Input and output organization of the supplementary motor area

Recent work on the supplementary motor area (SMA) in Macaca fascicularis led to the conclusion that this area is involved mainly in the preparation of self-paced movements. Results are presented indicating that the posterior portion of the SMA is also directly involved in movement execution and that it receives various sensory inputs. The main results are as follows: (1) The SMA has direct access to the spinal cord by way of corticospinal neurons, but the density of these neurons is lower than in the primary motor cortex (MI). (2) Intracortical microstimulation effects can be elicited in the SMA. Facilitatory effects on ongoing EMG activity can even be produced by single micropulses (8/s). The shortest latencies are compatible with an oligosynaptic or monosynaptic transmission. (3) SMA neurons respond (as do MI neurons) to external perturbations. (4) Anatomical tracing studies revealed that basal ganglia outflow to the SMA via the thalamus is important; our results suggest that dentate outflow contributes as well. (5) Many cells of the SMA may covary with conditioned movements in the same way as MI neurons do. It is argued that it is difficult to compare the lead-time of MI and SMA neurons since 'early' discharges may be coupled with anticipatory postural events.