The main pathways of the spinal cord and their functions. Functional anatomy of the spinal cord

The white matter of the spinal cord surrounds the gray matter and forms the columns of the spinal cord. Distinguish front, rear and side pillars. Pillars are tracts of the spinal cord formed by long axons of neurons going up towards the brain (ascending paths) or down from the brain to the lower segments of the spinal cord (descending paths).
The ascending pathways of the spinal cord carry information from receptors in the muscles, tendons, ligaments, joints, and skin to the brain. Ascending paths are also conductors of temperature and pain sensitivity. All ascending pathways cross at the level of the spinal (or brain) cord. Thus, the left half of the brain (the cerebral cortex and cerebellum) receive information from the receptors of the right half of the body and vice versa.

The main ascending pathways: from mechanoreceptors of the skin and receptors of the musculoskeletal system are muscles, tendons, ligaments, joints - Gaulle's and Burdach's bundles, or, respectively, the gentle and wedge-shaped bundles are represented by the posterior columns of the spinal cord (Fig. 17 A).
From these receptors, information enters the cerebellum along two pathways represented by the lateral columns, which are called the anterior and posterior spinal tracts. In addition, two more paths pass in the lateral columns - these are the lateral and anterior spinal thalamic paths, which transmit information from temperature and pain sensitivity receptors.
The posterior columns provide faster information about the localization of irritations than the lateral and anterior spinal thalamic pathways.
Descending paths, passing as part of the anterior and lateral columns of the spinal cord, are motor, as they affect the functional state of the skeletal muscles of the body. The pyramidal path begins mainly in the motor cortex of the hemispheres and passes through the medulla oblongata, where most of the fibers cross and pass to the opposite side. After that, the pyramidal path is divided into lateral and anterior bundles: respectively, the anterior and lateral pyramidal paths. Most of the pyramidal tract fibers terminate on interneurons, and about 20% form synapses on motor neurons. The pyramidal influence is exciting.
The reticulospinal tract, the rubrospinal tract, and the vestibulospinal tract (extrapyramidal system) start, respectively, from the nuclei of the reticular formation, the brain stem, the red nuclei of the midbrain, and the vestibular nuclei of the medulla oblongata. These pathways run in the lateral columns of the spinal cord, are involved in the coordination of movements and the provision of muscle tone. Extrapyramidal paths, as well as pyramidal ones, are crossed (Fig. 17 B).
Thus, the spinal cord performs two important functions: reflex and conduction. The reflex function is carried out due to the motor centers of the spinal cord: motor neurons

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1

A

Rice. 17 A-B

A - Ascending pathways of the spinal cord:

1. - Gaulle's bundle;
2. - Burdakh's bundle;
3. - dorsal spinal cerebellar tract;
4. - ventral spinal cerebellar tract;
5. - anterior spinal thalamic pathway;
6. - lateral dorsal-thalamic pathway.

B - Main descending spinal tracts:
pyramidal (lateral and anterior corticospinal tracts) and extrapyramidal (rubrospinal, reticulospinal and vestibulospinal tracts) systems.

And to the flexor muscles to the flexor muscles
and extensors and extensors

A - arcs of the flexion and cross extensor reflexes; B - an elementary scheme of an unconditioned reflex. Nerve impulses that occur when the receptor (P) is stimulated go along afferent fibers (aff. nerve, one such fiber is shown) to the spinal cord (1), where they are transmitted through the intercalary neuron to efferent fibers (eff. nerve), through which they reach effector. Dashed lines - the spread of excitation from the lower parts of the central nervous system to its higher parts (2, 3, 4) up to the cerebral cortex (5) inclusive. The resulting change in the state of the higher parts of the brain, in turn, affects (see arrows) the efferent neuron, affecting the final result of the reflex response.

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Rice. 19. Scheme of the pathways of the spinal cord:
Descending paths:
A - pyramidal or corticospinal;
B - extrapyramidal system
Rubrospinal and reticulospinal paths, which are part of the multineuronal extrapyramidal path, which goes from the cerebral cortex to the spinal cord;
Ascending pathways: B - anterior spinal thalamic tract
Along this path, information from pressure and touch receptors, as well as from pain and temperature receptors, enters the somatosensory cortex;
D - lateral spinal-thalamic tract This way information from pain and temperature receptors comes to vast areas of the cerebral cortex.

5

1. - motor cortex;
2. - midbrain;
3. - pyramidal path;
4. - medulla;
5. - lateral corticospinal tract;
6. - anterior corticospinal tract;
7. - diffuse projections on the cortex;
8. - interlamellar nuclei of the thalamus;
9. - lateral dorsal-thalamic pathway;
10. - somatosensory cortex;
11. - ventrobasal complex of the thalamus;
12. - medial loop;
13. - red core;
14. - bridge;
15. - reticular formation;
16. - rubrospinal path;
17. - reticulospinal path;
18. - spinal cord.

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their horns provide the work of the skeletal muscles of the body. At the same time, maintaining muscle tone, coordinating the work of the flexor-extensor muscles underlying movements, and maintaining the constancy of the posture of the body and its parts (see Fig. 18, p. 39). Motoneurons located in the lateral horns of the thoracic segments of the spinal cord provide respiratory movements (inhalation-exhalation), regulating the work of the intercostal muscles. Motoneurons of the lateral horns of the lumbar and sacral segments represent the motor centers of smooth muscles that make up the internal organs. These are the centers of urination, defecation, and the work of the genital organs.
The conduction function is performed by the spinal tracts (see Fig. 19, pp. 40 - 41).

The pathways in the CNS are divided into ascending and descending. The ascending pathways are formed by the axons of cells whose bodies are located in the gray matter of the spinal cord. These axons in the composition of the white matter are directed to the upper parts of the spinal cord, the brainstem and the cortex of the cerebral hemispheres. Descending pathways are formed by axons of cells whose bodies are located in different nuclei of the brain. These axons descend along the white matter to various spinal segments, enter the gray matter and leave their endings on its cells.

Ascending paths. The main ascending systems pass through the dorsal funiculi of the spinal cord and are axons of afferent neurons of the spinal ganglia. They pass throughout the spinal cord and end in the region of the medulla oblongata in the nuclei of the dorsal cord - the nuclei of Gaulle and Burdach. These paths are called Gaull's tract And Burdakh tract. The fibers located more medially in the cord carry afferent signals to the Gaulle's nucleus from the lower part of the body, mainly from the lower extremities. Lateral fibers go to Burdach's nucleus and transmit afferent signals from the receptors of the upper body and upper (in animals - forelimbs) limbs. The axons of the cells of the Gaulle and Burdach nuclei in the brain stem intersect and rise in the form of a dense bundle to the diencephalon. This bundle of fibers, formed by the cells of the nuclei of Gaulle and Burdach, is called medial loop. The cells of the nuclei of the diencephalon form the third link of neurons, the axons of which are sent to the cerebral cortex.

