Evidence for two organizational plans within the somatic sensory‐motor cortex of the rat

Abstract

Previous electrophysiological mapping of somatic sensory (SI) and motor (MI) cortex in the rat has shown that these functional regions are completely overlapping in the hindlimb (HL) area, partially separate in the forelimb area, and completely separate in the face area. The present studies were designed to show how the sensory and motor thalamic relay nuclei project onto separate or overlapping subdivisions of SI and MI cortex. The experimental sequence involved physiological mapping of one subdivision of SI or MI cortex and then injecting horseradish peroxidase (HRP) as a retrograde cell marker to back-fill the thalamic neurons that project to the identified area of cortex. The central issue was to compare the projections of the lemniscal-recipient ventrobasal nucleus (VB) and the cerebellar-recipient ventrolateral nucleus (VL) to the hindlimb and face subdivisions of SI and MI cortex. Various authors, however, have divided the ventral nucleus differently using Nisslstained transverse sections through the thalamus. Therefore, tritiated amino acids were also injected into the deep cerebellar nuclei and the distribution of cerebellar fiber terminals in the ventral nucleus was used as another criterion to identify the border between VB and VL.Following HRP injections in physiologically identified HL cortex, labelled neurons were present in both VB and VL. Face area SI injections back-filled cells in VB (but not VL), while face area MI injections back-filled cells in VL (but not VB). Neurons in the intralaminar nuclei and the posterior nuclear complex were labelled after all injections.These results demonstrate that SI and MI cortex in the rat shows a combination of overlapping and separated plans of organization. Other species, in which both the anatomical and physiological information is available, show other organizational plans. In the Virginia opossum, for example, overlap is present in all subregions of SI and MI cortex, while the Rhesus monkey shows separation of these areas. The results suggest that different mammals have evolved different degrees of separation of SI and MI cortex. Our results predict that in every case where there is evidence of sensory and motor overlap, there will be anatomical convergence of VB and VL projections to that single cortical area.In Woolsey's ('58) comparison of sensory and motor representations in the cortex of the rat, rabbit, cat and monkey, the first somatic sensory area (SI) and the first motor area (MI) were plotted as completely separate mirror image representations of the contralateral body structures. MI, the more anterior of the two representations, is the area of cortex where electrical stimulation a t the lowest threshold produces contractions of contralatera1 musculature. The more posterior SI cortex responds a t short latency to cutaneous stimulation. Anatomical studies on placental mammals have shown that the ventrobasal nucleus (VB) projects to SI cortex, and a few studies have concluded that VB does not project to MI cortex (Jones and Powell, '69, '7...

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