5. Somatosensory Processes
Part 2 of 10

Patrick Dougherty, Ph.D.
(Content provided by Chiyeko Tsuchitani, Ph.D.).

The Somatosensory Cortex

Somatosensory information converges in the parietal lobe of the cerebral cortex where it is processed to provide a cohesive perception of your body and your physical environment.

The primary somatosensory cortex, SI, includes the postcentral gyrus and the posterior paracentral lobule of the parietal lobe (Figure 5.4 & 5.5).

SI extends from the depths of the central sulcus up superiorly to form the posterior lip of the central sulcus.SI is considered the primary somatosensory cortex because it is the major site of termination of VPL and VPM axons:

• its neurons are responsive to somatosensory stimuli almost exclusively;
• lesions of SI produce severe somatosensory deficits; and
• electrical stimulation of SI produces somatosensory perceptions.
  
  
  

SI is somatotopically organized. The body and face are mapped in the contralateral cortex with the foot and leg represented in the posterior paracentral lobule and the trunk, chest, arm and hand in the upper half of the postcentral gyrus. The face is represented in the lower half of the postcentral gyrus (Figure 5.4).

Differential projections to the SI areas arise from the central core and shell of VPM and VPL. However, there is also convergence of somatotopic and modality specific information in SI. To appreciate the shape, texture, size, weight, and movement of a given object, the somatosensory cortex must integrate the parallel streams of information carried by the medial lemniscal pathway. To achieve this integration, the parallel streams converge at cortical levels, starting in SI. As a result of this convergence, receptive fields become larger, modality specificity diminishes, and the cortical neural responses become more complex.

SI neurons send their axons to the secondary somatosensory cortex, adjacent areas of the parietal lobe, and to cortical motor areas (Figure 5.6) as well as to subcortical nuclei, brain stem and spinal cord. Unilateral destruction of SI produces severe deficits in all aspects of discriminative touch and proprioception on the contralesional side of the body. In addition to deficits in the abilities to accurately localize and to recognize objects by shape, texture and size and to appreciate vibrating/moving stimuli, there are deficits in fine motor coordination.

The secondary somatosensory cortex, SII, is located inferiorly - in the pars opercularis of the parietal lobe, which forms part of upper lip of the lateral sulcus (Figure 5.4 & Figure 5.5). SII neurons send their axons to SI, association cortex, motor cortex, and insula (Figure 5.6). The latter projection, to the insula, influences structures such as the amygdala and hippocampus. These structures are important in tactile learning and memory. The projection to the somatosensory association cortex is involved in higher order processing required for recognizing hand-held objects by texture and size. Consequently, lesions in SII produce deficits in learning by object manipulation and in recognizing the texture and size of hand-held objects.

The somatosensory association cortex is located in the superior parietal lobe (a.k.a. posterior parietal cortex), which is posterior to SI. The highest degree of convergence of somatosensory information occurs in the posterior parietal cortex. The posterior parietal cortex receives the axons of SI and SII neurons and also receives input from the visual system and other systems involved in attention and motivation.

Neurons in the posterior parietal cortex are responsive to somatosensory and visual stimuli, have large somatic receptive fields in which responsiveness is based on stimulus context, and are often more responsive to stimulus movement.

Large lesions involving the posterior parietal cortex and the adjoining superior temporal gyrus may result in an attentional deficit called “neglect”, wherein there is a partial neglect (inattention) to tactile, proprioceptive and/or visual stimuli delivered contralateral to the lesion site. The patient is described as ignoring the contralesional half of her/his body and space. The perception of a "whole" body is lost and the body parts affected may be considered to belong to someone else. Visual stimuli on the contralesional side may also be ignored.

Pain information is processed in multiple pathways (see Table 1 in the chapter on Somatosensory Systems) involving multiple thalamic nuclei that project to multiple cortical areas. In addition to the somatosensory cortex, painful stimuli activate neurons in the rostral cingulate gyrus and the insula. Consequently, all pain sensation is not lost when the primary somatosensory cortex is damaged. Primary somatosensory cortex neurons that have small receptive fields and are selectively responsive to sharp, cutting painful stimuli are considered to provide the ability to accurately localize the exact point of contact with the painful stimulus. Lesions of the primary somatosensory cortex will affect the quality of pain sensations and the ability to localize the exact location of the painful stimulus.

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