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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 ganglia and are represented by pseudounipolar cells. Their peripheral dendritic processes run as part of the spinal nerves and begin from specialized receptors - Meissner's bodies, Merkel discs, Vater-Pacini bodies, located in the skin. Afferent fibers of the Ad and Ag types depart from these receptors. The speed of impulse conduction is low - 8–40 m/s. The central processes of the first neurons as part of the dorsal roots enter the spinal cord and are divided in a T-shape into ascending and descending branches, from which many collaterals arise. The terminal branches and collaterals of most of the fibers end at the apex of the dorsal horn of the spinal cord in the cells of the jellylike substance (laminas 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 are directed to the brain stem as part of the thin and cuneate fasciculi of the spinal cord.
The axons of the second neurons, the bodies of which are located in the substantia pulposum, form a decussation, passing through the anterior white commissure to the opposite side, and the level of the decussion is located 2-3 segments above the entry point of the corresponding dorsal root. They are then sent to the brain as part of the lateral cords, forming the anterior spinothalamic tract. This pathway passes through the medulla oblongata, then through the pontine tegmentum, where it goes along with the fibers of the medial lemniscus through the midbrain tegmentum, and ends in the ventrobasal ganglia of the thalamus.
The axons of the third neurons pass as part of the thalamo-cortical tract through the posterior leg of the internal capsule, and as part of the corona radiata they reach the postcentral gyrus and the superior parietal lobule (somatosensory cortical areas SI and SII).
Thus, the anterior spinothalamic tract is the pathway for tactile sensitivity.
Posterior funiculi (synonyms: fasciculus gracilis, fasciculus cuneatus, thin and wedge-shaped bundles, Gaulle and Burdach bundles, dorsolemniscal system, system
loops, medial lemniscus)
The Gaulle and Burdach bundles are fast-conducting pathways of spatial cutaneous sensitivity (sense of touch, touch, pressure, vibration, body weight) and sense of position and movement (articular-muscular (kinesthetic) sense).
The first neurons of the thin and cuneate fasciculi are represented by pseudounipolar cells, the bodies of which are located in the spinal ganglia. Dendrites pass as part of the spinal nerves, starting with quickly adapting receptors of the scalp (Meissner's corpuscles, Vater-Pacini corpuscles) and receptors of the joint capsules. Recently, the possibility of the participation of proprioceptors of muscles and tendons in the formation of a conscious proprioceptive sense has been shown.
The central processes of pseudounipolar cells as part of the dorsal roots enter the spinal cord segment by segment in the region of the posterior lateral sulcus and, having given off collaterals to plates II–IV, go in an ascending direction as part of the posterior cords of the spinal cord, forming a medially located thin fasciculus of Gaulle and a laterally located wedge-shaped Burdach's bundle (Fig. 5).
Gaulle bun
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 torso, upper limbs and neck, corresponding to the 12 upper spinal nodes (8 cervical and 4 upper thoracic).
The Gaulle and Burdach bundles, without interruption or crossing in the spinal cord, reach the cognate nuclei (gracilis and cuneate), located in the dorsal parts of the medulla oblongata, and here they switch to second neurons. The axons of the second neurons go to the opposite side, forming internal arcuate fibers (fibrae arcuatae internae) and, crossing the median plane, intersect with the same fibers of the opposite side, forming a decussation in the medulla oblongata between the olives. medial loop (decussatio lemniscorum)
External arcuate fibers (fibrae arcuatae externae) through the inferior cerebellar peduncles connect the loop system with the cerebellar cortex.
Next, the fibers follow through the tegmentum of the bridge, the tegmentum of the cerebral peduncles and reach the lateral nuclei of the thalamus (ventro-basal complex), where they switch to third neurons. In the pons, the spinothalamic tract (cutaneous sensory pathways of the neck, trunk and limbs) and the trigeminal loop, which conduct cutaneous and proprioceptive sensation from the face, join the medial lemniscus externally.
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).
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The conducting (descending and ascending) pathways are located at various points in the vicinity of the nuclei and roots of the cranial nerves. Knowledge of the spatial relationships between cranial nerves and pathways is of paramount importance for the topical diagnosis of a pathological focus.
