Excitation convergence principle(or the principle of a common final path, Sherrington's funnel). Convergence of nerve impulses means the convergence of two or more different excitations to one neuron at the same time.
This phenomenon was discovered by C. Sherrington. He showed that the same movement, such as reflex flexion of a limb at the knee joint, can be evoked by stimulating different reflexogenic zones. In this regard, he introduced the concept of a "common final path" or "funnel principle", according to which impulse flows from different neurons can converge on the same neuron (in this case, on the alpha motor neurons of the spinal cord). In particular, Ch. Sherrington found convergence to the same intermediate or efferent neurons of various afferents from different parts of the common receptive field (in the spinal cord and medulla oblongata) or even from different receptive fields (in the higher parts of the brain). It has now been shown that excitatory convergence, as well as excitatory divergence, is a very common phenomenon in the CNS.
The basis for convergence (as well as for irradiation) is a certain morphological and functional structure of various parts of the brain. Obviously, some of the convergent pathways are innate, while the other part (mainly in the cerebral cortex) is acquired as a result of learning in the process of ontogenesis. The formation of new convergent relationships for the neurons of the cerebral cortex during ontogenesis is largely associated with the formation of dominant foci of excitation in the cortex, which are able to "attract" excitation from other neurons.
Centers of vegetative nervous system
The centers of the autonomic nervous system are located in the spinal, medulla oblongata, midbrain, hypothalamus, cerebellum, reticular formation and cerebral cortex. Their interaction is based on the principle of hierarchy. The conditionally distinguished "lower floors" of this hierarchy, having sufficient autonomy, carry out local regulation of physiological functions. Each higher level of regulation provides more a high degree integration of autonomic functions.
1. Mesencephalic - fibers are part of the oculomotor nerve (parasympathetic)
2. Bulbar - fibers in the facial, glossopharyngeal and vagus nerves (parasympathetic)
3. Thoracolumbar - the nuclei of the god's horns from the 8th cervical to the 3rd lumbar segments (sympathetic)
4. Sacred - in the 2nd-4th segments of the sacral spinal cord (parasympathetic)
Divisions of the autonomic nervous system
sympathetic department. The bodies of the first neurons of the sympathetic division of the ANS are located mainly in the posterior nuclei of the hypothalamus, the midbrain and medulla oblongata and in the anterior horns of the spinal cord, starting from
1st thoracic and ending with 3rd, 4th segment of its lumbar department.
Parasympathetic department. The central neurons of the parasympathetic division of the autonomic nervous system are located mainly in the anterior hypothalamus, midbrain and medulla oblongata, in the 2nd-4th segments of the sacral spinal cord.
The sympathetic nervous system is activated during stress reactions. It is characterized by a generalized influence, while sympathetic fibers innervate the vast majority of organs.
It is known that parasympathetic stimulation of some organs has an inhibitory effect, while others have an excitatory effect. In most cases, the action of the parasympathetic and sympathetic systems is opposite.
Sympathetic synapse
Sympathetic synapses are formed not only in the region of numerous terminal branches of the sympathetic nerve, as in all other nerve fibers, but also in membranes varicose veins- numerous extensions of the peripheral sections of sympathetic fibers in the region of innervated tissues. Varicose veins also contain synaptic vesicles with a mediator, although in lower concentrations than the terminal endings.
The main mediator of sympathetic synapses is norepinephrine and such synapses are called adrenergic. The receptors that bind the adrenergic neurotransmitter are called adrenoreceptors. There are two types of adrenergic receptors - alpha and beta, each of which is divided into two subtypes - 1 and 2. A small part of the sympathetic synapses uses the mediator acetylcholine and such synapses are called cholinergic, and the receptors cholinergic receptors. Cholinergic synapses of the sympathetic nervous system are found in the sweat glands. In adrenergic synapses, in addition to norepinephrine, adrenaline and dopamine, also related to catecholamines, are contained in much smaller amounts, so the mediator substance in the form of a mixture of three compounds was previously called sympathin.
The action of postganglionic nerve fibers on the effector is ensured by the release of mediators into the synaptic cleft, which affect the postsynaptic membrane - the membrane of the cell of the working organ. Postganglionic parasympathetic fibers secrete acetylcholine, which binds to M-cholinergic receptors, i.e. muscari-
but similar to receptors (M - XP).
Parasympathetic synapse
Parasympathetic postganglionic or peripheral synapses use acetylcholine as a mediator, which is located in the axoplasm and synaptic vesicles of presynaptic terminals in three main pools or funds. It,
firstly, stable, strongly associated with the protein, not ready to release the mediator pool;
Secondly, mobilization, less firmly bound and suitable for release, pool;
third A ready to be deallocated spontaneously or actively allocated pool. In the presynaptic ending, pools are constantly moving in order to replenish the active pool, and this process is also carried out by moving synaptic vesicles to the presynaptic membrane, since the mediator of the active pool is contained in those vesicles that are directly adjacent to the membrane. The release of the mediator occurs in quanta, the spontaneous release of single quanta is replaced by an active one upon receipt of excitation impulses that depolarize the presynaptic membrane. The process of release of neurotransmitter quanta, as well as in other synapses, is calcium-dependent.
