Pyramidal tracts of the brain. pyramid system. The beginning and end of the pyramidal paths

The main efferent structure is the central motor neuron, represented by giant Betz pyramidal cells of the V layer of the projection motor cortex (prerolandic gyrus and paracentral lobule, 4th field). The set of processes of Betz cells is part of the pyramidal pathway. A significant part of its fibers originate from other parts of the cerebral cortex: the secondary motor cortex of the inner surface of the frontal lobe, the superior frontal gyrus, the premotor cortex (6th field), as well as the postcentral gyrus, and not only from the large pyramidal cells of layer V, but also from small pyramidal cells of layer III and from others. Most of the fibers of the pyramidal pathway terminate in the formations of the extrapyramidal system - the striatum, the pale ball, the substantia nigra, the red nucleus, and also in the reticular formation of the brain stem, carrying out the interaction of the pyramidal and extrapyramidal systems. Other fibers, especially the thickly myelinated ones, originate from the giant Betz cells of the projection motor cortex and end on the dendrites of the peripheral motor neuron.

The motor neuron is located in two places - the anterior horns of the spinal cord and in the motor nuclei of the cranial nerves, and therefore the pyramidal path consists of two paths - corticospinal and corticonuclear (Fig. 1.2.1).

The main part of the fibers of the corticospinal tract at the border of the medulla oblongata and spinal cord passes to the other side and there it goes in the lateral cords of the spinal cord, ending segmentally: most of the path is in the anterior horns of the cervical and lumbar thickening, the motor neurons of which innervate the limbs, its other part goes on its side in the anterior canal. Presumably the muscles of the trunk have a bilateral innervation.

The corticonuclear pathway ends in the brain stem on the dendrites of the motor nuclei of the cranial nerves. material from the site

The functional principle of somatotopic localization is implemented in the projection motor cortex: the representation of the muscles that perform the most complex and significant voluntary movements occupies the maximum area. This applies to the facial muscles (facial expression is a means of biocommunication), the muscles of the tongue, pharynx, larynx (articulation is the basis of motor speech), as well as the hands, especially the fingers of the hand and the hand itself, presented respectively in the lower and middle parts of the projection motor cortex. (Fig. 1.2.2). The latter occupies the back of the outer surface of the frontal lobe (precentral gyrus). Anterior to the projection motor cortex is the premotor cortex, which plays an important role in transforming movements into actions, and anterior to the premotor cortex is the prefrontal cortex, which is responsible for the implementation of holistic activities. The premotor cortex is also part of the extrapyramidal system. When complex motor skills are mastered, they are already performed automatically according to programs read from the premotor cortex.

Lesions of the projection motor cortex cause central paralysis, premotor - disturbances in action (praxis), and prefrontal - activity. The prefrontal cortex is also important in upright walking in humans, and its defeat leads to a disorder of standing and walking.

The descending pathways of the brain and spinal cord conduct impulses from the cerebral cortex, cerebellum, subcortical and stem centers to the underlying motor nuclei of the brain stem and spinal cord.

The highest motor center in humans is the cerebral cortex. It controls the motor neurons of the brain stem and spinal cord in two ways: directly through the cortical-nuclear, anterior and lateral cortical-spinal (pyramidal) pathways, or indirectly, through the underlying motor centers. In the latter case, the role of the cortex is reduced to the launch, maintenance, or termination of the execution of motor programs stored in these centers. Descending paths are divided into two groups:

pyramid system ensures the execution of precise purposeful conscious movements, adjusts breathing, ensuring the pronunciation of words. It includes the cortico-nuclear, anterior and lateral cortico-spinal (pyramidal) pathways.

Cortico-nuclear pathway begins in the lower third of the precentral gyrus of the brain. Pyramidal cells (1 neuron) are located here, the axons of which pass through the knee of the internal capsule to the brainstem and are directed in its basal part down to the motor nuclei of the cranial nerves of the opposite side (III–VII, IX–XII). Here are the bodies of the second neurons of this system, which are analogues of the motor neurons of the anterior horns of the spinal cord. Their axons go as part of the cranial nerves to the innervated muscles of the head and neck.