Ascending Paths. The path of deep sensitivity. Gaulle and Burdach bundles - conductors of deep sensitivity in the spinal cord, reaching the lower part of the medulla oblongata, are called f. gracilis (delicate tuft) - continuation of Gaulle's tuft and f. cuneatus (wedge-shaped fascicle) is a continuation of the Burdach fascicle. Here they gradually end in the nuclei of these bundles. The axons of the nuclear cells, which are the second neuron of deep sensitivity, tractus bulbo-thalamicus, pass to the opposite side (sensitive chiasm) in the form of a median loop, reach the visual thalamus and from there go to the cerebral cortex. Damage to the area where these pathways cross can cause impairment of deep sensitivity on both sides, and sometimes depending on the involvement of certain fibers in the form of cross anesthesia (arm on one side, leg on the other). Involvement of the loop in the pathological process at any level leads to disruption of deep sensitivity on the opposite half of the body.
The path of cutaneous sensitivity is located deep in the reticular formation. In the more oral parts of the hindbrain, this bundle is close to the medial lemniscus, with which it merges at the level of the midbrain. In practice, this means that damage to these levels already causes a violation of all types of sensitivity in the opposite half of the body.
The posterior direct cerebellar tract of Flegsig at the level of the medulla oblongata as part of the inferior cerebellar peduncle ends in the cerebellar vermis. On the periphery of the medulla oblongata, it stands out in the form of a roller and is located above the inferior olive. At this level, fibers from the posterior columns and vestibular nuclei join it.
In the depths of the reticular formation lies the crossed cerebellar tract of Govers. It is located between the olive and the rope body. Rising upward, the Govers' bundle through the pons reaches the superior cerebellar peduncle, within which it ends in the cerebellar vermis.
Descending Paths. The pyramidal tract in the midbrain is located in a compact bundle in the cerebral peduncle, occupying its middle third. At the base of the pons, pyramidal fibers lie in scattered small bundles, between which are the aforementioned pons own nuclei and corticopontine-cerebellar connections. In the remaining parts of the medulla oblongata, the pyramidal fibers again gather into two compact bundles on either side of the anterior cleft. Finally, at the border with the spinal cord, the pyramidal fibers cross into the spinal cord. Damage to the pyramidal tracts at the level of the entire brain stem above the chiasm causes central paralysis on the opposite half of the body with unilateral lesions and bilateral movement disorders with lesions of the pyramids on both sides. Damage to the brain stem is characterized by early bilateral involvement of the pyramids in the process. Damage to the pyramids at the base of the pons is distinguished by some features arising from what has been said about their location: incomplete hemiparesis, the prevalence of the disorder in one limb, and a combination of pyramidal signs with cerebellar disorders can occur here.
The presence of a pathological process in the area of the decussation of the pyramids causes various combinations of central paralysis, often bilateral, sometimes in a peculiar location: paralysis of the arm on one side, paralysis of the leg on the other.
Tractus cortico-bulbaris s. cortico-nuclearis - the path from the cerebral cortex (lower parts of the anterior central gyrus) to the nuclei of the motor cranial nerves. Passing through the knee of the internal capsule, the corticobulbar tract is located in the cerebral peduncle medially from the main pyramidal fasciculus and then gradually ends in the nuclei of the motor cranial nerves at different levels of the brain stem.
The corticomontine tract starts from various parts of the cerebral cortex, mainly from the frontal lobe, and passes through the internal capsule and cerebral peduncle. In the latter, the corticomontine tract is located as follows: the frontopontine tract occupies the medial section, and the occipital-parietal-temporopontine tract occupies its lateral sections.
In the tegmentum of the midbrain, the fascicle of Monaco begins in the red nuclei. Upon exiting them, it makes a cross (Trout) and goes through the brain stem to the spinal cord. In the trunk it is located deep in the reticular formation. Along this path, impulses from the cerebellum and subcortical nodes are carried to the spinal cord.
The posterior longitudinal fasciculus begins in Darkshevich's nucleus and passes through the entire brain stem to the spinal cord. It contains ascending and descending fibers and connects various levels of the trunk with individual segments of the spinal cord. Through the posterior longitudinal fasciculus, communication is carried out between the nuclei of all oculomotor nerves, between them, the vestibular apparatus and the spinal cord. Involvement of the posterior longitudinal fasciculus system in the brainstem in the pathological process causes a number of vestibular disorders.
Nystagmus. Depending on the level of damage to this system, the nature of nystagmus changes. When the caudal parts of the trunk are affected, the nystagmus is often rotatory in nature; when its middle parts are affected, it is horizontal; in the upper parts, it is vertical. Often there is a violation of the act of convergence (insufficiency, and sometimes absence of convergence), varying degrees of gaze paralysis. When the oral parts of the posterior longitudinal fasciculus system are involved in the process, vertical strabismus and upward gaze paresis are sometimes observed.