Charles Sherrington published the book: The Integrative Action of the Nervous System, where he outlined the principle of organizing an effector reaction, which he called the "Principle of a common final path." The term "Sherrington's funnel" is sometimes used in the literature.
“According to his ideas, the quantitative predominance of sensory and other incoming fibers over motor fibers creates an inevitable collision of impulses in a common final path, which is a group of motor neurons and the muscles innervated by them. Thanks to this collision blocking of all influences is achieved, except for one, which regulates the course of the reflex reaction. The principle of a common final path, as one of the principles of coordination, applies not only to the spinal cord, but also to any other part of the central nervous system.
Shcherbatykh Yu.V., Turovsky Ya.A., Physiology of the central nervous system for psychologists, St. Petersburg, "Peter", 2007, p. 105.
To explain this principle, a metaphor is often used: suppose five trains arrive at a railway station along five tracks, but only one track leaves the station and, accordingly, only one train leaves the station per unit of time ...
Thus, the very principles of the organization of the nervous system suggest that only some of the external influences, under conditions of their simultaneous influence on the body, will receive “access” to the muscles at the output. Some selection, selection of stimuli, rejection of some of them is the law of the activity of the nervous system. Myself Charles Sherrington believed that the most important factor ensuring the choice of one of several possible influences is the strength of the influence: a strong influence, as it were, suppresses, displaces weaker ones ...
Principles of coordination in the CNS
Coordination - this is concordance and conjugation nervous processes characteristic of the activity of the central nervous system (CNS).
1. The principle of reciprocal (conjugated, mutually exclusive) innervation.
2. The principle of a common final path (the principle of convergence, "Ch. Sherrington's funnel").
3. Dominant principle.
4. The principle of time O th connection.
5. The principle of self-regulation (direct and feedback).
6. The principle of hierarchy (subordination).
1. The principle of reciprocal (conjugated, mutually exclusive) innervation
The principle of reciprocal innervation of antagonist muscles was first discovered in 1896 by the outstanding Russian physiologist N.E. Vvedensky, a student of I.M. Sechenov.
The contraction of the flexor causes a decrease in the tone of the extensor on the same side, and on the opposite
side - on the contrary: it can cause an increase in the tone of the extensor.
The walking reflex is based on the reciprocal principle. Thus, walking is a conditioned reflex, based on the principle of reciprocal innervation, cyclic motor activity of the legs.
Excitation of the flexor causes conjugated inhibition and relaxation of the extensor: a cross-extension reflex occurs.
2. The principle of a common final path (principle of convergence)
This principle was discovered and investigated by the outstanding English physiologist Sir C.S. Sherrington (Charles Scott Sherrington) in 1896.
He found that in the nerve centers the number of afferent (bringing) cells is much greater than the number of efferent (carrying out) neurons that carry excitation to the muscles. It turns out that there is a struggle between neurons "for a common final path", i.e. for transferring their excitation to the effect neurons. This principle has also received the figurative name of "Sherrington's funnel".
Dominant (from Latin "to dominate") is a temporarily dominant reflex that subjugates the arcs of other reflexes. The dominant exists in the form of a stable focus of excitation, subjugating other excited foci.
The dominant can be humoral, or it can be artificially induced by causing depolarization of a part of the brain with the help of chemical or electrical influences.
Dominant examples:
The frog's attempts to get himself off the hook.
Features of the dominant focus (center):
- increased excitability,
- increased resistance (resistance to braking effects),
- inhibitory effect on other excited foci,
- the ability to summation of excitation from neighboring areas,
- the duration of the existence of this excited focus,
- inertia, i.e. prolonged retention of the excited state after the termination of the initial excitation and resistance to inhibitory action.
The dominant was discovered in 1924 by A.A. Ukhtomsky, a major domestic physiologist, a student of another major physiologist - N.E. Vvedensky.
The essence of this phenomenon lies in the fact that if there is a dominant focus that has excitation, then any other excitation will enhance the reaction of this particular dominant focus. And the reflex response will correspond exactly to the dominant focus (dominant nerve center), and not to the stimulus. We can say that the dominant disrupts the flow of classical conditioned and unconditioned reflexes. In addition, the dominant focus inhibits all other centers and suppresses their excitation. Thus, the dominant, as it were, filters the excitation coming from different sources. inhibits all extraneous unnecessary impulses.
In the 1960s, V.S. Rusinov obtained an artificial dominant by weak electrical stimulation of the 6th layer of the cerebral cortex.
Sometimes the basis of the dominant is a decrease in lability (mobility of nervous processes).
Dominant forms
1. Sensitive (touch).
2. Motor.
By mechanism:
1. Reflex.
2. Humoral (hungry, sexual).
Location level:
1. Spinal (spinal cord).
2. Bulbar (medulla oblongata).
3. Mesencephalic (midbrain).
4. Diencephalic (midbrain).
5. Cortical (cortical).
4. The principle of temporary connection
The highest form of temporal connection is a conditioned reflex.