All other ascending pathways do not start from the neurons of the spinal ganglia, but from neurons located in the gray matter of the spinal cord. Their fibers are fibers of the second order. The neurons of the spinal ganglia serve as the first link in these pathways, but in the gray matter of the spinal cord they leave their endings on the cells of the second link, and already these cells send their axons to the nuclei of the trunk and the cerebral cortex. The bulk of the fibers of these pathways runs in the lateral funiculus.

Spinal-thalamic the path begins at the base of the dorsal horn of the spinal cord. The axons of the neurons that form this tract pass to the opposite side, enter the white matter of the opposite lateral or ventral funiculus, and in it rise up through the entire spinal cord and brainstem to the nuclei of the diencephalon. Further, neurons of the third order (neurons of the diencephalon) transfer impulses to the cerebral cortex. The tracts of Gaulle and Burdach and the spinothalamic tract connect the receptive regions of each side of the body with the neurons of the cortex of the opposite hemisphere.

In the lateral cords, there are two more pathways that connect the spinal cord with the cerebellar cortex and form the spinal cerebellar tracts. The Flexig tract is located dorsally and contains fibers that do not pass to the opposite side of the brain. The Gowers pathway is ventral (ventral spinocerebellar tract), contains fibers that rise up the lateral funiculus of the opposite side of the body, but in the brainstem these fibers cross again and enter the cerebellar cortex from the side on which this path began.

Thus, if the cerebral cortex is always connected with the afferent fibers of the opposite side of the body, then the cerebellar cortex receives fibers mainly from the neural structures of the same side.

In addition to the paths leading to various structures of the brain, there are paths in the white matter of the spinal cord that do not go beyond it. These paths are located in the deepest part of the lateral and ventral cords, they connect various nerve centers. Such paths are called propriospinal.

Functions of ascending systems. Ascending systems provide various types of sensitivity, conducting impulses from receptors on the outer surface of the body, the motor apparatus and internal organs to the higher parts of the central nervous system.

Skin-mechanical sensitivity It is provided mainly by the ways of the dorsal funiculus (the bundles of Gaulle and Burdach). Afferent fibers pass through these tracts, transmitting impulses from mechanoreceptors that respond to the movement of hairs to a light or strong touch on the skin. These paths are the fastest. A significant part of the impulses from the skin receptors rises up the lateral cords to the cerebellar cortex (spinal-cerebellar tract), through the brainstem to the diencephalon and cerebral cortex (spinal-thalamic tract).

Another group of fibers of skin sensitivity goes to the upper cervical nucleus (spinal-cervical path), and from it, as part of the medial loop, rises to the forebrain. These systems have their own functional features. The tracts of Gaulle and Burdach are organized in such a way that each group of cells, activated by the endings of their axons, is excited only by impulses from a certain area of ​​the skin surface.

In the spinal-thalamic system, the spatial separation of signals from various skin receptors is poorly expressed; cellular reactions here are of a generalized nature. Each neuron in this system can receive impulses from large receptive fields. Thus, the spinal-thalamic system cannot transmit information about local irritations and serves to transmit general information about mechanical effects on the skin. The dorsal-cervical tract system and the medial loop are more accurate. The cells of the superior cervical nucleus perceive impulses only from limited receptive fields.

Ascending paths of temperature sensitivity pass along the lateral cords, impulses from temperature receptors rise along the fibers that go as part of the spinal-thalamic tract. Pathways are the same pathways for impulses from pain receptors. The transmission of impulses from the receptors of the motor apparatus (proprioreceptors) is carried out along the same paths along which impulses from skin receptors that perceive mechanical irritations go to the higher parts of the central nervous system. Impulses from proprioceptors are sent to the forebrain along the dorsal cord pathways, and to the cerebellum - along the spinal cerebellar pathways. Interoceptive impulses after synaptic switching on neurons of the spinal cord go to the higher parts of the CNS along the ascending pathways of the lateral cords. Specialized afferent pathways from the receptors of the internal organs to the brain stem also pass as part of the vagus nerve.

descending paths. Downward fibers are divided into several paths. The names of these pathways are based on the name of the CNS departments that they connect.

Corticospinal the path is formed by the axons of the pyramidal cells of the cerebral cortex (another name is pyramidal tract). Its fibers, without interruption, pass from the motor area and adjacent areas of the cortex through the stem structures to the medulla oblongata. In the region of the medulla oblongata, most of the fibers pass to the opposite side and, as part of the white matter of the lateral cords, descend to the caudal segments of the spinal cord. That part of the pyramidal fibers that has not crossed to the opposite side at the level of the medulla oblongata makes this transition at the level of those spinal segments to which they are directed.

Thus, the motor area of ​​the cerebral cortex is always associated with neurons on the opposite side of the spinal cord.

The main descending pathway of the midbrain begins in the red nucleus and is called rubro-spinal tract. The axons of the neurons of the red nucleus cross immediately below it and, as part of the white matter of the lateral funiculus of the opposite side, descend to the segments of the spinal cord, ending on the cells of the intermediate region of its gray matter. The rubro-spinal system, along with the pyramidal system, is the main system for controlling the activity of the spinal cord.

Two pathways originate from the medulla oblongata: vestibulo-spinal, starting from the vestibular nuclei, and reticulo-spinal, starting from the accumulation of cells of the reticular formation. The fibers of each of these pathways terminate on neurons in the medial part of the ventral horn. It is assumed that the fibers of the reticulospinal tract can influence the function of the spinal cord by pre-activating its cells.

In addition to long descending pathways, short intersegmental propriospinal fibers are present in the spinal cord. These fibers are included in the transmission of signals entering the spinal cord along long pathways.

Functions of downstream systems. The pyramidal (cortico-spinal) descending system is heterogeneous in its organization. It contains fast-conducting fibers (velocity of about 60 m/s) and slow-conducting fibers. One part of it provides fast (phasic) motor reactions and is represented by thick conductive fibers originating from large pyramidal cells of the cortex. Another part of the pyramidal system regulates the tonic reactions of the skeletal muscles. This influence is carried out mainly through thin fibers. With the defeat of the pyramidal system (transection of fibers), there is a violation of motor activity, mainly fine voluntary movements and a violation of the regulation of muscle tone. The volume of these disorders and their duration are small, since they are quickly compensated by the activity of the descending pathways that duplicate the functions of the pyramidal system. First of all, the cortico-rubro-spinal system. The speed of excitation in this system is 80 m/s, rubro-spinal fibers have a large diameter.

Pyramidal and rubro-spinal system in the central nervous system perform similar functions, they are combined into one group - the lateral descending systems. They pass in the lateral cords and are connected with the intercalary neurons of the lateral part of the gray matter, which send their axons mainly to the lateral motor nuclei that innervate the distal muscles of the limbs.

Vestibulo-spinal fibers are classified as very fast conductive (120 m/s). Their activation causes monosynaptic excitations of predominantly extensor motor neurons that innervate the muscles of the trunk and the proximal muscles of the extremities. In this case, reciprocal inhibitory processes occur in flexor neurons. Thus, the vestibulo-spinal system maintains the tonic tension of the extensor muscles.

