Conductive pathways of the brain and spinal cord

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 the chain.

By neural circuits, 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 a pulse in the centripetal direction - from the place of origin on the periphery (in the skin, mucous membranes, organs, walls of blood vessels) to the central nervous system (spinal cord and brain). The first in this chain is the sensitive (afferent) neuron that perceives the stimulus and transforms it into a nerve impulse. Other chains of neurons impulse in the centrifugal direction - from the brain or spinal cord to the periphery, to the working organ. The neuron that transmits the impulse to the working organ is efferent.

Chains of neurons in a living organism form reflex arcs.

The reflex arc is a chain of nerve cells, necessarily including the first - sensitive and last - motor (or secretory) neurons, through 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-neural arches, closing at the level of one segment of the spinal cord. In the three-neuron reflex arc, the first neuron is represented by a sensitive cell, through which the impulse from the place of origin in the sensitive nerve end (receptor) lying in the skin or in other organs moves first along the peripheral process (in the nerve). Then the impulse moves along the central process in the posterior root of the spinal nerve, heading for one of the nuclei of the posterior horn of the spinal cord, or along sensitive fibers of the cranial nerves to the corresponding sensitive nuclei. Here the impulse is transferred to the next neuron, the process of which is directed from the horn to the anterior, to the cells of the nuclei (motor) of the anterior horn. This second neuron performs a conductor (conductor) function. It transfers the impulse from the sensitive (afferent) neuron to the third - motor (efferent). The conductor neuron is an intercalary neuron, since it is located between the sensitive neuron, on the one hand, and the motor neuron (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 - in the anterior spine, 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 complicated. Multi-neural 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. The processes of nerve cells, which conduct nerve impulses from the spinal cord to the nuclei and the cortex of the brain and in the opposite direction, form bundles (fasciculi).

Bunches of nerve fibers connecting functionally homogeneous or different parts of gray matter in the central nervous system, occupying a definite place in the white matter of the brain and spinal cord and conducting the same impulse, have been called conductive pathways.

In the spinal cord and brain, three groups of conducting paths are distinguished in structure and function: associative, commissural and projection.

Associative nerve fibers (neurofibrae associations) connect the areas of gray matter, various functional centers (brain cortex, nuclei) within one half of the brain. Isolate short and long associative fibers (pathways). Short fibers connect nearby areas of gray matter and are located within one part of the brain (intra-lobe fiber bundles). Some associative fibers that connect the gray matter of neighboring gyrus do not extend beyond the cortex (intracortical). They arc curved in the form of the letter 0 and are called arcuate fibers of the large brain (fibrae arcuatae cerebri). Associative nerve fibers that emerge into the white matter of the hemisphere (outside the cortex) are called extracortical.

Long associative fibers bind gray areas far apart from each other, belonging to different parts (inter-fiber bundles of fibers). These are well-defined bundles of fibers that can be seen on the macro preparation of the brain. The long associative pathways include the following: the superior longitudinal fasciculus (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; the lower longitudinal fasciculus (fasciculus longitudinalis inferior), lying in the lower parts of the hemisphere and connecting the cortex of the temporal lobe with the occipital lobe; hooks, idyllic fasciculus (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 cell associative fibers connect gray matter belonging to different segments and form the front, lateral and rear own beams (beams intersegment) (fasciculi proprii ventrales, s. Anteriores lateralis, dorsrales, s. Posteriores). They are located directly near the gray matter. Short beams connect adjacent segments, spreading through 2-3 segments, long beams connect far apart segments of the spinal cord.

Commissural (adherent) 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. Comissural fibers pass from one hemisphere to another, forming spikes (corpus callosum, spike of the arch, anterior adhesion). In the corpus callosum, which is found only in mammals, there are fibers that connect new, younger, parts of the brain, cortical centers of the right and left hemispheres. In the white substance of the hemispheres, the fiber of the corpus callosum diverges fan-shaped, forming the radiance of the corpus callosum (radiatio corporis callosi).

