Corticospinal and corticorubral projections from the supplementary motor area in the monkey.

Neurons that were antidromically invaded from either pyramidal tract (PT) stimulation or red nucleus (RN) stimulation were studied in the anterior portion of the supplementary motor cortex of anaesthetized rhesus monkeys. The conduction velocity of 58 PT neurons ranged from 11 to 63 m/sec while those of the 29 RN neurons ranged from 10 to 38 m/sec. None of the neurons were found to be antidromically invaded from both stimulation sites. Thus, the corticorubral projection from the SMA does not appear to consist primarily of PT axon collaterals. There appeared to be a trend towards spatial segregation of the PT and RN neurons in the monkey cortex, the RN neurons tended to be located nearer the pial surface than the PT neurons, which tend to have somata nearer the subcortical white matter.