Conducting 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 - impulse conductors. The long process of one neuron - neurite (axon) comes into contact with short processes (dendrites) or the body of another neuron, the next in the chain.

Along neuron chains, nerve impulses move in a strictly defined direction, which is due to the structural features of nerve cells and synapses ("dynamic polarization"). Some neuron chains 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 a sensory (afferent) neuron, which perceives irritation and transforms it into a nerve impulse. Other neuron chains conduct an impulse in a centrifugal direction - from the brain or spinal cord to the periphery, to the working organ. A neuron transmitting an impulse to the 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 - sensory 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 ones, closing at the level of one segment of the spinal cord. In a three-neuron reflex arc, the first neuron is represented by a sensory cell, along which the impulse from the place of origin in the sensory nerve ending (receptor) located in the skin or 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 function. It transmits an impulse from the sensory (afferent) neuron to the third - motor (efferent). The conductive neuron is an 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 - as part of the anterior root, and then the spinal nerve extends to the working organ (muscle).

With the development of the spinal cord and brain, the connections in the nervous system also became more complex. Multi-neuron 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 conducting nerve impulses from the spinal cord to the nuclei and cortex of the brain and in the opposite direction form bundles (fasciculi).

Bundles of nerve fibers that connect functionally homogeneous or different areas of gray matter in the central nervous system, occupy a specific place in the white matter of the brain and spinal cord and conduct the same impulse, are called conducting pathways.

In the spinal cord and brain, three groups of conduction pathways are distinguished based on structure and function: associative, commissural and projection.

Association nerve fibers (neurofibrae associations) connect areas of gray matter, various functional centers (cerebral cortex, nuclei) within one half of the brain. Short and long association fibers (pathways) are distinguished. Short fibers connect adjacent areas of gray matter and are located within one lobe of the brain (intralobar fiber bundles). Some association fibers connecting the gray matter of neighboring convolutions do not go beyond the cortex (intracortical). They curve in an arcuate shape in the form of the letter 0 and are called arcuate fibers of the cerebrum (fibrae arcuatae cerebri). Association nerve fibers that go into the white matter of the hemisphere (beyond the cortex) are called extracortical.

Long association fibers connect areas of gray matter that are widely spaced from each other and belong to different lobes (interlobar fiber bundles). These are well-defined fiber bundles that can be seen on a macroscopic preparation of the brain. The long association pathways include the following: the superior 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; the inferior longitudinal bundle (fasciculus longitudinalis inferior), which lies in the lower parts of the hemisphere and connects the cortex of the temporal lobe with the occipital; the uncinate bundle (fasciculus uncinatus), which, arcuately bending in front of the insula, 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 the cells of the gray matter belonging to different segments and form the anterior, lateral and posterior proper bundles (intersegmental bundles) (fasciculi proprii ventrales, s. anteriores lateralis, dorsrales, s. posteriores). They are located directly next to the gray matter. Short bundles connect neighboring segments, crossing 2-3 segments, long bundles connect segments of the spinal cord that are far from each other.

Commissural (adhesive) nerve fibers (neurofibrae commissurales) connect the gray matter of the right and left hemispheres, analogous centers of the right and left halves of the brain in order to coordinate their functions. Commissural fibers pass from one hemisphere to the other, forming adhesions (corpus callosum, fornix commissure, anterior commissure). The corpus callosum, present only in mammals, contains fibers connecting new, younger parts of the brain, the 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 genu and beak of the corpus callosum connect parts of the frontal lobes of the right and left cerebral hemispheres with each other. Curving forward, the bundles of these fibers seem to embrace the anterior part of the longitudinal fissure of the cerebrum on both sides and form the frontal forceps (forceps frontalis). In the trunk of the corpus callosum pass nerve fibers connecting the cortex of the central convolutions, parietal and temporal lobes of the two cerebral hemispheres. The splenium of the corpus callosum consists of commissural fibers that connect the cortex of the occipital and posterior parts of the parietal lobes of the right and left cerebral hemispheres. Curving backward, the bundles of these fibers embrace the posterior parts of the longitudinal fissure of the cerebrum and form the occipital forceps (forceps occipitalis).