5. The principle of self-regulation (direct and feedback)
Direct and reverse links are ways of influence of the control object on the controlled object. Accordingly, the influence can be direct and reverse.
Feedbacks, in turn, are divided into positive (reinforcing) and negative (weakening).
6. The principle of hierarchy (subordination)
The principle of hierarchy is very simple - the underlying structures are subordinate to the overlying ones. This means that the overlying structures are able to both adjust and slow down the underlying structures.
There is also a functional hierarchy. So, highest place in the hierarchy of unconditioned reflexes, the defensive reflex occupies, then the food reflex, then the sexual reflex. But in more cases, leadership can be captured by the sexual reflex, pushing into the background eating behavior and even the instinct of self-preservation.
The principle of irradiation of excitations.
The neurons of different centers are interconnected by intercalary neurons, therefore, impulses that arrive with strong and prolonged stimulation of the receptors can cause excitation not only of the neurons of the center of this reflex, but also of other neurons. For example, if one of the hind legs is irritated in a spinal frog, slightly squeezing it with tweezers, then it contracts (defensive reflex), if the irritation is increased, then both hind legs and even the front legs contract. Irradiation of excitation
Fig.2.9. Scheme of the irradiation process.
provides with strong and biologically significant stimuli the inclusion of a larger number of motor neurons in the response. The basis of the irradiation of excitation is the phenomenon of divergence described above (Fig. 3.11).
Impulses coming to the CNS through different afferent fibers can converge (converge) to the same intercalary, or efferent, neurons. (Figure 3.12.). Sherrington called this phenomenon "the principle of a common final path". The same motor neuron can be excited by impulses coming from different receptors (visual, auditory, tactile), i.e. participate in many reflex reactions (include in various reflex arcs). So, for example, motor neurons innervating the respiratory muscles, in addition to providing inspiration, participate in such reflex reactions as sneezing, coughing, etc. On motor neurons, as a rule, impulses from the cerebral cortex and from many subcortical centers converge (through intercalary neurons or due to direct nerve connections).
|
Under the conditions of the physiological norm, the work of all organs and systems of the body is coordinated: on influences from external and internal environment the body reacts as a whole. The coordinated manifestation of individual reflexes that ensure the implementation of integral working acts is called coordination..
Phenomena of coordination play an important role in the activity of the motor apparatus. The coordination of such motor acts as walking or running is provided by the interconnected work of the nerve centers.
Due to the coordinated work of the nerve centers, the body is perfectly adapted to the conditions of existence. This occurs not only due to the activity of the motor apparatus, but also due to changes in the vegetative functions of the body (the processes of respiration, blood circulation, digestion, metabolism, etc.).
Row installed general patterns - principles of coordination: 1) the principle of convergence; 2) the principle of irradiation of excitation; 3) the principle of reciprocity; 4) the principle of sequential change of excitation by inhibition and inhibition by excitation; 5) the phenomenon of "recoil"; 6) chain and rhythmic reflexes; 7) the principle of a common final path; 8) the principle of feedback; 9) the principle of dominance.
Let's analyze some of them.
Convergence principle. This principle was established by the English physiologist Sherrington. Impulses coming to the central nervous system through different afferent fibers can converge (converge) to the same intercalary and efferent neurons. The convergence of nerve impulses is explained by the fact that there are several times more afferent neurons than efferent ones, therefore afferent neurons form numerous synapses on the bodies and dendrites of efferent and intercalary neurons.
Principle of irradiation. Impulses entering the central nervous system with strong and prolonged irritation of the receptors cause excitation not only of this reflex center, but also of other nerve centers. This spread of excitation in the central nervous system is called irradiation. The process of irradiation is associated with the presence in the central nervous system of numerous branches of axons, and especially dendrites of nerve cells and chains of intercalary neurons, which unite various nerve centers with each other.
The principle of reciprocity(conjugation) in the work of the nerve centers. This phenomenon was studied by I. M. Sechenov, N. E. Vvedensky, Sherrington. Its essence lies in the fact that when some nerve centers are excited, the activity of others can be inhibited. The principle of reciprocity was shown in relation to the nerve centers of the antagonist muscles - the flexors and extensors of the limbs. It manifests itself most clearly in animals with a removed brain and preserved dorsal (spinal animal). If the skin of the extremities of a spinal animal (cat) is irritated, a flexion reflex of this limb is noted, and an extension reflex is observed on the opposite side at this time. The described phenomena are related to the fact that when the flexion center of one limb is excited, reciprocal inhibition of the extension center of the same limb occurs. On the symmetrical side, there are reverse relationships: the extensor center is excited and the flexor center is inhibited. Only with such mutually combined (reciprocal) innervation is the act of walking possible.
Coupled, reciprocal inhibition of other reflexes can also occur. Under the influence of the brain, reciprocal relationships can change. A person or animal, if necessary, can bend both limbs, jump, etc.
The reciprocal relationships of the centers of the brain determine the ability of a person to master complex labor processes and no less complex special movements that occur during swimming, acrobatic exercises, etc.
