Common synaptic input across motor nuclei supplying intrinsic muscles involved in the precision grip.

To gain insight into the mechanism by which the nervous system orchestrates the activities of multiple muscles during voluntary motor behaviors, we measured the degree of synchrony in the discharge times across pairs of motor units recorded from two intrinsic hand muscles, the adductor pollicis (AdP) and first dorsal interosseous (FDI), in human subjects performing a precision grip. The magnitude of synchrony measured across muscles is thought to reflect the extent of divergent synaptic input delivered in common to the motor nuclei supplying the two muscles. Unlike the pronounced motor-unit synchrony observed across comparable extrinsic hand muscles during the precision grip, little synchrony was detected across these two intrinsic muscles. These results suggest that extrinsic and intrinsic muscles of the hand are controlled by descending pathways with distinctly different patterns of spinal connectivity--more widespread terminations across motor nuclei supplying extrinsic muscles whereas more focal input to individual motor nuclei innervating intrinsic muscles.