The bridge of the brain performs many important functions, they are associated with the fact that it contains the nuclei of the cranial nerves. This part of the hindbrain performs motor, sensory, conduction and integrative functions.
This department plays an important role, both in the connection of different departments, and in itself strongly influences the life of a person, it controls reflexes and conscious behavior.
Structure
The department is part of the hindbrain. The structure and functions of the bridge are very closely related, as in any other structure. He took a position in front of the cerebellum, being the department between the middle and medulla oblongata.
It is separated from the first by the beginning of the 4th pair of cranial nerves, and from the second by a transverse groove. Outwardly, it resembles a roller with a furrow, with nerves passing through it, they are responsible for the sensory abilities of the skin of the face. In the furrow there was a place for the basilar arteries, their features include the fact that they supply blood to the back of the brain.
This department has a special rhomboid fossa, located in the back of the pons varilla. From above, the fossa is limited by the brain strips, and above them are the facial mounds.
Above them there is a median eminence, and next to me is a blue spot, which is responsible for the feeling of anxiety, it includes many nerve endings of the norepinephrine type. The pathways look like thick fibers of nervous tissue that run from the pons to the cerebellum. Thus, they form the handles of the bridge and the peduncles of the cerebellum.
Among other things, the structure of the bridge has a "tire", which is an accumulation of gray matter. This gray matter is the centers of the cranial nerves and the parts that contain pathways. That is, the upper brain part is reserved for the centers that have a connection with the cranial nerves (fifth, sixth, seventh and eighth pairs).
Good to know: The cerebral cortex, structure and functions
Speaking of pathways, the medial loop and the lateral loop pass through this part. The same tire contains the reticular formation, it is part of six nuclei and contains structures that are responsible for auditory analyzers.
At the base are paths that run from the cerebral cortex to different parts:
- brain bridge;
- medulla;
- spinal cord;
- cerebellum.
And the blood supply occurs due to the arteries, which belong to the vertebrobasilar basin.
Conductor function
The Variliev Bridge was named for a reason. The thing is that absolutely all the paths that go both in the ascending and in the descending direction pass through this department.
They connect the forebrain and other structures such as the cerebellum, spinal cord and others.
Motor and sensory functions
Speaking in more detail about motor and sensory function, let's talk about the cranial nerves. Mentioning the cranial nerves, it should be noted the ternary or mixed nerve (V pair). This pair of nerves is responsible for the movement of the chewing muscles, as well as the muscles that are responsible for the tension of the tympanic membrane and the palatine curtain.
Afferent connections of nerve cells from receptors that are located in the skin of the human face, nasal mucosa, 60% of the tongue, eyeball and teeth go to the sensory part of the trigeminal nerve. The sixth pair, or the so-called abducens nerve, is responsible for the movement of the eyeballs, namely for its rotation to the outside.
One of the most important functions for human interaction is the 7th pair, it is responsible for the innervation of the muscles that allow the production of facial expressions. In addition, three glands are controlled by the facial nerve: salivary, sublingual, and submandibular. These glands provide reflexes such as salivation and swallowing.
The bridge also has a connection with the vestibulocochlear nerve. It is clear from the name that the cochlear part reaches the cochlear nuclei, but the vestibular part ends in the triangular nucleus. The eighth pair of nerves is responsible for the analysis of vestibular stimuli, it determines the degree of their severity and where they are directed.
Integrating function
These bridge functions connect parts of the brain called the cerebral hemispheres. Also, all other paths pass through the bridge, both ascending and descending, connecting it with many departments of the central nervous system. These include the spinal cord, cerebellum, and cerebral cortex.
Impulses passing through the bridge-cerebellar pathways of the cerebral cortex have their effect on the work of the cerebellum. The bark cannot influence directly, therefore it uses the bridge as an intermediary for these purposes. The bridge regulates the medulla oblongata, influencing the centers that are responsible for the respiratory process and its intensity.
Results
Now it became clear that the bridge is the most important part of the central nervous system, providing conscious control of the body together with the cerebellum.
In addition, it helps a person to perceive their own position in space. Under his responsibility is the sensitivity of the tongue, face, nasal mucosa and ocular conjunctiva.
The tissues of the brain are represented by a wide range of formations. In its structure, this is perhaps the most complex part of the human body, which determines the wide nature of the activity of the central nervous system. When assessing the structure, several areas of the CNS can be distinguished in this localization.
At the base of the cerebral structures is the so-called brain stem. It provides a group of vital functions: from breathing and cardiac activity to thermoregulation. Any injury or malfunction will result in severe disability or death.
The pons varolii is an integral part of the trunk, located between the middle and medulla oblongata, which ensures the normal conduction of nerve impulses and makes it possible to perform a number of arbitrary actions.
Responsible for some functions of higher activity. Its damage, for example, against the background of an injury, a stroke, leads to critical disorders in the functioning of the whole organism.
Diagnosis of lesions of this anatomical structure presents certain difficulties due to deep and "uncomfortable" localization. The only reliable method of examination is MRI or, less often, computed tomography.
The pons varolii is located at the level of the brain stem and conventionally two main areas are distinguished in its anatomy.
- Top part. Consists of gray matter and includes several pairs of cranial nerves (from the 5th to the 8th). This is actually a functional structure.
- bottom or base- provides conductivity of signals, acts as a transport way for natural impulses.
At the level of the upper part of the bridge is the reticular formation. It is represented by a large accumulation of fibers that allow the entire central nervous system to function in a harmonious manner.
At the base is a dense layer of thick conductive strands. They are three legs on each side, connected to the cerebellum and enable the extrapyramidal system to work.
Below the pons Varolieva is the medulla oblongata, in the upper part - the middle one.
There are some differences in the structure of this education for children and adults. When assessed in patients under 8 years of age, a complete absence of the myelin sheath is found, which is considered normal.
The formation of this part of the brain is observed in early adolescence. The anatomy of the bridge is complex, which is explained by the need to carry out a number of actions on the part of the central nervous system.
What is the function of the bridge
The pons of the brain is responsible for several important forms of activity.
Among those:
- Reflex automatic and voluntary movements of the eyes and eardrum in response to loud sounds, also the tissues of the oral cavity (palate). Any violation ends in problems.
- The ability to purposeful motor activity. Since the bridge in the brain provides the functioning of the cerebellum, with any damage, there are problems with the ability to control the body.
- Perception of vestibular stimuli. In this case, we are talking about the ability to perceive one's body as a whole, its orientation and location in space, respond to any changes in environmental conditions, and also extinguish unnecessary movements (for example, during sudden braking in public transport, stumbling, etc.). With the defeat, there is a violation of coordination. Ability to navigate in space.
- Ensuring olfactory function. The bridge has this ability partially. Other subcortical accumulations are also responsible for it.
- Normal innervation of the skin and mucous membranes of the face.
- The pons is involved in the formation of sleep. This is a complex and well-coordinated work of several cerebral formations at once. Any violations immediately lead to problems with night rest. The patient becomes lethargic, asthenic processes appear.
- The functions of the Varoliyev bridge include the acts of chewing and swallowing. Essential for nutrition and respiration.
- Actually, the body's ability to normal gas exchange also depends on the work of this structure. In the absence of adequate conduction of impulses, problems begin, up to disorders of a lethal nature.
Basic actions are performed by nerve tissues constantly. Even minor changes are noticeable immediately.
The pons is part of the brain stem, therefore deviations in its activity become an indirect cause of the dysfunction of this entire formation.
Possible complications up to rapid, fatal. High-quality medical care is not always possible due to the complex localization of structures and complex structure.
Pathologies that violate the functions of the bridge and their symptoms
There is a group of diseases for which a violation of the normal functioning of the body as a result of the destruction of the tissues in question is typical.
Among those:
Brissot-Sicard syndrome
Accompanied by a disorder of the activity of the cranial nerves. It is determined by unilateral paresis or complete paralysis of half of the body.
The ability to control the muscles of the facial region is also lost, ptosis (drooping of the eyelid) is possible with impaired visual function.
Such a disorder occurs against the background of an infectious, autoimmune or tumor lesion. Rarely becomes the result. After a transient attack or actually a full-fledged stroke.
Bonnier syndrome
It is characterized by damage to a group of cranial nerves. In this case, the auditory and vestibular nuclei ultimately suffer.
Symptoms are nonspecific. There are problems with the perception of sound stimuli. Patients constantly experience dizziness, nausea, weakness.
An integral part of the clinic is insomnia. The patient becomes irritable, there is an instability of the emotional background. Up to abrupt phase changes, as, for example, in bipolar affective psychosis.
Grene's syndrome
A typical feature of this pathological process is a violation of the sensitivity of facial muscles, which ultimately leads to problems with the manifestation of non-verbal signals and emotions.
There is partial paresis of the masticatory muscles on one side. On the other hand, controllability is also present, but to a much lesser extent.
Ventral syndrome
Extremely difficult condition. It is characterized by at least the loss of speech function. This is the easiest case.
The classical situation is determined by the complete loss of the ability to move. The person cannot move. Communication is possible only through the eyes.
This disorder persists for a long time. Quickly leads to stagnation, death of the patient. Recovery is not possible.
Raymond-Sestan syndrome
It is characterized by a key manifestation of the oculomotor nerves. A person loses the ability to arbitrarily focus his gaze and transfer it from one object to another.
Perhaps spontaneous relief of the condition and its subsequent return for unclear reasons.
Gubler syndrome
It does not have specific manifestations of blood paralysis of mimic muscles. The facial expression is characterized as a mask.
The patient is unable to adequately non-verbally express emotions and respond to environmental stimuli.
The sensitivity of the skin also falls, which is detected by the results of functional tests and physical examination.
