Somatotopic organization of the corticospinal tract in cat motor cortex

Abstract

The somatotopic organization of cat motor cortex (covering areas 4γ and 6aβ according to Hassler and Muhs-Clement) was examined by antidromic stimulation of the corticospinal tract from various spinal levels (high cervical, low cervical, thoracic and lumbar) in 15 cats anesthetized with chloralose. The stimulation parameters were adjusted to recruit the entire corticospinal tract and multiple recording electrode sites were used to localize as precisely as possible the cortical zones activated antidromically for each level of spinal stimulation. Both transcortical bipolar and monopolar depth electrodes were used for recording antidromic responses. In this study, only the population of fast conducting fibers is considered. (1) High cervical (C2) stimulation rarely produced activity in the medial anterior sigmoid gyrus and the gyrus proreus (area 6aβ) whereas a large number of activated sites were found in the middle and lateral anterior sigmoid gyrus, the lateral sigmoid gyrus and the lateral posterior sigmoid gyrus (area 4γ). Area 6aβ seems to contribute few, if any, fibers to the corticospinal tract. This observation is compared with the existing anatomical and electrophysiological data. (2) Different portions of area 4γ projected to different spinal levels; the middle and lateral parts of area 4 were associated with the cervical segments whereas the middle part of area 4 appeared to project to thoracic levels. An exploration of the depths of the cruciate sulcus indicated a focus for lumbar motor outflow in the caudal wall of the sulcus. (3) The overlap of the zones influencing the cervical and thoracic levels is interpreted as evidence for a zone of corticospinal tract origin common to several spinal levels. This zone is situated in the middle part of area 4. The functional role of this particular cortical zone is suggested with respect to a proximodistal distribution of the musculature.