriorly, capturing the region of the pons and midbrain. R. f. can affect various formations of the spinal cord, for example, alpha motor neurons that innervate the main (extrafusal) muscle fibers involved in voluntary movements. An increase in the latent periods of responses of motor neurons during stimulation of the inhibitory sections of R. t. suggests that the inhibitory effects of reticular structures on the motor responses of the spinal cord are carried out with the help of intercalary neurons, possibly Renshaw cells. The mechanism of influence of R. f. on muscle tone revealed by the Swedish neurophysiologist R. Granit, who showed that R. t. also affects the activity of gamma motor neurons, the axons of which go to the so-called intrafusal muscle fibers, playing an important role in the regulation of posture and phase movements of the body. Rising influences. Various departments of R. f. (from the diencephalon to the medulla oblongata) have excitatory generalized effects on the cerebral cortex, that is, they involve all areas of the cerebral cortex in the process of excitation ( rice. 2
). In 1949, the Italian physiologist J. Moruzzi and Magone, investigating the bioelectrical activity of the brain, found that stimulation of R. f. brainstem changes slow synchronous high-voltage oscillations characteristic of sleep to low-amplitude high-frequency activity characteristic of wakefulness. A change in the electrical activity of the cerebral cortex is accompanied in animals by external manifestations of awakening. R. f. is closely connected anatomically with the classical conduction pathways, and its excitation is carried out with the help of extero- and interoceptive afferent (sensitive) systems. On this basis, a number of authors attribute R. f. to the nonspecific afferent system of the brain. However, the use of various pharmacological substances in the study of the function of R. f., the discovery of the selective effect of chemicals on reactions carried out with the participation of R. f., allowed P. K. Anokhin to
to formulate a position on the specificity of the ascending influences of R. f. to the cerebral cortex. The activating influences of R. f. always have a certain biological significance and are characterized by selective sensitivity to various pharmacological substances (Anokhin, 1959, 1968). Drugs introduced into the body cause inhibition of the neurons of R. f., thereby blocking its ascending activating influences on the cerebral cortex. An important role in maintaining the activity of R. f., sensitive to various chemicals circulating in the blood, belongs to humoral factors: catecholamines, carbon dioxide, cholinergic substances, etc. This ensures the inclusion of R. f. in the regulation of some autonomic functions. The cerebral cortex, experiencing tonic activating influences from R. f., can actively change the functional state of reticular formations (change the rate of excitation in it, influence the functioning of individual neurons), i.e., control, according to I. P. Pavlov , the "blind force" of the subcortex. The discovery of the properties of R. f., its relationship with other subcortical structures and areas of the cerebral cortex made it possible to clarify the neurophysiological mechanisms of pain, sleep, wakefulness, active attention, the formation of integral conditioned reflex reactions, and the development of various motivational and emotional states of the body. R.'s research f. With the use of pharmacological agents, they open up the possibility of medical treatment of a number of diseases of the central nervous system, and determine a new approach to such important problems of medicine as anesthesia, etc. Lit.: Brodal A., Reticular formation of the brain stem, lane, from English, M., 1960; Rossi J. F., Tsanchetti A., Reticular formation of the brainstem, trans. from English, M., 1960; Reticular formation of the brain, trans. from English, M., 1962; Magun, G., The Waking Brain, trans. from English, 2nd ed., M., 1965; Anokhin P.K., Biology and neurophysiology of the conditioned reflex, M., 1968; Granit R., Fundamentals of regulation of movements, trans. from English, M., 1973; Moruzzi G., Magoun H. W., Brain stem reticular formation and activation of EEG, in Electroencephalography and clinical neurophysiology, v. 1, Boston, 1949 V. G. ZILOV Great Soviet Encyclopedia. - M.: Soviet Encyclopedia.
1969-1978
.
