Third Order Neuron

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The fibers of the medial lemniscus terminate in the ventroposterolateral (VPL) nucleus of the thalamus. This pathway is organized somatotopically such that the fibers

from the nucleus gracilis end most laterally within the VPL nucleus and those from the nucleus cuneatus end in the larger, medial part of the VPL nucleus. The VPL nucleus is the origin of the third-order sensory afferent neuron that sends projections to somatosensory cortex (Fig. 19-1 (Figure Not Available) ).

Primary somatosensory cortex consists of the postcentral gyrus and its medial extension into the paracentral lobule (Brodmann areas 1, 2, and 3). There is somatotopic organization such that the amount of tissue within the primary sensory cortex that represents a particular region of the body is related to the importance of that region in somatic sensation and not to its absolute size. For example, the thumb has a much greater area of cortical representation than does the proximal arm. In the region of the postcentral gyrus, the calf and the foot are represented on the medial surface of the hemisphere and then laterally follow the foci for the thigh, abdomen, thorax, shoulder, arm, forearm, hand, fingers, and face. The foci for the bladder, rectum, and genitalia are located on the lowest aspect of the medial surface of the hemisphere (Fig. 19-2 (Figure Not Available) ).

With regard to the subdivision of cortical regions by modality, Powell and Mountcastle have shown in monkey cortex that over 90 percent of the neurons in area 2 are related to receptors of the deep tissues of the body such as the joints, in area 3 most of the cells are activated by cutaneous stimuli, and area 1 has an intermediate position in terms of the modality specificity of its neurons. y

Adjacent to primary sensory cortex are smaller cortical zones that receive input from primary somatosensory cortex referred to as secondary sensory areas. Although contralateral representation of the body predominates there is some bilateral representation within these areas.

Figure 19-1 (Figure Not Available) The formation and course of the posterior columns in the spinal cord and the medial lemniscus in the brain stem. The posterior columns are formed from uncrossed ascending and descending branches of spinal ganglion cells. Ascending fibers in the fasciculi gracilis and cuneatus synapse on cells of the nucleus gracilis and cuneatus. Fibers forming the medial lemniscus arise from cells of the nuclei gracilis and cuneatus, cross in the lower medulla, and ascend to the thalamus. Impulses mediated by this pathway include proprioceptive, vibratory and discriminative touch. Spinal ganglia and afferent fibers entering the spinal cord at different levels are coded as in (Modified from Carpenter MB: Human Neuroanatomy. Baltimore, MD, Williams & Wilkins, 1983.)

Figure 19-2 (Figure Not Available) Sensory homunculus. This diagram shows the relative size and distribution of the parts of the cerebral cortex from which sensations localized to different parts of the body can be elicited on electrical stimulationFrom an. Penfield W, Rasmussen T: The Cerebral Cortex of Man. New York, Simon and Schuster, 1950.)

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