Reticulo-spinal fibers originating from the medial nuclei of the reticular formation and passing in the medial part of the anterior funiculus have a high speed of excitation - 130 m/s. Their irritation innervates predominantly flexor motor neurons innervating the muscles of the trunk and limbs. The vestibulo- and reticulo-spinal tracts have much in common. Their fibers pass side by side in the ventral cords and establish direct connections with motor neurons. The most pronounced effects upon their activation are observed in the motoneurons of the medial nuclei that innervate the axial muscles of the body. These two paths are combined into one group - medial descending systems, mainly associated with the implementation of positional reflexes. Unlike lateral systems, they are not in synergistic, but in antagonistic relations with each other, since they activate motor neurons of the opposite functional purpose.

The pyramidal tract is the path of voluntary movements. The remaining paths are extrapyramidal, their function is the implementation of reflex movements.

PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM

Spinal cord

Pathways of the spinal cord

The white matter of the spinal cord consists of myelin fibers, which are collected in bundles. These fibers can be short (intersegmental) and long - connecting different parts of the brain with the spinal cord and vice versa. Short fibers (they are called associative) connect neurons of different segments or symmetrical neurons of opposite sides of the spinal cord.

Long fibers (they are called projection) are divided into ascending, going to the brain, and descending - going from the brain to the spinal cord. These fibers form the pathways of the spinal cord.

Bundles of axons form the so-called cords around the gray matter: anterior - located medially from the anterior horns, posterior - located between the posterior horns of the gray matter, and lateral - located on the lateral side of the spinal cord between the anterior and posterior roots.

The axons of the spinal ganglia and gray matter of the spinal cord go to its white matter, and then to other structures of the central nervous system, thereby creating ascending and descending pathways.

Descending pathways are located in the anterior cords:

1) anterior cortical-spinal, or pyramidal, path (tractus corticospinalis ventralis, s.anterior), which is straight uncrossed;

2) posterior longitudinal bundle (fasciculus longitudinalis dorsalis, s.posterior);

3) tectospinal, or tectospinal, path (tractus tectospinalis);

4) pre-door-spinal, or vestibulospinal, path (tractus vestibulospinalis).

Ascending paths pass in the posterior cords:

1) a thin bundle, or Gaulle's bundle (fasciculus gracilis);

2) wedge-shaped bundle, or Burdach's bundle (fasciculus cuneatus).

Descending and ascending pathways run in the lateral cords.

Downstream paths include:

1) the lateral cortical-spinal, or pyramidal, path (tractus corticospinalis lateralis), is crossed;

2) red-nuclear-spinal, or rubrospinal, path (tractus rubrospinalis);

3) reticular-spinal, or reticulospinal, path (tractus reticulospinalis).

Ascending paths include:

1) spinal-thalamic (tractus spinothalamicus) path;

2) lateral and anterior dorsal-cerebellar, or Flexig and Govers bundles (tractus spinocerebellares lateralis et ventralis).

Associative, or propriospinal, pathways connect neurons of one or different segments of the spinal cord. They start from the neurons of the gray matter of the intermediate zone, go to the white matter of the lateral or anterior funiculus of the spinal cord and end in the gray matter of the intermediate zone or on the motoneurons of the anterior horns of other segments. These connections perform an associative function, which consists in coordinating the posture, muscle tone, and movements of different metameres of the body. The propriospinal tracts also include commissural fibers connecting functionally homogeneous symmetrical and asymmetric parts of the spinal cord.

Descending pathways (Fig. 4.10) connect parts of the brain with motor or autonomic efferent neurons.

Cerebrospinal descending pathways start from the neurons of the structures of the brain and end on the neurons of the segments of the spinal cord. These include the following pathways: anterior (straight) and lateral (crossed) cortical-spinal (from the pyramidal neurons of the pyramidal and extrapyramidal cortex, providing regulation of voluntary movements), red-nuclear-spinal (rubrospinal), vestibular-spinal (vestibulospinal), reticular-spinal ( reticulospinal) pathways are involved in the regulation of muscle tone. The unifying factor for all of these pathways is that their final destination is the motor neurons of the anterior horns. In humans, the pyramidal pathway terminates directly on motor neurons, while other pathways terminate predominantly on intermediate neurons.

The pyramidal path consists of two bundles: lateral and direct. The lateral bundle starts from the neurons of the cerebral cortex, at the level of the medulla oblongata passes to the other side, forming a decussation, and descends along the opposite side of the spinal cord. The direct bundle descends to its segment and there it passes to the motor neurons of the opposite side. Therefore, the entire pyramidal path is crossed.

The red nuclear-spinal, or rubrospinal, path (tractus rubrospinalis) consists of axons of neurons in the red nucleus. Immediately after leaving the nucleus, these axons pass to the symmetrical side and are divided into three bundles. One goes to the spinal cord, the other to the cerebellum, the third to the reticular formation of the brain stem.

The neurons that give rise to this pathway are involved in controlling muscle tone. Rubrocerebellar and rubroreticular pathways provide coordination of the activity of pyramidal neurons of the cortex and cerebellar neurons involved in the organization of voluntary movements.

The vestibular-spinal, or vestibulospinal, path (tractus vestibulospinalis) starts from the neurons of the lateral vestibular nucleus (Deiters' nucleus), which lies in the medulla oblongata. This nucleus regulates the activity of motor neurons of the spinal cord, provides muscle tone, coordination of movements, balance.

The reticular-spinal, or reticulospinal, path (tractus reticulospinalis) goes from the reticular formation of the brain stem to the motor neurons of the spinal cord, through which the reticular formation regulates muscle tone.

Damage to the conduction apparatus of the spinal cord leads to disturbances in the motor or sensory system below the site of injury.

The intersection of the pyramidal pathway causes muscle hypertonicity below the transection (spinal cord motor neurons are released from the inhibitory effect of the pyramidal cells of the cortex) and, as a result, to spastic paralysis.

When crossing sensitive paths, muscle, articular, pain and other sensitivity below the site of spinal cord transection is completely lost.

The spinocerebral ascending tracts (see Fig. 4.10) connect segments of the spinal cord to brain structures. These pathways are represented by the pathways of proprioceptive sensitivity, thalamic, spinal-cerebellar, spinal-reticular. Their function is to transmit information to the brain about extero-, intero- and proprioceptive stimuli.

The proprioceptive pathway (thin and wedge-shaped bundles) starts from the deep sensitivity receptors of the muscles of the tendons, periosteum, and joint membranes. A thin bundle starts from the ganglia, which collect information from the caudal parts of the body, pelvis, and lower extremities. The wedge-shaped bundle starts from the ganglia, which collect information from the muscles of the chest, upper limbs. From the spinal ganglion, axons go to the posterior roots of the spinal cord, to the white matter of the posterior cords, and rise to the thin and wedge-shaped nuclei of the medulla oblongata. Here the first switch to a new neuron occurs, then the path goes to the lateral nuclei of the thalamus of the opposite hemisphere of the brain, switches to a new neuron, i.e., the second switch occurs. From the thalamus, the path rises to the neurons of layer IV of the somatosensory cortex. The fibers of these tracts give off collaterals in each segment of the spinal cord, which makes it possible to correct the posture of the entire body. The speed of excitation along the fibers of this path reaches 60-100 m/s.