Anterior and lateral corticospinal(pyramidal) tracts conduct motor impulses from pyramidal cells located in the upper two-thirds of the precentral gyrus to the muscles of the trunk and limbs of the opposite side.

The axons of the first neurons of these pathways go together as part of the radiant crown, pass through the posterior leg of the internal capsule to the brainstem, where they are located ventrally. In the medulla oblongata they form pyramidal elevations (pyramids); and from this level these paths diverge. The fibers of the anterior pyramidal tract descend along the ipsilateral side in the anterior cord, forming the corresponding tract of the spinal cord (see Fig. 23), and then, at the level of their segment, they pass to the opposite side and end on the motor neurons of the anterior horns of the spinal cord (the second neuron of the system). The fibers of the lateral pyramidal pathway, in contrast to the anterior one, pass to the opposite side at the level of the medulla oblongata, forming the cross of the pyramids. Then they go in the back of the lateral cord (see Fig. 23) to their "own" segment and end on the motor neurons of the anterior horns of the spinal cord (the second neuron of the system).

Extrapyramidal system performs involuntary regulation and coordination of movements, regulation of muscle tone, maintenance of posture, organization of motor manifestations of emotions. Provides smooth movements, sets the initial posture for their implementation.

The extrapyramidal system includes:

cortico-thalamic pathway, conducts motor impulses from the cortex to the motor nuclei of the thalamus.

Radiation of the striatum- a group of fibers connecting these subcortical centers with the cerebral cortex and thalamus.

Cortical-red nuclear pathway, conducts impulses from the cerebral cortex to the red nucleus, which is the motor center of the midbrain.

Red nuclear-spinal tract(Fig. 58) conducts motor impulses from the red nucleus to the motoneurons of the anterior horns on the opposite side (for more details, see Section 5.3.2.).

Covering-spinal tract. Its passage in general terms is similar to the previous path, with the difference that it does not begin in the red nuclei, in the nuclei of the roof of the midbrain. The first neurons of this system are located in the tubercles of the quadrigemina of the midbrain. Their axons pass to the opposite side and, as part of the anterior cords of the spinal cord, descend to the corresponding segments of the spinal cord (see Fig. 23). Then they enter the anterior horns and end on the motor neurons of the spinal cord (the second neuron of the system).

Vestibulo-spinal tract connects the vestibular nuclei of the hindbrain (pons) and regulates the tone of the muscles of the body (see Section 5.3.2.).

Reticulospinal tract connects RF neurons and spinal cord neurons, providing regulation of their sensitivity to control impulses (see Section 5.3.2.).

Cortical-bridge-cerebellar pathway allow the cortex to control the functions of the cerebellum. The first neurons of this system are located in the cortex of the frontal, temporal, occipital or parietal lobe. Their neurons (cortical-bridge fibers) pass through the internal capsule and go to the basilar part of the bridge, to their own nuclei of the bridge. Here there is a switch to the second neurons of this system. Their axons (bridge-cerebellar fibers) pass to the opposite side and go through the middle cerebellar peduncle to the contralateral hemisphere of the cerebellum.

Main ascending paths.

A. Ascending to the hindbrain: Flexig's posterior spinal cerebellar tract, Gowers' anterior cerebellar tract. Both spinal cerebellar tracts conduct unconscious impulses (unconscious coordination of movements).

Ascending to the midbrain: lateral dorsal-middle cerebral (spinal-tectal) pathway

To the diencephalon: lateral dorsal-thalamic pathway. It conducts temperature irritations and pain; the anterior dorsal-thalamic is the way of conducting impulses of touch, touch.

