home life resources General principles of the coordination activity of the central nervous system. The principle of the functioning of the human nervous system What principle underlies the nervous activity

General principles of the coordination activity of the central nervous system. The principle of the functioning of the human nervous system What principle underlies the nervous activity

The functioning of the nervous system is based on reflex activity. Reflex (from lat. Reflexio - I reflect) is the body's response to external or internal irritation with the obligatory participation nervous system.

The reflex principle of the functioning of the nervous system

A reflex is the body's response to an external or internal stimulus. Reflexes are divided into:

unconditioned reflexes: innate reactions of the body to stimuli, carried out with the participation spinal cord or brain stem
conditioned reflexes: temporary reactions of the body acquired on the basis of unconditioned reflexes, carried out with the obligatory participation of the cerebral cortex, which form the basis of the higher nervous activity.

The morphological basis of the reflex is a reflex arc, represented by a chain of neurons that provide the perception of irritation, the transformation of the energy of irritation into a nerve impulse, the conduction of a nerve impulse to the nerve centers, the processing of incoming information and the implementation of a response.

Reflex activity presupposes the presence of a mechanism consisting of three main elements connected in series:

1. Receptors that perceive irritation and transform it into a nerve impulse; usually receptors are represented by various sensitive nerve endings in organs;

2. Effectors, which result in the effect of stimulating receptors in the form of a specific reaction; effectors include all internal organs, blood vessels and muscles;

3. chains connected in series neurons, which, by directionally transmitting excitation in the form of nerve impulses, ensure the coordination of the activity of effectors depending on the stimulation of the receptors.

A chain of neurons connected in series with each other forms reflex arc, which constitutes the material substratum of the reflex.

Functionally, the neurons that form the reflex arc can be divided into:

1. afferent (sensory) neurons that perceive stimulation and transmit it to other neurons. Sensory neurons are always located outside the central nervous system in the sensory ganglia of the spinal and cranial nerves. Their dendrites form sensitive nerve endings in the organs.

2. efferent (motor, motor) neurons, or motor neurons, transmit excitation to effectors (for example, muscles or blood vessels);

3. interneurons (interneurons) interconnect afferent and efferent neurons and thereby close the reflex connection.

The simplest reflex arc consists of two neurons - afferent and efferent. Three neurons are involved in a more complex reflex arc: afferent, efferent and intercalary. Maximum amount neurons involved in the reflex response of the nervous system is limited, especially in those cases when different parts of the brain and spinal cord are involved in the reflex act. At present, the basis of reflex activity is taken reflex ring. The classical reflex arc is supplemented by the fourth link - the reverse afferentation from the effectors. All neurons involved in reflex activity have a strict localization in the nervous system.

Nerve center

Anatomically, the center of the nervous system is a group of adjacent neurons that are closely related structurally and functionally and perform a common function in reflex regulation. In the nerve center, perception, analysis of incoming information and its transmission to other nerve centers or effectors take place. Therefore, each nerve center has its own system of afferent fibers, through which it is brought into an active state, and a system of efferent connections that conduct nervous excitement to other nerve centers or effectors. Distinguish peripheral nerve centers, represented by nodes ( ganglia ): sensitive and vegetative. In the central nervous system there are nuclear centers (nuclei)- local clusters of neurons, and cortical centers - extensive settlement of neurons on the surface of the brain.

Blood supply to the brain and spinal cord

I. Blood supply to the brain carried out by branches of the left and right internal carotid arteries and branches of the vertebral arteries.

internal carotid artery, entering the cranial cavity, it divides into the ophthalmic artery and the anterior and middle cerebral arteries. Anterior cerebral artery nourishes mainly the frontal lobe of the brain, middle cerebral artery - parietal and temporal lobes, and ophthalmic artery supplies blood to the eyeball. The anterior cerebral arteries (right and left) are connected by a transverse anastomosis - the anterior communicating artery.

Vertebral arteries (right and left) in the region of the brain stem unite and form an unpaired basilar artery, feeding the cerebellum and other parts of the trunk, and two posterior cerebral arteries supplying blood to the occipital lobes of the brain. Each of the posterior cerebral arteries is connected to the middle cerebral artery of its side by means of the posterior communicating artery.