Fauville syndrome
There is paralysis of mimic muscles and strabismus with visual impairment.
Gasperini's disease
Combined pathological process. It is characterized by mixed symptoms.
Diseases leading to the development of syndromes
The structure of the Varolii bridge suggests many possible lesions and an equally large number of manifestations. However, there is a group of diseases that become the foundation for the above syndromes.
This may include:
- Stroke. Acute violation of cerebral blood flow in a particular area with the death of nerve tissues and the loss of part of the functions of cerebral structures. If the brain stem itself suffers, in the most favorable case, this will end in a violation of higher activity.
- . Incorrectly called microstrokes. The same is observed, but there is no significant tissue death.
- . Violation of the patency of the arteries as a result of blockage of those with cholesterol plaques or spontaneous narrowing against the background, for example, of prolonged smoking, hypertension (increase in pressure).
- infectious processes. Especially those that affect cerebral tissue. encephalitis, meningitis.
- Demyelination. Multiple sclerosis.
The Varoliev bridge is responsible for a lot of important functions and has a systemic structure. The treatment of pathological conditions, when the activity of this structure is already impaired, is an extremely complex and sometimes impossible process.
Therefore, it makes sense to preventively influence all diseases that may become a source of problems in the future. This is an important preventive measure.
Bridge is part of the brain stem.
The neurons of the nuclei of the cranial nerves of the bridge receive sensory signals from auditory, vestibular, taste, tactile, pain thermoreceptors. The perception and processing of these signals form the basis of its sensory functions. Many neural pathways pass through the bridge, which ensure that it performs conductive and integrative functions. In the bridge there are a number of sensory and motor nuclei of the cranial nerves, with the participation of which the bridge performs its reflex functions.
Touch functions of the bridge
Sensory functions consist in the perception by neurons of the nuclei of the V and VIII pairs of cranial nerves of sensory signals from sensory receptors. These receptors can be formed by sensory epithelial cells (vestibular, auditory) or nerve endings of sensory neurons (pain, temperature, mechanoreceptors). The bodies of sensory neurons are located in the peripheral nodes. In the spiral ganglion there are sensitive auditory neurons, in the vestibular ganglion - sensitive vestibular neurons, in the trigeminal (lunate, Gasser) ganglion - sensitive neurons of touch, pain, temperature, proprioceptive sensitivity.
In the bridge, the analysis of sensory signals from the receptors of the skin of the face, mucous membranes of the eye, sinuses, nose, and mouth is carried out. These signals come along the fibers of the three branches of the trigeminal nerve - the ophthalmic maxillary and mandibular to the main nucleus of the trigeminal nerve. It analyzes and switches signals for transmission to the thalamus and further to the cerebral cortex (touch), the spinal nucleus of the trigeminal nerve (pain and temperature signals), the trigeminal nucleus of the midbrain (proprioceptive signals). The result of the analysis of sensory signals is an assessment of their biological significance, which becomes the basis for the implementation of reflex reactions controlled by the centers of the brain stem. An example of such reactions may be the implementation of a protective reflex to irritation of the cornea, manifested by a change in secretion, contraction of the muscles of the eyelids.
In the auditory nuclei of the pons, the analysis of the duration, frequency, and intensity of auditory signals begun in the organ of Corti continues. In the vestibular nuclei, the signals of acceleration of movement and the spatial position of the head are analyzed, and the results of this analysis are used for reflex regulation of muscle tone and posture.
Through the ascending and descending sensory pathways of the bridge, sensory signals are carried to the overlying and underlying parts of the brain for their subsequent more subtle analysis, identification and response. The results of this analysis are used to form emotional and behavioral reactions, some of the manifestations of which are realized with the participation of the bridge, medulla oblongata and spinal cord. For example, irritation of the vestibular nuclei at high acceleration can cause strong negative emotions and manifest itself as the initiation of a complex of somatic (eye nystagmus, ataxia) and vegetative (palpitations, increased sweating, dizziness, nausea, etc.) reactions.
Bridge Centers
Bridge Centers formed mainly by the nuclei of the V-VIII pairs of cranial nerves.
The nuclei of the vestibulocochlear nerve (n.vestibulocochlearis, VIII couple) subdivided into cochlear and vestibular nuclei. The cochlear (auditory) nuclei are divided into dorsal and ventral. They are formed by the second neurons of the auditory pathway, to which the first bipolar sensory neurons of the spiral ganglion converge with the formation of synapses, the axons of which form the auditory branch of the vestibulo-auditory nerve. At the same time, signals are transmitted to the neurons of the dorsal nucleus from the cells of the organ of Corti located on the narrow part of the main membrane (in the coils of the cochlea base) and perceiving high-frequency sounds, and to the neurons of the ventral nucleus - from cells located on the wide part of the main membrane (in the coils of the apex of the cochlea). ) and perceiving low-frequency sounds. The axons of the neurons of the auditory nuclei follow through the operculum to the neurons of the superior olivary complex, which then conduct auditory signals through the contralateral lamina to the neurons of the inferior colliculus. Part of the fibers of the auditory nucleus and the lateral lemniscus goes directly to the neurons of the medial geniculate body, without switching to the neurons of the inferior colliculi. Signals from the neurons of the medial geniculate body follow to the primary auditory cortex, in which a fine analysis of sounds is carried out.
With the participation of neurons of the cochlear nuclei and their neural pathways, reflexes of activation of cortical neurons are carried out under the action of sound (through connections between neurons of the auditory nuclei and nuclei of the Russian Federation); protective reflexes of the organ of hearing, realized through contraction m.tensortympani and m.stapedius when exposed to strong sounds.
Vestibular nuclei subdivided into medial (Schwalbs), lower (Roller), lateral (Deuters) and upper (Bekhterev). They are represented by the second neurons of the vestibular analyzer, to which the axons of sensitive cells located in the scarp ganglion converge. The dendrites of these neurons form synapses on the hair cells of the sac and uterus of the semicircular canals. Part of the axons of sensitive cells follows directly into the cerebellum.
The vestibular nuclei also receive afferent signals from the spinal cord, cerebellum, and vestibular cortex.
After processing and primary analysis of these signals, the neurons of the vestibular nuclei send nerve impulses to the spinal cord, cerebellum, vestibular cortex, thalamus, nuclei of the oculomotor nerves, and to the receptors of the vestibular apparatus.
The signals processed in the vestibular nuclei are used to regulate muscle tone and maintain posture, maintain body balance and reflex correction in case of loss of balance, control eye movements and form three-dimensional space.
The nuclei of the facial nerve (n.facialis, VII pair) represented by sensory motor and secretomotor neurons. Facial nerve fibers converge on sensory neurons located in the nucleus of the solitary tract, bringing signals from the taste cells of the anterior 2/3 of the tongue. The results of the taste sensitivity analysis are used to regulate the motor and secretory functions of the gastrointestinal tract.
The motor neurons of the nucleus of the facial nerve innervate with axons the mimic muscles of the face, the auxiliary masticatory muscles - the styloglossus and digastric, as well as the muscle of the stirrup in the middle ear. The motor neurons innervating the mimic muscles receive signals from the cerebral cortex via the corticobulbar pathways, the basal ganglia, the superior tubercles of the midbrain, and other areas of the brain. Damage to the cortex or the pathways connecting it with the nucleus of the facial nerve leads to paresis of facial muscles, changes in facial expression, and the inability to adequately express emotional reactions.
The secretomotor neurons of the nucleus of the facial nerve are located in the superior salivary nucleus of the pontine tegmentum. These nucleus neurons are the preganglionic cells of the parasympathetic nervous system and send fibers for innervation through the postganglionic neurons of the submandibular and pterygopalatine ganglia of the lacrimal, submandibular, and sublingual salivary glands. Through the release of acetylcholine and its interaction with M-ChR, the secretomotor neurons of the nucleus of the facial nerve control the secretion of saliva and lacrimation.
Thus, dysfunction of the nuclei or fibers of the facial nerve can be accompanied not only by paresis of the facial muscles, but also by loss of taste sensitivity of the anterior 2/3 of the tongue, impaired secretion of saliva and tears. This predisposes to the development of dry mouth, indigestion and the development of dental diseases. As a result of impaired innervation (paresis of the stapes muscle), patients develop increased auditory sensitivity - hyperacusis (Bell's phenomenon).
The nucleus of the abducens nerve (n.abducens, VI pair) located in the tire of the bridge, at the bottom of the IV ventricle. Represented by motor neurons and interneurons. Axons of motor neurons form the abducens nerve that innervates the lateral rectus muscle of the eyeball. Interneuron axons join the contralateral medial longitudinal fasciculus and terminate at the neurons of the subnucleus of the oculomotor nerve, which innervate the medial rectus muscle of the eye. The interaction carried out through this connection is necessary for the organization of the commonwealth of the horizontal gaze, when, simultaneously with the contraction of the muscle that abducts one eye, the medial rectus muscle of the other eye must contract to adduct it.
The neurons of the nucleus of the abducens nerve receive synaptic inputs from both hemispheres of the cerebral cortex through the corticorticulobulbar fibers; medial vestibular nucleus - through the medial longitudinal bundle, the reticular formation of the bridge and the prepositional hyoid nucleus.
Damage to the fibers of the abducens nerve leads to paralysis of the lateral rectus muscle of the eye on the ipsilateral side and the development of double vision (diplopia) when trying to exercise a horizontal gaze in the direction of the paralyzed muscle. In this case, two images of the object are formed in the horizontal plane. Patients with unilateral abducens nerve injury usually keep their head turned to the side of the disease in order to compensate for the loss of lateral eye movement.
