The corticothalamic projection from the second somatosensory cortical area in the cat. An experimental study with silver impregnation methods.

Abstract

The corticothalamic projection from the anterior ectosylvian gyrus in the cat has been studied with the silver impregnation method of Nauta. The second somatosensory cortical area (SII) projects upon the ipsilateral nucleus ventralis posterolateralis (VPL), nucleus ventralis posteromedialis (VPM), the posterior thalamic region (PO) and to a slight extent upon the reticular nucleus of the thalamus (R), the centrum medianum (CM), the parvocellular part of VPM (VPMpc) and the nucleus ventralis posterior inferior (VPI). A somatotopical arrangement in the projection upon the ventro-basal (VB) complex has been demonstrated and a topical arrangement in the corticothalamic fibers from SII to PO is also evident. The transitional area between SII and the second auditory cortex sends fibers mainly to the entire magnocellular part of the medial geniculate body (MGmc) and to a lesser degree to the principal division of this nucleus (MGp). The corticofugal fibers from SII follow various and rather complicated circuitous routes before they end in the different thalamic nuclei. The experimental findings are discussed in the light of recent anatomical and physiological observations. It is shown that zones B and C of SII which have been shown by Carreras and Andersson (1963) to possess a large number of place and modality specific neurons project upon the VB-complex. On the other hand, zone A which contains a majority of place and modality unspecific neurons sends its fibers exclusively to PO. Finally the problem of thalamocortical projections to SII is briefly discussed.