The pyramidal system of the woodchuck.

The pyramidal system of the woodchuck was examined anatomically and electrophysiologically. The pyramidal tract was found to originate entirely within the anterior half of the cerebral hemispheres and to follow a course typical of most rodents, decussating almost entirely and descending the length of the spinal cord in the ventral part of the dorsal funiculi. It decreased in size uniformly with distance along the spinal cord; most of its fibers terminated in the medial half of the dorsal horn, though they scattered widely and even appeared to terminate on motoneurons. Each tract contained 140,000 +/- 20,000 fibers, with 60-80% of the fibers being about 1 micrometer and 90% being less than 3 micrometer in diameter. Stimulation of the medullary pyramid evoked a minute antidromic potential (alpha wave) which was generally obscured by a large surface-positive response that reversed polarity deep in the cortex and that appeared to be synaptic in origin (r wave). It is proposed that the r wave results from intracortical pyramidal cell collateral activity. Though largest in the apparent region of origin of the pyramidal tract, the r wave also showed local maxima in the forepaw and hindpaw foci of somatosensory cortex. The somatosensory cortex was organized in a manner similar to other rodents, but an "association" area lacking topographical organization was found near the anterior pole of the hemispheres. In an allometric sense, the woodchuck was found to be a "normal" rodent and a "normal" mammal.