In addition to the nucleus of the abducens nerve, upon activation of the neurons of which the horizontal movement of the eyes is carried out, a group of neurons initiating these movements is located in the reticular formation of the bridge. The location of these neurons (anterior to the nucleus of the abducens nerve) was called the center of the horizontal gaze.
The nuclei of the trigeminal nerve (n.trigeminus, V pair) represented by motor and sensory neurons. The motor nucleus is located in the pontine tegmentum, the axons of its motor neurons form the efferent fibers of the trigeminal nerve, innervating the muscles of mastication, the muscle of the tympanic membrane, the soft palate, the anterior belly of the digastric and myelologoid muscles. The neurons of the motor nuclei of the trigeminal nerve receive synaptic inputs from the cortex of both hemispheres of the brain as part of the corticobulbar fibers, as well as from the neurons of the sensory nuclei of the trigeminal nerve. Damage to the motor nucleus or efferent fibers leads to the development of paralysis of the muscles innervated by the trigeminal nerve.
The sensory neurons of the trigeminal nerve are located in the sensory nuclei of the spinal cord, pons, and midbrain. Sensory signals come to sensory neurons but two types of afferent nerve fibers. Proprioceptive fibers are formed by dendrites of unipolar neurons of the semilunar (Gasser) ganglion, which are part of the nerve and end in the deep tissues of the face and mouth. Along the afferent proprioceptive fibers of the trigeminal nerve, signals from tooth receptors about pressure values, tooth movements, as well as signals from periodontal receptors, hard palate, articular capsules and from masticatory muscle stretch receptors are transmitted to its spinal and main sensory nucleus of the bridge. The sensory nuclei of the trigeminal nerve are analogous to the spinal ganglia, in which sensory neurons are usually located, but these nuclei are located in the CNS itself. Proprioceptive signals along the axons of the neurons of the trigeminal nucleus go further to the cerebellum, thalamus, RF, and the motor nuclei of the brainstem. Neurons of the sensory nucleus of the trigeminal nerve in the diencephalon are related to the mechanisms that control the force of jaw contraction during biting.
Fibers of general sensory sensitivity transmit signals of pain, temperature, touch from the surface tissues of the face and front of the head to the sensory nuclei of the trigeminal nerve. The fibers are formed by the dendrites of the unipolar neurons of the semilunar (Gasser) ganglion and form three branches of the trigeminal nerve on the periphery: mandibular, maxillary and ophthalmic. The sensory signals processed in the sensitive nuclei of the trigeminal nerve are used for transmission and further analysis (for example, pain sensitivity) to the thalamus, the cerebral cortex, as well as to the motor nuclei of the brain stem to organize reflex responses (chewing, swallowing, sneezing and other reflexes).
Damage to the nuclei or fibers of the trigeminal nerve may be accompanied by a violation of chewing, the appearance of pain in the lip area, innervated by one or more branches of the trigeminal nerve (trigeminal neuralgia). The pain occurs or worsens during eating, talking, brushing your teeth.
Along the midline of the base of the bridge and the rostral part of the medulla oblongata is located seam core. The nucleus consists of serotonergic neurons, the axons of which form a widely branched network of connections with neurons of the cortex, hippocampus, basal ganglia, thalamus, cerebellum, and spinal cord, which is part of the monoaminergic system of the brain. The neurons of the raphe nucleus are also part of the reticular formation of the brainstem. They play an important role in the modulation of sensory (especially pain) signals transmitted to the overlying brain structures. Thus, the raphe nucleus is involved in the regulation of the level of wakefulness, modulation of the sleep-wake cycle. In addition, the neurons of the raphe nucleus can modulate the activity of motor neurons of the spinal cord and thus affect its motor functions.
In the bridge there are groups of neurons that are directly involved in the regulation of breathing (pneumotaxic center), sleep and wake cycles, scream and laughter centers, as well as the reticular formation of the brain stem and other stem centers.
Signaling and integrative functions of the bridge
The most important pathways for signal transmission are fibers that begin in the nuclei of the VIII, VII, VI and V pairs of cranial nerves, and fibers that follow through the bridge to other parts of the brain. Since the bridge is part of the brainstem, many ascending and descending neural pathways pass through it, transmitting a variety of signals to the CNS.
Three tracts of fibers descending from the cerebral cortex pass through the base of the pons (its phylogenetically youngest part). These are the fibers of the corticospinal tract, following from the cerebral cortex through the pyramids of the medulla oblongata to the spinal cord, the fibers of the corticobulbar tract, descending from both hemispheres of the cerebral cortex directly to the neurons of the cranial nuclei of the brainstem or to the interneurons of its reticular formation, and the fibers of the corticospinal tract. The neural pathways of the last tract provide targeted communication of certain areas of the cerebral cortex with a number of groups of pontine and cerebellar nuclei. Most of the axons of the neurons of the pontine nuclei pass to the opposite side and follow the neurons of the vermis and cerebellar hemispheres through its middle legs. It is assumed that signals important for the rapid correction of movements come to the cerebellum through the fibers of the corticospinal tract.
Through the bridge cover tegmentum), which is a phylogenetically old part of the bridge, are ascending and descending pathways of signals. Afferent fibers of the spinothalamic tract pass through the medial lemniscus of the tegmentum, following from the sensory receptors of the opposite half of the body and from the interneurons of the spinal cord to the neurons of the thalamic nuclei. The fibers of the trigeminal tract also follow in the thalamus, which conduct sensory signals from the tactile, pain, temperature, and proprioreceptors of the opposite surface of the face to the neurons of the thalamus. Through the tire of the bridge (lateral lemniscus), axons of neurons of cochlear nuclei follow to the neurons of the thalamus.
The fibers of the tectospinal tract pass through the tire in a downward direction, controlling the movements of the neck and torso in response to signals from the visual system.
Among other tracts of the pontine tire, the rubrospinal tract descending from the neurons of the red nucleus to the neurons of the spinal cord is important for the organization of movements; ventral spinocerebellar tract, the fibers of which follow the cerebellum through its upper legs.
Through the lateral part of the tire of the bridge, the fibers of the sympathetic nuclei of the hypothalamus pass in a downward direction, following to the preganglionic neurons of the sympathetic nervous system of the spinal cord. Damage or rupture of these fibers is accompanied by a decrease in the tone of the sympathetic nervous system and a violation of the autonomic functions controlled by it.
One of the important ways of conducting signals about the balance of the body and the reaction to its changes is the medial longitudinal bundle. It is located in the pons operculum near the midline under the floor of the IV ventricle. The fibers of the longitudinal bundle converge on the neurons of the oculomotor nuclei and play an important role in the implementation of continuous horizontal eye movements, including the implementation of vestibulo-ocular reflexes. Injury to the medial longitudinal fasciculus may be accompanied by impaired adduction of the eye and nystagmus.
Numerous paths of the reticular formation of the brain stem pass through the bridge, which are important for regulating the general activity of the cerebral cortex, maintaining the state of attention, changing sleep-wake cycles, regulating breathing and other functions.
Thus, with the direct participation of the centers of the bridge and their interaction with other centers of the central nervous system, the bridge is involved in the implementation of many complex physiological processes that require the unification (integration) of a number of simpler ones. This is confirmed by examples of the implementation of a whole group of bridge reflexes.
Reflexes carried out at the level of the bridge
At the level of the bridge, the following reflexes are carried out.
chewing reflex It is manifested by contractions and relaxation of the chewing muscles in response to the receipt of afferent signals from the sensory receptors of the inner part of the lips and oral cavity through the fibers of the trigeminal nerve to the neurons of the nucleus of the trigeminal nerve. Efferent signals to the masticatory muscles are transmitted through the motor fibers of the facial nerve.
Corneal reflex It is manifested by the closing of the eyelids of both eyes (blinking) in response to irritation of the cornea of one of the eyes. Afferent signals from the sensory receptors of the cornea are transmitted along the sensory fibers of the trigeminal nerve to the neurons of the trigeminal nucleus. Efferent signals to the muscles of the eyelids and the circular muscle of the eye are transmitted through the motor fibers of the facial nerve.
salivation reflex manifested by the separation of a larger amount of liquid saliva in response to irritation of the receptors of the oral mucosa. Afferent signals from the receptors of the oral mucosa are transmitted along the afferent fibers of the trigeminal nerve to the neurons of its superior salivary nucleus. Efferent signals are transmitted from the neurons of this nucleus to the epithelial cells of the salivary glands through the glossopharyngeal nerve.
Tear reflex manifested by increased tearing in response to irritation of the cornea of the eye. Afferent signals are transmitted along the afferent fibers of the trigeminal nerve to the neurons of the superior salivary nucleus. Efferent signals to the lacrimal glands are transmitted through the fibers of the facial nerve.
Swallowing reflex It is manifested by the implementation of a coordinated contraction of the muscles that provide swallowing during irritation of the receptors of the root of the tongue, soft palate and posterior pharyngeal wall. Afferent signals are transmitted along the afferent fibers of the trigeminal nerve to the neurons of the motor nucleus and further to the neurons of other nuclei of the brainstem. Efferent signals from the neurons of the nuclei of the trigeminal, hypoglossal, glossopharyngeal and vagus nerves are transmitted to the muscles of the tongue, soft palate, pharynx, larynx and esophagus innervated by them.
Coordination of chewing and other muscles
The chewing muscles can develop a high degree of tension. A muscle with a cross section of 1 cm 2 develops a force of 10 kg during contraction. The sum of the cross section of the masticatory muscles that raise the lower jaw on one side of the face is on average 19.5 cm 2, and on both sides - 39 cm 2; the absolute strength of the masticatory muscles is 39 x 10 = 390 kg.
