home→Suspensions→ Afferent pathways. Conducting pathways of proprioceptive sensitivity of the cortical direction of the loop, medial lemniscus)

Afferent pathways. Conducting pathways of proprioceptive sensitivity of the cortical direction of the loop, medial lemniscus)

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Anterior spinothalamic tract (tr. spinothalamicus anterior)

- slow-conducting tract of discrete tactile sensitivity (sense of touch, touch, pressure).

The first neurons (receptor) are located in the spinal nodes and are represented by pseudo-unipolar cells. Their peripheral processes-dendrites are part of the spinal nerves and start from specialized receptors - Meissner bodies, Merkel discs, Vater-Pacini bodies located in the skin. Afferent fibers such as Ad and Ag depart from these receptors. The impulse conduction velocity is low, 8–40 m/s. The central processes of the first neurons as part of the posterior roots enter the spinal cord and divide in a T-shape into ascending and descending branches, from which many collaterals depart. The terminal branches and collaterals of most of the fibers end at the top of the posterior horn of the spinal cord in the cells of the gelatinous substance (plates I–III), which are the second neurons. Most of the axons of the first neurons of tactile sensitivity bypass the gray matter of the spinal cord and go to the brain stem as part of the thin and wedge-shaped bundles of the spinal cord.

The axons of the second neurons, whose bodies are located in the gelatinous substance, form a decussation, passing through the anterior white commissure to the opposite side, and the decussation level is located 2–3 segments above the entry point of the corresponding posterior root. Then they are sent to the brain as part of the lateral cords, forming the anterior spinal thalamic pathway. This path passes through the medulla oblongata, then through the pontine tire, where it goes along with the fibers of the medial loop through the midbrain tegmentum, and ends in the ventrobasal nuclei of the thalamus.

The axons of the third neurons pass through the thalamo-cortical tract through the posterior pedicle of the internal capsule, reach the postcentral gyrus and the superior parietal lobule (somatosensory areas of the SI and SII cortex) as part of the radiant crown.

Thus, the anterior spinothalamic tract is the conduction pathway for tactile sensitivity.

Posterior cords (synonyms: fasciculus gracilis, fasciculus cuneatus, thin and wedge-shaped bundles, bundles of Gaulle and Burdach, dorso-lemniscal system, system

loops, medial lemniscus)

The bundles of Gaulle and Burdach are fast-conducting pathways of spatial skin sensitivity (the sense of touch, touch, pressure, vibration, body mass) and the sense of position and movement (articular-muscular (kinesthetic) sense).

The first neurons of the thin and wedge-shaped bundles are represented by pseudo-unipolar cells, the bodies of which are located in the spinal nodes. Dendrites pass through the spinal nerves, beginning with rapidly adapting receptors in the scalp (Meissner bodies, Vater-Pacini bodies) and articular capsule receptors. Recently, the possibility of participation of proprioceptors of muscles and tendons in the formation of a conscious proprioceptive feeling has been shown.

The central processes of pseudo-unipolar cells as part of the posterior roots enter segmentally into the spinal cord in the region of the posterior lateral sulcus and, having given collaterals to the II-IV plates, go in an ascending direction as part of the posterior funiculi of the spinal cord, forming a medially located thin Gaulle's fascicle and laterally - a wedge-shaped Burdakh's bundle (Fig. 5).

Gaulle's bundle

conducts proprioceptive sensation from lower extremities and lower half of the body: from 19 lower spinal nodes, including 8 lower thoracic, 5 lumbar, 5 sacral and 1 coccygeal, and Burdach bundle

- from the upper body, upper limbs and neck, corresponding to 12 upper spinal nodes (8 cervical and 4 upper thoracic).

The bundles of Gaulle and Burdach, without interrupting or crossing in the spinal cord, reach the nuclei of the same name (thin and wedge-shaped) located in the dorsal sections of the medulla oblongata, and here they switch to the second neurons. The axons of the second neurons go to the opposite side, making up the internal arcuate fibers (fibrae arcuatae internae) and, crossing the median plane, intersect with the same fibers of the opposite side, forming a cross in the medulla oblongata between the olives. medial loop (decussatio lemniscorum)

External arcuate fibers (fibrae arcuatae externae) through the lower legs of the cerebellum connect the loop system with the cerebellar cortex.