See what the "Reticular formation" is in other dictionaries:
- (formatio reticularis; lat. reticulum network; synonymous with reticular substance) a complex of cellular and nuclear formations that occupy a central position in the brain stem and in the upper spinal cord. A large number of nerve fibers ... Wikipedia
Reticular formation- A complex network of neurons and cell nuclei that occupies the central part of the brain stem. Often referred to as the "reticular activation system" because of the role it plays in the activation process. Modern research allows ... ... Great Psychological Encyclopedia
A set of structures in the central parts of the brain that regulate the level of excitability and tone below and overlying parts of the central nervous system, including the cerebral cortex ... Big Encyclopedic Dictionary
RETICULAR FORMATION, a complex mechanism of the CENTRAL NERVOUS SYSTEM of vertebrates, located in the spinal cord stem. Consists of interconnected clusters of nerve cell bodies (gray matter) and is believed to affect many physiological... ... Scientific and technical encyclopedic dictionary
- (formatio reticularis), a set of nerve structures located in the spinal, medulla oblongata, midbrain and pons and forming a single funkt. complex. Phylogenetically ancient engine system. control. Well developed in all ... ... Biological encyclopedic dictionary
Reticular formation- (Latin rete network, formatio formation, education, compilation) a network-like nervous structure, consisting of more than 50 nuclei and an extensive network of neurons with complex and branched axonal and dendritic processes. Name suggested... Encyclopedic Dictionary of Psychology and Pedagogy
A set of structures located in the spinal, medulla oblongata and midbrain and the pons and forming a single functional complex. It has an activating and inhibitory effect on various parts of the central nervous system, increasing ... ... encyclopedic Dictionary
reticular formation- (formatio reticularis) a set of small but numerous nuclei located in the central parts of the brain stem. Neurons of the reticular formation have strongly branching processes going in different directions, resembling under a microscope ... Glossary of terms and concepts on human anatomy
Reticular formation- (from lat. reticulum mesh) a nervous structure located along the entire brain stem and consisting of cells whose processes branch out in vast areas of the cerebral cortex. The function of the reticular formation is to activate the cerebral cortex ... ... Human psychology: glossary of terms
I Reticular formation (formatio reticularis; lat. reticulum mesh; synonymous with reticular substance) is a complex of cellular and nuclear formations that occupy a central position in the brain stem and in the upper spinal cord. Big… … Medical Encyclopedia
Books
- network society. Necessity and possible construction strategies. Network (reticular) socio-economic formation: quasi-socialist principles and meritocracy. Issue No. 133, Oleskin A.V. This book is devoted to the prospects for the use of decentralized cooperative network structures (networks) in various areas of human society, with particular attention to their applications in…
Reticular formation of the brain stem is a complex of neurons that have extensive connections with different nerve centers, with each other and with the cerebral cortex. It runs rostral to the thalamus. Consider further its features.
Functions of the reticular formation
The tasks of the complex include the processing of sensory information. In addition, the reticular formation provides an activating effect on the cortex, controlling the activity of the spinal cord. Due to this, the tone of skeletal muscles, the work of the vegetative and reproductive systems of a person are regulated.
Mechanism of action
It was first identified by R. Granite. The scientist found that it can affect the activity of γ-motor neurons. As a result, γ-efferents (their axons) provoke contraction of muscle spindles and, accordingly, an increase in afferent impulses of muscle receptors. Signals entering the spinal cord provoke excitation of α-motor neurons. This determines the tone of the muscles. It was found that the neurons of the pons formation and the medulla oblongata are involved in the implementation of this function. Their behavior is diametrically opposed. The latter provoke the activation of α-motor neurons in the flexor muscles and, accordingly, inhibit them in the extensor muscles. Bridge neurons act in reverse. Reticular formation connected with the cerebellum and the cortex, from which information comes. This allows us to conclude that it acts as a collector of nonspecific sensory flux, which may be involved in the regulation of muscle activity. However, the need for a formation duplicating the tasks of neurons in the red and vestibular nuclei has not yet been clarified.
Structure
Reticular formation produced by scattered cells. Some of them are considered vital formations. In particular, the following centers can be distinguished:
- Respiratory and vasomotor. They are located in the medulla oblongata.