The commissural fibers running in the knee and beak of the corpus callosum connect the portions of the frontal lobes of the right and left hemispheres of the large brain to each other. Bending anteriorly, the bundles of these fibers, as it were, cover the anterior part of the longitudinal slit of the large brain from both sides and form the frontal forceps frontalis. In the trunk of the corpus callosum, nerve fibers pass through the cortex of the central gyri, parietal and temporal lobes of the two cerebral hemispheres. The corpus callosum consists of commissural fibers that connect the occipital cortex and the posterior parts of the parietal lobes of the right and left cerebral hemispheres. Bending backwards, the bundles of these fibers cover the posterior sections of the longitudinal slit of the large brain and form occipital forceps (forceps occipitalis).

The commissural fibers pass through the anterior commissure of the brain (commissura rostralis, s. Anterior) and the spikes of the arch (commissura fornicis). Most of the commissural fibers that form part of the anterior adhesion are bundles that connect the anterior medial cortex portions of the temporal lobes of both hemispheres in addition to the fibers of the corpus callosum. As part of the anterior soldering are also weakly expressed in human bundles of commissural fibers, going from the area of the olfactory triangle of one side of the brain to the same area of the other side. In the spike of the arch there are commissural fibers that connect portions of the cortex of the right and left temporal lobes of the cerebral hemispheres, the right and left hippocampi.

Projective nerve fibers (neurofibrae proectiones) connect the lower parts of the brain (spinal cord) to the brain, as well as the nuclei of the brain stem with the basal cores (striatum) and the cortex and, conversely, the cerebral cortex, basal nuclei with the nuclei of the brain stem and 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 a higher analysis of the impulses that have arrived here occurs, and their conscious evaluation. In the group of projection paths, ascending and descending systems of fibers are distinguished.

Ascending projective pathways (afferent, sensitive) are carried to the brain, to its subcortical and higher centers (to the cortex), impulses that have arisen as a result of influencing the body environmental factors, including sensory organs, as well as impulses from the organs of motion , internal organs, vessels. According to the nature of the impulses conducted, 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) that result from the influence of the external environment on the skin, as well as impulses from the higher sense organs (vision, hearing, taste , the sense of smell).
2. Proprioceptive pathways (from the Latin proprius - their own) conduct impulses from the organs of movement (from muscles, tendons, joint capsules, ligaments), carry information about the position of parts of the body, the scope of movements.
3. Interoceptive pathways (from the Latin interior - internal) conduct impulses from internal organs, vessels, where chemo-, baro- and mechanoreceptors perceive the state of the internal environment of the body, the intensity of metabolism, the chemistry of blood, tissue fluid, lymph, pressure in the vessels

Exteroceptive pathways. Conducting the path of pain and temperature sensitivity - the lateral spinal-thalamic path (tractus spinothalamicus lateralis) consists of three neurons. Sensitive conductive paths are usually given names in view of topography - the place of the beginning and end of the second neuron. For example, in the dorsal-thalamic path the second neuron extends from the spinal cord, where the body of the cell lies in the posterior horn, to the thalamus, where the axon of this neuron forms a synapse with the cell of the third neuron. The receptors of the first (sensitive) neuron, perceiving a sense of pain, temperature, are located in the skin, mucous membranes, and the third neuron neurite ends in the cortex of the postcentral gyrus, where the cortical end of the general sensitivity analyzer is located. The body of the first sensitive cell lies in the spinal node, and its central process as part of the posterior root is directed into the posterior horn of the spinal cord and ends with synapses on the cells of the second neuron. The axon of the second neuron, whose body lies in the posterior horn, is directed to the opposite side of the spinal cord through its anterior gray spike and enters the lateral cord, where it joins the lateral spinal-thalamic pathway. From the spinal cord, the bundle rises into the medulla oblongata and is located behind the olive core, and lies in the tire of the bridge and the midbrain at the outer edge of the medial loop. The second neuron of the lateral spinal-thalamic path 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 back leg of the inner capsule and in the composition of fan-shaped diverging bundles of fibers forming a radial crown (corona radiata). These fibers reach the cortex of the cerebral hemisphere, its postcentral gyrus. Here they end with synapses with cells of the fourth layer (internal granular plate). Fibers of the third neuron of a sensitive (ascending) pathway connecting the thalamus with the cortex form thalamocortical fascicles (fasciculi thalamocorticalis) - thalamotemeric fibers (fibrae thalamoparietales). The lateral dorsal-thalamic pathway is completely crossed by the conductive pathway (all fibers of the second neuron pass to the opposite side), so if one half of the spinal cord is damaged, the pain and temperature sensitivity on the opposite side of the lesion completely disappear.