The spinal thalamic pathway (tractus spinothalamicus) - the main pathway of skin sensitivity - starts from pain, temperature, tactile receptors and skin baroreceptors. Pain, temperature, tactile signals from skin receptors go to the spinal ganglion, then through the dorsal root to the dorsal horn of the spinal cord (first switch). Sensory neurons of the posterior horns send axons to the opposite side of the spinal cord and ascend along the lateral funiculus to the thalamus; the speed of conducting excitation along them is 1-30 m / s (second switching), from here - to the sensory area of ​​the cerebral cortex. Part of the skin receptor fibers goes to the thalamus along the anterior funiculus of the spinal cord.

The spinal cerebellar tract (tractus spinocerebellares) lies in the lateral cords of the spinal cord and is represented by non-crossing anterior, spinal cerebellar tract (Govers bundle) and doubly crossing posterior spinal cerebellar tract (Flexig bundle). Therefore, all spinal tracts start on the left side of the body and end in the left lobe of the cerebellum; likewise, the right lobe of the cerebellum receives information only from its side of the body. This information comes from the Golgi tendon receptors, proprioceptors, pressure and touch receptors. The speed of excitation along these tracts reaches 110-120 m/s.

Major pathways of the spinal cord

Without setting ourselves the task of listing all the pathways of the CNS, let us consider the basic principles of organizing these pathways using the most important of them as an example (Fig. 30). The pathways in the CNS are divided into:

ascending- are formed by axons of cells whose bodies are located in the gray matter of the spinal cord. These axons in the white matter are sent to the upper parts of the spinal cord, the brain stem and the cerebral cortex.

descending- are formed by axons of cells whose bodies are located in different nuclei of the brain. These axons descend along the white matter to various spinal segments, enter the gray matter and leave their endings on one or another of its cells.

A separate group is formed propriospinal conducting paths. They can be both ascending and descending, but they do not go beyond the spinal cord. After passing through several segments, they again return to the gray matter of the spinal cord. These paths are located in the deepest part lateral And ventral cords, they connect the various nerve centers of the spinal cord. For example, the centers of the lower and upper limbs.

Ascending pathways.

Tracts of Gaulle (thin bundle) and Burdakh (wedge-shaped bundle). The main ascending pathways pass through the dorsal funiculi of the spinal cord and represent the axons of afferent neurons. spinal ganglia. They pass throughout the spinal cord and end in the area oblong brain in the nuclei of the dorsal cord, which are called the nuclei of Gaulle and Burdach. That is why they are called Gaull's tract And tract Burdakh.

1. The first link of neurons:

a. The fibers located medially in the cord carry afferent signals to the Gaulle's nucleus from the lower part of the body, mainly from the lower extremities.

b. Lateral fibers go to Burdach's nucleus and transmit afferent signals from receptors in the upper body and forelimbs.

2. The second link of neurons:

In turn, the axons of the cells of the nuclei of Gaulle and Burdach in the brain stem cross and rise in the form of a dense bundle to intermediate brain. This bundle of fibers, already formed by the axons of the cells of the nuclei of Gaulle and Burdach, was called medial loop.

3. The third link of neurons:

The cells of the nuclei of the diencephalon give rise to axons that go to the cerebral cortex.

All other ascending paths do not begin from the neurons of the spinal ganglia, but from neurons located in gray matter of the spinal cord. Therefore, their fibers are fibers not of the first, but of the second order.

1. First link neurons of the spinal ganglia also serve in these pathways, but in the gray matter they leave their endings on the cells, as it were, of the “second link”.

Cells of this "second tier" send their axons to the nuclei of the brain stem and the cerebral cortex. The bulk of the fibers of these pathways runs in the lateral funiculus.

Spinal thalamic pathways (ventral and lateral).

2. The second link of neurons:

It originates at the base of the dorsal horn of the spinal cord. The axons of the neurons forming this path pass to the contralateral (opposite) side, enter the white matter of the opposite lateral or ventral funiculus and rise in it through the entire spinal cord And brain stem down to the core intermediate brain.

2. The third link of neurons:

The neurons of the diencephalon nuclei carry impulses to the cerebral cortex.

All of the above pathways (Gaulle, Burdach, and spinothalamic) connect receptive areas on each side of the body with cortical neurons. opposite hemisphere.

Spinal tracts. Two more pathways passing through the lateral cords connect the spinal cord with cerebellar cortex.

The Flexing Path - located dorsally and contains fibers that do not pass to the opposite side of the brain. This pathway in the spinal cord originates from Clarke's nucleus neurons, whose axons reach the medulla oblongata and enter the cerebellum via the inferior cerebellar peduncle.

Gower's Way - located more ventrally, contains fibers that rise up the lateral funiculus of the opposite side of the body, but in the brainstem these fibers cross again and enter the cerebellar cortex from the side on which this path began. In the spinal cord, it starts from the nuclei of the intermediate zone, axons enter the cerebellum through the superior cerebellar peduncle.

If the cerebral cortex is always connected with afferent fibers of the opposite side of the body, then the cerebellar cortex receives fibers mainly from neural structures. of the same name sides.

Descending pathways. Downward fibers are also subdivided into several paths. The names of these pathways are based on the names of those parts of the brain in which they originate.

Cortico-spinal (lateral and ventral) pathways formed by axons pyramidal cells lower layers of the motor cortex of the cerebral hemispheres. These paths are often called pyramidal. The fibers pass through white matter of the cerebral hemispheres, base of midbrain peduncles, by ventral divisions Varolieva bridge And oblong brain in dorsal brain.

o Lateral the path crosses in the lower part of the pyramids of the medulla oblongata and ends at the neurons of the base of the posterior horn.

o Ventral the path crosses the pyramids of the medulla oblongata without crossing. Before entering the anterior horn of the gray matter of the corresponding segment of the spinal cord, the fibers of this pathway pass to the opposite side and end on the motor neurons of the anterior horns of the contralateral side.

Thus, one way or another, but the motor area of ​​the cerebral cortex always turns out to be connected with neurons opposite sides of the spinal cord.

Rubro-spinal path - main descending path midbrain, starts at red core. The axons of the neurons of the red nucleus cross immediately below it and, as part of the white matter of the lateral funiculus, descend to the segments of the spinal cord, ending on the cells of the intermediate region of the gray matter. This is due to the fact that the rubrospinal system, along with the pyramidal system, is the main system for controlling the activity of the spinal cord.

Tectospinal path - Originates from neurons quadrigemina of the midbrain and reaches the motor neurons of the anterior horns.

Pathways originating in the medulla oblongata:

Vestibulo-spinal- starts from the vestibular nuclei, mainly from the cells of the nucleus of Deiters.

Reticulo-spinal- starts from an extensive accumulation of nerve cells of the reticular formation, which occupies the central part of the brain stem. The fibers of each of these pathways end on the neurons of the medial part of the anterior horn of the gray matter of the spinal cord. The main part of the endings are located on the intercalated cells.

