TOPOGRAPHICAL RELATIONS BETWEEN IPSILATERAL CORTICAL AFFERENTS AND CALLOSAL NEURONS IN THE 2ND SOMATIC SENSORY AREA OF CATS

Experiments were carried out on the second somatic sensory area (SII) of cats to study 1) the laminar distribution of axon terminals from th e ipsilateral first somatic sensory cortex (SI); and 2) the topographi cal relations between their terminal field and the callosal neurons pr ojecting to the contralateral homotopic cortex. To label simultaneousl y in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris-leucoaggluti nin (PHA-L) in the forepaw zone of ipsilateral SI, and pressure inject ions of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synapse callosal neurons was investigated with the electron microscope by com bining lesion-induced degeneration with retrograde HRP labelling. Fibe rs and terminations immunolabelled with PHA-L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concen trated in supragranular layers. Labelled fibers formed a very dense pl exus in layer III and ramified densely also in layers I and II. Labell ed axon terminals were both en passant and single-stalked boutons. Cou nts of 8,303 PHA-L-labelled terminals of either type showed that 82.40 % were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.32%) and I (8.09%). The remai ning terminals were distributed among layers IV (6.96%), V (4.93%), an d VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labe lled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangential ly in periodic patches. Patches of anterograde and retrograde labellin g either interdigitated or overlapped both areally and laminarly. In t he zones of overlap, numerous PHA-L-labelled axon terminals were seen in close apposition to HRP-labelled pyramidal cell dendrites. Combined HRP-electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP-l abelled dendrites and dendritic spines pertaining to callosal projecti on neurons. These results are discussed in relation to the layering an d function of the SI to SII projection, and to the evidence that SII n eurons projecting to the homotopic area of the contralateral hemispher e have direct access to the sensory information transmitted from ipsil ateral SI. (C) 1994 Wiley-Liss, Inc.