Next, the fibers follow through the pons operculum, the operculum of the legs of the brain and reach the lateral nuclei of the thalamus (ventrobasal complex), where they switch to third neurons. In the bridge, the spinal-thalamic tract (paths of skin sensitivity of the neck, trunk and limbs) and the loop trigeminal nerve, conducting cutaneous and proprioceptive sensitivity from the face.

Through the lower third of the posterior femur of the internal capsule, the loop system reaches the superior parietal lobule (5th, 7th cytoarchitectonic fields) and the postcentral gyrus of the cerebral cortex (SI).

General sensitivity pathways

1. The path of exteroceptive sensitivity. The path of pain, temperature and tactile sensitivity (ganglio-spinal-thalamo-cortical path) originates from the exteroreceptors of the skin of the trunk, limbs and neck (Fig. 4.2). Due to the fact that the skin makes up the covering of the body, this sensitivity is also called superficial, or exteroceptive.

Exteroceptors for various kinds surface sensitivity are specialized and are contact receptors. Pain is perceived by free nerve endings, heat by Ruffini's bodies, cold by Krause's flasks, touch and pressure by Meisner's bodies, Golgi-Mazzoni, Vater-Pacini and Merkel's disks.

From exteroceptors, impulses arrive through the peripheral processes of pseudounipolar neurons to their bodies, which are located in the sensory nodes of the spinal nerves (the bodies of the first neurons). The central processes of pseudo-unipolar cells in the composition of the posterior roots are sent to the spinal cord. The main part of the central processes ends in synapses on the cells of the own nucleus of the posterior horn. The tract from the sensitive node of the spinal nerve to the intercalary neuron can be called ganglio-spinal.

Rice. 4.2.

1 - postcentral gyrus; 2 - thalamus; 3 - own nucleus of the posterior horn; 4 - sensitive node of the spinal nerve; 5 - anterior spinal-thalamic path; 6 - lateral dorsal-thalamic pathway; 7 - dorsal-thalamic path; 8 - thalamo-cortical path

The axons of the neurons of the own nucleus of the posterior horn (second neurons) form bundles of fibers (spinal-thalamic tracts) that conduct nerve impulses to the thalamus.

In the spinal cord, the spinal-thalamic tracts have a number of characteristic features: all 100% of the fibers pass to the opposite side; the transition to the opposite side is carried out in the area of the white adhesion, while the fibers rise obliquely 2-3 segments above the initial level. The fibers that conduct pain and temperature sensitivity form the lateral spinothalamic tract, and the fibers that conduct tactile sensitivity form predominantly the anterior spinothalamic tract.

In the area of the medulla oblongata, the lateral and anterior spinal thalamic tracts are combined into a single spinal thalamic tract. At this level, the tract receives a second name - the spinal loop. Gradually, the dorsal-thalamic tract deviates in a dorsolateral direction, passing through the tegmentum of the pons and midbrain. The spinal-thalamic tract ends with synapses on the neurons of the ventrolateral nuclei of the thalamus (third neurons). The tract formed by the axons of these nuclei of the thalamus is called the thalamo-cortical tract.

The main part of the axons of the third neurons is directed through the middle part of the posterior pedicle of the internal capsule to the postcentral gyrus - the projection center of general sensitivity. Here they end on the neurons of the fourth layer of the cortex (fourth neuron), distributing along the gyrus, respectively, of the somatotopic projection (Penfield's sensory homunculus). A small part of the fibers (5–10%) ends on the neurons of the fourth layer of the cortex in the region of the intraparietal sulcus (the center of the body scheme).

Thus, the path of exteroceptive sensitivity consists of three successive tracts - ganglio-spinal, spinal-thalamic, thalamo-cortical.

Given the peculiarities of the location of the pathways, it is possible to determine the level of damage to the nervous structures. If the sensitive nodes of the spinal nerves, the posterior roots or the nucleus of the posterior horn are damaged, surface sensitivity disorders are noted on the side of the same name. In case of damage to the fibers of the spinal-thalamic tract, cells of the veitrolateral nuclei of the thalamus and fibers of the thalamo-cortical bundle, upset

Sensory properties are noted on the opposite side of the body.

2. The path of conscious proprioceptive sensitivity (deep sensitivity)(ganglio-bulbar-thalamo-cortical path) conducts nerve impulses from proprioceptors (Fig. 4.3).