Dizziness occurs mainly when moving the eyes. In clinical practice, a symptom of interest is known as the static phenomenon. If you put the patient in a position with his legs together and, gradually bringing the researcher’s finger closer to the eyes of the subject, force him to convert his eyeballs in this way, then in the presence of this symptom the patient will experience dizziness, staggering, often backwards, sometimes combined with a feeling of Fear and paleness of the face .
Central bundle of ankylosing spondylitis. This path begins in the diencephalon, passes through the tegmentum of the entire brain stem and ends in the inferior olive of the hindbrain. The axons of the cells of the inferior olive pass to the opposite side and, as part of the inferior cerebellar peduncle, end in the cerebellar hemisphere.
The central tegmental bundle is therefore one of the most important connections of the extrapyramidal system with the cerebellum. When the central tegmental bundle is damaged in combination with damage to the inferior olive and the dentate nucleus of the cerebellum, in some cases myoclonic twitching of the soft palate, tongue, pharynx, and larynx are observed. Sometimes these myoclonic twitches, which are rhythmic in nature, also affect other muscles (intercostal muscles, neck muscles, etc.).
The location of the most important pathways of the spinal cord is shown in Fig. 2.8. The diagram shows the relative area of individual tracts.
- 1. Posterior cord
- 1) thin beam (Gaull beam);
- 2) wedge-shaped bundle (Burdach bundle);
- 3) posterior own bundle;
- 4) radicular zone.
Thin Bun located in the medial part of the posterior cord. It is formed by the central processes of the pseudounipolar cells of the 19 lower sensory ganglia of the spinal nerves (coccygeal, all sacral and lumbar, as well as eight lower thoracic). These fibers enter the spinal cord as part of the dorsal roots and, without entering the gray matter, are directed to the posterior cord, where they take an ascending direction. Nerve fibers of the thin fasciculus carry impulses of conscious proprioceptive and partly tactile sensitivity from the lower extremities and lower torso. Proprioceptive (deep) sensitivity is information from muscles, fascia, tendons and joint capsules about the position of body parts in space, muscle tone, the feeling of weight, pressure and vibration, the degree of muscle contraction and relaxation.
Rice. 2.8.
1 – lateral corticospinal tract; 2 – red nucleus-spinal tract; 3 – olivospinal tract; 4 – vestibulospinal tract; 5 – medial longitudinal fasciculus; 6 – reticular-spinal tract; 7 – anterior corticospinal tract; 8 – roof-spinal tract; 9 – anterior own bundle; 10 – spinal reticular tract; 11 – anterior spinothalamic tract; 12 – anterior root of the spinal nerve; 13 – anterior spinocerebellar tract; 14 – lateral native bundle; 15 – lateral spinothalamic tract; 16 – posterior spinocerebellar tract; 17 – posterior root of the spinal nerve; 18 – posterior own bundle; 19 – wedge-shaped bundle; 20 – thin beam
Wedge-shaped bundle appears in the upper half of the spinal cord and is located lateral to the thin fasciculus. It is formed by the central processes of pseudounipolar cells of the 12 superior sensory ganglia of the spinal nerves (four upper thoracic and all cervical). It carries nerve impulses for conscious proprioceptive and partly tactile sensation from receptors in the muscles of the neck, upper limbs and upper torso.
Posterior own bundle represents axons of interneurons belonging to the segmental apparatus. They are located on the medial side of the posterior horn, oriented in the craniocaudal direction.
Radicular zone formed by the central processes of pseudounipolar cells located within the posterior funiculus (from the posterior lateral groove to the posterior horn). It is located in the posterolateral part of the cord.
Thus, the posterior cord contains sensory nerve fibers.
- 2. Lateral cord contains the following pathways:
- 1) posterior spinocerebellar tract (Flxxig bundle);
- 2) anterior spinocerebellar tract (Gowers bundle);
- 3) lateral spinothalamic tract;
- 4) lateral corticospinal tract;
- 5) red nuclear spinal tract (Monakov’s bundle);
- 6) olivo-spinal tract;
- 7) lateral own bundle.
Posterior spinocerebellar tract located in the posterolateral part of the lateral funiculus. It is formed by the axons of the cells of the thoracic nucleus only on its side. The tract carries impulses of unconscious proprioceptive sensitivity from the trunk, limbs and neck.
Anterior spinocerebellar tract located in the anterolateral part of the lateral funiculus. It is formed by the axons of the cells of the intermediate-medial nucleus, partly on its side and partly on the opposite side. Nerve fibers from the opposite side are part of the anterior white commissure. The anterior spinocerebellar tract plays the same role as the posterior one.