Commissural fibers pass through the anterior commissure of the brain (commissura rostralis, s. anterior) and the fornices commissure (commissura fornicis). Most of the commissural fibers that make up the anterior commissure are bundles that connect the anteromedial areas of the cortex of the temporal lobes of both hemispheres in addition to the fibers of the corpus callosum. The anterior commissure also contains bundles of commissural fibers, weakly expressed in humans, that run from the olfactory triangle on one side of the brain to the same area on the other side. The fornices commissure contains commissural fibers that connect areas of the cortex of the right and left temporal lobes of the cerebral hemispheres, and the right and left hippocampi.

Projection nerve fibers (neurofibrae projectes) connect the lower parts of the brain (spinal cord) with the cerebrum, as well as the nuclei of the brainstem with the basal nuclei (striated body) and the cortex and, conversely, the cerebral cortex, basal nuclei with the nuclei of the brainstem and with the spinal cord. With the help of projection fibers that reach the cerebral cortex, pictures of the external world are projected onto the cortex as if onto a screen, where the highest analysis of the impulses received here and their conscious evaluation take place. In the group of projection paths, ascending and descending fiber systems are distinguished.

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

1. Exteroceptive pathways (from the Latin exter. externus - external, outer) carry impulses (pain, temperature, touch and pressure) that arise as a result of the impact of the external environment on the skin, as well as impulses from the higher sense organs (organs of vision, hearing, taste, smell).
2. Proprioceptive pathways (from the 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 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 the blood, tissue fluid, lymph, and pressure in the vessels

Exteroceptive pathways. The pathway of pain and temperature sensitivity - the lateral spinothalamic tract (tractus spinothalamicus lateralis) consists of three neurons. Sensory pathways are usually named based on their topography - the place where the second neuron begins and ends. For example, in the spinothalamic tract, the 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 forms a synapse with the cell of the third neuron. The receptors of the first (sensory) neuron, which perceive pain and temperature, are located in the skin and mucous membranes, and the neuritis of the third neuron ends in the cortex of the postcentral gyrus, where the cortical end of the general sensitivity analyzer is located. The body of the first sensory 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, is directed 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 spinothalamic tract. From the spinal cord, the bundle ascends into the medulla oblongata and is located behind the olive nucleus, and in the tegmentum of the pons and midbrain it lies at the outer edge of the medial loop. The second neuron of the lateral spinothalamic tract ends with synapses on the cells of the dorsal lateral nucleus of the thalamus. The bodies of the third neuron are located here, the processes of whose cells pass through the posterior leg of the internal capsule and as part of fan-shaped diverging bundles of fibers that form the radiant 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). The fibers of the third neuron of the sensory (ascending) pathway connecting the thalamus with the cortex form thalamocortical bundles (fasciculi thalamocorticalis) - thalamoparietal fibers (fibrae thalamoparietales). The lateral spinothalamic tract is a completely crossed pathway (all fibers of the second neuron cross to the opposite side), therefore, when one half of the spinal cord is damaged, pain and temperature sensitivity on the side opposite the damage completely disappear.

The anterior spinothalamic tract (tractus spinothalamicus ventralis, s. anterior), which carries the sense of touch and pressure, carries impulses from the skin, where the receptors that perceive the sense of pressure and touch are located. The 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, are directed 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 cross to the opposite side of the spinal cord (through the anterior gray commissure), enter the anterior funiculus and, as part of it, are directed upward, to the brain. On their way in the medulla oblongata, the axons of this pathway join the fibers of the medial lemniscus on the lateral side 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 leg) and, as part of the corona radiata, reach layer IV of the cortex of the postcentral gyrus.