Conducting the path of touch and pressure, the forward dorsal-thalamic tract (tractus spinothalamicus ventralis, s. Anterior) carries impulses from the skin, where receptors lie, perceiving a feeling of pressure and touch. Impulses go to the cerebral cortex, to the postcentral gyrus - the location of the cortical end of the analyzer of general sensitivity. The cells of the first neuron cells lie in the spinal node, and their central processes in the posterior root of the spinal nerves are sent to the posterior horn of the spinal cord, where they terminate in synapses on the cells of the second neuron. Axons of the second neuron pass to the opposite side of the spinal cord (through the anterior gray spike), enter the anterior cord and in its composition go upward, to the brain. On their way to the medulla oblongata, the axons of this path join from the lateral side to the fibers of the medial loop and terminate in the thalamus, in its dorsal lateral nucleus, synapses on the cells of the third neuron. The fibers of the third neuron pass through the inner capsule (posterior pedicle) and in the composition of the radiant crown reach the fourth layer of the cortex of the postcentral gyrus.

It should be noted that not all fibers carrying touch and pressure impulses pass to the opposite side in the spinal cord. Part of the fibers of the conductive path of touch and pressure goes in the composition of the back cord of the spinal cord (its side) along with the axons of the conduction path 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 completely disappear, 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 impulses the musculo-articular senses to the cerebral cortex, to the postcentral gyrus. Sensitive endings (receptors) of the first neuron are located in the muscles, tendons, joint capsules, ligaments. Signals on 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, trunk, extremities) in space, and also during movement and to carry out deliberate conscious movements and their correction . The bodies of the first neurons lie in the spinal node. The central processes of these cells in the posterior root are directed into the posterior cord, passing the horn, and then go upward into the medulla oblongata to the thin and wedge-shaped nuclei. Axons bearing proprioceptive impulses enter the posterior cord beginning with the lower segments of the spinal cord. Each next bundle of axons is from the lateral side to the already 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 parts of the body and upper limbs. Axons occupying the inner part of the posterior cord (a thin bundle, the Gaull's bundle), carry proprioceptive impulses from the lower extremities and the lower half of the trunk. The central processes of the first neuron end with 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, bend forward and medially at the level of the lower corner of the rhomboid fossa and pass to the opposite side in the interlayer layer, forming a cross of medial loops (decussatio lemniscorum medialis). A bundle of fibers facing in the medial direction and passing to the other side is called the inner arc-shaped fibers (fibrae arcuatae internae), which are the initial section of the medial loop (lemniscus medialis). The fibers of the copper loop in the bridge are located in the rear part (in the tire), almost on the border with the front part (between the bundles of fibers of the trapezoid body). In the middle brain, the fiber bundle of the medial loop takes the place of the dorsolateral to the red nucleus, and ends in the dorsal lateral nucleus of the thalamus with synapses on the cells of the third neuron. Axons of cells of the third neuron through the back leg of the inner capsule and in the composition of the radial crown reach the postcentral gyrus.