Olivo-spinal- formed by the axons of the olive cells of the medulla oblongata, ends on the motor neurons of the anterior horns of the spinal cord.

Section 4

BRAIN

In the nervous system, nerve cells do not lie in isolation. They come into contact with each other, forming chains of neurons - conductors of impulses. The long process of one neuron - the neurite (axon) comes into contact with short processes (dendrites) or the body of another neuron following in the chain.

Along the chains of neurons, nerve impulses move in a strictly defined direction, which is due to the peculiarities of the structure of nerve cells and synapses. ("dynamic polarization"). Some chains of neurons carry an impulse in a centripetal direction - from the place of origin on the periphery (in the skin, mucous membranes, organs, vessel walls) to the central nervous system (spinal cord and brain). The first in this chain is sensory (afferent) neuron, perceives irritation and transforms it into a nerve impulse. Other chains of neurons conduct an impulse in a centrifugal direction - from the brain or spinal cord to the periphery, to the working organ. A neuron that transmits an impulse to a working organ is efferent.

Chains of neurons in a living organism form reflex arcs.

A reflex arc is a chain of nerve cells that necessarily includes the first - sensitive and the last - motor (or secretory) neurons, along which the impulse moves from the place of origin to the place of application (muscles, glands and other organs, tissues). The simplest reflex arcs are two- and three-neuron, closed at the level of one segment of the spinal cord. In a three-neuron reflex arc, the first neuron is represented by a sensitive cell, along which an impulse from the place of origin in a sensitive nerve ending (receptor) lying in the skin or in other organs moves first along the peripheral process (as part of the nerve). Then the impulse moves along the central process as part of the posterior root of the spinal nerve, heading to one of the nuclei of the posterior horn of the spinal cord, or along the sensory fibers of the cranial nerves to the corresponding sensory nuclei. Here, the impulse is transmitted to the next neuron, the process of which is directed from the posterior horn to the anterior, to the cells of the nuclei (motor) of the anterior horn. This second neuron performs a conductive (conductor) function. It transmits an impulse from a sensitive (afferent) neuron to a third - motor(efferent). The conductor neuron is intercalary neuron, since it is located between the sensory neuron, on the one hand, and the motor (or secretory) neuron, on the other. The body of the third neuron (efferent, effector, motor) lies in the anterior horn of the spinal cord, and its axon is part of the anterior root, and then the spinal nerve extends to the working organ (muscle).

With the development of the spinal cord and the brain, the connections in the nervous system became more complex. Multineuron complex reflex arcs were formed, in the construction and functions of which nerve cells located in the overlying segments of the spinal cord, in the nuclei of the brain stem, hemispheres and even in the cerebral cortex participate. The processes of nerve cells that conduct nerve impulses from the spinal cord to the nuclei and the cerebral cortex and in the opposite direction form bundles (fasciculi).

Bundles of nerve fibers connecting functionally homogeneous or different parts of the gray matter in the central nervous system, occupying a certain place in the white matter of the brain and spinal cord and conducting the same impulse, are called conducting paths.

In the spinal cord and brain, according to the structure and function, there are three groups of pathways: associative, commissural and projection.

Associative nerve fibers (neurofibrae associations) connect areas of gray matter, various functional centers (cerebral cortex, nuclei) within one half of the brain. Allocate short and long associative fibers (paths). Short fibers connect nearby areas of gray matter and are located within one lobe of the brain (intralobar fiber bundles). Some associative fibers connecting the gray matter of adjacent gyri do not extend beyond the cortex (intracortical). They arcuately bent in the form of the letter 0 and are called arcuate fibers of the large brain (fibrae arcuatae cerebri). Associative nerve fibers that extend into the white matter of the hemisphere (outside the cortex) are called extracortical.

Long associative fibers connect areas of gray matter that are far apart from each other, belonging to different lobes (interlobar fiber bundles). These are well-defined bundles of fibers that can be seen on a macroscopic specimen of the brain. The long associative pathways include the following: the upper longitudinal bundle (fasciculus longitudinalis superior), which is located in the upper part of the white matter of the cerebral hemisphere and connects the cortex of the frontal lobe with the parietal and occipital; lower longitudinal bundle (fasciculus longitudinalis inferior), lying in the lower parts of the hemisphere and connecting the cortex of the temporal lobe with the occipital; hooks, an id bundle (fasciculus uncinatus), which, arching in front of the island, connects the cortex in the region of the frontal pole with the anterior part of the temporal lobe. In the spinal cord, association fibers connect gray matter cells belonging to different segments and form anterior, lateral, and posterior bundles of their own. (intersegment bundles)(fasciculi proprii ventrales, s. anteriores lateralis, dorsrales, s. posteriores). They are located directly next to the gray matter. Short bundles connect neighboring segments, spreading over 2-3 segments, long bundles connect segments of the spinal cord that are far apart from each other.

Commissural (commissural) nerve fibers (neurofibrae commissurales) connect the gray matter of the right and left hemispheres, similar centers of the right and left halves of the brain in order to coordinate their functions. Commissural fibers pass from one hemisphere to another, forming commissures (corpus callosum, commissure fornix, anterior commissure). In the corpus callosum, which is found only in mammals, there are fibers connecting new, younger parts of the brain, cortical centers of the right and left hemispheres. In the white matter of the hemispheres, the fibers of the corpus callosum diverge fan-shaped, forming the radiance of the corpus callosum (radiatio corporis callosi).

Commissural fibers running in the knee and beak of the corpus callosum connect to each other sections of the frontal lobes of the right and left hemispheres of the brain. Curving anteriorly, the bundles of these fibers, as it were, cover the anterior part of the longitudinal fissure of the large brain on both sides and form frontal forceps (forceps frontalis). In the trunk of the corpus callosum, there are nerve fibers connecting the cortex of the central gyri, parietal and temporal lobes of the two hemispheres of the brain. The ridge of the corpus callosum consists of commissural fibers that connect the cortex of the occipital and posterior parietal lobes of the right and left hemispheres of the brain. Bending backwards, the bundles of these fibers cover the posterior sections of the longitudinal fissure of the large brain and form the occipital forceps (forceps occipitalis).

Commissural fibers run as part of the anterior commissure of the brain (commissura rostralis, s. anterior) and the commissure of the fornix (commissura fornicis). Most of the commissural fibers that make up the anterior commissure are bundles that connect the anteromedial cortex of the temporal lobes of both hemispheres to each other in addition to the fibers of the corpus callosum. The anterior commissure also contains bundles of commissural fibers, which are weakly expressed in humans, and go from the region of the olfactory triangle on one side of the brain to the same region on the other side. In the commissure of the fornix there are commissural fibers that connect the cortex of the right and left temporal lobes of the cerebral hemispheres, the right and left hippocampus.

Projective nerve fibers (neurofibrae proectiones) connect the underlying parts of the brain (spinal cord) with the brain, as well as the nuclei of the brain stem with the basal nuclei (striatum) and the cortex, and, conversely, the cerebral cortex, the basal nuclei with the nuclei of the brain stem and with the spinal cord. brain. With the help of projection fibers reaching the cerebral cortex, the pictures of the external world are projected onto the cortex as if on a screen, where the highest analysis of the impulses received here takes place, their conscious evaluation takes place. In the group of projection paths, ascending and descending fiber systems are distinguished.

