The Supplementary Motor Area — Control System for Posture?

Abstract

The supplementary motor area (SMA) is interconnected with the precentral motor cortex and receives its main afferents from the somatosensory areas I, II and 5; it is therefore most likely concerned with somatosensori-motor integration. In primates, there is no anatomical evidence of a thalamic relay to the SMA or of a direct projection to the spinal cord. Stimulation of corticospinal fibres evoked antidromic discharges in the precentral tail and hindleg areas (as defined by intracortical stimulation), but not in the SMA. Indirect influences on the motor apparatus may be exerted through the red nucleus, the pontine nuclei, and bilaterally via the precentral motor cortices and the striatum. Experiments in monkeys in which conventional serial surface (−) stimulation was used were complemented by experiments using also single pulse (+) and intracortical train (−) stimulation. Prolonged repetitive stimulation of the SMA at high intensities (1. 5–2. 5 ma) readily evoked movements of the contralateral upper limb. However, EMG bursts followed each pulse and occurred with the same latencies as those obtained with weak stimuli from the pre-central forelimb area.