Some of them are continuous fibers of primary afferent (sensory) neurons. These fibers - thin (Gaulle's bundle) and wedge-shaped (Burdach's bundle) bundles go as part of the dorsal funiculi of the white matter and end in the medulla oblongata near the neutron relay nuclei, called the nuclei of the dorsal cord, or the nuclei of Gaulle and Burdach. The fibers of the dorsal cord are conductors of skin-mechanical sensitivity.

The remaining ascending pathways start from neurons located in the gray matter of the spinal cord. Since these neurons receive synaptic inputs from primary afferent neurons, they are commonly referred to as second-order neurons, or secondary afferent neurons. The bulk of the fibers from the secondary afferent neurons pass through the lateral funiculus of the white matter. This is where the spinothalamic pathway is located. The axons of the spinothalamic neurons cross and reach without interruption through the medulla oblongata and midbrain to the thalamic nuclei, where they form synapses with thalamic neurons. The spinothalamic pathways receive impulses from skin receptors.

In the lateral cords, fibers of the spinal cerebellar tracts, dorsal and ventral, pass, conducting impulses from skin and muscle receptors to the cerebellar cortex.

As part of the lateral funiculus, there are also fibers of the spinocervical tract, the endings of which form synapses with relay neurons of the cervical spinal cord - neurons of the cervical nucleus. After switching in the cervical nucleus, this pathway is directed to the cerebellum and the brainstem nuclei.

The path of pain sensitivity is localized in the ventral columns of the white matter. In addition, the spinal cord's own pathways pass through the posterior, lateral, and anterior columns, ensuring the integration of functions and the reflex activity of its centers.

a) Pyramidal path (tr. pyramidalis) (Fig. 504). It is well developed in humans, since impulses are transmitted through it to striated muscles when performing purposeful, finely coordinated conscious movements. Pyramidal pathways exist in many animals, but function without conscious adjustment. The motor cells of the cortex do not innervate one or another muscle separately, but carry out a given program of movements for individual muscle groups. The pyramidal pathway takes its name from the two wedge-shaped protuberances lying on the ventral surface of the medulla oblongata. For many years it was believed that all fibers of the pyramidal tract originate from the cells of the cortex of the anterior central gyrus. It has now been established that only about 40% of the axons passing through the pyramids originate from the cells of the motor cortex, and 20% of the axons of the pyramidal pathway originate from the cells of the posterior central gyrus (somatosensory area). The remaining 40% of the fibers join the pyramidal pathway from the cells of various areas of the cerebral cortex.

504. Scheme of the pyramidal path (according to Sentagotai).
1 - gyrus precentralis; 2-tr. corticonuclearis; 3-tr. corticospinalis lateralis; 4-tr. corticospinalis anterior; 5 - hemisphere of the brain; 6 - midbrain; 7 - bridge; 8 - medulla oblongata; 9 - spinal cord; 10 - motor nucleus of the V pair; 11 - motor nucleus of the VII pair; 12 - motor nuclei of IX, X, XI pairs; 13 - the core of the XII pair.

The first neurons are located in the anterior central gyrus, precentral and paracentral lobules (fields 4-6), some neurons are scattered in other cortical fields (7-8-9-22-24, etc.). The essential point is that all the cortical fields of the pyramidal pathway are associated with neurons that, by their activity, suppress the motor activity of the motor zone and are located in fields 2 - 4 - 8-19. A similar inhibitory system is absent in other pathways. In addition, in field 4 there is a section 4S, from where special axons reach the nuclei of the reticular formation, which has an inhibitory or excitatory effect on arbitrary reflexes. Pyramid cell dendrites are connected with intercalary neurons that connect sensitive cells of all analyzers. These interneurons form short and long white matter association pathways.

In the anterior central gyrus and the paracentral lobule there are specialized areas of the cortex that carry out a program set for certain muscle groups: the muscles of the lower extremities are under the control of the cells of the upper sections (closer to the sagittal groove of the brain) of the anterior central gyrus and the paracentral lobule, the muscles of the upper extremities - cells the middle section of the central gyrus, the muscles of the face and organs of the head - the cells of the lower section.

