Patterns of synaptic input on corticocortical and corticothalamic cells in the cat visual cortex. II. The axon initial segment.

In the present study we examined the synaptology of the axon initial segments (AISs) of three populations of retrogradely labeled pyramidal cells: callosal, corticothalamic, and ipsilateral corticocortical projecting cells. The cells were labeled by horseradish peroxidase injected into the contralateral visual cortex, the ipsilateral lateral geniculate nucleus, or area 19 of the ipsilateral hemisphere. The AISs of these cells were completely reconstructed from tracings of serial electron micrographs and the number and type of synapses ending on them determined. These data were compared with that described in the companion paper (Fariñas and DeFelipe, J Comp Neurol 1991; 304:53-69), in which the ultrastructure and synaptology of the somata of callosal and corticothalamic cells was investigated. Together, these data permit comparisons to be made between the somatic and axonic innervation of the same cells. Our results show that each population of efferent cells examined received a characteristic and rather homogeneous number (in comparison with the number of axosomatic synapses) of axoaxonic synapses: 16 to 23 for AISs of callosal cells, 22 to 28 for ipsilateral corticocortical cells, and 1 to 5 for corticothalamic cells. Since most axoaxonic synapses are likely to be GABAergic, it is concluded that each population of pyramidal cells receives a distinct GABAergic inhibitory synaptic profile.