Proprioceptive sensitivity is information about the state of the proprioceptors of muscles, tendons, ligaments, joint capsules and periosteum, i.e. information about the functional state of the musculoskeletal system. It allows you to judge muscle tone, the position of body parts in space, a sense of pressure, weight and vibration. Proprioceptors constitute the most extensive group of receptor structures, represented by muscle spindles and encapsulated receptors. They also perceive tactile sensitivity, so the conscious proprioceptive sensitivity pathway partially conducts tactile impulses.

From proprioceptors, the nerve impulse enters through the peripheral processes of pseudo-unipolar cells to their bodies, which are located in the sensitive nodes of the spinal nerves (the bodies of the first neurons). The central processes of pseudo-unipolar cells as part of the posterior roots of the spinal nerves enter the spinal cord. In the spinal cord, they give off collaterals to the segmental apparatus. The main part of the fibers, bypassing the gray matter, is sent to the posterior funiculus.

In the posterior funiculus of the spinal cord, the central processes of pseudounipolar cells form two bundles: medially located - a thin bundle (Gaulle's bundle), and laterally located - a wedge-shaped bundle (Burdach's bundle).

Gaulle's bundle conducts impulses of conscious proprioceptive sensitivity from the lower extremities and the lower half of the body - from 19 lower sensory nodes of the spinal nerves of its side (1 coccygeal, 5 sacral, 5 lumbar and 8 thoracic). The Burdach bundle includes fibers from 12 upper sensory nodes of the spinal nerves, i.e. it conducts proprioceptive sensory impulses from the upper torso, upper limbs, and neck. Consequently, a thin bundle runs throughout the entire spinal cord, and the wedge-shaped one appears only from the level of the fourth thoracic segment. The area of each of the beams gradually increases in the cranial direction.

Rice. 4.3.

1 - nuclei of thin and wedge-shaped bundles; 2 - medulla oblongata; 3 - wedge-shaped bundle; 4 - sensitive node of the spinal nerve; 5 - thin beam; 6 - internal arcuate fibers; 7 - bulbar-thalamic path; 8 - inner capsule; 9 - thalamo-cortical path; 10 - precentral gyrus; 11 - thalamus

As part of the posterior funiculi of the spinal cord, the bundle of Gaulle and the bundle of Burdach rise to the nuclei of the thin and sphenoid tubercles of the medulla oblongata, where the bodies of the second neurons are located. The bundles of Gaulle and Burdach, formed by the central processes of the pseudo-unipolar cells of the sensitive nodes of the spinal nerves, can be called the ganglion-bulbar tract.

The axons of the nuclei of the thin and sphenoid tubercles of the medulla oblongata form two groups of fibers. The first group is internal arcuate fibers that intersect with the same fibers of the opposite side, bend in the form of a loop and go up.

The bundle of these fibers is called the bulbar-thalamic tract, or medial loop. A smaller part of the axons of the second neurons, constituting the second group (external arcuate fibers), is sent to the cerebellum through its lower pedicle, forming the bulbar-cerebellar tract. The fibers of this tract terminate on the neurons of the middle part of the cortex of the cerebellar vermis.

The bulbar-thalamic tract runs along the brainstem in the tegmentum, next to the spinal-thalamic tract and ends on the neurons of the ventrolateral nuclei of the thalamus (the body of the third neurons).

The axons of the neurons of the ventrolateral nuclei of the thalamus are sent to the projection centers of the cerebral cortex (fourth neuron). Basically, they end on the neurons of the fourth layer of the cortex of the precentral gyrus (60%) - in the center motor functions. A smaller part of the fibers goes to the cortex of the postcentral gyrus (30%) - the center of general sensitivity, and an even smaller part - to the interparietal sulcus (10%) - the center of the body schema. The somatotopic projection to these convolutions is carried out from the opposite side of the body, since the bulbar-thalamic tracts cross in the medulla oblongata.

The path from the ventrolateral nuclei of the thalamus to the projection centers of the cerebral cortex is called the thalamo-cortical tract. It passes through the internal capsule in the middle section of the hind leg.

The pathway of conscious proprioceptive sensitivity is phylogenetically more recent than other afferent pathways. When it is damaged, the perception of the position of body parts in space, the perception of posture, and the sensation of movements are disturbed. With closed eyes, the patient cannot determine the direction of movement in the joint, the position of body parts. The coordination of movements is also disturbed, the gait becomes uncertain, the movements are awkward, disproportionate.