The pyramidal path includes three bundles: a) the cortical-nuclear path (tr. corticonuclearis), which centrally encodes the movement program in the motor nuclei of the cranial nerves (III, IV, V, VI, VII, IX, X, XI, XII pairs); b) anterior corticospinal path (tr. corticospinal anterior); c) lateral cortical-spinal path (tr. corticospinalis lateralis). Both last bundles conduct impulses of the movement program to the motor neurons of the spinal cord.

The first neurons of the pyramidal tract are located in different areas of the cortex of the cerebral hemispheres. In layer V of the cerebral cortex there are pyramidal Betz cells, the axons of which take part in the formation of the radiant crown of the white matter of the cerebral hemispheres. These fibers converge downward, passing through the knee and into 2/3 of the posterior crus of the internal capsule. Pyramidal cells have long axons and a large number of collaterals that connect several motor cells of II neurons.

The fibers of the pyramidal tract, having passed the internal capsule, are located at the base of the brain stem, where the crossed fibers are separated from them to the nuclei of the oculomotor nerve (innervating, superior, inferior, medial rectus, inferior oblique muscles of the eyeball and the muscle that lifts the upper eyelid), to the nucleus of the block nerve (innervating the superior oblique muscle of the eyeball) and to the nucleus of the abducens nerve (innervating the lateral rectus muscle of the eyeball).

From the base of the brain stem, the pyramidal path descends to the ventral part of the bridge, at the level of which the crossed fibers are separated for contact with the motor nucleus of the trigeminal nerve (innervating the masticatory muscles), with the motor nucleus of the facial nerve (innervating the mimic muscles); some fibers give collaterals to the reticular formation. The bundle of the pyramidal tract is not compactly located in the pons, the fibers of the cortical-pontocerebellar tract pass through it transversely (described in the "Proprioceptive Pathways" section). In the medulla oblongata, the fibers of the pyramidal pathway are combined into a compact bundle and form pyramids on the ventral surface of the medulla oblongata. Each of the two tracts of the pyramidal pathways contains about 1 million fibers, mostly thin and poorly myelinated; about 3% of the fibers have a large diameter and are covered with a thick myelin sheath; they are the axons of Betz cells. In the medulla oblongata, the motor nuclei of the glossopharyngeal (IX pair), vagus (X pair), accessory (XI pair), hypoglossal (XII pair) nerves also come into contact with the fibers of the pyramidal pathway. The fibers of the pyramidal tract, heading to the nuclei of the motor cranial nerves, cross. These nuclei receive innervation from the fibers of their own and opposite sides. Therefore, with a central unilateral lesion of the cerebral cortex or pathways, there is no complete paralysis of the muscles innervated by III, IV, V, VI, VII, IXt X, XI pairs of cranial nerves. In the region of the pyramids of the medulla oblongata, a small part of the fibers of the pyramidal pathway, bending around the lower olive through the lower or middle cerebellar peduncle, enters it.

In the lower part of the medulla oblongata, the pyramidal tract is divided into two bundles. One large bundle (about 80% of the fibers) crosses (decussatio pyramidum) and passes into the lateral funiculus of the spinal cord, forming the lateral cortical-spinal tract (tr. corticospinalis lateralis). The fibers of this pathway terminate near the dendrites of the intercalated cells (II neuron) located in the posterior columns of the spinal cord. The axons of these cells transmit impulses to intercalary cells (III neuron) of the anterior column, and the latter - to large alpha neurons (IV neuron) of the anterior column, from which impulses are sent to small alpha neurons (V neuron), as well as to the muscles of the limbs and torso.

