Selective facilitation of different hand muscles by single corticospinal neurones in the conscious monkey.

Abstract

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.