- Eye coordination. It is located in the midbrain.
- Hunger, satiety and thermoregulation. They are located in the diencephalon.
The key tract is the reticulospinal tract. It passes to the neurons in the motor nuclei of the anterior spinal horns and cranial nerves along the trunk and to the intercalary elements of the nervous autonomic system. From them lie the thalamo-cortical fibers. They provide the activation of the cortex, which is necessary for the perception of specific stimuli. These thalamo-cortical fibers terminate in all cortical layers.
scientific observations
During the research, it was found that ticular formation has an activating effect on the cortex. This neural complex acts as a kind of "energy center". Without it, the nerve cells of the cortex, its various sections, as well as the entire brain as a whole, will not be able to perform all their diverse complex tasks. A complex of neurons is directly involved in the regulation of sleep and wakefulness. The results of the experiments made it possible to explain some of the observations of surgeons. So, in the process of operations on the brain, incisions can be made in the cortex of the hemispheres, and part of the tissue can be removed. In this case, the patient will not lose consciousness. However, if the scalpel is touched, the person will fall into a deep sleep.
Work specifics
Today, the specific nerve channels through which information is transmitted from the sense organs to the brain are well studied. This is how the cortex learns about the nature of the stimulus acting on the body. In accordance with this, it sends different impulses to systems and organs. Studies have shown that branches extend from all fibers directed from the periphery to the cortex. They end on the surface of the cells of the formation. External irritation of any nature has an exciting effect on it. At this moment there is a kind of "charging energy". Acting as a brain center, the formation determines the degree of performance of the cortex. By activating all departments, it provides an accurate synthesis and analysis of the variety of information that enters the cortex from the outside world.
Reaction to substances in the body
The reticular formation is sensitive not only to nerve signals, but also to compounds dissolved in the blood. In particular, we are talking about sugar, hormones, carbon dioxide, oxygen. Of particular importance among these substances is adrenaline. With emotional overstrain - with anger, fear, a state of passion, rage - a prolonged excitation of the formation is noted. It is supported by adrenaline, which is intensely secreted into the blood. The activity of the complex is largely determined by other chemical compounds. The first is carbon dioxide and oxygen. For example, if a person has difficulty breathing in a dream, then CO 2 begins to accumulate in the blood. Carbon dioxide activates the reticular formation, as a result of which a person wakes up.
Conclusion
Clinical studies and experimental data obtained in physiological laboratories have shown that the reticular formation is directly related to the emergence of emotions. The results of the study of its structure and the tasks that it implements are widely used in psycho- and neuropharmacology. It was found that lethargy, apathy, drowsiness or irritability, insomnia can be caused by a disorder in the work of the reticular formation. This neural complex also plays a role in the development of many CNS pathologies.
Phylogenetically very ancient neural structure and a well-developed part of the brain stem of reptiles. At first, it was a slow polysynaptic pathway closely associated with the olfactory and limbic regions. The progressive dominance of vision and hearing over the sense of smell led to a shift in sensory and motor functions inside the tegmentum of the midbrain. The direct dorsal and operculospinal tracts bypass the reticular formation, which is mainly responsible for autonomic regulation. In mammals, the tegmentum, in turn, began to play a secondary role in the transmission of excitation along very fast conducting fibers connecting the cerebral cortex with peripheral motor and sensory neurons.
In the human brain, the reticular formation retains its connection with the limbic system and continues to play an important role in autonomic and reflex regulation.
Term reticular formation referred only to the polysynaptic neuronal network of the brainstem, despite the fact that the network extends anteriorly into the thalamus and hypothalamus and posteriorly into the propriospinal tract of the spinal cord.
General structure shown in the figure below. The median reticular formation is formed by a number of suture nuclei (Greek - nucleii raphe). Most of the serotonergic pathways of the axial nervous system originate from the raphe nuclei.
Reticular formation (RF).(A) Departments. (B) Groups of aminergic and cholinergic cells.
