Retrograde cortical aand axonal changes following lesions of the pyramidal tract

Following lesions of the pyramidal tract in hamsters, retrograde changes were studied in the sensorimotor cortex and in the pyramidal tract axons proximal to the lesion, at survival times ranging from 2 weeks to 14 months. Severe cell shrinkage occurred in layer 5 pyramidal neurons as early as 2 weeks, but there was no cell loss among these neurons even with long survival times. Use of the Fink-Heimer method for degenerating axons revealed that the pyramidal tract proximal to the lesion had undergone a retrograde axon degeneration which, in some respects, resembled anterograde degeneration. The retrograde axon degeneration began at the lesion site and advanced slowly rostralwards with time involving increasingly greater numbers of fibers. However, even at the longest survival times the degeneration fell off markedly at pontine levels. The results indicate that this process represents a true retrograde fiber degeneration (as opposed to an indirect Wallerian degeneration) which appears to reach a point of equilibrium such that a partially shrunken pyramidal cell is maintaining a partially degenerated axon.