The chewing muscles ensure the closure of the jaws and maintain the closed state of the mouth, which do not require the development of significant tension in the muscles. At the same time, when chewing coarse food or increased closure of the jaws, the masticatory muscles are able to develop limiting stresses that exceed the endurance of the periodontium of individual teeth to the pressure exerted on them and cause pain.
From the above examples, it is obvious that a person must have mechanisms by which the tone of the chewing muscles is maintained at rest, contractions and relaxation of various muscles are initiated and coordinated during chewing. These mechanisms are necessary to achieve chewing efficiency and prevent the development of excessive muscle tension that could lead to pain and other adverse effects.
Chewing muscles belong to the striated muscles, so they have the same properties as other striated skeletal muscles. Their sarcolemma has excitability and the ability to conduct action potentials that occur during excitation, and the contractile apparatus ensures muscle contraction following their excitation. The masticatory muscles are innervated by axons of a-motoneurons, which form motor portions: the mandibular nerve - branches of the trigeminal nerve (masticatory, temporal muscles, anterior belly of the digastric and maxillohyoid muscles) and the facial nerve (auxiliary - stylohyoid and digastric muscles). Between the endings of the axons and the sarcolemma of the masticatory muscle fibers there are typical neuromuscular synapses, in which signal transmission is carried out using acetylcholine, which interacts with the n-cholinergic receptors of the postsynaptic membranes. Thus, to maintain tone, initiate contraction of masticatory muscles and regulate its strength, the same principles are used as in other skeletal muscles.
Keeping the mowing of the closed state of the mouth is achieved due to the presence of tonic tension in the masticatory and temporal muscles, which is supported by reflex mechanisms. Under the influence of this mass, the lower jaw constantly stretches the receptors of the muscle spindles. In response to stretching, afferent nerve impulses arise in the endings of the nerve fibers associated with these receptors, which are transmitted along the sensitive portion of the trigeminal fibers to the neurons of the mesencephalic nucleus of the trigeminal nerve and maintain the activity of motor neurons. The latter constantly send a stream of efferent nerve impulses to the extrafusal fibers of the chewing muscles, which create a tension of sufficient strength to keep the mouth closed. The activity of motor neurons in the trigeminal nucleus can be suppressed under the influence of inhibitory signals sent along the corticobulbar pathways from the region of the lower part of the primary motor cortex. This is accompanied by a decrease in the flow of efferent nerve impulses to the masticatory muscles, their relaxation and opening of the mouth, which occurs during voluntary opening of the mouth, as well as during sleep or anesthesia.
Chewing and other movements of the lower jaw are carried out with the participation of chewing, facial muscles, tongue, lips and other auxiliary muscles innervated by various cranial nerves. They can be arbitrary and reflex. Chewing can be effective and achieve its purpose, provided that the contraction and relaxation of the muscles involved are finely coordinated. The coordination function is performed by the chewing center, represented by a network of sensory, motor and interneurons located mainly in the brain stem, as well as in the substantia nigra, thalamus and cerebral cortex.
Information entering the structures of the masticatory center from taste, olfactory, thermo-, mechano- and other sensory receptors provides the formation of sensations of food present or entering the oral cavity. If the parameters of sensations about the incoming food do not correspond to the expected ones, then, depending on the motivation and feeling of hunger, a reaction of refusing to take it may develop. When the parameters of sensations coincide with the expected ones (extracted from the memory apparatus), a motor program of upcoming actions is formed in the center of chewing and other motor centers of the brain. As a result of the implementation of the motor program, it is ensured that the body is given a certain posture, exercise coordinated with the movement of the hands, opening and closing the mouth, biting and introducing food into the mouth, after which voluntary and reflex components of chewing are initiated.
It is assumed that in the neuronal networks of the mastication center there is a generator of motor commands formed in the process of evolution, sent to the motor neurons of the nuclei of the trigeminal, facial, and hypoglossal cranial nerves that innervate the masticatory and accessory muscles, as well as to the neurons of the motor centers of the trunk and spinal cord, initiating and coordinating hand movements, biting, chewing and swallowing food.
Chewing and other movements adapt to the texture and other characteristics of food. The leading role in this is played by sensory signals sent to the center of chewing and directly to the neurons of the nucleus of the trigeminal nerve along the fibers of the mesencephalic tract and, in particular, signals from the proprioceptors of the masticatory muscles and mechanoreceptors of the periodontium. The results of the analysis of these signals are used for the reflex regulation of masticatory movements.
With increased closure of the jaws, excessive deformation of the periodontium and mechanical irritation of the receptors located in the periodontium and (or) in the gums occur. This leads to a reflex weakening of pressure by reducing the force of contraction of the masticatory muscles. There are several reflexes by which chewing subtly adapts to the nature of the food being eaten.
masseter reflex initiated by signals from the proprioreceptors of the main masticatory muscles (especially m.masseter), leading to an increase in the tone of sensory neurons, activation of a-motor neurons of the mesencephalic nucleus of the trigeminal nerve, which innervates the muscles that lift the lower jaw. Activation of motor neurons, an increase in the frequency and number of efferent nerve impulses in the motor nerve fibers of the trigeminal nerves contribute to the synchronization of the contraction of motor units, and the involvement of high-threshold motor units in the contraction. This leads to the development of strong phasic contractions of the masticatory muscles, which provide lifting of the lower jaw, closing of the dental arches and an increase in masticatory pressure.
Periodontal muscle reflexes provide control over the force of chewing pressure on the teeth during contractions of the muscles that lift the lower jaw and compression of the jaws. They occur when the mechanoreceptors of the periodontium, sensitive to changes in masticatory pressure, are stimulated. Receptors are located in the ligamentous apparatus of the tooth (periodontium), as well as in the mucous membrane of the gums and alveolar ridges. Accordingly, two types of periodontal muscle reflexes are distinguished: periodontal muscle and o gingivomuscular.
Periodomuscular reflex protects the periodontium from excessive pressure. The reflex is carried out during chewing with the help of one's own teeth in response to irritation of periodontal mechanoreceptors. The severity of the reflex depends on the force of pressure and the sensitivity of the receptors. Afferent nerve impulses that arose in the receptors when they were mechanically stimulated by high chewing pressure developed when chewing solid food are transmitted along the afferent fibers of the sensory neurons of the gasser ganglion to the neurons of the sensory nuclei of the medulla oblongata, then to the thalamus and cerebral cortex. From the cortical neurons, efferent impulses along the corgico-bulbar pathway enter the masticatory center, motor nuclei, where it causes the activation of a-motoneurons that innervate the auxiliary masticatory muscles (lowering the lower jaw). At the same time, inhibitory interneurons are activated, which reduce the activity of a-motor neurons that innervate the main masticatory muscles. This leads to a decrease in the strength of their contractions and chewing pressure on the teeth. When biting into food with a very hard component (for example, nut shells or seeds), pain may occur and the act of chewing may stop to remove the solid from the oral cavity into the external environment or move it to teeth with more stable periodontium.
Gingival reflex is carried out in the process of sucking and (or) chewing in newborns or in the elderly after tooth loss, when the force of contraction of the main chewing muscles is controlled by mechanoreceptors of the mucous membrane of the gums and alveolar ridges. This reflex is of particular importance in people using removable dentures (with partial or complete adentia), when the chewing pressure is transferred directly to the gum mucosa.
Important in the regulation of contraction of the main and auxiliary masticatory muscles is articulatory-muscular a reflex that occurs when the mechano-receptors located in the capsule and ligaments of the temporomandibular joints are irritated.
The following parts are distinguished in the bridge (Fig.4.). These are the base (basis) (ventral part), the trapezoid body (corpus trapezoideum), the tire (dorsal part) (tegmentum).
The trapezoidal body (9) is the boundary between the base and the tire. Here are the neurons of the auditory pathway. The continuation of the trapezoid body at the exit from the bridge is the auditory loop, lemniscus lateralis (12).
The auditory or lateral loop consists of crossed and non-crossed nerve conductors of the auditory pathway. Axons of 2 neurons of the auditory pathway (cells of the vestibular nuclei) follow the surface of the rhomboid fossa from its angle to the median sulcus, forming the brain stripes, striae medullaris. Moving to the opposite side, these fibers join the fibers of the trapezoid body and form a lateral or auditory loop - lemniscus lateralis.
The base of the pons is composed of both white and gray matter.
The gray matter is represented by its own nuclei of the bridge (nuclei proprii pontis) (11). White matter - longitudinal and transverse fibers.
The longitudinal fibers of the bridge (fibrae pontis longitudinales) consist of pathways that run from the cerebral cortex to the nuclei of the bridge, cerebellum and spinal cord (tratus corticospinalis, tratus corticonuclearis, tratus cortico-ponto-cerebellaris).
The transverse fibers of the bridge (fibrae pontis transversus) form the bridge-cerebellar pathways (tratus ponto-cerebellaris) as part of the middle cerebellar peduncles. They follow from the nuclei of the bridge to the cerebellum. Thanks to these fibers, vestibular functions are regulated, namely, the coordination of movement and the position of the body in space are controlled.
The pontine tire, together with the medulla oblongata, is involved in the formation of the rhomboid fossa. Localized here: mesh formation, anterior spinal cerebellar tract, lateral and medial loops (10, 12), superior olive (6) (refers to the auditory analyzer), trigeminal nucleus (5), abducens (1), facial (2), vestibulocochlear nerves (4).
The fibers of the ascending sensory pathways (medial and spinal loops) pass through the pons operculum. At the level of the bridge, the fibers of the trigeminal (trigeminal) loop, formed by the processes of the second neurons that lie in the sensory nucleus of the trigeminal nerve, also join them.
