Observations on the process of degeneration of the afferent connections to the sensori-motor cortex of the monkey.

The ultrastructural features of degeneration of the afferent connections to the sensorimotor cortex of the primate have been studied with the electron microscope. Thalamo-cortical terminals show cytoplasmic darkening and swelling of vesicles, and neurofilaments and glycogen are often prominent in the larger terminals, although true neurofilamentous hypertrophy is rare. The terminals of the commissural and association projections similarly show darkening and some swelling of vesicles but are usually smaller and neurofilaments and glycogen are rarely seen. Ethanolic-phosphotungstic acid specifically stains degenerating axons and axon terminals. Degeneration occurs more rapidly in the terminals of the association projections than in those of the longer commissural projection although terminals at different stages of degeneration are present simultaneously in both instances. Micropinocytosis of degenerating terminals has been found and both glial and neuronal profiles are involved in this. Exposed post-synaptic thickenings have been seen and these induce subsurface cisternae in the neuronal structures to which they become apposed, including axon initial segments and unmyelinated axons.Damage to a part of the central nervous system thus causes consequential changes in other regions, and these changes are relevant to the interpretation of experimental studies and to the possibility of reorganization of connections following injury.