Thus, on the basis of the brain, an arterial circle of the cerebrum is formed.

Smaller ramifications of blood vessels in the pia mater

reach the brain, penetrate into its substance, where they are divided into numerous capillaries. From the capillaries, blood is collected in small, and then large venous vessels. Blood from the brain flows into the sinuses of the dura mater. Blood flows from the sinuses through the jugular foramina at the base of the skull into the internal jugular veins.

2. Blood supply to the spinal cord through the anterior and posterior spinal arteries. The outflow of venous blood goes through the veins of the same name to the internal vertebral plexus, located along the entire length of the spinal canal outside of the hard shell of the spinal cord. From the internal vertebral plexus, blood flows into the veins that run along the spinal column, and from them into the inferior and superior vena cava.

Liquor system of the brain

Inside the bone cavities, the brain and spinal cord are in suspension and are washed from all sides by cerebrospinal fluid - liquor. Liquor protects the brain from mechanical influences, ensures the constancy of intracranial pressure, is directly involved in the transport of nutrients from the blood to the brain tissues. Cerebrospinal fluid is produced by the choroid plexuses of the ventricles of the brain. CSF circulation through the ventricles is carried out according to the following scheme: from the lateral ventricles, the fluid enters through the foramen of Monro into the third ventricle, and then through the Sylvian aqueduct into the fourth ventricle. From it, the cerebrospinal fluid passes through the holes of Magendie and Luschka into the subarachnoid space. The outflow of cerebrospinal fluid into the venous sinuses occurs through the granulation of the arachnoid - pachyon granulations.

Between neurons and blood in the brain and spinal cord there is a barrier called blood-brain, which ensures the selective flow of substances from the blood to nerve cells. This barrier performs a protective function, as it ensures the constancy of the physico-chemical properties of the liquor.

Picks

Neurotransmitters (neurotransmitters, mediators) - biologically active chemical substances, through which the transmission of an electrical impulse from a nerve cell through the synaptic space between neurons is carried out. The nerve impulse entering the presynaptic ending causes the mediator to be released into the synaptic cleft. The mediator molecules react with specific receptor proteins of the cell membrane, initiating a chain of biochemical reactions that cause a change in the transmembrane current of ions, which leads to membrane depolarization and the emergence of an action potential.

Until the 1950s, mediators included two groups of low molecular weight compounds: amines (acetylcholine, adrenaline, norepinephrine, serotonin, dopamine) and amino acids (gamma-aminobutyric acid, glutamate, aspartate, glycine). Later, it was shown that neuropeptides constitute a specific group of mediators, which can also act as neuromodulators (substances that change the magnitude of a neuron's response to a stimulus). It is now known that a neuron can synthesize and release several neurotransmitters.

In addition, there are special nerve cells in the nervous system - neurosecretory, which provide a link between the central nervous system and the endocrine system. These cells have a typical neuron structural and functional organization. They are distinguished from a neuron by a specific function - neurosecretory, which is associated with the secretion of biologically active substances. Axons of neurosecretory cells have numerous extensions (Hering's bodies), in which neurosecretion temporarily accumulates. Within the brain, these axons are typically devoid of myelin sheath. One of the main functions of neurosecretory cells is the synthesis of proteins and polypeptides and their further secretion. In this regard, in these cells, the protein-synthesizing apparatus is extremely developed - the granular endoplasmic reticulum, the Golgi complex, and the lysosomal apparatus. By the number of neurosecretory granules in a cell, one can judge its activity.

Feedback can be classified according to various criteria. For example, according to the speed of action - fast (nervous) and slow (humoral), etc.

Many examples of feedback effects can be cited. For example, in the nervous system, the activity of motor neurons is regulated in this way. The essence of the process lies in the fact that excitation impulses propagating along the axons of motor neurons reach not only the muscles, but also specialized intermediate neurons (Renshaw cells), the excitation of which inhibits the activity of motor neurons. This effect is known as the rebound inhibition process.

An example of positive feedback is the process of generating an action potential. So, during the formation of the ascending part of the AP, the depolarization of the membrane increases its sodium permeability, which, in turn, increases the depolarization of the membrane.

