Somesthetic System of Body


The posterior funiculus, made up of the fasciculus gracilis and the fasciculus cuneatus, carries fibers that signal discriminative touch or pressure, muscle length and tension, and joint position. Some afferent fibers, principally those from quickly adapting cutaneous receptors, ascend the entire length of the spinal cord to synapse directly with neurons in the gracile and cuneate nuclei, which relate to the lower and upper parts of the body. Other fibers leave the posterior columns and either activate spinal neurons for reflex purposes or project upward in the posterolateral funiculus. In humans, most of these secondary ascending fibers also end in the gracile or cuneate nuclei, although a small number of axons sometimes terminate in the upper cervical segments. All of the above nuclei send their axons via the medial lemniscus to the contralateral ventral posterolateral (VPL) nucleus of the thalamus, which projects to the somatosensory regions of the cerebral cortex.


The relay neurons in the gracile, cuneate, and VPL nuclei and the neurons of the primary somatosensory cortex are activated by a single sensory modality over a restricted receptive field. The receptive fields of neurons within each nucleus are arranged in an orderly fashion and give rise to a somatotopic representation of the body surface. Thus a high degree of specificity and order is maintained throughout the pathway.


Anterolateral Funiculus. Two somatosensory pathways ascend in the anterolateral spinal white matter: the lateral and anterior spinothalamic tracts and the smaller spinoreticulothalamic pathway. The spinothalamic tracts arise from neurons in the regions of the posterior horn of the spinal cord that correspond to laminae I, IV, V, and VI of Rexed (see Plate 2-13). Most axons cross in the anterior white commissure at about the level of their cell bodies and ascend in the contralateral lateral and anterior funiculi, although a few fibers ascend ipsilaterally. The spinothalamic axons end principally in the VPL nucleus and in the posterior nuclear group and intralaminar nuclei. Some spinothalamic neurons (especially those in lamina I) respond only to strong, noxious stimuli, but most of these neurons are excited by the activity of a wide variety of afferent fibers related to touch, pressure, vibration, and temperature sense. All spinothalamic neurons have large, unilateral receptive fields and transmit information about a wide variety of peripheral stimuli but with less specificity than is shown by neurons in the posterior spinal pathways.


The spinoreticulothalamic pathway (not shown) begins with neurons in the regions corresponding to laminae I and V to VIII, which ascend in the lateral and anterior funiculi to activate neurons in the brainstem reticular formation, which, in turn, project to the intralaminar nuclei of the thalamus. The spinoreticulothalamic neurons respond to the same stimuli as spinothalamic neurons, but tend to have large, bilateral receptive fields. This fact, together with the nonspecific nature of the intralaminar nuclei, suggests that this pathway is involved with poorly localizable pain sensation and is more important in generalized arousal reactions than in discriminative processing of sensation.


Lesions. Because the principal pathways of the posterior and anterolateral columns cross in the medulla and in the spinal cord, and because each pathway transmits specific modalities, damage from spinal cord lesions presents specific and characteristic deficits. Posterior column destruction results in ipsilateral loss of discriminatory touch and vibration sense, as well as loss of position sense below the level of the lesion. Anterolateral column interruption produces contralateral loss of pain and temperature sense accompanied by diminished touch sense below the lesion.


Only gold members can continue reading. Log In or Register to continue

Stay updated, free articles. Join our Telegram channel

Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Somesthetic System of Body

Full access? Get Clinical Tree

Get Clinical Tree app for offline access