It should be noted that not all fibers carrying impulses of touch and pressure cross over to the opposite side in the spinal cord. Some fibers of the conductive pathway of touch and pressure go as part of the posterior funiculus of the spinal cord (their side) together with the axons of the conductive pathway of proprioceptive sensitivity of the cortical direction. In connection with this, when one half of the spinal cord is damaged, the cutaneous 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 proprioceptive pathway of cortical sensitivity (tractus bulbothalamicus - BNA) is so called because it conducts impulses of muscle-articular sense to the cerebral cortex, to the postcentral gyrus. The sensory endings (receptors) of the first neuron are located in muscles, tendons, joint capsules, ligaments. Signals about muscle tone, tendon tension, about 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 targeted conscious movements and their correction. The bodies of the first neurons are located in the spinal ganglion. The central processes of these cells as part of the posterior root are directed to the posterior funiculus, bypassing the posterior horn, and then go up into the medulla oblongata to the thin and cuneate nuclei. Axons carrying proprioceptive impulses enter the posterior funiculus starting from the lower segments of the spinal cord. Each subsequent bundle of axons is adjacent to the lateral side of the existing bundles. Thus, the outer sections of the posterior funiculus (the cuneate 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 funiculus (the thin bundle, Goll's bundle) conduct proprioceptive impulses from the lower limbs and the lower half of the body. The central processes of the first neuron end with synapses on their side, on the cells of the second neuron, the bodies of which lie in the thin and cuneate nuclei of the medulla oblongata. The axons of the cells of the second neuron emerge from these nuclei, arc forward and medially at the level of the inferior angle of the rhomboid fossa and in the interolivary layer pass to the opposite side, forming a decussation of the medial loops (decussatio lemniscorum medialis). The bundle of fibers facing in 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 pons are located in its posterior part (in the tegmentum), almost on the border with the anterior part (between the bundles of fibers of the trapezoid body). In the tegmentum of the midbrain, the bundle of fibers of the medial lemniscus 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 the cells of the third neuron reach the postcentral gyrus through the posterior leg of the internal capsule and as part of the corona radiata.

Some of the fibers of the second neuron, upon exiting the thin and cuneate nuclei, bend outward and divide into two bundles. One bundle, the posterior external arcuate fibers (fibrae arcuatae externae dorsales, s. posteriores), are directed to the inferior 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, cross to the opposite side, bend around the lateral side of the olivary nucleus and also go through the inferior cerebellar peduncle to the cortex of the cerebellar vermis. The anterior and posterior external arcuate fibers carry proprioceptive impulses to the cerebellum.

The proprioceptive pathway of the cortical direction is also crossed. The axons of the second neuron cross to the opposite side not in the spinal cord, but in the medulla oblongata. When the spinal cord is damaged on the side where the proprioceptive impulses originate (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 impaired.

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

The posterior spinocerebellar tract (tractus spinocerebellaris dorsalis, s. posterior; Flechsig's bundle) transmits proprioceptive impulses from muscles, tendons, and joints to the cerebellum. The cell bodies of the first (sensory) neuron are located in the spinal ganglion, and their central processes, as part of the posterior root, are directed to the posterior horn of the spinal cord and end in synapses on the cells of the thoracic nucleus (Clarke's nucleus), located in the medial part of the base of the posterior horn. The cells of the thoracic nucleus are the second neuron of the posterior spinocerebellar tract. The axons of these cells exit into the lateral funiculus of their side, into its posterior part, rise upward and through the inferior cerebellar peduncle enter the cerebellum, to the cells of the vermis cortex. Here the spinocerebellar tract ends.

It is possible to trace the systems of fibers along which the impulse from the vermis cortex reaches the red nucleus, the cerebellar hemisphere, and even the higher parts of the brain - the cerebral cortex. From the vermis cortex through the cork-shaped and spherical nuclei, the impulse is directed through the superior cerebellar peduncle to the red nucleus of the opposite side (cerebellar-tegmental tract). The vermis cortex is connected by association fibers with the cerebellar cortex, 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 of the cerebellum with the cortex also arose, carried out through the thalamus. Thus, from the dentate nucleus, the axons of its cells through the superior cerebellar peduncle exit into the tegmentum of the bridge, cross to the opposite side and head to the thalamus. Having switched to the next neuron in the thalamus, the impulse goes to the cerebral cortex, to the postcentral gyrus.