The principle of a common final path. This principle is associated with the peculiarity of the structure of the central nervous system. This feature, as already mentioned, is that there are several times more afferent neurons than efferent ones, as a result of which various afferent impulses converge to common output paths. The quantitative relationships between neurons can be schematically represented as a funnel: excitation flows into the central nervous system through a wide bell (afferent neurons) and flows out of it through a narrow tube (efferent neurons). Common paths can be not only terminal efferent neurons, but also intercalary ones.
Pulses converging in a common path "compete" with each other for the use of this path. This is how the ordering of the reflex response is achieved, the subordination of reflexes and the inhibition of less significant ones. At the same time, the body gets the opportunity to respond to various stimuli from the external and internal environment with the help of a relatively small number of executive organs.
Feedback principle. This principle was studied by I. M. Sechenov, Sherrington, P. K. Anokhin and a number of other researchers. With reflex contraction of skeletal muscles, proprioreceptors are excited. From proprioreceptors, nerve impulses again enter the central nervous system. This controls the accuracy of the movements made. Similar afferent impulses arising in the body as a result of the reflex activity of organs and tissues (effectors) are called secondary afferent impulses, or feedback.
Feedback can be positive or negative. Positive feedbacks contribute to the strengthening of reflex reactions, negative - to their oppression.
Due to positive and negative feedbacks, for example, the regulation of the relative constancy of the arterial pressure value is carried out.
With an increase in blood pressure, excitation of the mechanoreceptors of the aortic arch, carotid sinuses occurs. Impulses enter the vasomotor center and the center of cardiac activity, the vascular tone decreases reflexively, at the same time the activity of the heart slows down and the blood pressure decreases. With a decrease in blood pressure, irritation of the mechanoreceptors of these reflexogenic zones causes a reflex increase in vascular tone, an increase in the work of the heart. In this case, the value of blood pressure increases.
Secondary afferent impulses (feedback) also play an important role in the regulation of other autonomic functions: respiration, digestion, excretion.
Dominant principle. The principle of dominance was formulated by A. A. Ukhtomsky. This principle plays an important role in the coordinated work of the nerve centers. The dominant is a temporarily dominant focus of excitation in the central nervous system, which determines the nature of the body's response to external and internal stimuli.
The dominant focus of excitation is characterized by the following main properties: 1) increased excitability; 2) persistence of arousal; 3) the ability to sum up excitation; 4) inertia - the dominant in the form of traces of excitation can persist for a long time even after the cessation of the irritation that caused it.
The dominant focus of excitation is able to attract (attract) nerve impulses to itself from other nerve centers that are less excited in this moment. Due to these impulses, the activity of the dominant increases even more, and the activity of other nerve centers is suppressed.
Dominants can be of exogenous and endogenous origin. Exogenous dominant occurs under the influence of environmental factors. For example, when reading an interesting book, a person may not hear the music playing on the radio at that time.
Endogenous dominant arises under the influence of factors of the internal environment of the body, mainly hormones and other physiologically active substances. For example, with a decrease in the content of nutrients in the blood, especially glucose, the food center is excited, which is one of the reasons for the food installation of the organism of animals and humans.
The dominant can be inert (persistent), and for its destruction it is necessary to create a new, more powerful focus of excitation.
The dominant underlies the coordination activity of the organism, ensuring the behavior of humans and animals in environment, as well as emotional states, reactions of attention. The formation of conditioned reflexes and their inhibition is also associated with the presence of a dominant focus of excitation.
Spinal cord
Features of the structure of the spinal cord. The spinal cord is the most ancient and primitive part of the central nervous system. The gray matter is located in the central part of the spinal cord. It consists mainly of nerve cells and forms protrusions - posterior, anterior and lateral horns. Afferent nerve cells are located in the adjacent spinal ganglia. The long process of the afferent cell is located on the periphery and forms a receptive ending (receptor), while the short process ends at the cells of the posterior horns. The anterior horns contain efferent cells (motoneurons), whose axons innervate skeletal muscles; in the lateral horns - neurons of the autonomic nervous system. The gray matter contains numerous interneurons. Among them, special inhibitory neurons were found - Renshaw cells named after the author who first described them. Surrounded by gray matter white matter spinal cord. It is made up of nerve fibers. ascending and descending paths, connecting different parts of the spinal cord with each other, as well as the spinal cord with the brain (Fig. 75).
Functions of the spinal roots. The connection of the spinal cord with the periphery is carried out by means of nerve fibers passing in the spinal roots; along them, afferent impulses enter the spinal cord and efferent impulses pass from it to the periphery. There are 31 pairs of anterior and posterior roots on both sides of the spinal cord.
The functions of the spinal roots were elucidated using the methods of transection and irritation.
The outstanding Scottish anatomist and physiologist Bell and the French researcher Magendie found that with unilateral transection of the anterior roots of the spinal cord, paralysis of the limbs of the same side is noted, while sensitivity is completely preserved. Transection of the posterior roots leads to a loss of sensitivity, while the motor function is preserved.
Thus, it was shown that afferent impulses enter the spinal cord through the posterior roots (sensory), efferent impulses exit through the anterior roots (motor).
