Effects of intracortical microstimulation on the activity of limb muscles in alert rabbits

Motor and EMG responses of the limbs to intracortical microstimulation (IM) were studied in 14 rabbits. The motor representations (MR) of contralateral limbs are separately located in the neocortex. The zone of wrist control lies in the center the MR of the forepaw, surrounded by the zones of the forearm and shoulder. Efforts to obtain isolated responses of the digits to IM were not successful. We did not observe a geometrical pattern of organization in the more caudally placed MR of the hindleg, where IM elicited isolated responses in the muscles of the lower leg and the haunch. Minimal thresholds of IM reached 3-10 microA for the MR of the shoulder and forearm, but were as high as 30-200 microA in the MR of the haunch and lower leg. The latent periods of the EMG responses lasted 25-52 msec for the shoulder, 24-73 msec for the forearm, 32-54 msec for the haunch, and 36-71 msec for the lower leg. We conclude that the organization involved in the cortical regulation of limb movement in rabbits is primitive, especially where movement of the hindlimb is involved.