The anterior spinocerebellar tract (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 in synapses on the cells adjacent to the thoracic nucleus on 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 upward to the level of the isthmus of the rhombencephalon. At this point, the fibers of the anterior spinocerebellar tract return to their side and through the superior cerebellar peduncle enter the cortex of the vermis of their side, into its antero-superior sections. Thus, the anterior spinocerebellar tract, having made a complex, twice-crossed path, returns to the same side on which the proprioceptive impulses arose. The proprioceptive impulses that entered the cortex of the vermis through the anterior spinocerebellar proprioceptive tract are also transmitted to the red nucleus and through the dentate nucleus to the cerebral cortex (to the postcentral gyrus).

The diagrams of the structure of the conducting pathways of the visual, auditory analyzers, taste and smell are considered in the corresponding 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 brainstem and the motor nuclei of the anterior horns of the spinal cord. These pathways can be divided into two groups:

1. the main motor, or pyramidal tract (corticonuclear and corticospinal tracts), carries impulses of voluntary movements from the cerebral cortex to the skeletal muscles of the head, neck, trunk, and 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 tract (tractus pyramidalis) includes a system of fibers along which motor impulses from the cerebral cortex, from the precentral gyrus, from the gigantopyramidal 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. Taking into account the direction of the fibers, as well as the location of the bundles in the brainstem and the funiculi of the spinal cord, the pyramidal tract is divided into three parts:

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

The corticonuclear tract (tractus corticonuclearis) is a bundle of processes of gigantopyramidal neurons, which descend from the cortex of the lower third of the precentral gyrus to the internal capsule and pass through its genu. Further, the fibers of the corticonuclear tract go to the base of the cerebral peduncle, forming the medial part of the pyramidal tracts. The corticonuclear and corticospinal tracts occupy the middle 3/5 of the base of the cerebral peduncle. Starting from the midbrain and further, in the pons and medulla oblongata, the fibers of the corticonuclear tract cross to the opposite side to the motor nuclei of the cranial nerves: III and IV - in the midbrain; V, VI, VII - in the pons; IX, X, XI, XII - in the medulla oblongata. The corticonuclear tract ends in these nuclei. The fibers that make it up 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 directed to the skeletal muscles of the head and neck and innervate them.

The lateral and anterior corticospinal tracts (tractus corticospinales lateralis et ventralis, s.anterior) also originate from the gigantopyramidal neurons of the precentral gyrus, its upper 2/3. The axons of these cells are directed to the internal capsule, pass through the anterior part of its posterior leg (immediately behind the fibers of the corticonuclear tract), descend to the base of the cerebral leg, where they occupy a place lateral to the corticonuclear tract. Then the corticospinal fibers descend to the anterior part (base) of the pons, penetrate the transverse fiber bundles of the pons and exit into the medulla oblongata, where they form protruding ridges - pyramids - on its anterior (lower) surface. In the lower part of the medulla oblongata, some of the fibers cross to the opposite side and continue 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 tracts, participating in the formation of the pyramidal decussation (motor decussation), is called the lateral corticospinal tract. Those fibers of the corticospinal tract that do not participate in the formation of the pyramidal decussation and do not cross to the opposite side, continue their way downwards as part of the anterior funiculus of the spinal cord. These fibers make up the anterior corticospinal tract. Then these fibers also cross 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 corticospinal tract, located in the anterior funiculus, is evolutionarily younger 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 tracts are crossed, i.e. their fibers on the way to the next neuron sooner or later cross over to the opposite side. Therefore, damage to the fibers of the pyramidal tracts in case of unilateral damage to the spinal (or brain) cord leads to paralysis of the muscles on the opposite side, receiving innervation from the segments lying below the site of damage.

