Proliferation of thalamic afferents in cerebral cortex altered by callosal deafferentation

In area 41, the auditory region of rat neocortex, callosal afferents project to layers I through III and thalamic afferents project to deep layer III through IV. Thus, these two extrinsic systems of afferents project simultaneously to only a narrow lamina in mid to low layer III. For this study, this narrow region of overlap is quantitatively examined to determine the distribution of callosal and thalamic afferents by observing degenerating terminals produced by separate callosal and thalamic lesions. The results show that of all asymmetric synapses observed in the neuropil of this narrow zone, 84% are dendritic spines and the balance are dendritic shafts. Although both callosal and thalamic afferents prefer to synapse with dendritic spines in the neuropil, 78% of the thalamic afferents synapse with dendritic spines while 93% of the callosal afferents synapse with dendritic spines. Vaughan & Foundas (1982) have shown that 3 months after callosal lesions in 1-month-old animals, additional thalamic axons have grown into, and proliferated in, this part of mid to low layer III. Quantitative analysis of the distribution of the degenerating thalamic axon terminals in these long-term callosally lesioned animals has been used to determine whether the proliferating thalamic afferents demonstrate any specificity in the pattern of synapses they make or whether the callosally deafferented neurons determine the pattern of synapses. The results indicate that thalamic axons do exhibit axon specificity, for after they have proliferated into the callosal domain, 80% of the degenerating terminals synapse with dendritic spines and 20% synapse with shafts. This distribution is most comparable to the normal distribution of thalamic axons in this region.