Ascending projection pathways(afferent, sensitive) carry to the brain, to its subcortical and higher centers (to the cortex), impulses resulting from the impact on the body of environmental factors, including from the sense organs, as well as impulses from the organs of movement, internal organs , vessels. According to the nature of the conducted impulses, the ascending projection paths are divided into three groups.

1. Exteroceptive pathways (from Latin exter. externus - external, external) carry impulses (pain, temperature, touch and pressure) resulting from the influence of the external environment on the skin, as well as impulses from higher sense organs (organs of vision, hearing, taste , smell).
2. Proprioceptive pathways (from Latin proprius - own) conduct impulses from the organs of movement (from muscles, tendons, joint capsules, ligaments), carry information about the position of body parts, about the range of motion.
3. Interoceptive pathways (from lat. interior - internal) conduct impulses from internal organs, vessels, where chemo-, baro- and mechanoreceptors perceive the state of the internal environment of the body, metabolic rate, chemistry of blood, tissue fluid, lymph, pressure in the vessels

exteroceptive pathways. The pathway of pain and temperature sensitivity - the lateral spinal thalamic pathway (tractus spinothalamicus lateralis) consists of three neurons. It is customary to give names to sensitive conducting paths taking into account topography - the place of the beginning and end of the second neuron. For example, in the spinothalamic tract, a second neuron extends from the spinal cord, where the cell body lies in the posterior horn, to the thalamus, where the axon of this neuron synapses with the cell of a third neuron. The receptors of the first (sensory) neuron, which perceive the feeling of pain, temperature, are located in the skin, mucous membranes, and the neuritis of the third neuron ends in the cortex of the postcentral gyrus, where the cortical end of the analyzer of general sensitivity is located. The body of the first sensitive cell lies in the spinal ganglion, and its central process, as part of the posterior root, goes to the posterior horn of the spinal cord and ends in synapses on the cells of the second neuron. The axon of the second neuron, whose body lies in the posterior horn, goes to the opposite side of the spinal cord through its anterior gray commissure and enters the lateral funiculus, where it is included in the lateral spinal thalamic pathway. From the spinal cord, the bundle rises to the medulla oblongata and is located behind the nucleus of the olive, and in the tegmentum of the bridge and midbrain lies at the outer edge of the medial loop. The second neuron of the lateral spinal-thalamic pathway ends with synapses on the cells of the dorsal lateral nucleus of the thalamus. Here are the bodies of the third neuron, the processes of the cells of which pass through the posterior leg of the internal capsule and are part of fan-shaped diverging bundles of fibers that form a radiant crown (corona radiata). These fibers reach the cerebral cortex, its postcentral gyrus. Here they end in synapses with the cells of the fourth layer (inner granular lamina). The fibers of the third neuron of the sensitive (ascending) pathway connecting the thalamus with the cortex form thalamocortical bundles (fasciculi thalamocorticalis) - thalamoparietal fibers (fibrae thalamoparietales). The lateral spinal-thalamic pathway is a completely crossed pathway (all fibers of the second neuron pass to the opposite side), therefore, if one half of the spinal cord is damaged, pain and temperature sensitivity on the opposite side of the injury completely disappear.

Conducting the path of touch and pressure, the anterior spinothalamic path (tractus spinothalamicus ventralis, s. anterior) carries impulses from the skin, where the receptors that perceive the feeling of pressure and touch lie. Impulses go to the cerebral cortex, to the postcentral gyrus - the location of the cortical end of the general sensitivity analyzer. The cell bodies of the first neuron lie in the spinal ganglion, and their central processes, as part of the posterior root of the spinal nerves, go to the posterior horn of the spinal cord, where they end in synapses on the cells of the second neuron. The axons of the second neuron pass to the opposite side of the spinal cord (through the anterior gray commissure), enter the anterior funiculus and, in its composition, go up to the brain. On their way in the medulla oblongata, the axons of this path join from the lateral side to the fibers of the medial loop and end in the thalamus, in its dorsal lateral nucleus, with synapses on the cells of the third neuron. The fibers of the third neuron pass through the internal capsule (posterior pedicle) and, as part of the radiant crown, reach the fourth layer of the cortex of the postcentral gyrus.

It should be noted that not all fibers that carry impulses of touch and pressure pass to the opposite side in the spinal cord. Part of the fibers of the pathway of touch and pressure goes as part of the posterior funiculus of the spinal cord (its side) together with the axons of the pathway of the proprioceptive sensitivity of the cortical direction. In this regard, when one half of the spinal cord is affected, the skin sense of touch and pressure on the opposite side does not disappear completely, like pain sensitivity, but only decreases. This transition to the opposite side is partially carried out in the medulla oblongata.

proprioceptive pathways. The pathway of proprioceptive sensitivity of the cortical direction (tractus bulbothalamicus - BNA) is called so because it conducts impulses of the muscular-articular sense to the cerebral cortex, to the postcentral gyrus. Sensory endings (receptors) of the first neuron are located in muscles, tendons, joint capsules, ligaments. Signals about muscle tone, tendon tension, the state of the musculoskeletal system as a whole (impulses of proprioceptive sensitivity) allow a person to assess the position of body parts (head, torso, limbs) in space, as well as during movement and to carry out purposeful conscious movements and their correction . The bodies of the first neurons lie in the spinal ganglion. The central processes of these cells as part of the posterior root are sent to the posterior funiculus, bypassing the posterior horn, and then go up into the medulla oblongata to the thin and sphenoid nuclei. Axons carrying proprioceptive impulses enter the posterior cord starting from the lower segments of the spinal cord. Each next bundle of axons is adjacent from the lateral side to the existing bundles. Thus, the outer sections of the posterior cord (wedge-shaped bundle, Burdach's bundle) are occupied by axons of cells that carry out proprioceptive innervation in the upper thoracic, cervical sections of the body and upper limbs. Axons occupying the inner part of the posterior cord (thin bundle, Gaulle's bundle) conduct proprioceptive impulses from the lower extremities and the lower half of the trunk. The central processes of the first neuron end in synapses on their side, on the cells of the second neuron, whose bodies lie in the thin and wedge-shaped nuclei of the medulla oblongata. The axons of the cells of the second neuron emerge from these nuclei, arcuately bend forward and medially at the level of the lower angle of the rhomboid fossa and in the interstitial layer pass to the opposite side, forming a decussation of the medial loops (decussatio lemniscorum medialis). The bundle of fibers facing the medial direction and passing to the other side is called the internal arcuate fibers (fibrae arcuatae internae), which are the initial section of the medial loop (lemniscus medialis). The fibers of the medial loop in the bridge are located in its posterior part (in the operculum), almost on the border with the anterior part (between the fiber bundles of the trapezoid body). In the tegmentum of the midbrain, a bundle of fibers of the medial loop occupies a place dorsolateral to the red nucleus, and ends in the dorsal lateral nucleus of the thalamus with synapses on the cells of the third neuron. The axons of cells of the third neuron through the posterior pedicle of the internal capsule and as part of the radiant crown reach the postcentral gyrus.