3. The path of general sensitivity from the face area(ganglio-nuclear-thalamo-cortical path) conducts nerve impulses of pain, temperature, tactile and proprioceptive sensitivity from the face along the sensitive branches of the trigeminal nerve. From the proprioceptors of the mimic muscles, nerve impulses are conducted along the branches of the trigeminal nerve, and from the chewing muscles - along the mandibular seal. In addition to the facial area, the trigeminal nerve provides sensitive innervation (pain, temperature and tactile) of the mucous membranes, lips, gums, nasal cavity, paranasal sinuses, lacrimal sac, lacrimal gland and eyeball, as well as the teeth of the upper and lower jaws.

All three branches of the trigeminal nerve go to the trigeminal node (Gasser node), which is composed of pseudo-unipolar cells (the bodies of the first neurons).

The central processes of pseudounipolar cells enter the bridge as part of the sensory root of the trigeminal nerve and then go to the sensory nuclei (the bodies of the second neurons). Fibers are sent to the bridge nucleus, conducting impulses of tactile sensitivity from the skin of the face, impulses of pain, temperature and tactile sensitivity from deep tissues and organs of the head; to the nucleus of the spinal tract of the trigeminal nerve - fibers that conduct impulses of pain and temperature sensitivity from the skin of the face; to the midbrain nucleus - fibers that conduct impulses of proprioceptive sensitivity from the masticatory and facial muscles.

The axons of the second neurons pass to the opposite side and form the nuclear-thalamic tract, which ends in the ventrolateral nuclei of the thalamus. In the brainstem, this tract runs adjacent to the spinothalamic tract and is known as the trigeminal loop.

The axons of the third neurons located in the ventrolateral nuclei of the thalamus are sent through the posterior thigh of the internal capsule to the neurons of the cerebral cortex to the centers of general sensitivity, motor functions and body schema. They pass as part of the thalamo-cortical tract and end on the neurons of these centers in those areas of the cortex (the bodies of the fourth neurons) where the head area is projected.

The distribution of fibers of the thalamo-cortical bundle, which conducts impulses of general sensitivity from the head region, is as follows: 60% is sent to the postcentral gyrus, 30% to the precentral gyrus, and 10% to the interparietal sulcus.

A small part of the axons of the third neurons goes to the medial nuclei of the thalamus (the subcortical sensory center of the extrapyramidal system).

(Fleksig's bundle) provides impulses of unconscious proprioceptive sensitivity (Fig. 4.4). From the proprioceptors, along the fibers of the spinal nerves, impulses arrive at the pseudo-unipolar cells of the sensory nodes (the bodies of the first neurons). Their central processes, as part of the posterior roots, enter the spinal cord and penetrate into the gray matter, reaching the neurons of the thoracic nucleus. They pass as part of the gaiglio-spialial tract.

Rice. 4.4.

1 - lower cerebellar peduncle; 2 - thoracic nucleus; 3 - sensitive node of the spinal nerve; 4 - sacral segment; 5 - lumbar segment; 6 - cervical segment; 7 - posterior spinal-cerebellar path

The axons of the neurons of the thoracic nucleus (second neurons) are sent to the lateral funiculus of their side. In the posterolateral part of the lateral funiculus, they form the posterior spinal cerebellar tract. This tract, receiving fibers segment by segment, increases to the level of the seventh cervical segment; above this level, the area of the bundle does not change. In the region of the medulla oblongata, the posterior spinal cerebellar tract is located in the dorsal region and penetrates the cerebellum as part of its lower leg. In the cerebellum, this path ends on the neurons of the cortex of the lower part of the vermis (the third neuron).

(Govers bundle) also conducts impulses of unconscious proprioceptive sensitivity (Fig. 4.5).

The first link in the reflex arc of the Gowers and Flexig bundles is represented by similar nerve structures. The bodies of receptor neurons (pseudo-unipolar cells) are located in the sensory nodes of the spinal nerves (the first neuron). Their peripheral processes as part of the spinal nerves and their branches reach the proprioceptors. The central processes in the composition of the posterior roots of the spinal nerves penetrate the spinal cord, enter the gray matter and end on the neurons of the intermediate medial nucleus (second neuron). Most of its axons (90%) are sent to the opposite side through the anterior white commissure. A smaller part of the axons (10%) goes to the anterolateral part of the lateral funiculus of its side. Thus, in the lateral funiculus, an anterior spinal cerebellar path is formed, formed by the axons of the cells of the intermediate-medial nuclei of the predominantly opposite, in a small number - of its sides. It should be noted that fibers from the lower segments of the spinal cord occupy the medial part of the tract, from each overlying segment they join from the lateral side.