A smaller part of the pyramidal path in the medulla oblongata does not cross and descends in the anterior cord called the anterior cortical-spinal tract (tr. corticospinalis anterior). In each segment of the spinal cord, its axons pass to the opposite side, switching in the anterior columns with one part to the intercalary neurons (II neuron) and the other to motor neurons (II neuron). The axons of the intercalary neurons are connected to small alpha neurons (III neuron), the axons of which reach the muscles of the trunk and limbs (Fig. 505). Fibers of intercalary neurons can be traced in the cervical and upper thoracic segments of the spinal cord. Part of the fibers of the anterior cortical-spinal tract switches in the motor neuron pools of its side.

505. Scheme of switching of the corticospinal pathway (pyramidal) in the spinal cord.
1 - rear cord; 2 - rear pillar; 3 - lateral cord; 4 - anterior corticospinal path; 5 - large motor neurons of the anterior column; 5 - intercalary neurons of the anterior column; 7 - intercalary neurons of the posterior column; 8 - lateral cortical-spinal path.

506. Communication of the cerebral cortex with the basal nuclei, thalamus, reticular formation and nuclei of the subthalamic region.

1 - cortical fields;
2 - central furrow;
3 - fibers of the pyramidal path;
4 - lenticular body;
5 - Louis body;
6 - black substance;
7 - reticular formation;
8 - subthalamic nucleus;
9 - visual tubercle;
10 - tailed body.

Axons of the peripheral spinal nerve, which are processes of large motor neurons of the anterior columns of the gray matter of the spinal cord, innervate the extrafusal muscle fibers of the striated muscles. Each fiber has a chemically sensitive area - the end plate, where the motor axon ends; it is equivalent to the postsynaptic membrane of the neuron. When excited, the axon of the motor neuron releases acetylcholine, which acts on the end plate; at the same time, depolarization of the muscle fiber and the generation of an electrical impulse are observed, which propagate in both directions to the ends of the muscle fiber, causing its short-term contraction.

Consequently, the pyramidal pathway carries out mainly cross-innervation. The defeat of the lateral cortical-spinal tract causes a disorder in the movements of the limbs on the opposite side and almost does not impair the function of the muscles of the body due to the preservation of innervation due to the anterior corticospinal bundle. Not all muscle groups have such unilateral innervation. Most of the muscles, namely the muscles of the eyeball, chewing, mimic muscles of the upper face, pharynx, larynx, neck, trunk and perineum, have bilateral innervation due to the fibers of the cross and their side. Unilaterally innervated muscles of the limbs, tongue, facial muscles below the oral fissure. The defeat of the corresponding cells of the cortex causes complete paralysis.

Exist following descending pathways:
cortical-spinal pathway (pyramidal pathway);
reticulospinal pathway (extrapyramidal pathway);
vestibulo-spinal pathway;
tegmental-spinal pathway;
suture-spinal pathway;
pathways of aminergic systems of the CNS;
pathways of the autonomic nervous system.

Cortico-spinal tract

It is a major pathway for voluntary motor activity. About 40% of its fibers originate from the primary motor cortex of the precentral gyrus. The remaining fibers originate from the accessory motor area on the medial side of the hemisphere, the premotor cortex on the lateral side of the hemisphere, the somatic sensory cortex, the parietal cortex, and the cingulate cortex. The fibers from the two sensory centers mentioned above terminate at the sensory nuclei of the brainstem and spinal cord, where they regulate the transmission of sensory impulses.

Cortico-spinal tract descends through the radiant crown and the posterior leg of the internal capsule to the brain stem. It then passes in the peduncle (cerebrum) at the level of the midbrain and basilar part of the pons, reaching the medulla oblongata. Here it forms a pyramid (hence the name - the pyramidal pathway).

Passing through the brain stem, the corticospinal pathway gives off fibers that activate the motor nuclei of the cranial nerves, in particular those that innervate the muscles of the face, jaw, and tongue. These fibers are called cortical-bulbar. (The term "corticonuclear" is also used, since the term "bulbar" can be interpreted in different ways.)

