Areas of Cerebral Cortex concerned with Somatic Sensation
The primary somatosensory cortex (SI) is located in the post central gyrus, with the lower limb being represented on the medial surface of the hemisphere, and the head placed laterally near the Sylvian fissure. There is a representation of the body surface, the sensory homunculus, on the post-central gyrus, and the cortical representation of the body is distorted, based on the degree of sensory innervation. The most sensitive areas such as the lips and the fingertips are represented in large areas of S1.
The pathways between sensory receptors and SI are modality specific and are anatomically and electrophysiologically distinct. The afferent inputs from rapidly adapting and slowly adapting skin receptors, muscle afferents and joint receptors are handled by different strips of sensory cortex, and the columns of neurones within these strips are therefore modality specific.
Brodmann's cytoarchitectonic map of the cerebral cortex was the result of an analysis of the size and shape of cortical neurones. There is a strong correlation between structure and function in the cortex, and the somatosensory cortex is in Brodmann's areas 1,2 and 3a + 3b. These represent parallel strips along the length of the post-central gyrus.
The different Brodmann areas within the post central gyrus are strips in parallel with the central sulcus, and the functional aspect of each strip is that it is modality specific - i.e. it handles information from only only one class of sensory receptor.
Area 3 is sometimes divided into areas 3a and 3b: area 3a handles information from muscle receptors; areas
3b and 1 handle the input from rapidly and slowly adapting receptors in skin; and area 2 processes inputs from joint receptors. Areas 1 and 3b have larger numbers of columns dealing with the most sensitive areas of the skin; in contrast, in area 2, the input is from joint receptors: here the largest areas of cortex (number of cortical columns) are given to the large joints (hips, knees, etc) and smaller areas are concerned with smaller joints.
It has been found recently that pressure, flutter, and vibratory stimuli activate spatially distinct cortical columns in area 1. There is also modern evidence that nociceptive information may also handled by SI, but is represented in SII and the Insular Cortex.
The second somatosensory area (SII) also contains a somatotopic map; this is buried deep in the Sylvian fissure in Brodmann's area 40.
A third area, in the parietal lobe, the somatosensory association cortex, is concerned with tactile pattern recognition. Astereognosis is a condition in which the patient is unable to recognize objects using only his hands.
Cortical lesions in these areas elevate sensory thresholds and result in inabilities to make use of tactile information.