Cortical post-spike facilitations in elbow muscles during isometric contraction.

Averaged responses of elbow muscles to action potentials of 37 single motor cortex cells were analyzed in two Macaca fascicularis monkeys during performance of a submaximal but high level isometric contraction. Post-spike facilitations (PSFs) of the rectified electromyographic activity (EMG) were frequently (72%) observed. They consisted of PSFs largely distributed in agonistic (biceps brachii and brachioradialis) and antagonistic (triceps brachii) muscles. Some post-spike inhibitions (7%) were also noted. As compared with distal muscles PSFs, most of the proximal PSFs were of lower amplitude, probably because of the higher level of the contractions. These PSFs were also of longer latency. The data indicate functional connectivity between motor cortical cells and proximal limb muscles. Cortical cells with oligosynaptic and even, for a few of them, with monosynaptic connections to the alpha-motoneurones may contribute to recruitment of motor units (MUs) involved in proximal limb muscle contraction.