CORTICOMOTONEURONAL CONTRIBUTION TO THE FRACTIONATION OF MUSCLE-ACTIVITY DURING PRECISION GRIP IN THE MONKEY

1. During independent finger movements, the intrinsic muscles of the h and show a fractionated pattern of activity in which the timing and am plitude of electromyographic (EMG) activity varies considerably from o ne muscle to another. It has been suggested that, in the macaque monke y, corticomotoneuronal (CM) cells that produce postspike facilitation (PSF) of EMG in these muscles contribute to this fractionation. To tes t this hypothesis, we have investigated the relationship between the p attern of PSF exerted by a CM cell and the pattern of activity shown b y the cell and by its target muscles. 2. The activity of 15 identified CM cells was recorded from two monkeys that performed a precision gri p task. Spike-triggered averaging of rectified EMG during the hold per iod of this task showed that each cell produced PSF in at least two in trinsic hand muscles. 3. Segments of data were selected from the initi al movement period of the task in which the EMG activity in one target muscle was substantially greater than that of the other, and the mean firing rate of each CM cell was determined for these periods. 4. CM c ells showed bursts of activity in the movement period. Most of them (1 3/15) had a significantly (P < 0.001) higher firing rate when one of i ts target muscles was more active than the other. For nine of these ce lls (identified as set A), this muscle was the one receiving the large r PSF. In four cases (set B), the reverse was true. Two cells (set C), which produced PSF of equal size in their target muscles, showed no c hange in firing rate across the periods of fractionated EMG activity. 5. All set A and set B cells fired at significantly (P < 0.001) higher rates during the movement period, in association with fractionation o f EMG activity, than in the hold period, in which a cocontracted patte rn of muscle activity was observed. 6. There were pronounced differenc es in the strength of PSF exerted by the CM cells on their target musc les during the fractionation periods. One CM cell exerted PSF of a mus cle during one period of fractionation, but postspike suppression of t he same muscle during the other period. 7. It is suggested that change s in the firing rate of a CM cell and in the degree of facilitation it exerts could both contribute to the fractionation of activity in its target muscles. Cells of set A appear to be specifically recruited in a manner that directly reflects the pattern of facilitation they exert on the sampled target muscles. These results may explain why the CM s ystem is so important for the performance of relatively independent fi nger movements.