Lateral spinothalamic tract located medial to the anterior spinocerebellar tract. It is formed by the axons of the cells of the dorsal horn nucleus. They pass to the opposite side as part of the anterior white commissure, rising obliquely by 2–3 segments. The lateral spinothalamic tract carries impulses of pain and temperature sensitivity from the trunk, limbs and neck.
Lateral corticospinal tract located in the medial-posterior part of the lateral funiculus. In area it occupies about 40% of the lateral funiculus. The nerve fibers of the lateral corticospinal tract are axons of the pyramidal cells of the cerebral cortex of the opposite side, therefore it is also called the pyramidal tract. In the spinal cord, these fibers end segment by segment with synapses on the motor cells of the nuclei of the anterior horns. The role of this tract is manifested in the performance of conscious (voluntary) movements and in the inhibitory effect on the neurons of the intrinsic nuclei of the anterior horns of the spinal cord.
Red nuclear spinal tract located in the middle of the anterior part of the lateral cord. It is formed by the axons of the cells of the red nucleus of the midbrain on the opposite side. The axons move to the opposite side in the midbrain. The fibers in the spinal cord end on the neurons of the own nuclei of the anterior horns. The function of the tract is to ensure long-term maintenance of skeletal muscle tone (in a comfortable position) and to perform complex automatic conditioned reflex movements (running, walking).
Olive-spinal tract located in the anteromedial part of the lateral funiculus. The olivospinal tract is formed by the axons of the olive nuclei of the medulla oblongata on its side. The nerve fibers of these pathways end on the motor cells of the intrinsic nuclei of the anterior horns of the spinal cord. The function of this pathway is to ensure unconditioned reflex regulation of muscle tone and unconditioned reflex movements during changes in body position in space (during vestibular loads).
Lateral own bundle is a thin bundle of axons of interneurons belonging to the segmental apparatus. It is located in close proximity to the gray matter. These fibers ensure the transmission of nerve impulses to the neurons of the intrinsic nuclei of the anterior horns of the superior and underlying segments.
Thus, the lateral cord contains ascending (afferent), descending (efferent) and its own bundles, i.e. in terms of the composition of the pathways it is mixed.
- 3. Anterior cord contains the following paths:
- 1) roof-spinal tract;
- 2) anterior corticospinal tract;
- 3) reticular-spinal tract;
- 4) anterior spinothalamic tract;
- 5) medial longitudinal fasciculus;
- 6) vestibulospinal tract;
- 7) anterior own bundle.
Roof-spinal tract located in the medial part of the anterior cord, adjacent to the anterior median fissure. It is formed by the axons of the neurons of the superior colliculus of the midbrain on the opposite side. The crossing of fibers occurs in the midbrain. The fibers in the spinal cord end on the motor cells of the own nuclei of the anterior horns. The role of the tract is to perform unconditioned reflex movements in response to strong light, sound, olfactory and tactile stimuli - protective reflexes.
Anterior corticospinal tract located in the anterior part of the cord, lateral to the roof-spinal tract. The tract is formed by the axons of the pyramidal cells of the cerebral cortex, therefore this tract is called the same as the lateral corticospinal tract - pyramidal. In the spinal cord, its fibers end on the neurons of the own nuclei of the anterior horns. The function of this tract is the same as the lateral corticospinal tract.
Reticular-spinal tract located lateral to the anterior corticospinal tract. This tract is a collection of axons of neurons of the reticular formation of the brain (descending fibers). It plays an important role in maintaining muscle tone, and also produces differentiation of impulses (strengthening or weakening) passing through other tracts.
Anterior spinothalamic tract located lateral to the previous one. It is formed, like the lateral spinothalamic tract, by the axons of the cells of the intrinsic nucleus of the dorsal horn of the opposite side. Its function is to conduct impulses primarily of tactile sensitivity.
Medial longitudinal fasciculus located in the posterior part of the anterior cord. It is formed by the axons of the cells of the Cajal and Darkshevich nuclei located in the midbrain. The axons end in the spinal cord on the cells of the own nuclei of the anterior horns of the cervical segments. The function of the beam is to ensure combined (simultaneous) rotation of the head and eyes.
vestibulospinal tract located on the border of the anterior and lateral funiculi. The path is formed by the axons of the vestibule nuclei of the bridge on its side. It ends on the motor cells of the own nuclei of the anterior horns of the spinal cord. The function of this pathway is to ensure unconditioned reflex regulation of muscle tone and unconditioned reflex movements when the body position in space changes (during vestibular loads).