Thus, the nerve fibers that make up the spinal, medial and trigeminal loops carry sensory information to the diencephalon and telencephalon and are called lemniscal tracts.
The cranial nerves from the V to VIII pair emerge from the bridge.
V pair, trigeminal nerve, n. trigeminus, mixed.
Motor fibers are axons of the motor nucleus of the trigeminal nerve located in the bridge. Sensitive - represented by the central processes of pseudo-unipolar cells located in the sensitive node of the crescent shape - the trigeminal, Gasser node (ganglion trigeminale). This node lies on the anterior surface of the pyramid of the temporal bone, the central processes of its cells end on the neurons of three nuclei: the midbrain (nucleus mesencephalicus), the bridge (nucleus pontinus), the nucleus of the spinal cord of the trigeminal nerve, (nucleus tractus spinalis n. trigemini). The trigeminal nerve leaves the substance of the bridge on the border with the middle cerebellar peduncle with two roots - sensory and motor. The sensitive root represents the totality of all the central processes of the cells of the trigeminal node. They form 3 branches: ophthalmic, maxillary and mandibular nerves. The motor fibers attach only to the mandibular nerve.
The ophthalmic nerve enters the orbit through the superior orbital fissure, innervates the contents of the orbit, the upper eyelid, the skin of the forehead and crown, the mucous membrane of the upper part of the nasal cavity and the paranasal sinuses. The maxillary nerve exits through a round opening into the pterygopalatine fossa. It innervates the gums and teeth of the upper jaw, the mucous membrane of the palate, nasal cavity and maxillary sinus, the skin of the nose and cheeks. The mandibular nerve contains sensory and motor fibers, passes through the foramen ovale, innervates the gums and teeth of the lower jaw, the mucous membrane of the tongue, the skin of the cheeks, chin, the lower part of the auricle and the external auditory canal. Motor fibers innervate the masticatory muscles.
VI pair - abducens nerve (n.abducens ), motor. It is formed by the axons of the neurons of the motor nucleus located in the bridge. The nerve emerges from the transverse groove between the pons and the pyramid of the medulla oblongata and goes to the orbit. There it passes through the superior orbital fissure. This nerve innervates the lateral rectus muscle of the eyeball.
VII pair - facial nerve (n. facialis), mixed.
Motor fibers are axons of the motor nucleus, located deep in the bridge under the facial tubercle. Sensory fibers are the central processes of pseudo-unipolar nerve cells of the sensory ganglion (ganglion geniculi) located in the bend of the facial nerve canal (in the thickness of the temporal bone pyramid). In the bridge, sensory fibers terminate at the neurons of the nucleus of the solitary pathway (nucleus tractus solitarius). The preganglionic parasympathetic fibers of the facial nerve originate from two parasympathetic (secretory) nuclei - the superior salivary nucleus (nucleus salivatorius superior) and the lacrimal nucleus (nucleus lacrimalis), which lie in the pons operculum. The facial nerve exits the pons at the cerebellopontine angle. The cranial cavity leaves through the canalis stylo-mastoideum. Innervates all facial muscles, some muscles of the neck, stapedius muscle, taste buds in the anterior 2/3 of the tongue, submandibular and sublingual salivary glands, mucous glands of the palate, nasal cavity, lacrimal gland.
VIII pair, vestibulo-cochlear nerve (n.vestibulo-cochlearis)- a nerve of special sensitivity (auditory and vestibular), consists of two parts: cochlear and vestibular. Each part has its own sensitive node. The cochlear node (cochlear node) is located in the spiral canal of the cochlea. The peripheral processes of the cells of this node end on the cells of the spiral (Korti) organ, and the central processes go to the ventral and dorsal cochlear nuclei of the pons. The totality of the central processes of the bipolar cells of the cochlear node is the cochlear part (pars cochlearis) of the VIII pair. The vestibular node is located at the bottom of the internal auditory meatus. The peripheral processes of the cells of this node form nerves ending at the vestibular receptors of the auditory crests and spots. The central processes of the bipolar cells of the vestibular ganglion make up the vestibular part of the VIII pair and end on the vestibular nuclei of the pons. From the receptors of the inner ear, the vestibulocochlear nerve goes to the internal auditory canal, exits it, enters the substance of the bridge in the region of the cerebellopontine angle, lateral to the facial nerve.
Bridge functions:
1. Conductor function - fibers pass in the ascending and descending direction.
2. Place of exit of the cranial nerves from the V-VIII pair.
Rice. 4. Cross section of the bridge
1. Nucleus nervi abducens (abducens nucleus)
2. Nucleus nervi facialis (nucleus of the facial nerve)
3. Stria medullaris (brain strips)
4. Nucleus cochlearis dorsalis (posterior auditory nucleus)
5. Nucleus tractus spinalis nervi trigemini (spinal nucleus of the trigeminal nerve)
6. Oliva superior (top olive)
7. Nucleus cochlearis ventalis (anterior auditory nucleus)
8. Tractus pyramidalis (pyramidal tract)
9. Corpus trapezoideum (trapezoid body)
10. Lemniscus medialis (medial loop)
11. Nucleus proprius pontis (pontis own nucleus)
12. Lemniscus lateralis (lateral loop)
The bridge of Varolii performs motor, sensory, integrative and conductive functions. Important functions of the bridge are associated with the presence of cranial nerve nuclei in it.
V pair – trigeminal nerve(mixed). The motor nucleus of the nerve innervates the chewing muscles, the muscles of the palatine curtain and the muscles that strain the eardrum. The sensory nucleus receives afferent axons from the receptors of the skin of the face, the nasal mucosa, teeth, 2/3 of the tongue, the periosteum of the bones of the skull, and the conjunctiva of the eyeball.
VI pair - abducens nerve(motor), innervates the rectus extrinsic muscle, which abducts the eyeball outwards.
VII pair - facial nerve(mixed), innervates the mimic muscles of the face, sublingual and submandibular salivary glands, transmits information from the taste buds of the anterior part of the tongue.
VIII pair - vestibulocochlear(sensitive). The cochlear part of the vestibulocochlear nerve ends in the brain in the cochlear nuclei; its vestibular part is in the triangular nucleus, Deiters' nucleus, Bekhterev's nucleus. Here is the primary analysis of vestibular stimuli of their strength and direction.
In the bridge is pneumotaxic center, triggering the exhalation center of the medulla oblongata, as well as a group of neurons that activate the inhalation center.
All ascending and descending paths pass through the bridge, connecting the bridge with the cerebellum, spinal cord, cerebral cortex and other structures of the central nervous system. Through the bridge-cerebellar pathways through the bridge, the controlling influence of the cerebral cortex on the cerebellum is carried out.
2. 3. 3 Cerebellum (cerebellum). The cerebellum (small brain) is located behind the bridge and the medulla oblongata. It consists of a middle, unpaired, phylogenetically old part - a worm - and paired hemispheres, characteristic only of mammals. The cerebellar hemispheres develop in parallel with the cerebral cortex and reach a significant size in humans. The worm on the underside is sunk deep between the hemispheres; its upper surface passes into the hemispheres gradually (Fig. 20).
In general, the cerebellum has extensive efferent connections with all motor systems of the brain stem: corticospinal, rubrospinal, reticulospinal, and vestibulospinal. No less diverse are the afferent inputs of the cerebellum.
The cerebellum is connected to the brain stem by three pairs of peduncles. The thickest medium legs expanding, they pass into the pons. upper legs begin in the dentate nuclei of the cerebellum (see below) and go to the quadrigemina of the midbrain. Third pair of legs (lower) descends, merging with the medulla oblongata. Afferent fibers entering the cerebellum are predominantly part of the middle and lower peduncles, while efferent fibers are collected mainly in the upper cerebellar peduncles.
The entire surface of the cerebellum is divided into lobes by deep grooves. In turn, each lobe is divided into convolutions by parallel grooves; groups of convolutions form the lobules of the cerebellum. Each lobule is designated both by the classical name (tongue, central, apex, etc.) and by the Latin numbering (I - X) in accordance with the common nomenclature.
The hemispheres and the cerebellar vermis consist of a gray matter lying on the periphery - the cortex - and a white matter located deeper, in which clusters of nerve cells are laid that form the nuclei of the cerebellum - the tent nuclei, spherical, corky and serrated (Fig. 21).
The cerebellar cortex has a specific structure that is not repeated anywhere in the central nervous system. It is represented by three layers (Fig. 22). The most superficial layer molecular, it consists of parallel fibers and branchings of dendrites and axons of neurons of the underlying layers. In the lower part of the molecular layer, the bodies of basket cells are located, the axons of which braid the bodies and initial segments of the axons of Purkinje cells. Here, in the molecular layer, there is a certain amount of stellate cells.
In the second layer of the cortex - ganglionic- the bodies of Purkinje cells are concentrated. These large cells are oriented vertically with respect to the surface of the cerebellar cortex. Their dendrites rise up and branch widely in the molecular layer. The dendrites of Purkinje cells contain many spines on which parallel fibers of the molecular layer form synapses. Axons of Purkinje cells descend to the nuclei of the cerebellum. Some of them end on the vestibular nuclei. Axons of Purkinje cells are practically the only exit from the cerebellar cortex.
Under the ganglionic layer lies granular (granular) a layer that contains a large number of granule cell bodies, or granule cells. According to some estimates, their number can reach 10 billion. The axons of granule cells rise vertically upwards into the molecular layer and branch there in a T-shape. The branches run parallel to the surface of the cortex and form synapses on the dendrites of other cells. Here, in the granular layer, there are Golgi cells, the axons of which approach the granule cells.
All cells of the cerebellar cortex are inhibitory, with the exception of granule cells, which have an excitatory effect.