Functions and centers of the spinal cord. The spinal cord has two functions: reflective and conductive.
Reflex function of the spinal cord. Afferent impulses enter the spinal cord from skin receptors, proprioreceptors of the motor apparatus, interoreceptors of blood vessels, the digestive tract, excretory and genital organs. Efferent impulses from the spinal cord go to the skeletal muscles (with the exception of the muscles of the face), including the respiratory ones - the intercostals and the diaphragm. In addition, impulses from the spinal cord along the autonomic nerve fibers go to all internal organs, blood vessels, sweat glands, etc.
The motor neurons of the spinal cord are excited by afferent impulses coming to them from various receptors in the body. However, the level of activity of motoneurons depends not only on this afferentation, but also on complex intracentral relationships. A large role in the regulation of the activity of motor neurons belongs to the descending influences of the brain (the cerebral cortex, the reticular formation of the brain stem, cerebellum, etc.), as well as the intraspinal influences of numerous intercalary neurons. Among intercalary neurons, Renshaw cells play a special role. These cells form inhibitory synapses on motor neurons. When Renshaw cells are excited, the activity of motor neurons slows down, which prevents overexcitation and controls their work. The activity of the motor neurons of the spinal cord is also controlled by the flow of impulses coming from the proprioreceptors of the muscles (reverse afferentation).
Spinal reflexes, i.e., reflexes inherent in the spinal cord itself, can be studied in their pure form only after the separation of the spinal cord from the brain (spinal animal). The first consequence of transection between the medulla oblongata and the spinal cord is spinal shock, which lasts from several minutes to several weeks, depending on the level of development of the central nervous system. Spinal shock is manifested by a sharp drop in excitability and inhibition of the reflex functions of all nerve centers located below the site of transection. In spinal shock great importance has the elimination of nerve impulses coming to the spinal cord from the overlying sections of the central nervous system, including from neurons of the reticular formation of the brain stem.
Upon termination of the spinal shock, the reflex activity of the skeletal muscles, the magnitude of blood pressure, the reflexes of urination, defecation, and a number of sexual reflexes are gradually restored. In a spinal animal, voluntary movements, sensitivity and body temperature, as well as breathing are not restored. Spinal animals can live only under the condition of artificial respiration. Consequently, the centers that regulate these functions are located in the overlying parts of the central nervous system..
reflex centers of the spinal cord. In the cervical region of the spinal cord there is the center of the phrenic nerve and the center of pupillary constriction, in the cervical and thoracic regions - the centers of the muscles of the upper extremities, the muscles of the chest, back and abdomen, in the lumbar region - the centers of the muscles of the lower extremities, in the sacral region - the centers of urination, defecation and sexual activity, in the lateral horns of the thoracic and lumbar spinal cord - sweating centers and spinal vasomotor centers.
By studying the disturbances in the activity of certain muscle groups or individual functions in sick people, it is possible to establish which section of the spinal cord is damaged or the function of which section is impaired.
Reflex arcs of individual reflexes pass through certain segments of the spinal cord. The excitation that has arisen in the receptor, along the centripetal nerve, enters the corresponding section of the spinal cord. Centrifugal fibers emerging from the spinal cord as part of the anterior roots innervate strictly defined areas of the body. The scheme in fig. 76 shows which segments innervate each part of the body.
The conduction function of the spinal cord. The ascending and descending tracts pass through the spinal cord.
ascending nerve pathways transmit information from tactile, pain, temperature receptors of the skin and from muscle proprioceptors through the neurons of the spinal cord and other parts of the central nervous system to the cerebellum and cerebral cortex.
Descending nerve pathways(pyramidal and extrapyramidal) connect the cerebral cortex, subcortical nuclei and formations of the brain stem with the motor neurons of the spinal cord. They provide the influence of the higher parts of the central nervous system on the activity of skeletal muscles.
Medulla
The direct continuation of the spinal cord in all vertebrates and humans is the medulla oblongata.
The medulla oblongata and the pons (bridge of the brain) are combined under the general name of the hindbrain. The hindbrain together with the midbrain and diencephalon form the brainstem. The brain stem contains a large number of nuclei, ascending and descending tracts. Of great functional importance is located in the brain stem, in particular in the hindbrain, reticular formation.
In the medulla oblongata, compared with the spinal cord, there is no clear segmental distribution of gray and white matter.
The accumulation of nerve cells leads to the formation of nuclei, which are the centers of more or less complex reflexes. Of the 12 pairs of cranial nerves that connect the brain with the periphery of the body - its receptors and effectors, eight pairs (V-XII) originate in the medulla oblongata.
The medulla oblongata performs two functions - reflex and conduction.
Reflex function of the medulla oblongata. In the medulla oblongata there are centers of both relatively simple and more complex reflexes. Due to the medulla oblongata, the following are carried out: 1) protective reflexes (blinking, tearing, sneezing, cough reflex and vomiting reflex); 2) adjusting reflexes, providing muscle tone necessary to maintain posture and perform work acts; 3) labyrinth reflexes that contribute to the correct distribution of muscle tone between individual muscle groups and the establishment of a particular body posture; 4) reflexes associated with the functions of the respiratory, circulatory, and digestive systems.