The importance of feedback mechanisms in maintaining homeostasis is great. So, for example, maintaining a constant level is carried out by changing the impulse activity of baroreceptors of vascular reflexogenic zones, which change the tone of vasomotor sympathetic nerves and thus normalize blood pressure.

5. The principle of reciprocity (combination, conjugation, mutual exclusion). It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limb, etc.). For example, activation of the proprioreceptors of the flexor muscle simultaneously excites the motor neurons of the flexor muscle and inhibits the motor neurons of the extensor muscle through intercalary inhibitory neurons (Fig. 18). Reciprocal inhibition plays an important role in the automatic coordination of motor acts,
6. The principle of a common final path. The effector neurons of the CNS (primarily the motor neurons of the spinal cord), being the final ones in the chain consisting of afferent, intermediate and effector neurons, can be involved in the implementation of various reactions of the body by excitations coming to them from a large number afferent and intermediate neurons, for which they are the final path (the path from the CNS to the effector). For example, on the motoneurons of the anterior horns of the spinal cord, which innervate the muscles of the limb, the fibers of afferent neurons, neurons of the pyramidal tract and extrapyramidal system (nuclei of the cerebellum, reticular formation and many other structures) terminate. Therefore, these motor neurons, which provide the reflex activity of the limb, are considered as the final path for common implementation on the limb of many nervous influences.

What principle underlies the work of the nervous system? What is called a reflex? Name the links of the reflex arc, their position and functions.

The reflex principle is the basis of the work of the nervous system.

Reflex - the body's response to irritation of receptors, carried out with the participation of the central nervous system (CNS). The path along which the reflex is carried out is called the reflex arc. The reflex arc consists of the following components:

Receptor that perceives irritation;

Sensitive (centripetal) nerve pathway through which excitation is transmitted from the receptor to the central nervous system;

Nerve center - a group of intercalary neurons located in the central nervous system and transmitting nerve impulses from sensory nerve cells to motor ones;

The motor (centrifugal) nerve pathway that transmits excitation from the central nervous system to the executive organ (muscle, etc.), the activity of which changes as a result of the reflex.

The simplest reflex arcs are formed by two neurons (knee reflex) and contain sensory and motor neurons. The reflex arcs of most reflexes include not two, but a larger number of neurons: sensory, one or more intercalary and motor. Through intercalary neurons, communication is carried out with the overlying parts of the central nervous system and information is transmitted about the adequacy of the response of the executive (working) organ to the received stimulus.

To implement complex reactions, it is necessary to integrate the work of individual nerve centers. Most reflexes are complex, sequentially and simultaneously occurring reactions. Reflexes in the normal state of the body are strictly ordered, since there are general arrangements their coordination. Excitations arising in the central nervous system radiate through its centers.

Coordination is ensured by selective excitation of some centers and inhibition of others. Coordination is the unification of the reflex activity of the central nervous system into a single whole, which ensures the implementation of all body functions. The following basic principles of coordination are distinguished:

1. 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 of a spinal frog is irritated by 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 provides, with strong and biologically significant irritations, inclusion in the response more motoneurons.

2. The principle of a common final path. Impulses coming to the CNS through different afferent fibers can converge (converge) to the same intercalary, or efferent, neurons. 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 that innervate 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 cortex converge hemispheres and from many subcortical centers (through intercalary neurons or through direct nerve connections).

On the motoneurons of the anterior horns of the spinal cord, innervating the muscles of the limb, the fibers of the pyramidal tract, extrapyramidal pathways, from the cerebellum, the reticular formation and other structures end. The motoneuron, which provides various reflex reactions, is considered as their common final path. In which specific reflex act the motor neurons will be involved depends on the nature of the stimuli and on the functional state of the organism.

3. The principle of dominance. It was discovered by A.A. Ukhtomsky, who discovered that irritation of the afferent nerve (or cortical center), which usually leads to contraction of the muscles of the limbs during overflow in the animal intestine, causes an act of defecation. In this situation, the reflex excitation of the defecation center "suppresses, inhibits the motor centers, and the defecation center begins to respond to signals that are foreign to it.