In the medulla oblongata, the anterior spinal cerebellar tract is located in the dorsal region between the olive and the lower cerebellar peduncles. Then it rises into the tire of the bridge. At the level of the border of the bridge and the midbrain, the anterior spinal cerebellar tract turns sharply in the dorsal direction. In the region of the superior medullary sail, the fibers that crossed in the spinal cord return to their side and then, as part of the superior cerebellar peduncles, reach the upper part of the cortex of the cerebellar vermis (the third neuron).

Rice. 4.5.

1 - superior cerebellar peduncle; 2 - sensitive node of the spinal nerve; 3 - intermediate-medial nucleus; 4 - sacral segment; 5 - lumbar segment; 6 - cervical segment; 7 - anterior spinal cerebellar path

Due to the fact that the nerve fibers that make up the Gowers bundle form decussations twice (in the anterior white commissure of the spinal cord and in the superior medullary velum), impulses of unconscious proprioceptive sensitivity are transmitted to the cerebellum from the same side of the body.

1. Pathways of proprioceptive (deep) sensitivity. Consist of bundles of Gaulle and Burdakh (Fig. 502). With the help of these paths, movements are made that are evaluated by consciousness. Controllability of movements is carried out due to afferent impulses from the muscles and joints of the moving parts of the body. Impulses reach the postcentral gyrus of the parietal cortex. This feedback provides gradualness and coordination of movements. If the pathways of proprioceptive sensitivity are damaged, the patient cannot perform precise, proportionate, dexterous movements.

502. Scheme of the proprioceptive pathways of the trigeminal nerve, Gaulle and Burdakh (according to Sentagotai).
1 - Gaull's way; 2 - the path of Burdakh; 3 - nucl. cuneatus; 4 - nucl. gracilis; 5 - sensitive path of the trigeminal nerve; 6- midbrain; 7-sensitive nucleus of the V pair; 8 - bridge; 9 - medulla oblongata; 10 - spinal cord; 11 - proprioreceptors of the Gaull and Burdach pathways.

The first unipolar sensory neurons of the Gaull and Burdach pathways are located in the spinal nodes (Fig. 502). Their receptors - the fusiform Kuehne bodies - begin in the muscles, then forming the peripheral nerve. Axons form a posterior root, which enters segmentally into the white matter of the posterior funiculus, uniting into thin (Gaulle) and wedge-shaped (Burdach) bundles. The thin bundle is closer to the medial sulcus and is composed of axons of the coccygeal, sacral, lumbar, XII-VII thoracic segments. The wedge-shaped bundle is located lateral to the thin bundle and unites axons from the VIII - I thoracic and VIII - I cervical segments.

The thin and wedge-shaped bundles end not in the nuclei of the spinal cord, but in the thin and wedge-shaped nuclei of the medulla oblongata. The axons of the cells of the thin and sphenoid nuclei (II neuron) at the border with the bridge form a medial loop that contacts the cells of the ventrolateral nucleus of the thalamus. From the lateral side, fibers of the spinothalamic pathway join the medial loop. Axons from the nuclei of the thalamus (III neuron), passing through the back of the internal capsule, terminate in the cortex of the superior parietal lobule (fields 5 and 7) and in the anterior central gyrus (fields 4-6).

Part of the fibers of II neurons of the proprioceptive sensory pathways is sent to the cerebellum through its lower legs, participating in the mechanism of coordination of movements.

There are proprioceptive sensory pathways that connect the nuclei of the spinal cord, the medulla oblongata, the pons, subcortical formations, the extrapyramidal subsystem with the cerebellum, which are involved in the mechanisms of automatic movement coordination and muscle tone, in addition to pathways that close in the cerebral cortex. These mechanisms, as a rule, manifest themselves with sudden imbalances or automatic movements (walking, dancing, writing, etc.) that are developed during exercise and under the influence of social moments. Unconditional reflex impulses from all the formations listed above are integrated in the cerebellum, which coordinates and determines movements of various accuracy. Impulses from the cerebellum have a regulatory inhibitory effect on the nuclei of the vestibular analyzer and the reticular formation. Since the vestibulo-spinal path arises from the vestibular nuclei, then along it and the reticulospinal path, inhibition or facilitation of the function of alpha and gamma motor neurons of the anterior columns of the spinal cord and muscle spindles of motor peripheral nerves occurs. Thus, thanks to the mechanisms feedback through the vestibulospinal and reticulospinal pathways, the cerebellum coordinates fast and slow contractions of all muscles. The cerebellum resembles a control unit based on the feedback principle. The cerebellar vermis coordinates movement when walking and standing. In the hemisphere of the cerebellum there are mechanisms for very precise coordination of movements, mainly for performing movements of the upper limb. The worm is subordinate to the cerebellar cortex, and it functions under the influence of the cerebral cortex.