Part of the fibers of the second neuron, upon exiting the thin and wedge-shaped nuclei, bends outward and is divided into two bundles. One bundle - the posterior external arcuate fibers (fibrae arcuatae externae dorsales, s. posteriores), are sent to the lower cerebellar peduncle of their side and end in the cortex of the cerebellar vermis. The fibers of the second bundle - the anterior external arcuate fibers (fibrae arcuatae externae ventrales, s. anteriores) go forward, go to the opposite side, go around the olive nucleus from the lateral side and also go through the lower cerebellar pedicle to the cortex of the cerebellar vermis. Anterior and posterior external arcuate fibers carry proprioceptive impulses to the cerebellum.

proprioceptive pathway the cortical direction is also crossed. The axons of the second neuron pass to the opposite side not in the spinal cord, but in the medulla oblongata. If the spinal cord is damaged on the side of proprioceptive impulses (in case of brain stem injury - on the opposite side), the idea of ​​the state of the musculoskeletal system, the position of body parts in space is lost, and coordination of movements is disturbed.

Along with the proprioceptive pathway that carries impulses to the cerebral cortex, the proprioceptive anterior and posterior spinal cerebellar pathways should be mentioned. Through these pathways, the cerebellum receives information from the lower sensitive centers (spinal cord) about the state of the musculoskeletal system, participates in the reflex coordination of movements that ensure the balance of the body without the participation of the higher parts of the brain (the cerebral cortex).

The posterior spinocerebellar path (tractus spinocerebellaris dorsalis, s. posterior; Flexig's bundle) transmits proprioceptive impulses from muscles, tendons, joints to the cerebellum. The cell bodies of the first (sensitive) neuron are located in the spinal ganglion, and their central processes, as part of the posterior root, go to the posterior horn of the spinal cord and end in synapses on the cells of the thoracic nucleus (Clark nucleus), which lies in the medial part of the base of the posterior horn. The cells of the thoracic nucleus are the second neuron of the posterior spinal cerebellar tract. The axons of these cells exit into the lateral funiculus of their side, into its back part, rise up and through the lower cerebellar peduncle enter the cerebellum, to the cells of the cortex of the worm. This is where the spinal cord ends.

It is possible to trace the fiber systems along which the impulse from the cortex of the worm reaches the red nucleus, the cerebellar hemisphere, and even the overlying parts of the brain - the cerebral cortex. From the cortex of the worm, through the corky and spherical nuclei, the impulse through the superior cerebellar peduncle is directed to the red nucleus of the opposite side (cerebellar-tegmental path). The cortex of the worm is connected by associative fibers with the cortex of the cerebellar hemisphere, from where impulses enter the dentate nucleus of the cerebellum.

With the development of higher centers of sensitivity and voluntary movements in the cortex of the cerebral hemispheres, connections between the cerebellum and the cortex also arose, through the thalamus. Thus, from the dentate nucleus, the axons of its cells through the superior cerebellar peduncle exit into the tegmentum operculum, pass to the opposite side and go to the thalamus. Switching in the thalamus to the next neuron, the impulse follows in the cerebral cortex, in the postcentral gyrus.

The anterior dorsal cerebellar path (tractus spinocerebellaris ventralis, s. anterior; Gowers bundle) has a more complex structure than the posterior one, since it passes in the lateral funiculus of the opposite side, returning to the cerebellum on its side. The cell body of the first neuron is located in the spinal ganglion. Its peripheral process has endings (receptors) in muscles, tendons, and joint capsules. The central process of the cell of the first neuron as part of the posterior root enters the spinal cord and ends with synapses on cells adjacent to the thoracic nucleus from the lateral side. The axons of the cells of this second neuron pass through the anterior gray commissure into the lateral funiculus of the opposite side, into its anterior part, and rise up to the level of the isthmus of the rhomboid brain. At this point, the fibers of the anterior spinal cerebellar tract return to their side and through the superior cerebellar peduncle enter the cortex of the vermis of their side, into its anterior superior sections. Thus, the anterior spinal cerebellar path, having completed a complex, doubly crossed path, returns to the same side on which the proprioceptive impulses arose. Proprioceptive impulses that have entered the cortex of the worm along the anterior spinal-cerebellar proprioceptive pathway are also transmitted to the red nucleus and through the dentate nucleus to the cerebral cortex (to the postcentral gyrus).

Schemes of the structure of the pathways of the visual, auditory analyzers, taste and smell are considered in the relevant sections of anatomy (see "Sense Organs").

Descending projection pathways (effector, efferent) conduct impulses from the cortex, subcortical centers to the underlying sections, to the nuclei of the brain stem and motor nuclei of the anterior horns of the spinal cord. These paths can be divided into two groups:

1. main engine, or pyramidal path(cortical-nuclear and cortical-spinal tracts), carries impulses of voluntary movements from the cerebral cortex to the skeletal muscles of the head, neck, trunk, limbs through the corresponding motor nuclei of the brain and spinal cord;
2. extrapyramidal motor pathways(tractus rubrospinalis, tractus vestibulospinalis, etc.) transmit impulses from the subcortical centers to the motor nuclei of the cranial and spinal nerves, and then to the muscles.

The pyramidal pathway (tractus pyramidalis) includes a system of fibers along which motor impulses from the cerebral cortex, from the precentral gyrus, from giant pyramidal neurons (Betz cells) are directed to the motor nuclei of the cranial nerves and the anterior horns of the spinal cord, and from them to the skeletal muscles . Given the direction of the fibers, as well as the location of the bundles in the brain stem and cords of the spinal cord, the pyramidal path is divided into three parts:

1. cortical-nuclear - to the nuclei of the cranial nerves;
2. lateral cortical-spinal - to the nuclei of the anterior horns of the spinal cord;
3. anterior cortical-spinal - also to the anterior horns of the spinal cord.

The cortical-nuclear pathway (tractus corticonuclearis) is a bundle of processes of giant-pyramidal neurons that descend from the cortex of the lower third of the precentral gyrus to the internal capsule and pass through its knee. Further, the fibers of the cortical-nuclear pathway go to the base of the brain stem, forming the medial part of the pyramidal pathways. Cortico-nuclear, as well as cortical-spinal tracts occupy the middle 3/5 of the base of the brain stem. Starting from the midbrain and further, in the bridge and the medulla oblongata, the fibers of the cortical-nuclear pathway pass to the opposite side to the motor nuclei of the cranial nerves: III and IV - in the midbrain; V, VI, VII - in the bridge; IX, X, XI, XII - in the medulla oblongata. In these nuclei, the cortical-nuclear pathway ends. Its constituent fibers form synapses with the motor cells of these nuclei. The processes of the mentioned motor cells leave the brain as part of the corresponding cranial nerves and are sent to the skeletal muscles of the head and neck and innervate them.