The cerebellum receives two types of fibers. This is, firstly, climbing, or liana-shaped coming from the inferior olives of the medulla oblongata. The lower olive receives afferent impulses from skin receptors, muscles, articular membranes, periosteum through the so-called spinal cerebellar tracts: posterior (dorsal) and anterior (ventral). Climbing fibers branch widely and, like vines, braid the dendrites of Purkinje cells, forming synapses on them. Exciting Purkinje cells, they increase the inhibitory effect of these cells on the nuclei of the bridge.
The second system of afferent fibers is it is mossy or mossy fibers coming from the nuclei of the bridge (which also receives information from the receptors of the muscles, tendons, vestibular nuclei) and ending on the grain cells. Grain cells send nerve impulses to basket cells, stellate cells, Golgi cells, exciting them, which leads to inhibition of the activity of Purkinje cells (removal of the inhibitory effect of the cortex on the nuclei of the cerebellum).
Thus, the path leading to Purkinje cells through liana-like fibers enhances the inhibitory effect of Purkinje cells on the cerebellar nuclei, and the path through mossy fibers, on the contrary, removes this inhibitory effect.
Consequently, the activity of the entire neuronal system of the cerebellar cortex is reduced to the inhibition of the nuclei, over which the cortex is built up. According to the hypothesis put forward by J. Eccles, a large number of inhibitory neurons in the cerebellar cortex prevents the long-term circulation of excitation through neural circuits. Any excitatory impulse, arriving in the cerebellar cortex, turns into inhibition in a time of about 100 ms. This is how an automatic erasure of previous information takes place, which allows the cerebellar cortex to participate in the regulation of fast movements.
From the point of view of the functions that the cerebellum performs, it is divided into three parts: archiocerebellum (ancient cerebellum), paleocerebellum (old cerebellum) and neocerebellum (new cerebellum).
Bark archiocerebellum(the inner part - the cerebellar vermis) is associated with the nucleus of the tent, which regulates the activity of the vestibular nuclei. Therefore, the archiocerebellum is a vestibular regulator. Damage to the ancient structures of the cerebellum leads to imbalance.
Function paleocerebellum, or the middle part of the cortex - mutual coordination of posture and purposeful movement, as well as correction of the performance of relatively slow movements by the feedback mechanism - is implemented with the participation of corky and spherical nuclei. If the structures of the old cerebellum are damaged, it is difficult for patients to stand and walk, especially in the dark, in the absence of visual correction.
neocerebellum(the lateral part of the cerebellar cortex), together with the dentate nucleus, plays an important role in programming complex movements, the implementation of which occurs without the use of a feedback mechanism. As a result, a purposeful movement occurs, performed at high speed, for example, playing the piano. When neocerebellum structures are disturbed, complex sequences of movements are disturbed, they become arrhythmic and slowed down.
The functions of the cerebellum were studied in the clinic with its lesions in humans, as well as in animals by removal (extirpation of the cerebellum) (L. Luciani, L. A. Orbeli). As a result of the loss of cerebellar functions, movement disorders occur (on the side of the lesion), which the Italian physiologist L. Luciani characterized by the famous triad A - astasia, atony and asthenia. Subsequent researchers added another symptom - ataxia (tetrad A).
astasia- loss of the ability for prolonged muscle contraction, which makes it difficult to stand, sit, etc., the inability to maintain a fixed position, continuous rocking movements, trembling of the head, trunk and limbs.
Atony- a sharp drop and improper distribution of muscle tone.
Asthenia- decrease in the strength of muscle contraction, rapid muscle fatigue.
Ataxia- 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, drunken staggering 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 (walking, writing, etc.). When the function of the cerebellum is disturbed, the movements become inaccurate, inharmonious, scattered, and often do not reach the goal.
Dysmetria- a discrepancy between the intensity of muscle contraction and the task of the performed movement, which can be expressed in a decrease or increase in the intensity of reflex reactions. For example, a person, when climbing stairs, raises his knees too high ("cock's gait"), or vice versa, stumbles over every step.
dysarthria- a disorder of the organization of speech motor skills. When the cerebellum is damaged, the patient's speech becomes stretched, words are sometimes pronounced as if in shocks (scanned speech).
Over time, movement disorders are smoothed out. Only biased observation reveals some disturbances (compensation phase). As E. A. Asratyan showed, the compensation of functions occurs due to the cerebral cortex. The cerebellum is involved in the regulation of movements, making them smooth, precise, proportionate.
The cerebellum also influences a number of autonomic functions, such as the gastrointestinal tract, blood pressure, and blood composition.
For a long time, the cerebellum was considered a structure responsible solely for the coordination of movements. Today, scientists are increasingly talking about its participation in the processes of perception and cognitive activity. Thus, neuroscientists studying the cognitive functions of the brain have found that in humans this structure remains highly active during various forms of activity that are not directly related to movements.
2. 3. 4 Midbrain (mesencephalon). The midbrain is located above the bridge and is represented by the legs of the brain and the quadrigemina. The legs of the brain consist of a base and a tire, between which there is a black substance containing highly pigmented cells. The nuclei of the trochlear (IV pair) and oculomotor (III pair) nerves are located in the tegmentum of the brain. The cavity of the midbrain is represented by a narrow canal - the Sylvian aqueduct, which connects the III and IV cerebral ventricles. The length of the midbrain in an adult is about 2 cm, weight - 26 g. In the process of embryonic development, the midbrain is formed from the midbrain bladder, the lateral protrusions of which move laterally and form the retina of the eye, which structurally and functionally represents the nerve center of the midbrain placed on the periphery .
The largest nuclei of the midbrain are the red nuclei, the substantia nigra, the nuclei of the cranial (oculomotor and trochlear) nerves, and the nuclei of the reticular formation. Through the midbrain, there are ascending pathways to the thalamus, cerebral hemispheres, and cerebellum, and descending pathways to the medulla oblongata and spinal cord.
In the midbrain there is a large number of neurons of the reticular formation. In the quadrigemina, the upper and lower colliculi are isolated (Fig. 23).
The midbrain performs a number of functions: conductive, motor and reflex.
Conductor function lies in the fact that all ascending paths to the overlying sections pass through it: the thalamus (medial loop, spinothalamic path), the cerebrum and the 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 implemented due to the nucleus of the block nerve (n. trochlearis), the nuclei of the oculomotor nerve (n. oculomotorius), the red nucleus (nucleus ruber), the black substance (substantia nigra).
Red cores are located in the tire (Fig. 23. - B, 4). Being an integral part of the extrapyramidal system organizing movement, the red nuclei receive nerve impulses from the motor cortex, subcortical nuclei, cerebellar nuclei, substantia nigra of the midbrain and give rise to the rubrospinal (red-nuclear-spinal) tract, which, as a common pathway, provides regulation of skeletal muscle tone, reducing tone extensor muscles. This is evidenced by the classic experience of Ch. Sherrington with the transection of the brain stem. If the transection is made at the level of the anterior edge of the posterior colliculi of the quadrigemina and thus separates the red nucleus from the hindbrain, then the cat develops 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 from the red nucleus, overlying structures, and 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 .
The implementation of rectifying and statokinetic reflexes is associated with the midbrain. Rectifier reflexes consists of two phases: lifting the head and then lifting the body. The first phase is carried out due to reflex influences from the receptors of the vestibular apparatus and skin, the second - from the proprioreceptors of the muscles of the neck and trunk. Statokinetic reflexes aimed at returning the body to its original position when moving the body in space, during rotation.
Another functionally important midbrain nucleus is black substance(Semmering) (Fig. 23. - 5). It is connected with the basal ganglia lying at the base of the forebrain hemispheres - the striatum and the pale ball - and regulates the acts of chewing, swallowing (their sequence), provides accurate 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.
Damage to the substantia nigra, causing degeneration of dopaminergic pathways to the striatum, is associated with severe neurological disease, Parkinson's disease. Parkinsonism is manifested in the violation of fine friendly movements, the function of mimic muscles and the appearance of involuntary muscle contractions, or tremors. This painful syndrome can be relieved with the introduction of L-dioxyphenylalanine, a substance from which dopamine is synthesized in the body.
Thus, by replenishing the mediator deficiency, it became possible to stop the neurological disease and at the same time provide actual evidence of the role of the substantia nigra of the midbrain in sensorimotor coordination of movements.
Neurons of nuclei oculomotor and trochlear nerves regulate the movement of the eye up, down, out, towards the nose, and down towards 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 the 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 the light source, etc. colliculus) or sound (lower colliculus) 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 or 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.
2. 3. 5 Diencephalon. The diencephalon is located under the corpus callosum and fornix, growing together on the sides with the cerebral hemispheres. It includes: thalamus (visual tubercles), hypothalamus (hypothalamus), epithalamus (supratuberous region) and metathalamus (extratuberous region). The cavity of the diencephalon is the third ventricle of the brain.
Epithalamus includes the endocrine gland - the pineal gland (pineal gland). In the dark, it produces the hormone melatonin, which inhibits puberty and also affects the growth of the skeleton.
Metathalamus represented by lateral and medial geniculate bodies. Lateral, or the outer, geniculate body - this subcortical center of vision has direct efferent connections with the occipital lobe of the cerebral cortex and afferent connections with the retina and with the anterior tubercles of the quadrigemina. The neurons of the lateral geniculate bodies react differently to color stimuli, turning on and off the light, i.e., they can perform a detector function.