Characteristics of the fibers of the cortical-spinal tract above the level of the spinal junction:

About 80% (70-90%) of the fibers pass to the opposite side at the level of the pyramidal decussation;

These fibers descend on the opposite side of the spinal cord and make up the lateral cortical-spinal pathway (crossing cortical-spinal pathway); the remaining 20% ​​of the fibers do not cross and continue down in the anterior part of the spinal cord;

Half of these non-decussing fibers enter the anterior/ventral corticospinal pathway and are located in the ventral/anterior funiculus of the spinal cord at the cervical and upper thoracic levels; these fibers pass to the opposite side at the level of the white commissure and innervate the muscles of the anterior and posterior walls of the abdominal cavity;

The other half enters the lateral cortico-spinal pathway on its own half of the spinal cord.

It is believed that the cortical-spinal pathway contains about 1 million nerve fibers. The average conduction velocity is 60 m/s, which indicates an average fiber diameter of 10 µm (the "rule of six"). About 3% of the fibers are very large (up to 20 microns); they depart from giant neurons (Betz cells), located mainly in the region of the motor cortex, which is responsible for the innervation of the lower extremities. All fibers of the cortical-spinal tract are excitatory and use glutamate as a mediator.

Target cells of the lateral corticospinal tract:

a) Motoneurons of the distal limbs. In the anterior horns of the gray matter of the spinal cord, axons of the lateral corticospinal tract can directly synapse on the dendrites of α- and γ-motoneurons innervating the muscles of the extremities, especially the upper ones (however, as a rule, this occurs through interneurons within the gray matter of the spinal cord). Individual axons of the lateral corticospinal tract can activate "large" or "small" motor units.

A motor unit is a complex consisting of a neuron of the anterior horn of the spinal cord and all the muscle fibers that this neuron innervates. Small motor unit neurons selectively innervate a small number of muscle fibers and are involved in performing fine and precise movements (for example, when playing the piano). The neurons of the anterior horn that innervate large muscles (for example, the gluteus maximus) can individually cause hundreds of muscle cells to contract at once, since these muscles are responsible for gross and simple movements.

The unique property of these corticomoneuronal fibers of the lateral corticospinal tract is demonstrated by the concept of "fractionation" referring to the variable activity of interneurons, whereby small groups of neurons can be selectively activated to perform a specific general function. This is easily shown in the index finger, which can be flexed or extended regardless of the position of the other fingers (although three of its long tendons have a common origin with the muscle beds of all four fingers).

Fractionation is of great importance when performing habitual movements, such as buttoning up a coat or tying shoelaces. Traumatic or other damage to the cortical motor neuron system at any level entails the loss of the skills to perform habitual movements, which are then rarely recoverable.

When performing these movements, α- and γ-motor neurons are activated together through the lateral cortical-spinal pathway in such a way that the spindles of the muscles primarily involved in the movement send impulses about active stretching, and the spindles of antagonist muscles - about passive stretching.

b) Renshaw Cells. The functions of the synapses of the lateral cortical-spinal tract on Renshaw cells are quite numerous, since inhibition on some cell synapses mainly occurs due to type Ia interneurons; on other synapses, this function is performed by Renshaw cells. Probably the most important function is to control the joint contraction of the main motive muscles and their antagonists to fix one or more joints, for example when working with a kitchen knife or a shovel. Joint contraction occurs due to the inactivation of inhibitory Ia interneurons by Renshaw cells.

in) Excitatory interneurons. The lateral cortical-spinal pathway influences the activity of motor neurons located in the middle part of the gray matter and at the base of the anterior horn of the spinal cord, innervating the axial (vertebral) muscles and muscles of the proximal limbs through excitatory interneurons. d) la-inhibiting interneurons. These neurons are also located in the middle part of the gray matter of the spinal cord and are activated by the lateral corticospinal tract, primarily during voluntary movements.