Nearby is the paramedian reticular formation. This section consists entirely of large cell neurons; in the lower part of the bridge and the upper part of the medulla oblongata (up to the level of fusion of the reticular formation with the central reticular nucleus of the medulla oblongata), giant cell neurons can also be found.
The most anterior division is considered lateral small cell reticular formation. Long dendrites of small cell neurons branch out at regular intervals. The dendrites have a predominantly transverse direction, and long pathways to the thalamus pass through the gaps between them. The lateral section is formed mainly by afferent neurons. They are approached by fibers from all sensitive pathways, including the sense organs.
The olfactory fibers pass through the medial forebrain bundle, located next to the hypothalamus.
Visual pathways pass through the superior colliculus.
Auditory fibers come from the upper core of the olive.
The vestibular fibers come from the medial vestibular nucleus.
Somatic sensory fibers pass through the spinal-reticular tracts from the spinal and own (main or main pontine) nuclei of the trigeminal nerve.
Most of the axons of small cell neurons branch intensively between the dendrites of neurons of the paramedian reticular formation. However, some of them form synapses with the nuclei of cranial nerves and participate in the creation of movement programs.
Paramedian reticular formation- predominantly efferent system. The axons are relatively long, and some rise up to form synapses with the reticular formation of the brainstem or thalamus. Both ascending and descending branches depart from others, forming a polysynaptic network. Large cell neurons are approached by fibers from the premotor cortex, which give rise to the reticulospinal tracts of the pons and medulla oblongata.
a) Aminergic neurons in the brainstem. Groups of aminergic (or monoaminergic) neurons scattered along the reticular formation are neurons whose mediators are formed from aromatic amino acids and have a number of effects on the cell. One group produces the neurotransmitter serotononin, the other three produce catecholamines (dopamine, norepinephrine and adrenaline), and one group produces histamine.
Serotonergic pathways from the midline brainstem (raphe).
Serotonergic neurons- the most common neurons in any part of the central nervous system (CNS). These include midbrain neurons, the fibers of which rise to the cerebral hemispheres; pontine neurons branching in the brainstem and cerebellum; medulla oblongata descending into the spinal cord.
All departments of the gray matter of the CNS are penetrated by serotonin-secreting axonal branches. An increase in serotonergic activity is used in clinical practice for the treatment of such a common disease as major depressive disorder.
Dopaminergic neurons in the midbrain represented by two groups. A black substance is located at the junction of the tire with the legs. Medial to it are the ventral nuclei of the tegmentum, from which mesocortical fibers extend to the frontal lobe and mesolimbic fibers that go directly to the nucleus accumbens.
Noradrenergic (norepinephrine) neurons slightly less numerous than serotonergic ones. About 90% of the bodies of neurons are concentrated in the blue spot (locus ceruleus) in the bottom of the IV ventricle at the upper end of the bridge. Paths in all directions start from the blue spot, as shown in the figure below.
Noradrenergic pathways from the pons and medulla of that brain.
Adrenaline-secreting (epinephrine-secreting) neurons relatively few in number and located predominantly in the rostral/caudal medulla oblongata. One part of the fibers ascends to the hypothalamus, the other goes down, forming synapses with the preganglionic sympathetic neurons of the spinal cord.
In the cerebral hemispheres, the ionic and electrical activity of aminergic neurons differs significantly. First, there is more than one type of postsynaptic receptor for each amine. Secondly, some aminergic neurons also release protein substances that can regulate the action of the neurotransmitter - as a rule, increasing its duration. Third, the larger cortical neurons receive many excitatory and inhibitory influences from local networks of circulating excitation and also have many different types of receptors. Activation of one type of aminergic receptor can lead to a strong or weak effect, depending on the initial excited state of the neuron.
Our knowledge of the physiology and pharmacodynamics of aminergic neurons is far from complete, but their importance in a wide variety of behavioral functions is beyond doubt.
Part of a transverse section through the upper part of the bridge, showing elements of the reticular formation.