Anterior own bundle located in the anterior cord on the medial side of the anterior horn. This bundle is formed by the axons of interneurons belonging to the segmental apparatus. It ensures the transmission of nerve impulses to the neurons of the intrinsic nuclei of the anterior horns of the superior and underlying segments.
Thus, the anterior cord contains predominantly efferent fibers.
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. Information received by receptors is transmitted along ascending pathways.
Impulses from muscles, tendons and ligaments pass into the overlying parts of the central nervous system, partly along the fibers of the Gaul and Burdach bundles located in the posterior columns spinal cord, partly along the fibers of the spinocerebellar tracts of Govers and Flexig, located in the lateral columns. The Gaulle and Burdach 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 located segments of the spinal cord. These branches form synapses on intermediate and effector neurons that are part of the spinal reflex arcs. The Gaulle and Burdach bundles end in the nuclei of the medulla oblongata, where the second neuron of the afferent pathway begins, heading after the chiasm 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 Gaulle and Burdach bundles and go, without interruption, to the medulla oblongata, all other afferent nerve fibers of the dorsal roots enter the gray matter of the spinal cord and are interrupted here, i.e. they form synapses on various nerve cells . From the so-called columnar, or clarke, cells of the dorsal horn and partly from the commissure, or commissural, cells of the spinal cord, the nerve fibers of the Govers and Flexig bundles originate. Disruption of the conduction of afferent impulses along the spinocerebellar pathways entails a disorder of complex movements, in which disturbances in muscle tone and ataxia are observed, as with lesions of the cerebellum. Rice. 228. Diagram of the pathways of the posterior columns of the spinal cord. 1 - tactile receptors of the skin; 2 - gentle Gaulle bundle (fasciculus gracilis); 3 - wedge-shaped bundle of Burdach (fasciculus cuneatus); 4 - medial loop (lemniscus medians); 5 - cross of the medial loop; 6 - Burdach's nucleus in the medulla oblongata; 7 - Gaulle's nucleus in the medulla oblongata; SM - spinal cord (segments C8 and S1); PM - medulla oblongata; VM - pons; ZB - visual hillocks (nuclei are visible, especially the posterior ventral one, where the fibers of the medial lemniscus end). |
Impulses from proprioceptors propagate along high-velocity (up to 140 m/sec) thick myelin fibers of group Aα, forming the spinocerebellar tracts, and through slower-conducting (up to 70 m/sec) fibers of the Gaulle and Burdach bundles. The high speed of impulse transmission from muscle receptors in joints and tendons is obviously associated with the importance for the body of quickly obtaining information about the nature of the motor act being performed, which ensures its continuous control.
Impulses from pain and temperature receptors enter the cells of the dorsal horns of the spinal cord; this is where the second neuron of the afferent pathway begins. 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 form part of the lateral spinothalamic tract ( see fig. 227) go to the visual thalamus, where the third neuron begins, conducting impulses to the cerebral cortex. Impulses from pain and temperature receptors are partially carried along fibers that are directed upward along the posterior horns of the gray matter of the spinal cord. Conductors of pain and temperature sensitivity are thin myelinated fibers of group AΔ and non-myelinated fibers, characterized by low conduction speed.
With some lesions of the spinal cord, disorders of only pain or only temperature sensitivity may be observed. Moreover, sensitivity to only heat or only to cold may be impaired. This proves that impulses from the corresponding receptors are carried out in the spinal cord along nerve fibers.
Impulses from the tactile receptors of the skin arrive at the cells of the dorsal horns, the processes of which ascend through the gray matter into several segments, move 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 thalamus, where the third neuron is located. , transmitting 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 tracts, as well as in the speed of impulse propagation along both. Impulses from touch receptors are transmitted along the ascending pathways of the posterior columns, making it possible to accurately localize the site of irritation. The fibers of these pathways also conduct high-frequency impulses that arise from the action of vibration on the receptors. Pulses from pressure receptors are also carried out here, making it possible to accurately determine the intensity of stimulation. The spinothalamic tract carries impulses from touch and pressure receptors, as well as from temperature and pain receptors, which do not provide precise differentiation of the localization and intensity of irritation.
Fibers passing in the Gaulle and Burdach bundles, transmitting more differentiated information about current stimuli, conduct impulses at a higher speed, and the frequency of these impulses can vary within significant limits. The fibers of the spinothalamic tracts have a low conduction velocity; with different strengths of stimulation, the frequency of impulses passing through them changes little.