Connection spinal cord with the overlying parts of the central nervous system (brain stem, cerebellum and cerebral hemisphere is carried out through ascending and descending pathways. The information received by the receptors is transmitted along the ascending pathways.

Impulses from muscles, tendons and ligaments pass into the overlying parts of the central nervous system partly along the fibers of the bundles of Gaulle and Burdach located in the back columns spinal cord, partly along the fibers of the spinal-cerebellar tracts of Gowers and Flexig, located in the lateral columns. Gaulle's and Burdach's bundles are formed by processes of receptor neurons, the bodies of which are located in the spinal ganglia ( rice. 227).

These processes, entering spinal cord, go in an ascending direction, giving short branches to the gray matter of several higher and lower segments of the spinal brain. These branches form synapses on the intermediate and effector neurons that are part of the spinal reflex arcs. The bundles of Gaulle and Burdakh end in the nuclei of the medulla oblongata, from where the second neuron of the afferent pathway begins, heading after the cross to the thalamus; here is the third neuron, the processes of which conduct afferent impulses to the cerebral cortex ( rice. 228).

With the exception of those fibers that are part of the bundles of Gaulle and Burdach and go without interruption to the medulla oblongata, all other afferent nerve fibers of the posterior roots enter the gray matter of the spinal cord and are interrupted here, that is, they form synapses on various nerve cells . From the so-called columnar, or clarke, cells of the posterior horn and partly from the spike, or commissural, cells of the spinal cord, the nerve fibers of the Gowers and Flexig bundles originate.

Violation of the conduction of afferent impulses along the spinal-cerebellar pathways entails a disorder of complex movements, in which there are violations of muscle tone and ataxia phenomena, as in lesions of the cerebellum.

Rice. 228. Scheme of the pathways of the posterior columns of the spinal cord. 1 - tactile receptors of the skin; 2 - gentle bundle of Gaulle (fasciculus gracilis); 3 - wedge-shaped bundle of Burdakh (fasciculus cuneatus); 4 - medial loop (lemniscus medians); 5 - intersection of the medial loop; 6 - Burdakh's nucleus in the medulla oblongata; 7 - Gaulle's nucleus in the medulla oblongata; CM - spinal cord (segments C8 and S1); PM - medulla oblongata; VM - varoli bridge; ZB - visual tubercles (nuclei are visible, especially the posterior ventral one, where the fibers of the medial loop end).

Impulses from proprioreceptors propagate along the thick myelin fibers of the Aα group, which have a high conduction velocity (up to 140 m / s), which form the spino-cerebellar pathways, and along the slower conductive (up to 70 m / s) fibers of the Gaulle and Burdach bundles. The high rate of conduction of impulses from the receptors of the muscles of the joints and tendons is obviously associated with the importance for the body of quickly obtaining information about the nature of the performed motor act, which ensures its continuous control.

Impulses from pain and temperature receptors arrive at the cells of the posterior horns of the spinal cord; from here begins the second neuron of the afferent pathway. The processes of this neuron at the level of the same segment, where the body of the nerve cell is located, pass to the opposite side, enter the white matter of the lateral columns and are part of the lateral spinothalamic pathway ( see fig. 227) go to the thalamus, where the third neuron begins, conducting impulses to the cerebral cortex. Impulses from pain and temperature receptors are partially carried out along the fibers, heading up the posterior horns of the gray matter of the spinal cord. The conductors of pain and temperature sensitivity are thin myelinated fibers of the AΔ group and non-myelinated fibers, characterized by a low conduction velocity.

In some lesions of the spinal cord, disorders of only pain or only temperature sensitivity can be observed. Moreover, the sensitivity to only heat or only to cold may be impaired. This proves that the impulse from the corresponding receptors is carried out in the spinal cord along the nerve fibers.