Conductor function of the medulla oblongata. Through the medulla oblongata pass ascending paths from the spinal cord to the brain and descending paths connecting the cerebral cortex with the spinal cord. The medulla oblongata and the pons have their own pathways connecting the nucleus and olive of the vestibular nerve with the motor neurons of the spinal cord.
Through the ascending tracts and cranial nerves, the medulla oblongata receives impulses from the receptors of the muscles of the face, neck, limbs and torso, from the skin of the face, mucous membranes of the eyes, nasal and oral cavity, from hearing receptors, vestibular apparatus, larynx, trachea, lungs, and digestive interoreceptors. apparatus and cardiovascular system.
The functions of the medulla oblongata have been studied in bulbar animals, in which the medulla oblongata is separated from the middle one by a transverse section. Consequently, the life of bulbar animals is carried out due to the activity of the spinal cord and medulla oblongata. Such animals lack voluntary movements, there is a loss of all types of sensitivity, the regulation of body temperature is disturbed (a warm-blooded animal turns into a cold-blooded animal). In bulbar animals, the reflex reactions of the body are preserved and the regulation of the functions of internal organs is carried out.
reflex centers in the medulla oblongata. There are a number of vital centers in the medulla oblongata. These include the respiratory, cardiovascular and nutritional centers. Due to the activity of these centers, the regulation of respiration, blood circulation and digestion is carried out. Thus, the main biological role medulla oblongata is to ensure the constancy of the composition of the internal environment of the body.
Due to connections with proprioreceptors, the medulla oblongata acts as a regulator of skeletal muscle tone, primarily providing tonic tension in the extensor muscles designed to overcome the body's gravity.
The medulla oblongata regulates the functioning of the spinal cord. This coordination function is aimed at the functional unification of all segments of the spinal cord, at providing conditions for its integral activity. The medulla oblongata carries out more subtle forms of adaptive reactions of the organism to the external environment in comparison with the spinal cord.
midbrain
The formations of the midbrain include the legs of the brain, nuclei III (oculomotor) and IV (trochlear) pairs of cranial nerves, quadrigemina, red nuclei and substantia nigra. In the legs of the brain are ascending and descending nerve pathways.
In the structure of the midbrain, segmental features are completely lost. In the midbrain, cellular elements form complex clusters in the form of nuclei. Nuclear formations relate directly to the midbrain, as well as to the reticular formation that is part of it.
Anterior colliculi receive impulses from the retina. In response to these signals, the pupillary lumen is regulated and the eye is accommodated. Accommodation is the adaptation of the eye to a clear vision of objects at different distances by changing the curvature of the lens.
Posterior tubercles of the quadrigemina receive impulses from the nuclei of the auditory nerves located in the medulla oblongata. Due to this, there is a reflex regulation of the muscle tone of the middle ear, and in animals - the turn of the auricle to the sound source. Thus, with the participation of the anterior and posterior tubercles of the quadrigemina, adjusting, orienting reflex reactions to light and sound stimuli are carried out (eye movements, turning the head and even the body towards the light or sound stimulus). With the destruction of the nuclei of the quadrigemina, vision and hearing are preserved, but there are no orienting reactions to light and sound.
The function of the nuclei of the III and IV pairs of cranial nerves is closely related to the activity of the tubercles of the quadrigemina, the excitation of which determines the movement of the eyes up, down, to the sides, as well as the convergence (convergence) and dilution of the eye axes when transferring gaze from distant objects to close ones and vice versa,
Red cores participate in the regulation of muscle tone and in the manifestation of adjusting reflexes that ensure the preservation of the correct position of the body in space. When the hindbrain is separated from the middle brain, the tone of the extensor muscles increases, the limbs of the animal tense and stretch, and the head throws back. Consequently, in a healthy animal and human, red nuclei somewhat slow down the tone of the extensor muscles.
black substance also regulates muscle tone and maintaining posture, participates in the regulation of chewing, swallowing, blood pressure and breathing, i.e. the activity of the black substance, like the red nuclei, is closely related to the work of the medulla oblongata.
Thus, the midbrain regulates muscle tone, distributes it appropriately, which is a necessary condition for coordinated movements. The midbrain regulates a number of vegetative functions of the body (chewing, swallowing, blood pressure, respiration). Due to the midbrain, the reflex activity of the body expands and becomes more diverse (orienting reflexes to sound and visual stimuli).
The formations of the brain stem ensure the correct distribution of tone between individual muscle groups. The reflexes that provide muscle tone are called tonic. The motor neurons of the spinal cord, the vestibular nuclei of the medulla oblongata, the cerebellum, and the formations of the midbrain (red nuclei) participate in the implementation of these reflexes. In the whole organism, the manifestation of tonic reflexes is controlled by the cells of the motor zone of the cerebral cortex.
Tonic reflexes occur when the position of the body and head changes in space due to the excitation of muscle proprioceptors, receptors of the vestibular apparatus of the inner ear and tactile receptors of the skin.