Impulses that are carried along afferent pathways typically generate 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 more important information for the body through other afferent conductors.
Along the descending pathways of the spinal cord, impulses from overlying effector centers arrive to it. Receiving impulses along descending pathways 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, tracts passing in the anterior lateral columns of the spinal cord, impulses come to it directly from the large pyramidal cells of the cerebral cortex. Fibers of the pyramidal tracts form synapses on intermediate and motor neurons (direct communication between pyramidal neurons and motor neurons is found only in humans and monkeys). The corticospinal tract contains about a million nerve fibers, of which about 3% are thick fibers with a diameter of 16 microns, belonging to type Aα and having a high conduction speed (up to 120-140 m/sec). 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 speed. A significant number of these fibers conduct impulses to the spinal neurons of the autonomic nervous system.
The corticospinal tracts of the lateral columns intersect 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 move to the opposite side near the segment where they end. Due to this crossover of corticospinal tracts, 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 pyramidal neurons or the nerve fibers of the corticospinal tract coming from them, some pathological reflexes arise. A typical symptom of damage to the pyramidal tract is the perverted Babinski cutaneous-plantar reflex. It manifests itself in the fact that line irritation of the plantar surface of the foot causes extension of the big toe and fan-shaped divergence of the remaining toes; such a reflex is also obtained in newborns, in whom the pyramidal tracts have not yet completed their development. In healthy adults, line irritation of the skin of the sole causes reflex flexion of the fingers.
At synapses formed by fibers of the corticospinal tract, both excitatory and inhibitory postsynaptic potentials can arise. As a result, excitation or inhibition of motor neurons may occur.
The axons of pyramidal cells, forming the corticospinal tracts, give off collaterals that end in the nuclei of the striatum, hypothalamus, and red nucleus, in the cerebellum, in the reticular formation of the brain stem. From all of these nuclei, impulses travel through descending pathways called extracorticospinal or extrapyramidal to the interneurons of the spinal cord. The main of these descending tracts are the reticulospinal, rubrospinal, tectospinal and vestibulospinal tracts. Along the rubro-spinal tract (Monakov's bundle), impulses from the cerebellum, quadrigeminal and subcortical centers arrive to the spinal cord. Impulses passing along this path are important in coordinating movement and regulating muscle tone.
The vestibulospinal tract runs from the vestibular nuclei in the medulla oblongata to the cells of the anterior horn. Impulses coming along this path ensure the implementation of tonic reflexes of body position. The reticulospinal tracts transmit the activating and inhibitory influences of the reticular formation on the neurons of the spinal cord. They influence both motor and interneurons. In addition to all these long descending pathways (in the white matter of the spinal cord), there are also short pathways connecting the overlying segments with the underlying ones.
Conductor function The spinal cord is that ascending and descending tracts pass through it.
TO upward paths relate:
- the system of posterior cords (tender and wedge-shaped bundles), which are conductors of skin-mechanical sensitivity in;
- spinothalamic pathways along which impulses from receptors arrive to;
- spinocerebellar tracts (dorsal and ventral) are involved in conducting impulses coming from skin receptors and proprioceptors in.
TO descending paths relate:
- pyramidal, or corticospinal, tract;
- extrapyramidal tracts, including rubrospinal, reticulospinal, vestibulospinal tracts. These descending pathways provide the influence of the higher parts of the central nervous system on the function of skeletal muscles.
|
Name |
Characteristic |
|
Thin Gaulle Bun |
Proprioceptors of tendons and muscles, part of the tactile receptors of the skin, from the lower part of the body |
|
Wedge-shaped bundle of Burdach |
Propronoceptors of tendons and muscles, part of the tactile receptors of the skin from the upper body |
|
Lateral spinothalamic tract |
Pain and temperature sensitivity |
|
Ventral spinothalamic tract |
Tactile sensitivity |
|
Flexig's dorsal spinocerebellar tract |
Not crossed - proprioception |
|
Ventral spinocerebellar tract of Gowers |
Double crossed proprioception |
Classification of descending tracts of the spinal cord
|
Name |
Characteristic |
|
Lateral corticospinal pyramidal |
|
|
Straight anterior corticospinal pyramidal |
|
|
Rubrospinal (Monakova) |
|
|
Vestibulospinal |
|
|
Reticulospinal |
|
|
Tectospinal |
|
Signal functions
Nerve fibers of the spinal cord form its white matter and are used to conduct many signals from sensory receptors in the central nervous system, signals between neurons of the spinal cord itself and between neurons of the spinal and other parts of the central nervous system, as well as from neurons of the spinal cord to effector organs. A significant part of the spinal cord pathways consists of axons of so-called propriospinal neurons. The fibers of these neurons create connections between spinal segments and do not extend beyond the spinal cord.
