The synaptic organization of lemniscal projections to the ventrobasal thalamus of the cat

Abstract

The fine structure of degenerating synapses in VB following ablation of the dorsal column nuclei has been studied with the electron microscope. Within 24 h after the lesion, large synaptic knobs forming asymmetrical synapses are seen to fill with neurofilaments. The neurofilamentous degenerative changes become more marked 2–3 days following the lesion. The degenerating knobs exhibit an electrondense appearance after 3–5 days, and then are phagocytosed and most disappear after 5–10 days. The degenerating lemniscal synapses are found to synapse principally upon dendrites, and also upon other synaptic knobs or dendritic profiles giving rise to synapses. The fine structure of degeneration is compared to degeneration seen with the light microscope. Neurofilamentous synaptic degeneration corresponds to theformation of neurofibrillar rings stained by the Glees method. Electron-dense axons and synapses are correlated with staining of degeneration with Nauta methods. The morphological findings support physiological postulates concerning secure relations between lemniscal terminals and VB neurons, but are at variance with physiological models used to explain presynaptic inhibition.