Neuronal activity in cortical efferent zones projecting to wrist extensors during voluntary wrist extension in the monkey.

Three monkeys were trained to make rapid wrist extension on presentation of a cue light. After training the animal sufficiently, unit activity was recorded from the cortical efferent zones corresponding to the wrist extensors. Most neurons in the wrist extensor zone were increased in their discharge rate during the wrist extension. Their activation pattern, especially the onset time, varied among different neurons. The earliest neuron was activated 65 msec before the EMG onset of the wrist extensors, while the latest did 30 msec after the EMG onset. About 80% of them activated before the EMG onset. In contrast, variation of the discharge onset of each neuron was rather small and 5-15 msec in quartile deviation. These findings may suggest that each neuron in the cortical efferent zone functions in different timing in the voluntary phasic contraction of the target muscle.