Fine structural aspects of the synaptic organization of spinal trigeminal nucleus (pars interpolaris) of the cat

Abstract

Pars interpolaris of the spinal trigeminal nuclear complex has been studied electron microscopically in normal adult cats and after total ipsilateral retrogasserian rhizotomy. Small cell body profiles are numerous and have few axo-somatic synapses. Larger cell body profiles are few in number but have several axo-somatic synapses. Axo-dendritic synaptic contacts are numerous and axo-axonic serial synapses are seen at all levels. Presynaptic profiles containing predominantly round vesicles are usually characterized by an asymmetrical postynaptic specialization and those with many flattened vesicles usually have a symmetrical specialization. A small proportion of profiles with flattened vesicles are associated with an asymmetrical specialization. Axo-axonic synapses have flattened vesicle profiles presynaptic to round vesicle ones. Degeneration of axons and presynaptic processes are seen at 30 h survival and become rare after 7 days. Most degenerated processes are electron-dense and associated with an asymmetrical specialization. In serial synapses the round vesicle profile degenerates. After 3 days many degenerated endings are replaced by glial and other processes which are applied to the denervated postsynaptic specialization. After 7 days survival some unaltered flattened vesicle profiles appear to migrate into the denervated postsynaptic sites. Later survivals show such endings applied to both symmetrical and asymmetrical specializations on the same dendritic profile. Other origins for unaltered presynaptic processes are considered. A correlation between denervated postsynaptic sites and neuronal hyperactivity is discussed, as is the apparent synaptic reorganization or re-innervation of these denervated sites.