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The first neurons of the pathways of pain, temperature, and some part of tactile sensitivity are represented by nerve cells of the spinal node with their T-shaped dividing processes. Their dendrites end with receptors embedded in the skin. Axons form the spinal nerve and the posterior root. Entering the substance of the spinal cord, they end at the second neurons - the base cells of the posterior horns, giving rise to the anterior and lateral spinal-thalamic pathways.

The axons of the second neurons, having crossed the front white commissure, fall into the front and lateral cords of the opposite side, rise up and, passing through the dorsal part (cap) of the trunk, end in the ventrolateral nucleus of the thalamus (third neuron).

The anatomical features of the dorsal thalamic tracts, which have diagnostic value, are as follows.

1. The fibers do not cross in the horizontal plane, at the level of a given segment, but at a certain angle and higher, therefore, when the anterior and lateral cords are damaged, the level of sensitivity disorder is found on the opposite side 1-2 segments below the localization of the lesion (.27).

2. Fibers that conduct temperature and pain irritations from the lower extremities are located laterally in the lateral cords, and medially from the upper extremities (the law of the eccentric arrangement of longer conductors). Therefore, as the extramedullary pathological focus spreads, the disorders of pain and temperature sensitivity will spread from bottom to top (foot, lower leg, thigh, trunk, then arm) on the opposite side (ascending type of sensitivity disorder). With intramedullary localization of the pathological focus, sensitive disorders spread from top to bottom (arm, trunk, then leg) on ​​the opposite side (a descending type of sensitivity disorder).

3. A part of the fibers conducting tactile sensitivity comes with conductors of deep sensitivity, that is, bilaterally (without crossing). This has diagnostic value.

The axons of the third neurons form the thalamo-parietal fibers (fibrae thalamoparietales), which pass through the posterior third of the posterior leg of the inner capsule (capsula interna), then, as part of the radiant crown, approach the postcentral and partially precentral gyrus, where they end. Due to the cross within the front white commissure, impulses from the pain, temperature and part of the tactile receptors of the right

half of the body reaches the cortical end of the analyzer in the left hemisphere, and from the indicated receptors of the left half of the body - in the right.

The cells of the first neurons of the pathways of deep (proprioceptive) and tactile sensitivity are located in the spinal node. Their dendrites end with receptor endings that receive proprioceptive impulses from muscles (Kühne bodies) and tendons (Golgi-Mazzoni receptors) and tactile impulses (Merkel plates, Meissner bodies). Axons in the structure of the posterior roots, approaching the spinal cord and not entering the gray matter, are sent to the posterior cord of their side, where they form thin and wedge-shaped bundles (. 28).

In a thin bundle located medially in the posterior cords, there are fibers from the underlying segments (below Tr ^) that carry the corresponding impulses from the lower extremities and the lower body in a wedge-shaped bundle located laterally — fibers from cells of the spinal nodes lying above Th4, which carry pulses from the upper limbs and upper body. This feature has topical diagnostic value: with the spread of the extramedullary pathological process in the cervical spinal cord, disorders of these types of sensitivity increase in a descending manner (first in the upper limb, then in the trunk and lower limb); however, intramedullary processes cause an increase in these disorders in the reverse order, i.e., in an ascending type of development of sensitivity disorders.

The axons of the second neurons located in the sphenoid and thin nuclei of the medulla oblongata conduct impulses to the thalamus.
Part of the fibers does not go to the thalamus, but to the cerebellar worm (through its lower legs). Then they make a cross in the inter-olive layer of the medulla oblongata, within the bridge, occupying a medial position in relation to tractus spinothalamicus lateralis and forming with it a medial loop (lemniscus medialis). In addition, fibers of the second neurons connecting impulses of all types of sensitivity from the opposite side of the sensory nuclei of the trigeminal, vagus and glossopharyngeal nerves are connected to the medial loop. The defeat of the medial loop is accompanied by the loss of all types of sensitivity on the opposite side of the body.

The medial loop ends in the ventrolateral nucleus of the optic tubercle, from the cells of which pulses along the thalamoteminal fibers are directed through the inner capsule and radial crown to the postcentral and partially to the precentral gyrus. However, the analysis and synthesis of pain, temperature, tactile and proprioceptive impulses occurs not only in these areas, but also in much wider areas of the cortex, due to which feelings are formed that give ideas about the outside world. The sensitive pathways of extero- and proprioreceptors to the cerebral cortex also consist of three neurons.

The first neurons of sensitive conductors for the face, paranasal sinuses, mouth and nose are located in the trigeminal ganglion, for the larynx, root of the tongue and nasal pharynx - in the nodes of the vagus and glossopharyngeal nerves (gangll. Superius et inferius), for the front two-thirds of the tongue - in cranked node (gangl. geniculi) of the facial nerve.

If, within the spinal cord, the posterior root enters the posterior horns, then, within the trunk of the stem, the roots of the cranial nerves are directed to the corresponding sensitive nuclei, where the second neurons are located, the axons of which after intersection join to the outer part of the medial loop and end in the thalamus, from where the axons of the third neurons into the composition of the thalamotemic fibers are sent to the sensitive area of ​​the cortex.

Brief conclusions.

1. The pathways from receptors to the cortex for all types of sensitivity consist of three neurons.

2. The first neurons of the pathways of all types of sensitivity are located outside the brain - in the spinal nodes and nodes of the cranial nerves.

3. The third neurons of the pathways of all types of sensitivity are in the thalamus; thus, only through tala-mus — the collector of all kinds of sensitivity — do the sensitive pathways reach the cortex.

4. The second neurons of sensitive analyzers lie separately, namely: surface sensitivity analyzers (pain, temperature, partially tactile) - in the back horns of the spinal cord; analyzers of deep sensitivity (muscular-articular, vibrational, etc.) and partially tactile - in the sphenoid and thin nuclei of the medulla oblongata.

5. Before entering the spinal cord, the conductors of all types of sensitivity go together, therefore, when the posterior roots are damaged, all types of sensitivity fall out on the affected side.

6. From the medial loop to the cerebral cortex, the conductors of all kinds of sensitivity go together; damage to the conductors from the bridge to the cortex is accompanied by the loss of all types of sensitivity according to the conductor type on the opposite side.

7. Throughout the spinal cord, the conductors of the sensitive systems go separately, namely: pain and temperature and partially tactile in the anterior and lateral cords, proprioceptive and partially tactile in the posterior. Therefore, the pathological process in the spinal cord causes an isolated loss of certain types of sensitivity.

8. The fibers of the second neurons of the pathways of all types of sensitivity undergo a cross: paths of surface sensitivity - at the level of segments of the spinal cord, paths of pro-prioceptive and partially tactile - within the medulla oblongata.

9. Sensitive pathways enter the spinal cord only through the posterior roots.

10. The transformation of reception into sensation occurs in the cortical part of the analyzer.
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