MORPHOLOGY OF CALLOSAL AXONS INTERCONNECTING AREA-17 AND AREA-18 OF THE CAT

Seventeen callosally projecting axons originating near the border betw een areas 17 and 18 in adult cats were anterogradely labelled with bio cytin and reconstructed in 3-D from serial sections. All axons termina ted near the contralateral 17/18 border. However, they differed in the ir diameter, tangential and radial distributions, and overall geometry of terminal arbors. Diameters of reconstructed axons ranged between 0 .45 and 2.25 mu m. Most of the axons terminated in multiple terminal c olumns scattered over several square millimetres of cortex. Thus in ge neral callosal connections are not organized according to simple, poin t-to-point spatial mapping rules. Usually terminal boutons were more n umerous in supragranular layers; some were also found in infragranular layers, none in layer IV. However, a few axons were distributed only or mainly in layer IV, others included this layer in their termination . Thus, different callosal axons may selectively activate distinct cel l populations. The geometry of terminal arbors defined two types of ar chitecture, which were sometimes represented in the same axon: paralle l architecture was characterized by branches of considerable length wh ich supplied different columns or converged onto the same column; seri al architecture was characterized by a tangentially running trunk or m ain branch with radial collaterals to the cortex. These architectures may relate to temporal aspects of inter-hemispheric interactions. In c onclusion, communication between corresponding areas of the two hemisp heres appears to use channels with different morphological and probabl y functional properties.