Mriganka Sur

Regional segregation of neurons responding to quickly adapting, slowly adapting, deep and Pacinian receptors within thalamic ventroposterior lateral and ventroposterior inferior nuclei in the squirrel monkey (Saimiri sciureus).

Abstract The ventroposterior nuclei and the ventroposterior inferior nucleus of the thalamus were mapped in squirrel monkeys ( Saimiri sciureus ) using low-impedance platinum-iridium microelectrodes. The body site and receptor category from which the optimum response was obtained was recorded every 50 μm in orthogonal penetrations through these nuclei. The results suggest that the neurons are grouped according to submodality in this area and that discrete and spatially separate volumes of the thalamus are devoted to input from deep, Pacinian, cutaneous rapidly adapting and cutaneous slowly adapting receptors. The Pacinian responses were limited to the ventroposterior inferior nucleus and deep responses were limited to a region just above the ventroposterior nuclei. Within ventroposterior nuclei the cutaneous rapidly adapting and cutaneous slowly adapting volumes were interspersed in a pattern that could not be readily defined. The results indicate that there is more than one representation of the body in this area of the thalamus. Discontinuities in receptive field sequences make it difficult to see how any representation can be a spatially continuous map of the body.

Representations of the body surface in areas 3b and 1 of postcentral parietal cortex of cebus monkeys

The somatotopic organization of postcentral parietal cortex was determined with microelectrode mapping methods in a New World monkey, Cebus albifrons. As in previous studies in macaque, squirrel and owl monkeys, two separate representations of the body surface were found in regions corresponding to the architectonic fields 3b and 1. The two representations were roughly mirror-images of each other, with receptive field locations matched for recording sites along the common border. As in other monkeys, the glabrous digit tips of the hand and foot pointed rostrally in the Area 3b representation and caudally in the Area 1 representation. Both representations proceeded in parallel from the tail on the medial wall of the cerebral hemisphere to the teeth and tongue in lateral cortex along the Sylvian fissure. Compared with the other monkeys, the tail of the cebus monkey, which is prehensile, was represented in a very large region of cortex in Areas 3b and 1. Like its close relative, the squirrel monkey, the representation of the trunk and parts of the limbs were reversed in orientation in both Area 3b and Area 1 in cebus monkeys as compared to owl and macaque monkeys. The reversals of organization for some but not all parts of the representations in cebus and squirrel monkeys suggest that one line of New World monkeys acquired a unique but functionally adequate pattern of somatotopic organization for the two adjoining fields.

The Postcentral Somatosensory Cortex

Many of our present day concepts of the organization of somatosensory cortex stem from the landmark study of Woolsey, Marshall and Bard, first published in 1937 (7) and then more completely in 1942 (21). These investigators documented important features of the overall organization of the postcentral parietal cortex in Macaca mulatta by recording evoked potentials from the surface of the brain. The basic procedure was to determine the portion of the body surface (the receptive field), where movements of hairs or punctate stimulations of glabrous surfaces with a cat’s vibrissa were capable of evoking responses at a given recording site and to repeat the procedure for a grid of closely spaced recording sites. By removing parts of the brain, they were even able to explore cortical surfaces buried in the central sulcus and on the medial wall of the cerebral hemisphere. These studies led to several important conclusions: (a) the region of postcentral cortex activated by tactile somatic stimuli includes the separate architectonic fields 3 (3a and 3b), 1 and 2 of Brodmann; (b) the cortex is activated almost exclusively from the contralateral half of the body; (c) there is an orderly representation of body parts, with a medial to lateral sequence across cortex from tail to tongue along the body; (d) the cortical organization does not exactly reflect the body surface, so that there are displacements in the cortical map; one clear example given was the separation in cortex of the representation of the face from the caudal head; and (e) the skin surfaces with the greatest tactile acuity have the largest cortical projection areas.

Organization of the S I Cortex

Recent electrophysiological mapping studies have led to a significant redefinition of the internal organization of “S I” of primates (11, 17). Classically, “SI” has been described as a single and at least largely continuous representation of the body surface spanning four distinct architectonic regions, areas 3a, 3b, 1 and 2, in a number of New World and Old World species of primates (1, 2, 4, 5, 8, 14, 27–31, 36-42). However, these new studies (extending earlier, more limited mapping studies of Paul et al., 23–25) have revealed that there are two complete (or nearly complete) representations of the body surface and two representations of deep body structures within the classical primate “S I” (11,17). The two large cutaneous representations are coincident with cytoarchitectonic areas 3b and 1. These two cutaneous representations have striking differences in internal topography, and in proportional representation of different skin surfaces.