Impulses from the tactile receptors of the skin come to the cells of the posterior horns, the processes of which ascend through the gray matter into several segments, pass to the opposite side of the spinal cord, enter the white matter and in the ventral spinothalamic tract carry an impulse to the nuclei of the visual tubercles, where the third neuron is located. , which transmits the information it receives to the cerebral cortex. Impulses from skin touch and pressure receptors also partially pass through the Gaulle and Burdach bundles.

There are significant differences in the nature of the information delivered by the fibers of the Gaulle and Burdach bundles and the fibers of the spinothalamic pathways, as well as in the speed of propagation of impulses along both. The ascending pathways of the posterior pillars transmit impulses from touch receptors, which provide the possibility of precise localization of the site of irritation. The fibers of these pathways also conduct impulses of high frequency, arising from the action of vibration on receptors. Impulses from pressure receptors are also conducted here, making it possible to accurately determine the intensity of irritation. The spinothalamic pathways carry impulses from touch, pressure, temperature and pain receptors, which do not provide accurate differentiation of the localization and intensity of stimulation.

The fibers passing in the bundles of Gaulle and Burdach, transmitting more differentiated information about the existing stimuli, conduct impulses at a higher speed, and the frequency of these impulses can vary significantly. The fibers of the spinothalamic pathways have a low conduction velocity; at different strengths of stimulation, the frequency of the impulses passing through them changes little.

Impulses that are carried along the afferent pathways generate, as a rule, an excitatory postsynaptic potential strong enough to cause a propagating impulse to occur in the next neuron of the ascending afferent pathway. However, impulses passing from one neuron to another can be inhibited if at the moment the central nervous system receives some information more important for the body through other afferent conductors.

The descending paths of the spinal cord receive impulses from the overlying effector centers. Receiving impulses along descending paths from the centers of the brain and transmitting these impulses to the working organs, the spinal cord performs a conductor-executive role.

Along the corticospinal, or pyramidal, pathways, passing in the anterior lateral columns of the spinal cord, impulses come to it directly from the large pyramidal cells of the cerebral cortex. The fibers of the pyramidal pathways form synapses on intermediate and motor neurons (a direct connection between pyramidal neurons and motor neurons is available only in humans and monkeys). There are about a million nerve fibers in the corticospinal tracts, among which about 3% are thick fibers with a diameter of 16 microns, belonging to the Aα type and have a high conduction speed (up to 120-140 m / s). These fibers are processes of large pyramidal cells of the cortex. The remaining fibers have a diameter of about 4 microns and have a much lower conduction velocity. A significant number of these fibers conduct impulses to the spinal neurons of the autonomic nervous system.

The corticospinal tracts of the lateral columns cross at the level of the lower third of the medulla oblongata. The corticospinal tracts of the anterior columns (the so-called direct pyramidal tracts) do not cross in the medulla oblongata; they pass to the opposite side near the segment where they end. In connection with this intersection of the corticospinal pathways, disturbances in the motor centers of one hemisphere cause paralysis of the muscles of the opposite side of the body.

Some time after damage to the pyramidal neurons or the nerve fibers of the corticospinal tract coming from them, some pathological reflexes occur. A typical symptom of the defeat of the pyramidal tract is a perverted skin-plantar Babinski reflex. It manifests itself in the fact that the dashed irritation of the plantar surface of the foot causes extension thumb and fan-shaped divergence of the remaining toes; such a reflex is also obtained in newborns, in whom the pyramidal pathways have not yet completed their development. In healthy adults, dashed irritation of the skin of the sole causes reflex flexion of the fingers.

In synapses formed by the fibers of the corticospinal tract, both excitatory and inhibitory postsynaptic potentials can occur. As a result, excitation or inhibition of motor neurons may occur.

The axons of the pyramidal cells, forming the corticospinal pathways, give off collaterals that end in the nuclei of the striatum, hypothalamus, and the red nucleus, in the cerebellum, in the reticular formation of the brain stem. From all of these nuclei, impulses travel down descending pathways, called extracorticospinal or extrapyramidal, to the intercalary neurons of the spinal cord. The main descending tracts are the reticulospinal, rubrospinal, tectospinal, and vestibulospinal tracts. The rubro-spinal tract (Monakov's bundle) sends impulses to the spinal cord from the cerebellum, quadrigemina, and subcortical centers. The impulses passing along this path are important in the coordination of movement and the regulation of muscle tone.