The most well-known examples of the simplest neural networks conducting signals in the spinal cord and their use to control the work of effector organs are neural networks of somatic and autonomic reflexes. The sensory neuron and its fibers, intercalary and motor neurons take part in the conduction of the signal (nerve impulse), which initially arises in the receptor nerve ending.
The signal is not only carried by neurons within the segment in which they are located, but is processed and used to carry out a reflex response to receptor stimulation.
Signals arising in the receptors of the body surface, muscles, tendons, and internal organs are also transmitted to the overlying structures of the central nervous system but to the fibers of the cords (columns) of the spinal cord, called ascending (sensitive) pathways(Table 1). These pathways are formed by fibers (axons) of sensory neurons, the bodies of which are located in the spinal ganglia, and interneurons, the bodies of which are located in the dorsal horns of the spinal cord.
Table 1. Main ascending sensory pathways of the central nervous system
|
Name |
Start, 1st neuron |
Localization in the spinal cord |
Ending |
Function |
|
Medial and posterior funiculi |
Somatosensory cortex of the opposite hemisphere. fields 1. 2. 3 |
|||
|
Wedge-shaped |
Axons of sensory neurons |
Lateral and posterior funiculi |
Somatosensory cortex of the opposite hemisphere, fields 1, 2,3 |
Proprioceptive signals (conscious) |
|
Dorsal spinocerebellar |
Ipsilateral Clark's nucleus |
Lateral funiculus |
Cortex of the inlateral cerebellar hemisphere |
Proprioceptive signals (unconscious) |
|
Ventral spinocerebellar |
Contralateral dorsal horn |
Lateral funiculus |
Cortex of the contralateral cerebellar hemisphere |
Proirnoceptive signals (unconscious) |
|
Lateral spinothalamic |
Contralateral dorsal horn |
Lateral funiculus |
Thalamus, somatosensory cortex |
Signals of pain temperature sensitivity |
|
Anterior spinothalamic |
Contralateral dorsal horn |
Thalamus, somatosensory cortex |
Touch |
The course of fibers conducting signals from receptors of different sensitivity (modality) is not the same. For example, pathways from proprioceptors carry signals about the condition of muscles, tendons, and joints to the cerebellum and cerebral cortex. The fibers of this pathway are axons of sensory neurons of the spinal ganglia. Having entered the spinal cord through the dorsal roots, they, along the same side of the spinal cord (without crossing), as part of the thin and wedge-shaped fasciculi, ascend to the neurons of the medulla oblongata, where they end in the formation of a synapse and transmit information to the second afferent neuron of the pathway (Fig. 1 ).
This neuron carries the processed information along the axon passing to the opposite side to the neurons of the thalamic nuclei. After switching on the neurons of the thalamus, information about the state of the motor system is transmitted to the neurons of the postcentral region of the cerebral cortex and is used to form sensations about the degree of muscle tension, the position of the limbs, the angle of flexion in the joints, passive movement, and vibration.
The thin bundle also contains some fibers from skin receptors that conduct information used to form conscious tactile sensitivity in the form of touch, pressure, and vibration.
Other spinal sensory pathways are formed by the axons of second afferent (intercalary) neurons, the bodies of which are located in the dorsal horns of the spinal cord. The axons of these neurons within their segment make a cross and on the opposite side of the spinal cord as part of the lateral spinothalamic tract they go to the neurons of the thalamus.
Rice. 1. Diagram of the pathways from proprioceptors, tactile, temperature and pain receptors to the brain stem and cortex
This pathway contains fibers that conduct signals of pain and temperature sensitivity, as well as part of the fibers that conduct signals of tactile sensitivity (see Fig. 1).
The anterior and posterior spinocerebellar tracts also pass through the lateral funiculi. They conduct signals from proprioceptors to the cerebellum.
Signals along the ascending sensory pathways are also carried to the centers of the ANS, the reticular formation of the brain stem and other structures of the central nervous system.
The spinal cord neurons receive signals from neurons in higher brain structures. They follow the axons of nerve cells that form descending(mainly motor) pathways, used to control muscle tone, form posture and organize movements. The most important among them are the corticospinal (pyramidal), rubrospinal, reticulospinal, vestibulospinal and tectospinal tracts (Table 2).