Part of the fibers of the second neuron on the way out of the thin and wedge-shaped nuclei bends outward and divides into two beams. One bundle - the posterior outer arc-shaped fibers (fibrae arcuatae externae dorsales, S. Posteriores), are sent to the lower cerebellar peduncle of their side and terminate in the cortex of the cerebellum worm. Fibers of the second fascicle - the front outer arc-shaped fibers (fibrae arcuatae externae ventrales, S. Anteriores) go forward, pass to the opposite side, go around the lateral side of the olive nucleus and also go through the lower cerebellar pedicle to the cortex of the cerebellum worm. The front and back outer arcuate fibers carry proprioceptive impulses to the cerebellum.

The proprioceptive path of 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 oblong brain. If the spinal cord is damaged on the side of the appearance of proprioceptive impulses (when the brainstem is traumatized on the opposite side), the idea of the state of the musculoskeletal system, the position of parts of the body in space is lost, coordination of movements is disrupted.

Along with the proprioceptive conducting pathway, which carries impulses to the cerebral cortex, proprioceptive anterior and posterior spinal-cerebellar pathways should be mentioned. According to these conducting paths, the cerebellum receives information from the below-located sensory centers (spinal cord) about the state of the musculoskeletal system, participates in the reflex coordination of movements that ensure the equilibrium of the body without the participation of higher brain regions (cerebral cortex cerebral cortex).

The posterior spinal path (tractus spinocerebellaris dorsalis, S. Posterior, Fleksig's bundle) transmits proprioceptive impulses from the muscles, tendons, joints to the cerebellum. The bodies of the cells of the first (sensitive) neuron are in the spinal node, and their central processes in the posterior root are directed to the horn of the spinal cord and terminate in synapses on the cells of the thoracic nucleus (the Clark nucleus) lying 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 path. The axons of these cells emerge into the lateral cord of their side, into its posterior part, rise upward and through the lower cerebellar peduncle enter the cerebellum, to the cells of the cortex of the worm. Here the spinal-cerebellar path ends.

It is possible to trace the fiber systems through which the impulse from the worm cortex reaches the red nucleus, the hemisphere of the cerebellum and even the overlying parts of the brain - the cortex of the cerebral hemispheres. From the cortex of the worm through the cork-shaped and globular nucleus, the pulse through the upper cerebellar pedicle is directed to the red nucleus of the opposite side (the cerebellar-lining path). The bark of the worm is connected by associative fibers to the cerebral cortex, wherece the impulses enter the jagged nucleus of the cerebellum.

With the development of higher centers of sensitivity and arbitrary movements in the cortex of the cerebral hemispheres, the connections of the cerebellum with the cortex, which are realized through the thalamus, also arose. Thus, from the dentate nucleus, the axons of its cells through the upper cerebellar pedicle emerge into the bridge cover, pass to the opposite side and are sent to the ta-lamus. Switching in the thalamus to the next neuron, the impulse follows the cerebral cortex, the postcentral gyrus.

The anterior spinal path (tractus spinocerebellaris ventralis, s. Anterior, the Hover's bundle) has a more complex structure than the posterior one, as it passes in the side cord of the opposite side, returning to the cerebellum to its side. The cell body of the first neuron is located in the spinal node. Its peripheral process has endings (receptors) in muscles, tendons, joint capsules. The central outgrowth of the cell of the first neuron as part of the posterior root enters the spinal cord and ends with synapses on the cells adjacent to the lateral side to the thoracic nucleus. The axons of the cells of this second neuron pass through the anterior gray spike in the lateral cord 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 cord path return to their side and through the upper cerebellar pedicle enter the cortex of their side, into its anterior regions. Thus, the anterior spinal-cerebellar path, having made a complex, double-crossed path, returns to the same side as the proprioceptive impulses appeared. Proprioceptive impulses that enter the cortex of the worm along the anterior spinal cerebellum proprioceptive pathway are also transmitted to the red nucleus and through the dentate nucleus into the cerebral cortex (into the postcentral gyrus).