The lateral and anterior cortico-spinal tracts (tractus corticospinales lateralis et ventralis, s.anterior) also start from the giant-pyramidal neurons of the precentral gyrus, its upper 2/3. The axons of these cells go to the internal capsule, pass through the anterior part of its posterior pedicle (immediately behind the fibers of the cortical-nuclear tract), descend to the base of the brain peduncle, where they occupy a place lateral to the cortical-nuclear tract. Further, the cortical-spinal fibers descend into the anterior part (base) of the bridge, penetrate the bundles of fibers of the bridge going in the transverse direction and exit into the medulla oblongata, where on its anterior (lower) surface they form protruding ridges - pyramids. In the lower part of the medulla oblongata, part of the fibers passes to the opposite side and continues into the lateral funiculus of the spinal cord, gradually ending in the anterior horns of the spinal cord with synapses on the motor cells of its nuclei. This part of the pyramidal pathways, which is involved in the formation of the pyramidal decussation (motor decussation), is called lateral corticospinal tract. Those fibers of the cortical-spinal tract that do not participate in the formation of the pyramidal decussation and do not cross to the opposite side continue their way down as part of the anterior funiculus of the spinal cord. These fibers make up anterior cortico-spinal tract. Then these fibers also pass to the opposite side, but through the white commissure of the spinal cord and end on the motor cells of the anterior horn of the opposite side of the spinal cord. The anterior cortical-spinal tract, located in the anterior cord, is younger in evolutionary terms than the lateral one. Its fibers descend mainly to the level of the cervical and thoracic segments of the spinal cord.

It should be noted that all pyramidal paths are crossed, i.e. their fibers on the way to the next neuron sooner or later switch to the opposite side. Therefore, damage to the fibers of the pyramidal pathways with a unilateral lesion of the spinal (or brain) cord leads to paralysis of the muscles on the opposite side, which receive innervation from the segments lying below the injury site.

The second neurons of the descending voluntary motor pathway (cortical-spinal) are the cells of the anterior horns of the spinal cord, the long processes of which exit the spinal cord as part of the anterior roots and are sent as part of the spinal nerves to innervate the skeletal muscles.

extrapyramidal pathways, united in one group, unlike the newer pyramidal pathways, they are evolutionarily older, having extensive connections in the brain stem and with the cerebral cortex, which has taken over the functions of controlling and managing the extrapyramidal system. The cerebral cortex, which receives impulses both along direct (cortical direction) ascending sensory pathways and from subcortical centers, controls the body's motor functions through extrapyramidal and pyramidal pathways. The cerebral cortex influences the motor functions of the spinal cord through the cerebellum-red nuclei system, through the reticular formation, which has connections with the thalamus and striatum, through the vestibular nuclei. Thus, the centers of the extrapyramidal system include red nuclei, one of the functions of which is to maintain muscle tone, which is necessary to keep the body in a state of balance without effort of will. The red nuclei, which also belong to the reticular formation, receive impulses from the cerebral cortex, the cerebellum (from the cerebellar proprioceptive pathways) and themselves have connections with the motor nuclei of the anterior horns of the spinal cord.

The red nuclear-spinal path (trdctus rubrospinalis) is part of the reflex arc, the bringing link of which is the spinal-cerebellar proprioceptive pathways. This path originates from the red nucleus (Monakov's bundle), passes to the opposite side (Forel's cross) and descends in the lateral funiculus of the spinal cord, ending on the motor cells of the spinal cord. The fibers of this path pass in the back (tire) of the bridge and the lateral sections of the medulla oblongata.

An important link in the coordination of the motor functions of the human body is the vestibulo-spinal tract (tractus vestibulospinalis). It connects the nuclei of the vestibular apparatus with the anterior horns of the spinal cord and provides the body's adjusting reactions in case of imbalance. Axons of the cells of the lateral vestibular nucleus take part in the formation of the vestibulo-spinal tract. (Deiters kernel), as well as the lower vestibular nucleus (descending root) of the vestibulocochlear nerve. These fibers descend in the lateral part of the anterior funiculus of the spinal cord (on the border with the lateral one) and end on the motor cells of the anterior horns of the spinal cord. The nuclei that form the pre-door-spinal path are in direct connection with the cerebellum, as well as with the posterior longitudinal bundle (fasciculus longitudinalis dorsalis, s. posterior), which in turn is associated with the nuclei of the oculomotor nerves. The presence of connections with the nuclei of the oculomotor nerves ensures that the position of the eyeballs (the direction of the visual axis) is maintained when the head and neck are turned. In the formation of the posterior longitudinal bundle and those fibers that reach the anterior horns of the spinal cord (reticular-spinal tract, tractus reticulospinalis), cell clusters of the reticular formation of the brain stem, mainly the intermediate nucleus (nucleus intersticialis, Cajal core) the nucleus of the epithalamic (posterior) commissure, the nucleus of Darkshevich, to which fibers come from the basal nuclei of the cerebral hemispheres.

The control of the functions of the cerebellum, which is involved in the coordination of movements of the head, trunk and limbs and is in turn connected with the red nuclei and the vestibular apparatus, is carried out from the cerebral cortex through the bridge along the corticopontocerebellar path (tractus corticopontocerebellaris). This pathway consists of two neurons. The cell bodies of the first neuron lie in the cortex of the frontal, temporal, parietal and occipital lobes. Their processes - cortical backbone fibers (fibrae corticopontinae) are sent to the internal capsule and pass through it. Fibers from the frontal lobe, which can be called the frontal bridge fibers (fibrae frontopontinae), pass through the anterior leg of the internal capsule. Nerve fibers from the temporal, parietal and occipital lobes pass through the posterior leg of the internal capsule. Further, the fibers of the cortical-bridge path go through the base of the brain stem. From the frontal lobe, the fibers pass through the most medial part of the base of the brain stem, medially from the cortical-nuclear fibers. From the parietal and other lobes of the cerebral hemispheres go through the most lateral part, outward from the cortical-spinal tract. In the anterior part (at the base) of the pons, the fibers of the cortical-bridge pathway terminate in synapses on the cells of the pontine nucleus on the same side of the brain. The cells of the pontine nuclei with their processes constitute the second neuron of the cortical-cerebellar pathway. The axons of the cells of the nuclei of the bridge are folded into bundles - the transverse fibers of the bridge (fibrae pontis transversae), which pass to the opposite side, cross the descending bundles of fibers of the pyramidal pathways in the transverse direction and through the middle cerebellar pedicle are sent to the cerebellar hemisphere of the opposite side.

Thus, the pathways of the brain and spinal cord establish connections between afferent and efferent (effector) centers, participate in the formation of complex reflex arcs in the human body. Some conducting paths (systems of fibers) begin or end in evolutionarily older nuclei lying in the brain stem, providing functions that have a certain automatism. These functions (for example, muscle tone, automatic reflex movements) are carried out without the participation of consciousness, although under the control of the cerebral cortex. Other pathways transmit impulses to the cerebral cortex, to the higher parts of the central nervous system, or from the cortex to the subcortical centers (to the basal ganglia, nuclei of the brain stem and spinal cord). Conducting pathways functionally unite the body into a single whole, ensure the consistency of its actions.