The vestibulospinal tract runs from the vestibular nuclei in the medulla oblongata to the cells of the anterior horn. The impulses coming along this path ensure the implementation of tonic reflexes of body position. The reticulo-spinal pathways transmit the activating and inhibitory effects of the reticular formation on the neurons of the spinal cord. They affect both motor and intermediate neurons. In addition to all these long descending paths (in the white matter of the spinal cord), there are also short paths connecting the overlying segments with the underlying ones.

Gaulle's bundle conducts proprioceptive sensitivity from the lower extremities and the lower half of the body: from 19 lower spinal nodes, including 8 lower thoracic, 5 lumbar, 5 sacral and 1 coccygeal, and Burdach bundle- from the upper body, upper limbs and neck, corresponding to 12 upper spinal nodes (8 cervical and 4 upper thoracic).

Next, the fibers follow through the pons operculum, the operculum of the legs of the brain and reach the lateral nuclei of the thalamus (ventrobasal complex), where they switch to third neurons. In the bridge, the spinal-thalamic tract (paths of skin sensitivity of the neck, trunk and limbs) and the loop of the trigeminal nerve, which conduct skin and proprioceptive sensitivity from the face, join the medial loop from the outside.

1.5.1.2.3. spinocervical tract

(spinal-cervical-thalamic tract, lateral tract of Morin)

Pathway of spatial skin sensitivity (pressure and deformation of the skin) and sense of position.

Almost no attention is paid to this tract in textbooks on the physiology of the spinal cord. This is probably due to the fact that the dorsal-cervical tract is most pronounced in carnivorous mammals. However, the significance of this tract is quite large for primates as well. The spinocervical tract begins with slowly adapting receptors in the skin and articular capsules (Merkel's discs and Ruffini's bodies). It is hypothesized that high-threshold muscle afferents also activate the spinocervical tract. The afferents of this tract are thick, myelinated, fast conducting (more than 100 m/s). Further, axon-like dendrites enter the spinal ganglia, where the bodies of the first neurons of the tract are located. The receptor field of these neurons is very small. Then, predominantly at the level of the lumbar and sacral segments, the axons of the first neurons enter the spinal cord and form a synapse with a second-order neuron in plate IV. Rising in the lateral funiculus on their side, their axons reach the lateral cervical nucleus (C I -C II), where the third-order neuron is located. Further, the axons of the third neurons cross and follow along with the axons of the second-order neurons of the posterior cords as part of the medial loop.

The fourth neuron is located in the ventrobasal region of the thalamus. The final projection is to the somatosensory area of the SII cortex.

Despite the higher number of switches (four switches instead of the usual three), the signal along the spinocervical tract reaches the somatosensory cortex even a few milliseconds earlier than in the medial lemniscus. This is due to the fact that the fibers of the spinal-cervical tract are more fast-conducting (more than 100 m/s).

The spinal-cervical tract is activated during severe deformities of the skin and joint capsules due to slowly adapting skin and joint receptors. The physiological significance of this pathway is interpreted in different ways. Some authors believe that the spinocervical tract simply duplicates the medial loop, and in a diffuse variant. However, there is every reason to believe that this tract is specialized in the rapid conduction of signals associated with a sense of position and precise localization of tactile stimulation.

In general, the lemniscal system is characterized by following functions:

accurate localization of touch;

Exact discrimination of the intensity of irritation;

Vibration sensitivity

skin and joint sensitivity of movement (kinesthesia);

sense of position

· stereognosis;

sense of mass

two-dimensional-spatial sensitivity;

discrimination sensitivity.

The lemniscal system is a three-neuron sensory system (with the exception of the spinal-cervical tract) with small receptor fields, an accurate description of the place, intensity and time of stimulation, is characterized by a contralateral projection into the ventrobasal nuclei of the thalamus (the presence of a decussation), a topical projection into the somatosensory areas of the cortex, a fast holding.

Proprioceptive and exteroceptive pathways of the cortical direction carry conscious information about the state of the musculoskeletal system. Based on this information, due to associative links with the precentral gyrus, it becomes possible to perform purposeful, conscious movements and make additional corrections during their implementation. Thus, the feedback mechanism is activated, which is necessary to ensure conscious coordination and correction of movements.