Thalamocortical relationships in the somatic sensory system as revealed by silver impregnation techniques.

Abstract

Most of our knowledge of the organization of thalamocortical projections has been derived from cortical ablation-retrograde degeneration studies. This paper, however, reports the results of placement of small electrolytic lesions within the feline thalamus, using a recording-lesioning electrode, and subsequent study of intracortical axonal degeneration revealed by the Fink-Heimer I stain. Prior to lesioning, the functional properties of thalamic neurons to be destroyed were determined by evoking responses in the ventrobasal complex (VB) or posterior group (PO) by natural somesthetic stimulation. This approach permitted more exact structural-functional correlations, description of intracortical organization of thalamic afferents, and revelation of topographic relationships within a large area of cortex hidden in sulci. VB projected somatotopically to both S-I and S-II. Following VB lesions, an extremely dense patch of terminal degeneration was seen in laminae IV-III of S-I. This patch, termed the primary focus, was sharply demarcated from surrounding cortex containing far less degeneration. Small and large VB lesions projected in an equally dense fashion, indicating little overlap in the projection of VB neurons to S-I. These observations have led to the conclusion that thalamocortical projections contribute significantly to the functional columnar organization in S-I. Similar studies of the ventrolateral nuclear (VL) projections to motor cortex by Strick(1970) revealed an overlap of projections from different VL zones and thus provided no anatomical evidence that thalamocortical projections determine the columnar organization in motor cortex. The VB projection zone in S-II lay rostrally in the anterior ectosylvian gyrus and contained degeneration less dense than that of the primary focus. PO did not project to S-I; and the lighter, less focal cortical degeneration observed was concentrated caudally in S-II in the caudal anterior ectosylvian gyrus. In addition, PO projected to auditory areas of the middle ectosylvian and anterior Sylvian gyri. Significant degeneration was found in the banks and fundi of various sulci following all thalamic lesions. These findings indicate that sulci do not necessarily lie between physiologically distinct regions and that they are not thin 'athalamic' zones enveloped by more richly innervated gyral crowns, a condition suggested as a causative factor in sulcal development.