Further investigations of the efferent linkage of the supplementary motor area (SMA) with the spinal cord in the monkey.

Intracortical microstimulation of the supplementary motor area (SMA) was studied in awake monkeys. With short trains of micropulses, contralateral muscle twitches mainly in shoulder and proximal arm muscles were elicited. There was an indication of a rostro-caudal representation of distal to proximal forelimb, trunk, and proximal to distal hindlimb muscles. However, an intermingling of efferent zones was much more prominent as compared to the precentral motor cortex (MI). All efferent zones to the spinal cord were clustered in the caudal half of the SMA, and we failed to detect face and ocular movements (except at one stimulation site) when microstimulating the rostral portions of the SMA. Single micropulses were also injected in efferent zones of the microexcitable cortex in order to investigate post-pulse facilitation of sustained EMG activity. For motor cortex (MI) stimulation, post-pulse facilitation was prominent and observed in 14 of 17 tested stimulation sites. The incidence of facilitation in comparable muscles obtained with SMA stimulation in comparable muscles obtained with SMA stimulation was only 20 out of 54 tests. The onset latencies of EMG modulation obtained from the two areas were in the same range but the amount of modulation in the SMA was less conspicuous than in MI. These results indicate that the SMA has oligo- or possibly even monosynaptic connections with motoneurones, but that these connections are less dense than those from MI.