A.A. Ukhtomsky believed that in each this moment life, a determining (dominant) focus of excitation arises, subordinating the activity of the entire nervous system and determining the nature of the adaptive reaction. Excitations from different areas of the central nervous system converge to the dominant focus, and the ability of other centers to respond to signals coming to them is inhibited. Due to this, conditions are created for the formation of a certain reaction of the body to a stimulus that has the greatest biological significance, i.e. satisfying a vital need.

In the natural conditions of existence, the dominant excitation can cover entire systems of reflexes, resulting in food, defensive, sexual and other forms of activity. The dominant excitation center has a number of properties:

1) its neurons are characterized by high excitability, which contributes to the convergence of excitations to them from other centers;

2) its neurons are able to summarize incoming excitations;

3) excitation is characterized by persistence and inertness, i.e. the ability to persist even when the stimulus that caused the formation of the dominant has ceased to act.

Despite the relative stability and inertness of excitation in the dominant focus, the activity of the central nervous system under normal conditions of existence is very dynamic and changeable. The central nervous system has the ability to restructure dominant relationships in accordance with the changing needs of the body. The doctrine of the dominant has found wide application in psychology, pedagogy, the physiology of mental and physical labor, and sports.

4. The principle of feedback. The processes occurring in the central nervous system cannot be coordinated if there is no feedback, i.e. data on the results of function management. Feedback allows you to correlate the severity of changes in system parameters with its operation. The connection of the output of the system with its input with a positive gain is called positive feedback, and with a negative gain - negative feedback. Positive feedback is mainly characteristic of pathological situations.

Negative feedback ensures the stability of the system (its ability to return to its original state after the influence of disturbing factors ceases). There are fast (nervous) and slow (humoral) feedbacks. Feedback mechanisms ensure the maintenance of all homeostasis constants. For example, saving normal level blood pressure is carried out by changing the impulse activity of the baro-receptors of the vascular reflexogenic zones, which change the tone of the vagus and vasomotor sympathetic nerves.

5. The principle of reciprocity. It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limbs), and lies in the fact that the neurons of one center, being excited, inhibit the neurons of the other and vice versa.

6. The principle of subordination (subordination). The main trend in the evolution of the nervous system is manifested in the concentration of the functions of regulation and coordination in the higher parts of the central nervous system - cephalization of the functions of the nervous system. There are hierarchical relationships in the central nervous system - the cerebral cortex is the highest center of regulation, the basal ganglia, the middle, medulla and spinal cord obey its commands.

7. The principle of function compensation. The central nervous system has a huge compensatory ability, i.e. can restore some functions even after the destruction of a significant part of the neurons that form the nerve center (see plasticity of the nerve centers). If individual centers are damaged, their functions can be transferred to other brain structures, which is carried out with the obligatory participation of the cerebral cortex. Animals that had their cortex removed after restoration of lost functions experienced their loss again.

With local insufficiency of inhibitory mechanisms or with excessive intensification of excitation processes in one or another nerve center, a certain set of neurons begins to autonomously generate pathologically increased excitation - a generator of pathologically increased excitation is formed.

With a high generator power, a whole system of functioning in single mode non-ironal formations, which reflects a qualitatively new stage in the development of the disease; rigid connections between the individual constituent elements of such a pathological system underlie its resistance to various therapeutic effects. The study of the nature of these connections allowed G.N. Kryzhanovsky to discover new form intracentral relations and integrative activity of the central nervous system - the principle of the determinant.

Its essence lies in the fact that the structure of the central nervous system, which forms a functional premise, subjugates those departments of the central nervous system to which it is addressed and forms a pathological system together with them, determining the nature of its activity. Such a system is characterized by the lack of constancy and inadequacy of functional premises, i.e. such a system is biologically negative. If, for one reason or another, the pathological system disappears, then the formation of the central nervous system, which played the main role, loses its determinant significance.

Neurophysiology of movements

The relationship of individual nerve cells and their totality form the most complex ensembles of processes that are necessary for the full life of a person, for the formation of a person as a society, defines him as a highly organized being, which puts a person on more high level development in relation to other animals. Thanks to the highly specific relationships of nerve cells, a person can produce complex actions and improve them. Consider below the processes necessary for the implementation of arbitrary movements.

