Single and multiple-unit analysis of cortical stage of pyramidal tract activation.

THE EFFECT of exciting motor cortical areas is usually studied by recording muscle contractions or observing limb movements. Such studies yield little information about the central excitatory events which ultimately funnel into the final common motor pathway. Adrian and Moruzzi (1) were the first to recognize the importance of recording activity in the pyramidal tract. Their results were significantly supplemented by Lloyd’s (21) analysis of the effects of pyramidal volleys on spinal cord neurons. The present studies stemmed from a project in which cortically induced pyramidal discharges were to be used to assay cortical inhibitory mechanisms. As an initial step (31), we recorded from the region of pyramidal decussation and confirmed that single units isolated at this site have many of the properties described by Adrian and Moruzzi. Surprisingly, however, essentially different responses were recorded from the bulbar pyramid or the lateral column of the cervical spinal cord (31, 32, 33)~-i.e., above and below pyramidal decussation. The data indicate that records from the “region of pyramidal decussation” are contaminated with, and often dominated by, activity in extrapyramidal neurons. This report presents an analysis of cortically evoked pyramidal tract discharges recorded from sites where such distortion is minimized or absent. The analysis and terminology reported here and previously (31, 32, 33) have recently been confirmed by Wall et al. (37) and by Bertrand (4).