On the thalamo-cortical connexions of the general sensory cortex of Macaca

Abstract

The ascending thalamic connexions of the general sensory cortex of Macaca have been studied by recording the retrograde cell degeneration in the posterior ventral nucleus of the thalamus following selective lesions involving the cytoarchitectonic areas o f the postcentral gyrus. Area 3 (of Brodmann) receives connexions from the entire rostro-caudal extent of the nucleus; they are derived from most of the cells in the rostral half and from at least 50% of the cells in the caudal half. Area 1 receives the exclusive projection of a proportion of the cells (up to 30% in the caudal half of the nucleus), and probably also collaterals from other axons which terminate mainly in area 3. The retrograde degeneration following lesions of area 2 provides no evidence that this area receives the exclusive projection of any of the cells of the posterior ventral nucleus, but there is evidence that it receives collaterals of axons terminating mainly in areas 3 and 1. Destruction of any one of the cytoarchitectonic areas of the postcentral gyrus results in a partial diffuse degeneration over the whole cross-sectional area of the ventro-lateral nucleus at different levels. Only when a complete sector of the postcentral gyrus is destroyed, involving all three cytoarchitectonic areas, is there a sharply circumscribed lamina of retrograde degeneration extending throughout the rostro-caudal extent of the nucleus. The nucleus lateralis posterior of the thalamus projects to the parietal cortex; there is no evidence that it has any connexion with the general sensory cortex. The results of these experiments are discussed in relation to clinical observations of dissociated sensory loss following lesions of the postcentral gyrus in man.