The ipsilateral cortical connexions of the somatic sensory areas in the cat

Abstract

The ipsilateral association connexions of the somatic sensory cortex have been investigated in the cat using the Nauta technique. These connexions are restricted to a small number of adjoining cortical regions all of which have been previously defined in both functional and architectural terms. The first and second somatic sensory areas display certain similarities and one significant difference. A highly organized set of reciprocal connexions joins the first and second somatic sensory areas to one another and to the motor cortex so that all the representations of a given part of the periphery are interconnected. Further connexions join both areas to a portion of area 6 which probably represents the supplementary motor area, while others pass in an organized manner from the first somatic area, only, to area 5, a portion of which seems to represent the ‘third somatic sensory area’ of Darian-Smith et al. Other fibres pass from the second somatic area to the suprasylvian fringe of Rose 45 and Woolsey 55, a region of somatic sensory and auditory convergence. Intrinsic connexions link points in each of the major subdivisions of the first somatic sensory area to one another but not to points in other subdivisions, while all points in the second somatic area appear to be interconnected. Intrinsic connexions are effected by intracortical fibres which terminate mainly in layers III and IV. Connexions with other cortical areas are by cortico-cortical fibres travelling in the white matter but terminating in a similar manner. A constant feature of lesions in all parts of the neocortex is the presence of a number of degenerating fibres radiating out in the molecular layer for distances of up to 7 mm from the lesion.