The very act of movement begins to form in the motor area of the cloak cortex. Distinguish between primary and secondary motor cortex. In the primary motor cortex (precentral gyrus, field 4) there are neurons that innervate the motor neurons of the muscles of the face, trunk and limbs. It has an accurate topographic projection of the muscles of the body. In the upper parts of the precentral gyrus, the projections of the lower extremities and torso are focused, in the lower parts - the upper limbs of the head, neck and face, occupying most of the gyrus (Penfield's "motor man"). This area is characterized by increased excitability. The secondary motor zone is represented by the lateral surface of the hemisphere (field 6), it is responsible for planning and coordinating voluntary movements. It receives the bulk of the efferent impulses from the basal ganglia and the cerebellum, and is also involved in recoding information about complex movements. Irritation of the cortex of field 6 causes more complex coordinated movements (turning the head, eyes and torso to the opposite side, friendly contractions of the flexor-extensor muscles on the opposite side). In the premotor zone with coordinated motor centers responsible for the social functions of a person: cent writing in the posterior part of the middle frontal gyrus, Broca's motor speech center (field 44) in the posterior part of the inferior frontal gyrus, which provides speech praxis, as well as the musical motor center (field 45), which determines the tone of speech and the ability to sing.

In the motor cortex, a layer of large pyramidal Betz cells is better expressed than in other areas of the cortex. Motor cortex neurons receive afferent inputs through the thalamus from muscle, joint, and skin receptors, as well as from the basal ganglia and the cerebellum. Pyramidal and associated intercalary neurons are located vertically in relation to the cortex. Such adjacent neuronal complexes that perform similar functions are called functional motor columns. Pyramidal neurons of the motor column can inhibit or excite motor neurons of the stem or spinal centers, for example, innervating one muscle. Neighboring columns functionally overlap, and pyramidal neurons that regulate the activity of one muscle, as a rule, are located in several columns.

The pyramidal tracts consist of 1 million fibers of the corticospinal tract, starting from the cortex of the upper and middle third of the precentral gyrus, and 20 million fibers of the corticobulbar tract, starting from the cortex of the lower third of the precentral gyrus (projection of the face and head). The fibers of the pyramidal tract terminate on the alpha motor neurons of the motor nuclei of 3-7 and 9-12 cranial nerves (corticobulbar tract) or on the spinal motor centers (corticospinal tract). Arbitrary simple movements and complex purposeful motor programs (professional skills) are carried out through the motor cortex and pyramidal pathways, the formation of which begins in the basal ganglia and cerebellum and ends in the secondary motor zone. Most of the fibers of the motor pathway are crossed, but a small part of them go to the same side, which contributes to compensation for unilateral lesions.

The cortical extrapyramidal pathways include the corticorubral and corticoreticular pathways, starting approximately from the zones in which the pyramidal pathways begin. The fibers of the corticorubral pathway terminate on the neurons of the red nuclei of the midbrain, from which the rubrospinal pathway then begins. The fibers of the corticoreticular pathway terminate at the medial nuclei of the pontine reticular formation (beginning of the medial reticular pathway), and at the neurons of the giant cells of the reticular pathway of the medulla oblongata, from which the lateral reticulospinal pathways begin. Through these pathways, the regulation of tone and posture is carried out, providing precise movements. These extrapyramidal pathways are constituent elements of the extrapyramidal system, which also includes the cerebellum, basal ganglia, motor centers of the brain stem; it regulates the tone, balance posture, the performance of learned motor acts, such as walking, running, speaking, writing, etc.

Assessing in general the role of various structures of the brain in the regulation of complex purposeful movements, it can be noted that the impulse to move is created in the limbic system, the idea of movement is in the associative zone of the cerebral hemispheres, motion programs-in basal ganglia, cerebellum and premotor cortex, and the execution of complex movements occurs through the motor cortex, motor centers of the brain stem and spinal cord.