Tonic reflexes are divided into two groups: static and statokinetic. Static reflexes arise when the position of the body, especially the head, in space changes. Statokinetic reflexes appear when the body moves in space, when the speed of movement (rotational or rectilinear) changes.
Thus, tonic reflexes prevent the possibility of imbalance, the loss of an active posture and contribute to the restoration of the disturbed posture.
diencephalon
The diencephalon is part of the anterior brain stem. The main formations of the diencephalon are the visual tubercles (thalamus) and hypothalamus (hypothalamus).
Visual tubercles- a massive paired formation, they occupy the bulk of the diencephalon. The visual tubercles reach the largest size and the highest complexity of the structure in humans.
Visual hillocks are the center of all afferent impulses. Through the visual tubercles, information from all the receptors of our body, with the exception of the olfactory ones, enters the cerebral cortex. In addition, nerve impulses are transmitted from the visual tubercles to various formations of the brain stem. A large number of nuclear formations were found in the visual mounds. Functionally, they can be divided into two groups: specific and nonspecific nuclei.
Specific nuclei receive information from receptors, process it and transmit it to certain areas of the cerebral cortex, where the corresponding sensations arise (visual, auditory, etc.).
Non-specific nuclei do not have a direct connection with the receptors of the body. They receive impulses from receptors through a large number of switches (synapses). Impulses from these formations through the subcortical nuclei go to many neurons located in different areas of the cerebral cortex, causing an increase in their excitability.
If the optic tubercles are damaged, a person experiences a complete loss of sensitivity or its decrease on the opposite side, there is a contraction of facial muscles that accompanies emotions, sleep disorders, hearing loss, vision loss, etc. can also occur.
Hypothalamic (hypothalamic) area participates in the regulation of various types of metabolism (proteins, fats, carbohydrates, salts, water), regulates heat generation and heat transfer, the state of sleep and wakefulness. In the nuclei of the hypothalamus, a number of hormones are formed, which are then deposited in the posterior pituitary gland. The anterior hypothalamus is the highest center of the parasympathetic nervous system, the posterior - the sympathetic nervous system. The hypothalamus is involved in the regulation of many autonomic functions of the body.
Basal nuclei
The subcortical, or basal, nuclei include three paired formations: the caudate nucleus, the shell, and the pale ball. The basal nuclei are located inside the cerebral hemispheres, in their lower part, between the frontal lobes and the diencephalon. The development and cellular structure of the caudate nucleus and the shell are the same, therefore they are considered as unified education- striatal body.
striatum is in charge of complex motor functions, participates in the implementation of unconditioned reflex reactions of a chain nature - running, swimming, jumping. The striatum performs this function through the pale ball, slowing down its activity. In addition, the striatum through the hypothalamus regulates the autonomic functions of the body, and also, together with the nuclei of the diencephalon, provides for the implementation of complex unconditioned reflexes of a chain nature - instincts.
pale ball is the center of complex motor reflex reactions (walking, running), forms complex facial reactions, participates in ensuring the correct distribution of muscle tone. The pale ball performs its functions indirectly through the formations of the midbrain (red nuclei and black matter). With irritation of the pale ball, a general contraction of the skeletal muscles of the opposite side of the body is observed. With the defeat of the pale ball, the movements lose their smoothness, become clumsy, constrained.
Consequently, the activity of the subcortical nuclei is not limited to their participation in the formation of complex motor acts. Due to their connections with the hypothalamus, they are involved in the regulation of metabolism and the functions of internal organs.
Thus, the basal nuclei are the highest subcortical centers of unification (integration) of body functions. In humans and higher vertebrates, the activity of the subcortical nuclei is controlled by the cerebral cortex.
Reticular formation of the brain stem
Structural features. The reticular formation is a cluster of special neurons that form a kind of network with their fibers. Neurons of the reticular formation in the region of the brainstem were described in the last century by the German scientist Deiters. V. M. Bekhterev found similar structures in the region of the spinal cord. The neurons of the reticular formation form clusters, or nuclei. The dendrites of these cells are relatively long and slightly branched; on the contrary, the axons are short and have many branches (collaterals). This feature causes numerous synaptic contacts of neurons of the reticular formation.
The reticular formation of the brain stem occupies a central position in the medulla oblongata, pons varolii, midbrain and diencephalon (Fig. 77).
The neurons of the reticular formation do not have direct contacts with the body's receptors. When the receptors are excited, nerve impulses arrive at the reticular formation along the collaterals of the fibers of the autonomic and somatic nervous system.
Physiological role. The reticular formation of the brain stem has an ascending effect on the cells of the cerebral cortex and a descending effect on the motor neurons of the spinal cord. Both of these influences of the reticular formation can be activating or inhibitory.
