Somatosensory System

Sensory information (proprioception, touch, pain, temperature, and vibration) from receptors in the skin, joints, and muscles throughout the body is relayed to layer 4 of the primary somatosensory cortex (SI) from the ventral posterior nuclei of the thalamus. SI is located along the postcentral gyrus of the parietal cortex and is divided into four parts based on cytoarchitecture and somatic inputs (Brodmann areas 3a, 3b, 2, and 1). The way in which sensory information (the location and intensity of a stimulus) is encoded and represented in SI is determined primarily by three principles of cortical organization: cortical receptive fields, preservation of somatotopy, and the columnar organization of the cortex.

The pyramidal cells in SI are continuously active, and this activity can be enhanced or inhibited by a stimulus acting at a specific location on the skin. This location in the periphery, where stimulation maximally activates a given cell in SI, is termed the receptive field for that cell. The receptive field for each cell in the cortex has a gradient of excitation, such that stimulation at the center of the field maximally excites the cell, and this excitation diminishes toward the outer edge of the field. There is also a less prominent inhibitory gradient that may extend beyond that of the excitatory one (inhibitory surround). This organization of the receptive field is the basis for lateral inhibition of sensory input, which enhances the ability to detect the precise location of a stimulus (Fig. 5).

Preservation of somatotopy refers to the fact that at all levels of the central nervous system, the organization of somatosensory input from the periphery is maintained. For instance, somatosensory information for discriminative tactile sensation enters the dorsal column of the white matter of the spinal cord in a medial-lateral manner, such that sensory information

Peripheral receptive field

Figure 5 Schematic model of a peripheral excitatory receptive field with inhibitory surround. This provides an anatomical basis for the process of lateral inhibition of sensory input. Stimulation at the center of the receptive field maximally excites the sensory cells, which relay this excitation to a cortical column in the primary somatosen-sory area. Stimulation of cells in the outer edge of the receptive field suppresses the ascending sensory input by activating cells that synapse upon inhibitory interneurons. This pattern of firing is preserved as the information ascends the neuraxis, such that cortical neurons innervated by input from the center of the receptive field will be maximally activated, whereas the immediately adjacent cells in the cortex will be suppressed. This lateral inhibition acts to enhance the contrast between the stimulation site and peripheral areas to enable precise localization of the stimulus.

from the lower body lies medially and that ofthe upper body is organized laterally in the dorsal column. This organization is preserved throughout the brain stem, at the thalamic relay nuclei and ultimately in the primary somatosensory area of the cerebral cortex. The somatotopic organization in SI dictates that sensory input from the lower body is represented along the medial surface of the postcentral gyrus, whereas the upper body and head are represented along the lateral convexity of the gyrus. An interesting feature of somatotopy in SI is that those areas of the body that have large numbers of sensory receptors, such as the hands and mouth, have disproportionately much larger areas of SI cortex devoted to them. Moreover, the receptive fields in these areas of the skin are much smaller and more numerous than areas such as the trunk that are not concerned with precise discrimination of the stimulus.

Finally, in addition to its laminar organization as described previously, the cerebral cortex also possesses columnar organization, such that the cells in any given column of cortical tissue (from the cortical surface to the underlying white matter) respond maximally to a specific modality of stimulation. For instance, an edge placed on the skin with a specific orientation will optimally activate one cortical column, whereas the adjacent column will maximally respond to a slightly different orientation.

The primary somatosensory cortex uses these features in discriminating the intensity and location of a stimulus or detecting the shape or texture of an object. For example, the activation of a population of neurons on the medial surface of the postcentral gyrus indicates that a precise area of the lower body is being stimulated. The intensity of the stimulus is determined by the frequency of firing of the neurons and by the number of neurons recruited. Finally, the precise nature of the stimulus (pressure, touch, and direction of hair movement) is determined by which cortical columns respond.

Projection fibers from layers 2 and 3 of the trunk representation of SI innervate the contralateral SI as well as the secondary somatosensory cortex (SII). SII is located in the upper bank of the lateral sulcus, just inferior to the central sulcus (Fig. 6A). It is believed to have poor somatotopy, and the cells within SII have bilateral receptive fields, suggesting more of an inte-grative rather than a discriminative function. Information from SII is relayed to the posterior parietal cortex, which is considered to be the somatosensory association area (Brodmann areas 5 and 7). This area is involved in high-level integration of somatosensory information to enable learning and memory of the surrounding tactile and spatial environment. SI also has direct projections to subcortical structures (the basal ganglia, brain stem, and spinal cord) via pyramidal cells from layer 5 and to the ventral posterior nuclei of the thalamus from layer 6.

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