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1. 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. 2. 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. 3. Spikes recorded from these cells were grouped according to the occurrence of a previous spike in the periods 0‐10 ms, 10‐20 ms, and so on up to 60‐70 ms before the trigger spike. The post‐spike period in which no additional spikes were allowed to fall was kept at either 12.5 or 25 ms. 4. Spikes selected in this way produced a transient facilitation of their target muscle EMG activity. The peak amplitude of this facilitation was normalized as a percentage of modulation of the background EMG level. The background level was determined from a period in the average to which the cell could not have contributed, because of the post‐trigger spike interval. We verified that the percentage of modulation was not influenced by the overall level of EMG activity, since, for a given interval, the modulation was the same whether the relevant spikes were selected during periods of high‐ or low‐level EMG activity. 5. The relative amplitude of the post‐spike facilitation (i.e. the percentage of modulation) showed marked variation with interspike interval. A full analysis was completed for seventeen neurones. Spikes with the shortest intervals (less than 10 ms) usually produced the strongest effects, and evidence is presented that this was due to temporal summation and facilitation at the corticomotoneuronal synapse. Mid‐range intervals (10‐40 ms) were generally far less effective, although they constituted the highest proportion of cell activity. 6. A striking finding was the strong facilitation generated by the longer interspike intervals (40‐70 ms). Although the absolute size of this post‐spike effect was much smaller than that of the shortest intervals, its percentage of modulation was similar. It is suggested that this enhanced facilitation results from a combination of lower frequency discharge among the active motoneurones, and increased synchrony in the corticomotoneuronal input to them. 7. All of the above results were confirmed by examining cross‐correlations between single corticomotoneuronal cells and single motor units in their target muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

1. The method of spike‐triggered averaging has been used to detect a direct influence of pyramidal tract neurones on the activity of hand and forearm muscles in conscious monkeys trained to perform repetitive movements of the hand and fingers. Gross electromyograms (e.m.g.s) from individual muscles were rectified and synchronously averaged with respect to the discharge of single, antidromically identified pyramidal tract cells in the 'hand' area of the pre‐central gyrus. 2. The presence in an average of a post‐spike facilitation which could be revealed reproducibly from successive epochs of recording and was clearly larger than the biggest fluctuations seen in pseudo‐randomly triggered averages of the same e.m.g. data, was taken to indicate a direct cortico‐motoneuronal excitatory influence. 3. 55% of cortical neurones analysed showed post‐spike facilitation in one or more recorded muscle and 7% showed post‐spike suppression. In terms of the total number of muscle‐neurone combinations analysed, the proportions showing post‐spike effects were 18 and 1% respectively. These figures have been influenced by the pre‐selection of neurones for analysis according to restrictive criteria. The neurones selected (a) were recorded at cortical loci where weak intracortical microstimulation could evoke finger movements, (b) could be activated antidromically at short latency by medullary pyramidal tract stimulation, (c) showed natural discharge activity which was clearly modulated in relation to voluntary finger movements, and (d) were located in the anterior bank of the central sulcus. The results provide some evidence to vindicate these criteria. 4. The strongest post‐spike facilitation observed had a peak which was 42% higher than the average pre‐spike level of e.m.g. activity, but most were within the range 5‐20%. Facilitation peaks below about 3% could not have been resolved from the 'noise' in the averages. The mean latency from cell discharge in the cortex to the start of the post‐spike facilitation was 11.2 ms (range 7.4‐17.2) for intrinsic hand muscles and 9.8 ms (range 4.1‐15.0) for forearm muscles. These latencies were compared with the latencies of responses to intracortical microstimulation and to stimulation of the medullary pyramidal tract. 5. Evidence was obtained suggesting that the latency for cortico‐motoneuronal activation of an individual motor unit was commonly subject to considerable variability and that different motor units of a muscle could be facilitated by the one cortical neurone at different latencies. These factors are thought to contribute to an elongation of the time course of post‐spike facilitation.(ABSTRACT TRUNCATED AT 400 WORDS)