Medial geniculate body subcortical, thalamic center of hearing, it receives afferent impulses from the lateral loop and from the inferior tubercles of the quadrigeminae. Efferent paths from the medial geniculate bodies go to the temporal lobe of the cerebral cortex, reaching the primary auditory zone there. The medial geniculate body has a clear tonotopicity. Consequently, already at the level of the diencephalon, the spatial distribution of the sensitivity of all sensory systems of the body, including sensory transmissions from the interoreceptors of blood vessels, organs of the abdominal and thoracic cavities, is ensured.
thalamus(thalamus, visual tubercle) - a paired egg-shaped organ (Fig. 24), the anterior part of which is pointed (anterior tubercle), and the posterior expanded part (pillow) hangs over the geniculate bodies. The medial surface of the thalamus faces the cavity of the third ventricle of the brain.
thalamus – sensitive nucleus of the subcortex. It is called the "collector of sensitivity", since afferent (sensory) paths from all receptors converge to it, except for the olfactory ones.
In the nuclei of the thalamus, the information coming from the extero-, proprio- and interoceptors is switched to the thalamocortical pathways that begin here.
The main function of the thalamus is the integration (unification) of all types of sensitivity. To analyze the external environment, signals from individual receptors are not enough. In the thalamus, the information received through various channels is compared and its biological significance is assessed. There are about 40 pairs of nuclei in the visual hillock, which are divided into specific (ascending afferent pathways end on the neurons of these nuclei), non-specific (nuclei of the reticular formation) and associative. Through associative nuclei the thalamus is connected with all the motor nuclei of the subcortex - the striatum, the pale ball, the hypothalamus and with the nuclei of the midbrain and medulla oblongata.
From specific nuclei information about the nature of sensory stimuli enters strictly defined areas of III-IV layers of the cerebral cortex (somatotopic localization). Violation of the function of specific nuclei leads to the loss of specific types of sensitivity, since the nuclei of the thalamus, like the cerebral cortex, have somatotopic localization. Individual neurons of specific nuclei of the thalamus are excited by receptors of only their own type. Signals from the receptors of the skin, eyes, ear, and muscular system go to the specific nuclei of the thalamus. Signals from the interoreceptors of the projection zones of the vagus and celiac nerves, the hypothalamus also converge here.
Neurons non-specific nuclei form their connections according to the reticular type. Their axons rise to the cerebral cortex and contact with all its layers, forming not local, but diffuse connections. Nonspecific nuclei receive connections from the reticular formation of the brain stem, hypothalamus, limbic system, basal ganglia, and specific thalamic nuclei. Excitation of nonspecific nuclei causes the generation of specific spindle-shaped electrical activity in the cortex, indicating the development of a sleepy state. Violation of the function of nonspecific nuclei makes it difficult for spindle-shaped activity to appear, i.e., the development of a sleepy state. The complex structure of the thalamus, the presence of interconnected specific, nonspecific and associative nuclei in it, allows it to organize such motor reactions as sucking, chewing, swallowing, and laughing. Motor reactions are integrated in the thalamus with autonomic processes that provide these movements.
The visual hillock is the center of the organization and realization of instincts, drives, emotions. The ability to receive information about the state of many body systems allows the thalamus to participate in the regulation and determination of the functional state of the body as a whole. The convergence of sensory stimuli to the thalamus causes the emergence of so-called thalamic intractable pains that occur during pathological processes in the thalamus itself.
In the clinic, the symptoms of damage to the visual tubercles are severe headache, sleep disorders, sensitivity disorders, both upward and downward, movement disorders, their accuracy, proportionality, the occurrence of violent involuntary movements.
Hypothalamus (hypothalamus, hypothalamus)- the structure of the diencephalon, which is part of the limbic system, organizing the emotional, behavioral, homeostatic reactions of the body (Fig. 25).
The hypothalamus has a large number of nerve connections with the cerebral cortex, basal ganglia, thalamus, midbrain, pons, medulla oblongata and spinal cord.
The hypothalamus consists of a gray tubercle, a funnel with a neurohypophysis, and mastoid bodies. Morphologically, in the neuronal structures of the hypothalamus, about 50 pairs of nuclei can be distinguished, which have their own specific function. Topographically, these nuclei can be combined into 5 groups: 1) the anterior group, which includes the supraoptic, paraventricular nuclei; 2) the posterior group is formed from the medial and lateral nuclei of the mastoid bodies and the posterior hypothalamic nucleus; 3) the middle group consists of the lower medial and upper medial nuclei; 4) the preoptic group has pronounced connections with the telencephalon and is divided into medial and lateral preoptic nuclei; 5) the outer group includes the lateral hypothalamic field and the serotuberous nuclei.
The nuclei of the hypothalamus have a powerful blood supply, which is confirmed by the fact that a number of nuclei of the hypothalamus have an isolated duplicating blood supply from the vessels of the arterial circle of the large brain (circle of Willis). There are up to 2600 capillaries per 1 mm 2 of the area of the hypothalamus, while on the same area of the V layer of the precentral gyrus (motor cortex) there are 440 of them, in the hippocampus - 350, in the pale ball - 550, in the occipital lobe of the cerebral cortex (visual cortex ) - 900. The capillaries of the hypothalamus are highly permeable for large molecular protein compounds, which include nucleoproteins, which explains the high sensitivity of the hypothalamus to neuroviral infections, intoxications, and humoral changes.
In humans, the hypothalamus finally matures by the age of 13-14, when the formation of the hypothalamic-pituitary neurosecretory connections ends. Due to powerful afferent connections with the olfactory brain, basal ganglia, thalamus, hippocampus, cerebral cortex, the hypothalamus receives information about the state of almost all brain structures. At the same time, the hypothalamus sends information to the thalamus, the reticular formation, the autonomic centers of the brain stem and spinal cord.
The neurons of the hypothalamus have features that determine the specifics of the functions of the hypothalamus itself. These features include the sensitivity of neurons to the composition of the blood washing them, the absence of the blood-brain barrier between neurons and blood, the ability of neurons to neurosecrete peptides, neurotransmitters, etc.
Influence on sympathetic and parasympathetic regulation allows the hypothalamus to influence the autonomic functions of the body through humoral and nervous pathways.
Irritation of nuclei front group accompanied by parasympathetic effects. Irritation of nuclei rear group causes sympathetic effects in the work of organs. Nucleus stimulation middle group leads to a decrease in the influence of the sympathetic division of the autonomic nervous system. The specified distribution of functions of the hypothalamus is not absolute. All structures of the hypothalamus are capable of inducing sympathetic and parasympathetic effects to varying degrees. Consequently, there are functional complementary, mutually compensating relationships between the structures of the hypothalamus.
In general, due to the large number of connections, polyfunctionality of structures, the hypothalamus performs an integrating function of autonomic, somatic and endocrine regulation, which is also manifested in the organization of a number of specific functions by its nuclei. So, in the hypothalamus there are centers of homeostasis, thermoregulation, hunger and satiety, thirst and its satisfaction, sexual behavior, fear, rage. All these centers realize their functions by activating or inhibiting the autonomic (vegetative) division of the nervous system, the endocrine system, brainstem and forebrain structures.
A special place in the functions of the hypothalamus is occupied by the regulation of the activity of the pituitary gland. In the hypothalamus and pituitary gland, neuroregulatory peptides are also formed - enkephalins, endorphins, which have a morphine-like effect and help reduce stress, etc.
Neurons of the nuclei of the anterior group The hypothalamus produces vasopressin, or antidiuretic hormone (ADH), oxytocin, and other peptides that travel along the axons to the posterior pituitary gland, the neurohypophysis.
Neurons of the nuclei of the middle group hypothalamus produce so-called releasing factors, stimulating (liberins) and inhibiting (statins) the activity of the anterior pituitary gland - adenohypophysis. It produces substances such as somatotropic, thyroid-stimulating and other hormones. The presence of such a set of peptides in the structures of the hypothalamus indicates their inherent neurosecretory function.
The neurons of the hypothalamus also have a detecting function: they respond to changes in blood temperature, electrolyte composition and plasma osmotic pressure, the amount and composition of blood hormones.
Research Delgado (Delgado) during surgical operations showed that in humans, irritation of certain areas of the hypothalamus caused euphoria, erotic experiences. The clinic has also shown that pathological processes in the hypothalamus may be accompanied by an acceleration of puberty, menstrual irregularities, and sexual function.
Irritation of the anterior hypothalamus can cause a passive-defensive reaction in animals, rage, fear, and irritation of the posterior hypothalamus causes active aggression. In addition, irritation of the posterior hypothalamus leads to exophthalmos, dilated pupils, increased blood pressure, narrowing of the lumen of arterial vessels, and contractions of the gallbladder and bladder. There may be outbursts of rage with the described sympathetic manifestations. Injections in the hypothalamus cause glucosuria, polyuria. In a number of cases, irritation caused a violation of thermoregulation: the animals became poikilothermic, they did not develop a feverish state.
The hypothalamus is also the center of regulation of the wake-sleep cycle. At the same time, the posterior hypothalamus activates wakefulness, stimulation of the anterior one causes sleep. Damage to the posterior hypothalamus can cause so-called lethargic sleep.
2. 3. 6 End brain (telencephalon). The telencephalon is the youngest in phylogenetic terms. It consists of two hemispheres, each of which is represented by a cloak, an olfactory brain, and basal or subcortical ganglia (nuclei). The length of the hemispheres is on average 17 cm, height - 12 cm. The cavity of the telencephalon is the lateral ventricles located in each of the hemispheres. The hemispheres of the cerebrum are separated from each other by the longitudinal fissure of the cerebrum and are connected by the corpus callosum, the anterior and posterior commissures, and the commissure of the fornix. The corpus callosum consists of transverse fibers that continue laterally into the hemispheres, forming the radiance of the corpus callosum.