Afferent impulses to the cerebral cortex come in two ways: specific and nonspecific. specific neural pathway necessarily passes through the visual tubercles and carries nerve impulses to certain areas of the cerebral cortex, as a result, any specific activity is carried out. For example, when the photoreceptors of the eyes are stimulated, impulses through the visual tubercles enter the occipital region of the cerebral cortex and visual sensations arise in a person.
non-specific neural pathway necessarily passes through the neurons of the reticular formation of the brain stem. Impulses to the reticular formation come through the collaterals of a specific nerve pathway. Due to numerous synapses on the same neuron of the reticular formation, impulses of different values (light, sound, etc.) can converge (converge), while they lose their specificity. From the neurons of the reticular formation, these impulses do not arrive in any particular area of the cerebral cortex, but spread like a fan through its cells, increasing their excitability and thereby facilitating the performance of a specific function (Fig. 78).
In experiments on cats with electrodes implanted in the region of the reticular formation of the brainstem, it was shown that stimulation of its neurons causes the awakening of a sleeping animal. With the destruction of the reticular formation, the animal falls into a long sleepy state. These data indicate the important role of the reticular formation in the regulation of sleep and wakefulness. The reticular formation not only affects the cerebral cortex, but also sends inhibitory and excitatory impulses to the spinal cord to its motor neurons. Due to this, it is involved in the regulation of skeletal muscle tone.
In the spinal cord, as already mentioned, there are also neurons of the reticular formation. It is believed that they maintain a high level of activity of neurons in the spinal cord. The functional state of the reticular formation itself is regulated by the cerebral cortex.
Cerebellum
Features of the structure of the cerebellum. Connections of the cerebellum with other parts of the central nervous system. The cerebellum is an unpaired formation; it is located behind the medulla oblongata and the pons, borders on the quadrigemina, is covered from above by the occipital lobes of the cerebral hemispheres. In the cerebellum, the middle part is distinguished - the worm and two hemispheres located on the sides of it. The surface of the cerebellum is made up of gray matter called the cortex, which includes the nerve cell bodies. Inside the cerebellum is white matter, which is the processes of these neurons.
The cerebellum has extensive connections with various parts of the central nervous system due to three pairs of legs. The lower legs connect the cerebellum with the spinal cord and medulla oblongata, the middle ones with the pons and through it with the motor area of the cerebral cortex, the upper ones with the midbrain and hypothalamus.
Functions of the cerebellum were studied in animals in which the cerebellum was removed partially or completely, as well as by recording its bioelectrical activity at rest and during stimulation.
When half of the cerebellum is removed, an increase in the tone of the extensor muscles is noted, therefore, the limbs of the animal are extended, a bend of the body and a deviation of the head to the operated side are observed, sometimes rocking movements of the head. Often the movements are made in a circle in the operated direction ("manege movements"). Gradually, the marked violations are smoothed out, but some awkwardness of movements remains.
When the entire cerebellum is removed, more pronounced movement disorders occur. In the first days after the operation, the animal lies motionless with its head thrown back and elongated limbs. Gradually, the tone of the extensor muscles weakens, trembling of the muscles appears, especially the cervical ones. In the future, motor functions are partially restored. However, until the end of life, the animal remains a motor invalid: when walking, such animals spread their limbs wide, raise their paws high, i.e., they have impaired coordination of movements.
Movement disorders during the removal of the cerebellum were described by the famous Italian physiologist Luciani. The main ones are: atony- the disappearance or weakening of muscle tone; asthenia- decrease in the strength of muscle contractions. Such an animal is characterized by rapidly onset muscle fatigue; astasia- loss of the ability to fused tetanic contractions. In animals, trembling movements of the limbs and head are observed. The dog after removal of the cerebellum cannot immediately raise its paws, the animal makes a series of oscillatory movements with its paw before lifting it. If you put such a dog, then its body and head sway all the time from side to side.
As a result of atony, asthenia and astasia, the animal's coordination of movements is disturbed: a shaky gait, sweeping, awkward, inaccurate movements are noted. The whole complex of motor disorders in the lesion of the cerebellum is called cerebellar ataxia(Fig. 79).
Similar disorders are observed in humans with damage to the cerebellum.
Some time after the removal of the cerebellum, as already indicated, all movement disorders are gradually smoothed out. If the motor area of the cerebral cortex is removed from such animals, then the motor disturbances increase again. Consequently, compensation (restoration) of movement disorders in case of damage to the cerebellum is carried out with the participation of the cerebral cortex, its motor area.
The studies of L. A. Orbeli showed that when the cerebellum is removed, not only a drop in muscle tone (atony), but also its incorrect distribution (dystonia) is observed. L. A. Orbeli found that the cerebellum also affects the state of the receptor apparatus, as well as autonomic processes. The cerebellum has an adaptive-trophic effect on all parts of the brain through the sympathetic nervous system, it regulates the metabolism in the brain and thereby contributes to the adaptation of the nervous system to changing conditions of existence.
Thus, the main functions of the cerebellum are the coordination of movements, the normal distribution of muscle tone, and the regulation of autonomic functions. The cerebellum realizes its influence through the nuclear formations of the middle and medulla oblongata, through the motor neurons of the spinal cord. A large role in this influence belongs to the bilateral connection of the cerebellum with the motor zone of the cerebral cortex and the reticular formation of the brain stem (Fig. 80).
