Experimental degeneration of motor and sensory cortical terminals in the cuneate nucleus of the monkey (Macaca fascicularis).

A total of nine monkeys (Macaca fascicularis) of both sexes was used for the present study. Eight monkeys were used for either motor or sensory cortical lesions, and were allowed to survive for 1--7 days post-operatively. No degenerating nerve terminals were observed 1 day following either motor or sensory cortical lesions. After a motor cortical lesion, most degenerating terminals were seen on the third and only a few on the fifth day. Following a sensory cortical lesion, most degenerating terminals were seen on the fifth and fewer on the third and seventh days. Two types of degenerative changes were observed: granular and electron-dense. Granular change was more evident after the shorter survival periods, while the electron-dense type of degeneration was more commonly seen in animals which were allowed to survive longer after operation. The cortical axon terminals were small and contained either round or flattened synaptic vesicles. Most of those axons containing flattened vesicles originated from the sensory cortex, but a few came from the motor cortex. Vice versa, most of those axons containing round vesicles appeared to arise from the motor cortex and only a few from the sensory cortex. Cortical axon terminals containing round synaptic vesicles formed asymmetrical synapses with small to medium-sized dendrites poor in organelles. In a few instances such cortical terminals were observed to be postsynaptic to other axon terminals containing flattened vesicles. No cortical axon terminal was observed to terminate presynaptically on another axon terminal. Cortical terminals containing flattened vesicles formed symmetrical synapses with dendritic profiles of varying diameters, some of which were the proximal dendrites of Group II neurons. No cortical axon terminals containing flattened vesicles have been observed to form axo-axonal synapses in the cuneate neuropil.