The activity of Ia-interneurons promotes relaxation of antagonist muscles before agonists begin to contract. In addition, they cause refractoriness of motor neurons of antagonist muscles to stimulation of the neuromuscular spindle by afferents when they are passively stretched during movement. The sequence of processes for voluntary flexion of the knee joint is shown in the figure below.

(Note the terminology: in a relaxed standing position, the person's knees are "closed" in slight hyperextension, and the quadriceps femoris is inactive, as evidenced by the "free" position of the patella. When trying to bend one or both knees, the quadriceps femoris twitches in a response to passive stretching of dozens of muscle spindles in it.Since flexion is resisted in this way, the reflex is called a resistance reflex.

On the other hand, during voluntary flexion of the knee joint, the muscles contribute to this movement using the same mechanism, but through the help reflex. The change in sign from negative to positive is called the reversal reflex.)

e) Presynaptic inhibitory neurons that mediate the stretch reflex. Consider the movements of the sprinter. With each step, gravity pulls his body down onto the straightened quadriceps knee. At the moment of contact with the ground, all neuromuscular spindles in the contracted quadriceps muscle are sharply stretched, as a result of which there is a danger of muscle rupture. The Golgi tendon organ provides some protection through internal inhibition, but the main defense mechanism is provided by the lateral corticospinal pathway through presynaptic inhibition of spindle afferents near their contact with motor neurons.

At the same time, the lengthening of the pause to the Achilles reflex serves as an advantage in this situation, since the motor neurons that innervate the back of the leg are restored for the next jerk. It is assumed that the degree of suppression of the stretch reflex from the side of the lateral cortical-spinal tract depends on the specific movements.

e) Presynaptic inhibition of first-order sensory neurons. In the posterior horn of the gray matter of the spinal cord, there is some suppression of the transmission of sensory impulses to the spinothalamic pathway during voluntary movements. It does this by activating synapses formed by inhibitory interneurons and primary sensory nerve endings.

Even finer regulation is observed at the level of the subtle and wedge-shaped nuclei, where the fibers of the pyramidal tract (after crossing) are able to increase the transmission of sensitive impulses during slow, accurate movements or weaken it during fast movements.

Video lesson anatomy of the pyramidal tract - tractus corticospinalis et corticonuclearis

pyramid system, pyramidal path(lat. tractus pyramidales, PNA) - a system of nervous structures. Supports complex and fine coordination of movements.

The pyramidal system is one of the late acquisitions of evolution. The lower vertebrates do not have a pyramidal system; it appears only in mammals, and reaches its greatest development in monkeys and especially in humans. The pyramidal system plays a special role in bipedal locomotion.

pyramid path

The fibers cross at the border of the brain and (most - in the medulla oblongata, the smaller - in the spinal cord). Then they pass through the spinal cord (anterior and lateral columns of the spinal cord). In each segment of the spinal cord, these fibers form synaptic endings (see), which are responsible for a specific part of the body (the cervical spinal cord for the innervation of the arms, the thoracic for the trunk, and the lumbar for the legs). These fibers transmit impulses from the cerebral cortex either directly or through intercalary neurons.

Projection zones of the cerebral cortex

Direct stimulation of certain parts of the cerebral cortex leads to muscle spasms corresponding to the part of the cortex - the projection motor zone. When the upper third of the anterior central is irritated, a spasm of the muscles of the leg occurs, the middle one - the arm, the lower one - the face, moreover, on the side opposite to the focus of irritation in the hemisphere. These seizures are called partial (Jacksonian). They were discovered by the English neurologist D. H. Jackson (1835-1911). In the projection motor zone of each hemisphere of the brain, all the muscles of the opposite half of the body are represented.

Types of nerve fibers

The human pyramidal system contains about 1 million nerve fibers. There are the following types of fibers:

The largest number of pyramidal cells (Betz cells) innervates small muscles responsible for fine differentiated hand movements, facial expressions and speech act. A significantly smaller number innervates the muscles of the trunk and lower extremities.