Patterns of Marchi degeneration in the monkey pyramidal tract following small discrete cortical lesions

THE PYRAMIDAL TRACT was one of the first models used to study cerebral corticifugal pathways. Yet controversy persists regarding the location and degree of specific aggregation (or of overlap and diffusion) of fibers from different regions at successive rostrocaudal l e ~ e l s . ~ f ~ In the years that followed demonstration of the electrical excitability of cortex, a large amount of experimental work was done in the effort to define functional patterns of cortical areas with respect to their cytoarchitectural characteri~tics.l9-~3 Most of the associated Marchi degeneration studies included only a few animals with variably defined lesions. It was our purpose to define the distribution of corticospinal tract degeneration following a variety of systematically placed single, small cortical lesions. Although the Nauta and Gygax34 method provides more detailed information concerning small fiber and terminal degeneration than does the Marchi method, the latter may be more appropriate for correlation with cortical electrical stimulation since in low-threshold areas it is the largest, fastest conducting fibers which provide the major excitation of the anterior horn cells.35-41 In the present work, the location of cortical lesions was based upon the motor maps defined in electrophysiological studies of Woolsey and his colleagues.32~33 Corticospinal tract degeneration was studied at several levels of the neuroaxis in order to define the somatotopic distribution and overlap of projection fibers. In addition, we were interested in a postulated somatotopic medullary decussation assumed to explain in man the syndrome of “cruciate” hemiplegia (paralysis of one upper limb and of the opposite lower extremity).