1. Principle dominants was formulated by A. A. Ukhtomsky as the basic principle of the work of nerve centers. According to this principle, the activity of the nervous system is characterized by the presence in the central nervous system of the dominant (dominant) foci of excitation in a given period of time, in the nerve centers, which determine the direction and nature of body functions during this period. The dominant focus of excitation is characterized by the following properties:

Increased excitability;

Persistence of excitation (inertness), since it is difficult to suppress other excitation;

The ability to summation of subdominant excitations;

The ability to inhibit subdominant foci of excitation in functionally different nerve centers.

2. Principle spatial relief. It manifests itself in the fact that the total response of the organism with the simultaneous action of two relatively weak stimuli will be greater than the sum of the responses obtained with their separate action. The reason for the relief is due to the fact that the axon of an afferent neuron in the CNS synapses with a group of nerve cells in which a central (threshold) zone and a peripheral (subthreshold) "border" are isolated. Neurons located in the central zone receive from each afferent neuron a sufficient number of synaptic endings (for example, 2 each) (Fig. 13) to form an action potential. The neuron of the subthreshold zone receives from the same neurons a smaller number of endings (1 each), so their afferent impulses will be insufficient to cause the generation of action potentials in the "border" neurons, and only subthreshold excitation occurs. As a result, with separate stimulation of afferent neurons 1 and 2, reflex reactions occur, the total severity of which is determined only by the neurons of the central zone (3). But with simultaneous stimulation of afferent neurons, action potentials are also generated by neurons of the subthreshold zone. Therefore, the severity of such a total reflex response will be greater. This phenomenon has been named central relief. It is more often observed when weak stimuli act on the body.

Rice. 13. Scheme of the phenomenon of relief (A) and occlusion (B). The circles indicate the central zones (solid line) and the subthreshold "border" (dotted line) of the neuron population.

3. Principle occlusion. This principle is the opposite of spatial facilitation, and it consists in the fact that two afferent inputs jointly excite a smaller group of motor neurons compared to the effects when they are activated separately. The reason for occlusion is that afferent inputs, due to convergence, are partly addressed to the same motor neurons, which are inhibited when both inputs are activated simultaneously (Fig. 13). The phenomenon of occlusion is manifested in cases of application of strong afferent stimuli.

4. Principle feedback. The processes of self-regulation in the body are similar to technical ones, which involve automatic regulation of the process using feedback. The presence of feedback allows you to correlate the severity of changes in the parameters of the system with its work as a whole. The connection of the output of the system with its input with a positive gain is called positive feedback, and with a negative coefficient - negative feedback. In biological systems, positive feedback is realized mainly in pathological situations. Negative feedback improves the stability of the system, i.e., its ability to return to its original state after the influence of disturbing factors ceases.

Feedback can be classified according to various criteria. For example, according to the speed of action - fast (nervous) and slow (humoral), etc.

The importance of feedback mechanisms in maintaining homeostasis is great. For example, maintaining a constant level of blood pressure is carried out by changing the impulse activity of the baroreceptors of the vascular reflexogenic zones, which change the tone of the vasomotor sympathetic nerves and thus normalize blood pressure.

5. Principle reciprocity(combinations, conjugations, mutual exclusions). It reflects the nature of the relationship between the centers responsible for the implementation of opposite functions (inhalation and exhalation, flexion and extension of the limb, etc.). For example, activation of the proprioreceptors of the flexor muscle simultaneously excites the motor neurons of the flexor muscle and inhibits the motor neurons of the extensor muscle through intercalary inhibitory neurons (Fig. 18). Reciprocal inhibition plays an important role in the automatic coordination of motor acts.

6. Principle common end path. The effector neurons of the central nervous system (primarily the motor neurons of the spinal cord), being the final ones in the chain consisting of afferent, intermediate and effector neurons, can be involved in the implementation of various body reactions by excitations coming to them from a large number of afferent and intermediate neurons, for which they are the final path (by way from the CNS to the effector). For example, on the motoneurons of the anterior horns of the spinal cord, which innervate the muscles of the limb, the fibers of afferent neurons, neurons of the pyramidal tract and extrapyramidal system (nuclei of the cerebellum, reticular formation and many other structures) terminate. Therefore, these motor neurons, which provide the reflex activity of the limb, are considered as the final path for the general implementation of many nerve influences on the limb.