Synaptic loci on visual cortical neurons of the rabbit: the specific afferent radiation.

Abstract In newborn rabbits, the visual projection was interrupted unilaterally, by enucleation or lesion of the lateral geniculate body. Analysis of the relevant cortical area prepared 1 month later by the rapid Golgi technique revealed clear-cut changes in the number of dendrite spines limited to the apical shafts of pyramids. Following enucleation there was consistent diminution in spine counts of 30% or more over the central three quartiles of shaft length. Spine loss was more severe with lateral geniculate lesions but loss distribution was similar for both. Little or no change was detected along the apical shaft arches or tips in layer I, nor along oblique shaft branches or basilar dendrites. Normal appearance and distribution of stellate (short axoned, class II) cells in layer IV indicates that the changes on pyramidal dendrites were direct rather than transneuronal. The data are interpreted as indicating that specific visual projection fibers synapse directly on vertically oriented dendrites of cortical pyramids in rabbits. The significance of this distribution pattern is discussed in the light of previous ideas about the arrangement of primary afferent terminals and the problem of input-output functions of sensory cortex. The study underlines the sensitivity of postsynaptic components to changes in level of presynaptic function. It focuses attention upon the dendrite spine as an accurate index of presynaptic geometry, thereby providing a more effective measure of terminal axonal patterns throughout the nervous system.