Olfactory brain represented by olfactory bulbs, olfactory tubercle, transparent septum and adjacent areas of the cortex (preperiform, periamygdala and diagonal). This is the smallest part of the telencephalon, it provides the function of the first sense organ that appeared in living beings - the function of smell, and, in addition, is part of the limbic system. Damage to the structure of the limbic system causes profound impairment of emotions and memory.
Basal ganglia(kernels of gray matter) are located in the depths of the cerebral hemispheres. They make up about 3% of their volume. The basal ganglia form numerous connections both between the structures that make up them and other parts of the brain (cerebral cortex, thalamus, substantia nigra, red nucleus, cerebellum, motor neurons of the spinal cord). The basal ganglia include a strongly elongated and curved caudate nucleus(Fig. 26.–1) and embedded in the thickness of the white matter lenticular nucleus. With two white plates, it is divided into the largest, lying laterally shell and pale ball(Fig. 26.–2, 3). The caudate nucleus and putamen are united under the name striatum, they are anatomically related and are characterized by alternating white and gray matter.
striatum takes part in the organization and regulation of movements and ensuring the transition of one type of movement to another. Stimulation caudate nucleus inhibits the perception of visual, auditory and other types of sensory information, inhibits the activity of the cortex, subcortex, unconditioned reflexes (food, defensive, etc.) and the development of conditioned reflexes, leads to the onset of sleep. With a lesion of the striatum, retroanterograde amnesia is observed - loss of memory for events preceding the injury. Bilateral damage to the striatum induces the desire to move forward, unilateral - leads to arena movements. With a violation of the functions of the striatum, a disease of the nervous system is associated - chorea (auxiliary and mimic movements are intensified). Shell provides organization of eating behavior. When it is damaged, trophic skin disorders are observed, and its irritation causes salivation and a change in respiration.
Functions pale ball consist in provoking an orienting reaction, movement of the limbs, eating behavior (chewing, swallowing).
After the destruction of the pale ball, there are stiffness of movements, impoverishment of facial expressions (mask-like face), hypodynamia, emotional dullness, tremor of the head, limbs during movement, monotonous speech. When the pale ball is damaged, twitching of individual muscles of the face and trunk may appear, the synergism of the movement of the limbs when walking is disturbed. A person with globus pallidus dysfunction has difficulty initiating movements, auxiliary and reactive movements disappear when standing up, friendly hand movements when walking are disturbed, a symptom of propulsion appears: prolonged preparation for movement, then rapid movement and stop. Such cycles in patients are repeated many times.
Raincoat in man is presented bark, i.e., a plate of gray matter separated from the cavity of the ventricles white matter, which contains a huge number of nerve fibers, divided into three groups.
1. association fibers, or paths, connect different parts of the cerebral cortex within the same hemisphere. There are short or arcuate associative fibers that connect two adjacent gyruses, and long ones that stretch from one lobe to another, remaining within the same hemisphere.
2. commissural e or adhesive fibers connect the cortex of both hemispheres. The largest commissure in the brain is the corpus callosum.
3. projection paths connect the cerebral cortex with the periphery. There are centrifugal (efferent, motor) fibers that carry nerve impulses from the cortex to the periphery, and centripetal (afferent, sensory) fibers that carry impulses from the periphery to the cerebral cortex.
The highest department of the CNS is the cerebral cortex (cerebral cortex). It provides a perfect organization of animal behavior on the basis of congenital and ontogenesis-acquired functions.
The cerebral cortex is divided into ancient (archicortex), old (paleocortex) and new (neocortex). The ancient cortex, along with other functions, is related to the sense of smell and ensuring the interaction of brain systems. The old cortex includes the cingulate gyrus, the hippocampus. In the new cortex, the greatest development of size, differentiation of functions is noted in humans. The thickness of the new cortex ranges from 1.5 to 4.5 mm and is maximum in the anterior central gyrus.
The cortex of the hemispheres is covered with furrows and convolutions (Fig. 27). Due to this, the surface of the cortex is significantly increased. Distinguish the deepest primary furrows, which divide the hemispheres into lobes. Lateral furrow(Silviev) separates the frontal lobe from the temporal, central sulcus(Rolandova) - frontal from the parietal. Parietal-occipital the sulcus is located on the medial surface of the hemisphere and separates the parietal and occipital lobes; on the upper lateral surface, there is no clear border between these lobes. On the medial surface there is a cingulate sulcus, which passes into the hippocampal sulcus, which limit the olfactory brain from the rest of the lobes.
In the depths of the lateral furrow (Fig. 28) is located insula. It is surrounded on three sides by a circular furrow, its surface is indented with furrows and convolutions. Functionally, the insula is associated with the olfactory medulla.
The secondary furrows are less deep, they divide the lobes into convolutions and are located outside the convolutions of the same name. Tertiary (nameless) furrows give the convolutions an individual shape, increase the area of their cortex.
The functions of individual zones of the new cortex are determined by the features of its structural and functional organization, connections with other brain structures, participation in the perception, storage and reproduction of information in the organization and implementation of behavior, regulation of the functions of sensory systems, internal organs.
Features of the structural and functional organization of the cerebral cortex are due to the fact that in evolution there was a corticalization of functions, i.e., the transfer of the functions of the underlying brain structures to the cerebral cortex. However, this transfer does not mean that the cortex takes over the functions of other structures. Its role is reduced to the correction of possible violations of the functions of the systems interacting with it, more perfect, taking into account individual experience, analysis of signals and organization of an optimal response to these signals, the formation in one’s own and in other interested brain structures of memory traces about the signal, its characteristics, meaning and the nature of the reaction to it. In the future, as automation proceeds, the reaction begins to be carried out by subcortical structures.
Cytoarchitectonics of the cerebral cortex. The total area of the human cerebral cortex is about 2200 cm 2, the number of cortical neurons exceeds 10 billion. The cortex contains pyramidal, stellate, spindle-shaped neurons.
pyramidal neurons are of different sizes, their dendrites carry a large number of spines; the axon of the pyramidal neuron, as a rule, goes through the white matter to other areas of the cortex or to the structures of the central nervous system.
stellate cells have short, well-branched dendrites and a short ascon, which provides connections between neurons within the cerebral cortex itself.
Fusiform neurons provide vertical or horizontal interconnections of neurons of different layers of the cortex.
The cerebral cortex has a predominantly six-layer structure (Fig. 29).
Layer I - the upper molecular layer, is represented mainly by branching of the ascending dendrites of pyramidal neurons, among which there are rare horizontal cells and granule cells, fibers of the nonspecific nuclei of the thalamus come here, regulating the level of excitability of the cerebral cortex through the dendrites of this layer.
Layer II - outer granular, consists of stellate cells that determine the duration of the circulation of excitation in the cerebral cortex, that is, related to memory.
Layer III - outer pyramidal, is formed from pyramidal cells of small size and, together with layer II, provide cortical-cortical connections of various convolutions of the brain.
Layer IV is internal granular and contains predominantly stellate cells. Here the specific thalamocortical pathways end, that is, the pathways starting from the receptors of the analyzers.
Layer V is the inner pyramidal (ganglion) layer of large pyramids, which are the output neurons, their axons go to the brainstem and spinal cord. In the motor zone in this layer there are giant pyramidal cells discovered by Betz (Betz cells).
Layer VI is a layer of polymorphic cells; most of the neurons in this layer form corticothalamic pathways.
The cellular composition of the cortex in terms of diversity of morphology, function, and forms of communication is unparalleled in other parts of the CNS. The neuronal composition, the distribution of neurons in layers in different areas of the cortex are different, which made it possible to distinguish 53 cytoarchitectonic fields (Brodmann fields) in the human brain. The division of the cerebral cortex into cytoarchitectonic fields is more clearly formed as its function improves in phylogenesis.
In humans and higher mammals, along with primary, secondary and tertiary cortical fields are distinguished, which ensure the association of the functions of a given analyzer with the functions of other analyzers. All analyzers are characterized by the somatotopic principle of organization
A feature of cortical fields is the screen principle of their functioning. This principle lies in the fact that the receptor projects its signal not on one neuron of the cortex, but on the field of neurons, which is formed by their collaterals and connections. As a result, the signal is focused not point to point, but on a variety of different neurons, which ensures its complete analysis and the possibility of transferring it to other interested structures. Thus, one fiber entering the visual cortex can activate a zone 0.1 mm in size. This means that one axon distributes its action to more than 5,000 neurons.
Input (afferent) impulses enter the cortex from below, rise to the stellate and pyramidal cells of the III-V layers of the cortex. From the stellate cells of layer IV, the signal goes to the pyramidal neurons of layer III, and from here along the associative fibers to other fields, areas of the cerebral cortex. The stellate cells of field 3 switch the signals going to the cortex to the pyramidal neurons of layer V, from here the processed signal goes from the cortex to other brain structures.
In the cortex, input and output elements, together with stellate cells, form the so-called columns - functional units of the cortex, organized in a vertical direction. This is evidenced by the following: if the microelectrode is immersed perpendicularly into the cortex, then on its way it encounters neurons that respond to one type of stimulation, but if the microelectrode is inserted horizontally along the cortex, then it encounters neurons that respond to different types of stimuli.
The column diameter is about 500 µm and is determined by the area of distribution of collaterals of the ascending afferent thalamocortical fiber. Neighboring columns have interrelations that organize sections of a plurality of columns in the organization of a particular reaction. The excitation of one of the columns leads to the inhibition of the neighboring ones.
As already mentioned, different areas of the cerebral cortex have different fields, determined by the nature and number of neurons, the thickness of the layers, etc. The presence of structurally different fields also implies their different functional purpose (Fig. 30). Indeed, sensory, motor and associative areas are distinguished in the cerebral cortex.