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Nervous system

الكلية كلية طب الاسنان     القسم  العلوم الاساسية     المرحلة 2
أستاذ المادة حيدر حميد عباس الحيدري       09/09/2020 23:40:53
The course of the somatic sensations through the spinal cord
(the somatosensory system)
Almost all the afferent sensory somatic information of the body enters the spinal cord through the dorsal roots of the spinal nerves or the brain stem via the cranial nerves. On entering the spinal cord the sensory signals are carried to the brain by one of two sensory pathways:

1— the dorsal column pathway (leminiscal system): In which:
[A] first order neurons (dorsal root sensory fibers) enter the dorsal column of the spinal cord and then pass up on the same side of its entrance in the spinal cord to the medulla, where they synapse in the cuneate and gracile nuclei.
[B] From the cuneate and gracile nuclei the second order neurons are originated and decussate immediately to the opposite side and then pass upward to the thalamus through medial leminisci pathways which is joined by additional fibers from the sensory nucleus of the trigeminal nerve.
[C] From thalamus, third order neurons project mainly to the somatic sensory area located at postcentral gyrus and occupy the cerebral cortex of the anterior portion of the parietal lobe.


The dorsal column carries the following sensations: fine touch and pressure (including weight, shape, Size), and synthetic sensations (two-point discrimination, vibration, proprioception).
Collaterals from the fibers that enter the dorsal columns pass to the substantia gelatinosa of dorsal horn of the spinal cord. These collaterals may modify the input signals from other cutaneous sensory systems including the pain system. Therefore, the dorsal horn represents a gate in which impulses in the sensory nerve fibers are modified. This gate is also affected by impulses in descending tracts from the brain.
2— the anterolateral pathways (spinothalamic pathway): In which:
[A] First order neurons (dorsal root sensory fibers) enter the dorsal horns of the spinal cord and synapse with the second order neurons.
[B] The second order neurons cross to the opposite anterolateral white column where they turn upward toward the thalamus through anterior and lateral spinothalamic tracts. Some of the second order neurons of the anterolateral system, which carry signals from slow C pain fibers, pass to the reticular formation of the medulla, pons, and mesencephalon through a spinoreticular pathway and through spinotectal tract. From these areas, higher order neurons are transmitted from reticular formation to the thalamus.
[C] From thalamus, third order neurons project mainly to the somatic sensory area of the cortex along with the neurons of the dorsal column.
The anterolateral system carries the following sensations: crude touch and pressure, pain, thermal. tickle, itch, and sexual sensations.
In general, the sensations that transmitted rapidly, and with fine gradations of intensity and highly localized to exact points in the body are transmitted in the dorsal system. While those sensations which do not transmit rapidly, and lack of fine gradations, and poorly localized to exact points in the body are transmitted in the anterolateral system.

Some of the first or second order neurons of the dorsal column pathway and the anterolateral pathways during their ascending course give off multiple terminals to intermediate and anterior portions of the cord gray matter. These terminals serve two purposes; A- some of them elicit local spinal cord reflexes. B- some of them synapse with the neurons of the spinocerebellar and spinocervical tracts.

Signs of lesions of the central sensory pathways:
[1] A lesion confined to the posterior column of the spinal cord will cause:
? Loss of position and vibration sense on the same side, but the sensation of pain, touch, temperature will be preserved.
? The loss of the sense of the position causes sensory ataxia (muscle incoordination) and the patient has difficulty on standing in upright balanced position with the feet close together without swaying (Romberg’s test) due to loss of proprioceptive sensations. This type of ataxia is more marked when the eyes are closed. The same symptoms will be found if the first order neurons of the proprioceptive nerve fiber are damaged peripherally but they will then be associated with other signs of peripheral nerve disease.
[2] Lesions of the spinothalamic tracts cause impairment of the ability to appreciate pain and temperature on the contralateral side of the body below the level of the lesion. Touch is usually modified (it feels different) but not abolished because of its alternative pathway in the posterior columns.
[3] In the brain stem, the spinothalamic tract and medial lemniscus run close together. Therefore, lesion of the upper brain stem usually affect all forms of sensation on the contralateral side of the body.
[4] Lesions of the main sensory nuclei of the thalamus may cause:
? Loss of various modalities of sensation on the opposite side of the body.
? And spontaneous pain of most unpleasant quality in the opposite side of the body which often causes considerable emotional reaction.



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