Table 2. Main descending efferent pathways of the central nervous system
|
Path name |
Start, 1st neuron |
Localization in the spinal cord |
Ending |
Function |
|
Lateral corticospinal |
Contralateral cortex |
Lateral funiculus |
Ineulateral ventral and dorsal horns |
|
|
Anterior corticospinal |
Ipsilatsral cortex |
Anterior cord |
Contralateral ventral and dorsal horns |
Movement control and sensitivity modulation |
|
Rubrospinal |
Contralateral red nucleus of the midbrain |
Lateral funiculus |
Movement control |
|
|
Lateral vestibulospinal |
Ipsilateral, lateral vestibular nucleus |
Lateral funiculus |
Ipsilateral ventral horn |
Control of muscles that support body posture and balance |
|
Medial vestnbulospinal |
Ipsi- and contralateral medial vestibular nuclei |
Anterior cord |
Ipsilateral ventral horn |
Head position on vestibular signals |
|
Regulospinal |
Reticular formation of the bridge and medulla oblongata |
Lateral and anterior funiculi |
Ipsilateral ventral horn and intermediate zone |
Movement and posture control, sensitivity modulation |
|
Tectospinal |
Contralateral superior colliculus |
Anterior cord |
Ipsilateral ventral horn |
Head position associated with eye movements |
The corticospinal tract is divided into the lateral one, whose fibers run in the lateral cords of the white matter of the spinal cord, and the anterior one, in the anterior cords. The corticospinal tract is formed by the axons of pyramidal neurons of the motor areas of the cerebral cortex, which end with synapses mainly on interneurons of the spinal cord. A small part of the fibers of the lateral corticospinal tract ends in synapses directly on the a-motoneurons of the spinal cord, innervating the muscles of the hand and distal muscles of the limbs.
The rubrospinal, reticulospinal, vestibulospinal and tectospinal tracts are formed by the axons of neurons of the corresponding nuclei of the brain stem and are also called extrapyramidal. Along these pathways, efferent nerve impulses are carried primarily to interneurons and γ-motoneurons of the spinal cord, used to maintain muscle tone, posture and involuntary movements, performed due to congenital or acquired reflexes. Through these pathways, conditions are formed for the effective execution of voluntary movements initiated by the cerebral cortex.
Through the spinal cord, signals are carried from the higher centers of the ANS to the preganglionic neurons of the sympathetic nervous system, located in the lateral horns of its thoracolumbar region, and to the neurons of the parasympathetic nervous system, located in the sacral part of the spinal cord. Through these pathways of the spinal cord, the tone of the sympathetic nervous system and its influence on the functioning of the heart, the state of the lumen of blood vessels, the functioning of the gastrointestinal tract and other internal organs, as well as the parasympathetic nervous system and its influence on the functions of the pelvic organs are maintained.
Starting from the level of the intersection of the motor fibers of the corticospinal tract of the medulla oblongata to the level of the northwestern part of the cervical spinal cord, the spinal nucleus of the trigeminal nerve is located, to the neurons of which the axons of sensory neurons located in the trigeminal ganglion descend through the medulla oblongata. Through them, the nucleus receives signals from pain sensitivity of teeth, other tissues of the jaws and oral mucosa, pain, temperature and touch signals from the surface of the face, tissues of the eye and orbit.
The axons of the neurons of the spinal nucleus of the trigeminal nerve intersect and follow in the form of a diffuse bundle to the neurons of the thalamus and to the neurons of the reticular formation of the brain stem. With damage to the afferent fibers of the trigeminal tract and the spinal nucleus of the trigeminal nerve, a decrease or loss of pain and temperature sensitivity on the ipsilateral side of the face may be observed.
If the integrity of the pathways for conducting afferent and (or) efferent signals at the level of the spinal cord or other levels of the central nervous system is disrupted, a person decreases or loses a certain type of sensitivity and (or) movements. Knowing the morphological features of the structure of the intersection of the fibers of the pathways, it is possible, taking into account the nature of the disturbance of sensitivity and (or) movements, to establish the level of damage to the central nervous system that caused these disturbances.
Signals from the neurons of the locus coeruleus and the raphe nucleus of the brain stem are carried to the intercalary and motor tracts along descending pathways. They are used to control muscle activity associated with sleep and wakefulness states. Signals from neurons of the periaqueductal gray matter are carried to the interneurons of the spinal cord along descending pathways. These signals and the neurotransmitters released from the axons of these neurons are used to control pain sensitivity.
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