Corticospinal facilitation of hand muscles during voluntary movement in the conscious monkey.

Abstract

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 effect 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 per-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. 6. A muscle receiving post-spike facilitation from a given pyramidal tract neurone continued to display such facilitation during different phases (finger movement cf. steady holding of position) of the precision grip task. 7. The results help further to establish the spike-triggered averaging technique as a powerful tool for the identification, in the awake performing animal, of not only individual cortical output neurones but also the target muscles to which these neurones direct their influence, and confirm the existence of such functional corticomotor influences to intrinsic muscles of the hand.