The anterior border zones of primary somatic sensory (S I) neocortex and their relation to cerebral convolutions, shown by micromapping of peripheral projections to the region of the fourth forepaw digit representation in raccoons

Abstract

In raccoons the somatic sensory neocortex is greatly expanded, with separate gyral crowns devoted to and intervening sulci separating, sensory representations of separate body parts, most strikingly those of the volar surfaces of individual forepaw digits. Most of the cortex in this region is buried in widely ramifying sulcal walls, wherein sensory projections have not been studied. We have determined mechanosensory projections to the fourth digit representation region including all neighboring sulcal walls, using tungsten microelectrodes for fine-grain 3-dimensional micromapping. We found no significant alteration in the location and pattern of projections when the following different anesthetics were used: dial-urethane, chloralose, or methoxyflurane with nitrous oxide. The precisely organized somatotopic representation of the distal volar surface of the fourth digit, on the caudal aspect of its gyral crown, continues down the anterior bank of the triradiate sulcus. This meets, at the fundus, projections from the proximal volar surface of the digit which occupy the posterior sulcus wall; they in turn meet projections from the volar palm at the gyral crown. In the anterior part of the crown containing the representation of the distal volar digit, across the crown of the gyral bridge intervening between the medial and lateral segments of the central sulcus, throughout the posterior walls of the central sulci, and in the walls of the interbrachial sulcus, we found a distinctive border-zone of projections from heterogeneous receptive fields. Within a roughly somatotopic basic pattern of organization we found intermingled projections from single and multiple claws and dorsal hairy surfaces of digits and proximal hand, along with additional projections from volar surfaces. These projections can be construed as forming something of a distorted mirror-image of the representation of the volar hand. Beyond this was a second zone of distinctive projections from afferents of the forelimb muscles, in the anterior walls of the central sulci. These projections are interrupted where the sulci are interrupted. The zone of muscle afferent projections corresponds to those seen between sensory and motor regions in other species; its strict association with sulcal folding here and in other species suggests a general relationship of these projections to central sulci. The zone of heterogeneous projections resembles similar zones seen at other levels of this system in raccoons, in the cortex of other species, and it may relate to some of the multiple representations reported in other species. It also may be related to the formation of sulci in this region and may be a specialized zone for cortico-cortical connections.