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

Descending projection ways (effector, efferent) conduct impulses from the cortex, subcortical centers to the underlying parts, 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. the main motor or pyramidal path (cortico-nuclear and cortico-spinal pathways), bears the impulses of arbitrary movements from the cerebral cortex to the skeletal muscles of the head, neck, trunk, extremities through the corresponding motor nuclei of the brain and spinal cord;
2. extrapyramidal motor tracts (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 tract (tractus pyramidalis) includes a system of fibers through which motor impulses from the cerebral cortex, from the precentral gyrus, from giant-pyramidal neurons (cells of the Betz) 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 beams in the brainstem and the cord of the spinal cord, the pyramidal path is divided into three parts:

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

The cortical-nuclear path (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 inner capsule and pass through its knee. Next, 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 and spinal cord pathways occupy the middle 3/5 base of the brain stem. Starting from the middle brain and further, in the bridge and oblong brain, 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 middle brain; V, VI, VII - in the bridge; IX, X, XI, XII - in the medulla oblongata. In these nuclei the cortical-nuclear pathway ends. The fibers that make up it form synapses with the motor cells of these nuclei. The sprouts of these motor cells exit the brain in the corresponding cranial nerves and are directed to the skeletal muscles of the head and neck and innervate them.

The lateral and anterior cortical and spinal tracts (tractus corticospinales lateralis et ventralis, s.anterior) also start from the giant-pyramidal neurons of the precentral gyrus, its upper 2/3. Axons of these cells go to the inner capsule, pass through the front part of its posterior leg (just behind the fibers of the cortical-nuclear pathway), descend into the base of the brain stem, where they take the place lateral to the cortical-nuclear pathway. Further, the cortical and spinal fibers descend into the anterior part (base) of the bridge, the bundles of fiber of the bridge that run in the transverse direction penetrate into the medulla oblongata, where the forward (lower) surface of the bridge is formed by projecting ridges- pyramids. In the lower part of the medulla oblongata, a part of the fibers passes to the opposite side and continues into the lateral cord of the spinal cord, gradually ending in the anterior horns of the spinal cord by synapses on the motor cells of its nuclei. This part of the pyramid pathways involved in the formation of the cross of the pyramids (motor crossover) is called the lateral cortical-spinal cord. Those fibers of the cortex and spinal cord, which do not participate in the formation of the pyramid crosshairs and do not pass to the opposite side, continue their way downward in the anterior cord of the spinal cord. These fibers constitute the anterior cortex-spinal path. Then these fibers also pass to the opposite side, but through the white spike of the spinal cord and terminate on the motor cells of the anterior horn of the opposite side of the spinal cord. Located in the anterior cord, the anterior cortical and cerebrospinal path 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 pyramid paths are crossed; their fibers on the way to the next neuron sooner or later pass to the opposite side. Therefore, damage to the fibers of the pyramidal pathways with unilateral damage to the spinal (or brain) brain leads to paralysis of the muscles on the opposite side, receiving innervation from the segments lying below the injury site.

The second neurons of the descending arbitrary motor pathway (cortical and spinal marrow) are the cells of the anterior horns of the spinal cord, the long processes of which leave the spinal cord in the anterior roots and are sent as a part of the spinal nerves for the innervation of skeletal muscles.

