Callosal and ipsilateral cortical connections of the body surface representations in SI and SII of tree shrews.

Abstract

Injections of horseradish peroxidase (HRP) were used to study the connections of the first and second somatosensory areas (SI and SII) in tree shrews. The locations of callosally projecting neurons in SI were determined by placing large injections of HRP in the SI region of one cerebral hemisphere and determining the organization of SI of the other cerebral hemisphere with microelectrode mapping. Many callosally projecting neurons were revealed in lateral SI representing the face, especially the glabrous nose. A sparse scattering of callosally projecting neurons were located more centrally in SI in portions representing the forepaw; these neurons tended to be in cortex devoted to the dorsal hand and pads of the palm rather than the digits. Part of medial SI, representing the forelimb and trunk, had a moderately dense distribution of callosally projecting neurons. More restricted injections in SI indicated that callosally projecting neurons were largely within comparable portions of contralateral SI, although a few neurons projecting callosally to SI were located in SII and cortex caudal and rostral to SI. Large injections of HRP in SII labeled neurons throughout contralateral SII, including representations of the forepaw and hindpaw. More restricted injections in SII labeled neurons in somatotopically comparable parts of the contralateral SII. A few labeled neurons were also seen in somatotopically matched parts of contralateral SI. The results also demonstrated strong somatotopically organized connections between SI and SII of the same hemisphere, and connections of SI and SII with adjoining subdivisions of parietal and frontal cortex. The major thalamic projections to both SI and SII originated in the ventroposterior nucleus.