Knowledge of Motor Commands and the Recruitment of Human Motoneurons

This study examined the extent to which subjects can direct internal motor commands to specific pools of motoneurons. These commands were subthreshold for muscle activity and were presumably not associated with any change in background afferent activity. Percutaneous stimuli were delivered to the motor cortex using the technique of Merton and Morton (1980) while the discharge of the first-recruited motor units from two intrinsic hand muscles was recorded with selective needle electrodes. In the absence of movement or detectable electromyographic activity subjects learned to focus their internal command upon one of the pair of hand muscles such that a liminal cortical stimulus activated motoneurons in the 'focused' but not the 'unfocused' muscle. This ability was not acquired for pairs of muscles in the forearm. When the cortical stimuli activated only one motor unit in a muscle, the same unit was the first recruited during a voluntary contraction. These results suggest that motor commands can be precisely monitored and fractionated for individual intrinsic muscles of the human hand without recourse to afferent feedback. This ability may be useful for organizing and learning fine manipulative tasks.