FUNCTIONAL LINKAGES BETWEEN MOTOR CORTICAL-CELLS AND ELBOW FLEXOR MUSCLES - EVIDENCE FOR AND CHARACTERISTICS OF POSTSPIKE FACILITATION

1. Two monkeys (Macaca fascicularis) making high-level but submaximal isometric flexions of the elbow were investigated for the output effect of motor cortical cells on the electromyogram (EMG) activity of two main elbow flexors using the method of spike-triggered averaging of rectified EMGs (STAs). 2. Monkeys were trained to perform individual isometric contractions for >2 s, and two series of greater than or equal to 20 contractions, the second series being at a greater force. EMG electrodes pairs were implanted in the biceps brachii and brachioradialis. A total of 257 cortical cells were found that discharged with the active and passive movements of the elbow. We examined the EMG postspike facilitations (PSFs) produced in either one or the two flexors for only those cells that discharged during the isometric contraction, and provoked PSFs in the two series of contractions. 3. The main characteristics of the EMG isometric contractions in the agonists were analyzed. Spectral analysis showed that the increases in the EMG median frequency with force stabilized at the force levels performed by monkeys. Cross correlation methods showed no cross talk between agonists. 4. The 26 selected cortical cells had a regular discharge frequency. Ten cells did not change frequency with a 22-30% force increase, 14 cells discharged at a higher frequency, and 2 cells discharged at a lower frequency. For single-cell frequencies of 5-65 Hz, interspike intervals <10 ms were rare: the median and modal intervals were 20-30 ms. 5. The significance of PSFs with respect to the EMG background noise was estimated statistically. STAs from successive epochs under identical load conditions, and STAs performed at a distance from the trigger, showed that PSFs were authentic postspike effects and not sudden EMG changes synchronized by chance with the triggering cell. The features distinguishing PSF from secondary postspike EMG changes or coactivation and task-related effects were studied in simultaneous STAs of flexors and autocorrelogram of cortical spikes. 6. The magnitude of the PSF was expressed as the percent peak amplitude above the mean EMG baseline. The mean percent amplitude of the 90 PSFs produced in both muscles and series was 4.0 +/- 2.4% (mean +/- SD). There was no difference in the average amplitude of PSFs in the two flexors, although the baseline voltages in the biceps brachii were higher. Neither was there any significant change with force while the baseline level increased by 29 +/- 10%, indicating that the absolute PSF amplitude increased in the same proportion as baseline. 7. The mean onset latency was 9.4 +/- 4.2 ms for all PSFs, but it was 6.9 +/- 1.8 ms for two thirds of the PSFs. The differences in flexors were not significant. There was a weak correlation between the PSF amplitudes and latencies. 8. Nineteen cells cofacilitated the two flexors. Fifteen cells facilitated the two flexors at the same latency within a 4-ms range at each force level, and 11 did it for the two levels of force. 9. We conclude that motor cortical cells exert an excitatory effect on elbow flexor muscles via monosynaptic or disynaptic linkages.