The second neurons of the descending voluntary motor pathway (corticospinal) are the cells of the anterior horns of the spinal cord, the long processes of which emerge from the spinal cord as part of the anterior roots and are directed as part of the spinal nerves to innervate the skeletal muscles.

The extrapyramidal pathways, united in one group, unlike the newer pyramidal pathways, are evolutionarily older, having extensive connections in the brainstem and with the cerebral cortex, which has taken over the functions of control and management of the extrapyramidal system. The cerebral cortex, receiving impulses both along direct (cortical direction) ascending sensory pathways and from the subcortical centers, controls the motor functions of the body through the 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 the red nuclei, one of whose functions is to maintain muscle tone, necessary to keep the body in a state of equilibrium without an 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 rubrospinal tract (trdctus rubrospinalis) is part of the reflex arc, the afferent link of which is the spinocerebellar proprioceptive pathways. This tract originates from the red nucleus (Monakow's bundle), crosses to the opposite side (Forel's decussation) and descends in the lateral funiculus of the spinal cord, ending in the motor cells of the spinal cord. The fibers of this tract pass in the posterior part (tegmentum) of the pons and the lateral parts of the medulla oblongata.

An important link in the coordination of motor functions of the human body is the vestibulospinalis tract. It connects the nuclei of the vestibular apparatus with the anterior horns of the spinal cord and ensures the body's corrective reactions in case of imbalance. The axons of the cells of the lateral vestibular nucleus (Deiters' nucleus) and the inferior vestibular nucleus (descending root) of the vestibulocochlear nerve participate in the formation of the vestibulospinalis tract. 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 vestibulospinalis tract 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) when turning the head and neck. In the formation of the posterior longitudinal fasciculus and those fibers that reach the anterior horns of the spinal cord (reticular-spinal tract, tractus reticulospinalis), cellular clusters of the reticular formation of the brainstem take part, mainly the intermediate nucleus (nucleus intersticialis, the nucleus of Cajal), the nucleus of the epithalamic (posterior) commissure, the nucleus of Darkshevich, to which fibers from the basal nuclei of the cerebral hemispheres come.

The functions of the cerebellum, which is involved in coordinating movements of the head, trunk, and limbs and is in turn connected with the red nuclei and the vestibular apparatus, are controlled from the cerebral cortex through the bridge along the corticopontocerebellar tract (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, the cortical spinal fibers (fibrae corticopontinae), are directed to the internal capsule and pass through it. Fibers from the frontal lobe, which can be called frontopontinae fibers (fibrae frontopontinae), pass through the anterior leg of the internal capsule. Nerve fibers from the temporal, parietal, and occipital lobes go through the posterior leg of the internal capsule. Then the fibers of the corticopontocerebellar tract go through the base of the cerebral leg. From the frontal lobe, the fibers pass through the most medial part of the base of the cerebral peduncle, inward from the corticonuclear fibers. From the parietal and other lobes of the cerebral hemispheres, they pass through the most lateral part, outward from the corticospinal tracts. In the anterior part (at the base) of the pons, the fibers of the corticopontine tract end in synapses on the cells of the pontine nucleus of the same side of the brain. The cells of the pontine nuclei with their processes constitute the second neuron of the corticocerebellar tract. The axons of the cells of the pontine nuclei are folded into bundles - the transverse fibers of the pons (fibrae pontis transversae), which pass to the opposite side, cross in the transverse direction the descending bundles of fibers of the pyramidal tracts and through the middle cerebellar peduncle are directed to the cerebellar hemisphere of the opposite side.

Thus, the conduction pathways of the brain and spinal cord establish connections between the afferent and efferent (effector) centers, participate in the formation of complex reflex arcs in the human body. Some conduction pathways (fiber systems) begin or end in evolutionarily older nuclei located in the brain stem, providing functions with 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 conduction 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 nuclei, nuclei of the brain stem and spinal cord). Conduction pathways functionally unite the organism into a single whole, ensure the coordination of its actions.

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