Convergence on the same neurons in the feline ventrobasal thalamus of terminals from the dorsal column and the lateral cervical nuclei: an ultrastructural study combining orthograde degeneration and anterograde axonal transport of lectin conjugated horseradish peroxidase.

After electrocoagulation of the dorsal column nuclei (DCN) and injections of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) into the lateral cervical nucleus (LCN), two kinds of changes in fibers and boutons were observed in the dorsolateral part of the thalamic ventroposterolateral nucleus (VPL). The axons and boutons from neurons in the DCN demonstrated dark degeneration, while the fibers and synaptic terminals from the LCN-neurons contained the characteristic needle-shaped peroxidase reaction products following incubation with tetramethylbenzidine (TMB). Occasionally dark degenerating boutons and boutons with TMB-positive reaction products showed synaptic contact with the same dendritic profiles. After consideration of the possibility of an axoaxonal transfer of the tracer it is concluded that the findings demonstrate convergence of synaptic input both from the DCN and from the LCN on the same postsynaptic VPL neurons. The present results are in accordance with earlier light microscopic and neurophysiological findings. It is also concluded that the ultrastructural technique employed in this study is well suited to reveal convergence of different afferent systems on the same postsynaptic neurons.