Extrapyramidal pathways, united in one group, unlike the newer pyramidal pathways, are evolutionarily older, having extensive connections in the cerebral trunk and with the cerebral cortex that has assumed the functions of monitoring and controlling the extrapyramidal system. The cerebral cortex, receiving impulses both in the direct (cortical direction) ascending sensory pathways and from the subcortical centers, controls the motor functions of the organism through extrapyramidal and pyramidal pathways. The cerebral cortex influences the motor functions of the spinal cord through the cerebellum-red nucleus system, through the reticular formation, which has connections with the thalamus and striatum, through the vestibular nuclei. Thus, the number of centers of the extrapyramidal system includes red nuclei, one of the functions of which is to maintain the muscle tone necessary to keep the body in a state of equilibrium 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 cord (trdctus rubrospinalis) is part of the reflex arc, the resulting link of which are spinal-cerebellar proprioceptive pathways. This path originates from the red core (the bundle of Monakova), goes to the opposite side (Cross of the Trout) and descends into the lateral cord of the spinal cord, ending in the motor cells of the spinal cord. The fibers of this pathway pass in the posterior part (the tire cover) 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 pre-spinal cord (tractus vestibulospinalis). It connects the nuclei of the vestibular apparatus with the anterior horns of the spinal cord and provides the adjusting reactions of the body in the event of imbalance. Axons of cells of the lateral vestibular nucleus (the nucleus of Deiters), as well as of the inferior vestibular nucleus (descending root) of the pre-vertebral nerve, take part in the formation of the pre-vertebral-spinal path . These fibers descend in the lateral part of the anterior cord of the spinal cord (on the border with the lateral) and terminate on the motor cells of the anterior horns of the spinal cord. The nuclei that form the anterior-spinal cord are in direct connection with the cerebellum, as well as with the posterior longitudinal fasciculus (fasciculus longitudinalis dorsalis, S. Posterior), which in turn is connected with the nuclei of the oculomotor nerves. The presence of connections with the nuclei of the oculomotor nerves ensures the preservation of the position of the eyeballs (the direction of the visual axis) during the rotation of the head and neck. In the formation of the posterior longitudinal fascicle and those fibers that reach the anterior horns of the spinal cord (reticular-spinal tract, tractus reticulospinalis), the cellular assemblages of the reticular formation of the brain stem part, mainly the intermediate nucleus (nucleus intersticialis, the nucleus of Kahal), the nucleus of the epithalamic ( back) spikes, the core of Darksevic, to which fibers from the basal nuclei of the cerebral hemispheres come.

Management of the functions of the cerebellum, involved in the coordination of movements of the head, trunk and extremities and associated in turn with red nuclei and the vestibular apparatus, is carried out from the cerebral cortex through the bridge along the cortico-companion bridge (tractus corticopontocerebellaris). This pathway consists of two neurons. The bodies of the cells of the first neuron lie in the cortex of the frontal, temporal, parietal and occipital lobes. Their processes - cortex spine fibers (fibrae corticopontinae) are directed to the inner capsule and pass through it. Fibers from the frontal lobe, which can be called frontal-bridge fibers (fibrae frontopontinae), pass through the front leg of the inner capsule. Nerve fibers from the temporal, parietal and occipital lobes go through the back leg of the inner capsule. Next, the fibers of the cortex-bridge run 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, inward from the cortical-nuclear fibers. From the parietal and other parts of the cerebral hemispheres go through the most lateral part, outside of the cortical and spinal cord. In the front part (at the base) of the bridge, the fibers of the cortical-bridge path end in synapses on the cells of the core of the bridge on the same side of the brain. The cells of the nuclei of the bridge with their processes form the second neuron of the cortex-cerebellar path. 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 in the transverse direction the descending bundles of fibers of the pyramidal pathways and through the middle cerebellar pedicle to the hemisphere of the cerebellum of the opposite side.

Thus, the conducting paths of the brain and spinal cord establish the connections between the afferent and efferent (effector) centers, participate in the formation of complex reflex arcs in the human body. Some conducting paths (fiber systems) 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 performed without the participation of consciousness, although under the control of the cerebral cortex. Other pathways transmit impulses to the cerebral cortex, to the higher CNS, or from the cortex to the subcortical centers (to the basal nuclei, to the nuclei of the brain stem and spinal cord). Conductive ways functionally unite the body into one whole, ensure the consistency of its actions.

[1], [2], [3]