The influence of changes in discharge frequency of corticospinal neurones on hand muscles in the monkey.

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Abstract
The possibility that the discharge pattern of monkey corticomotoneuronal cells influences the degree to which they facilitate their target hand muscles was tested by compiling spike-triggered averages of EMG recorded from these muscles. Records were made from area 4 corticomotoneuronal cells in three conscious macaque monkeys while they performed a precision grip between index finger and thumb. Simultaneous EMG recordings were made from up to six different intrinsic hand muscles. Twenty cells which produced clear post-spike facilitation of one or more muscles were selected for further analysis. The results suggest that, in addition to overall discharge rate, the firing pattern of corticomotoneuronal cells is of considerable significance for their target motoneurones. Short interval spikes (2-20 ms) generally occurred before or during the burst of EMG activity at the onset of the precision grip movement, while longer intervals (20-70 ms) occurred during steady, low-level EMG activity associated with maintenance of a steady grip. Because these associations are consonant with the post-spike influence of these interspike intervals, we suggest a causal relationship between discharge interval and EMG level. Long-interval spikes may exert a particularly strong influence on the level of muscular activity during the maintenance of gentle grip forces by the hand.