Patterns of the somesthetic messages transferred through the corpus callosum.

Abstract

1. In the rostral part of the corpus callosum (somesthetic callosal region, SCR) fibres were identified, through which the callosally-projecting cells of the somatosensory areas transmit to the other hemisphere signals originated in the body surface. 2. With seriate macroelectrode penetrations it was found that, to some extent, the body surface is represented somatotopically in the rostrocaudal extent of the SCR. The strongest mass potentials to trigeminal, fore- and hindlimb stimulation were recorded from the rostral, middle and caudal portions of the SCR. The whisker region and the forelimb (both paws and proximal segments) appeared to have the widest callosal representation. 3. Ablation experiments showed that callosal somesthetic fibres originate in both SI and SII areas and that only impulses set up in the contralateral hemibody are relayed in these areas. Direct stimulation of the latter evoked within the SCR mass potentials whose rostrocaudal distribution parallels that of the peripherally evoked responses. 4. Exploring the SCR with microelectrodes, 43 spontaneously active fibres were isolated, all reactive to electrical and physiological stimulation of the related peripheral receptive fields. These were located in trigeminal (31 fibres), segmental (10 fibres) or both in trigeminal and segmental regions (2 fibres). The extent of the receptive fields and the reactivity characteristics of almost all the fibres sampled were lemniscal in type, and similar to those of the somatotopic neurones of cortical somatosensory areas.