1. Post-spike facilitation of e.m.g. activity by monkey motor cortex neurones has been investigated in different hand and forearm muscles. 2. Seventy-eight neurones were recorded concurrently with between five and ten different muscles. Forty-seven neurones were identified as cortico-motor by the presence of post-spike facilitation in the spike-triggered average of at least one of the tested muscles. 3. All forty-seven cortico-motor neurones showed clear increases in activity during performance of a precision grip task by the monkey, and all of them were co-activated with the sampled muscles. 4. To assess the divergence of facilitation from a single cortico-motor neurone to different muscles, spike-triggered averages were constructed with all of the concurrently recorded muscles. The number of muscles in the sample, and the number of muscles showing post-spike facilitation, were corrected by excluding any post-spike facilitation which could have arisen by cross-talk between the different pairs of e.m.g. electrodes. 5. Most cortico-motor neurones produced post-spike facilitation in a restricted number of tested muscles. The mean number of post-spike facilitation-bearing muscles per cortico-motor cell rose from 1.4 +/- 0.5 (S.D.) when five muscles were sampled to 2.0 +/- 1.5 when ten were sampled. On average, each cortico-motor neurone produced post-spike facilitation in 27% of the tested muscles. Only three of forty-seven cortico-motor neurones gave post-spike facilitation in half or more of the tested muscles. 6. The distribution pattern of post-spike facilitation among the muscles sampled with a given cortico-motor neurone was not altered when the spike-triggered averages were constructed from cortico-motor cell and e.m.g. activity recorded during two different phases of the precision grip task, or during performance of a quite different, power grip, task. 7. Cortico-motor cells which produced post-spike facilitation in two or more different muscles often did so in muscles with synergistic functions. 8. It is suggested that cortico-motor neurones may contribute to relatively independent finger movements by virtue of their selective facilitation of hand muscles leading to a fractionated pattern of muscle activity.
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