Quantitative evidence for multiple widespread representations of individual muscles in the cat motor cortex.

We sought to determine why a given muscle appears represented in widespread loci in the motor cortex (MCx). To this end, we microstimulated every 500 microm along medio-lateral rows and recorded the evoked electromyographic (EMG) responses of up to a dozen forelimb muscles of the cat. A consistent finding in all animals studied was that along a given row, distal muscle responses could be elicited from medially situated cortical loci and conversely, proximal muscles responses from laterally situated cortical loci. In many such cases, the evoked EMG responses were such that the largest responses from a distal muscle were obtained by stimulation at a medially situated point and those of a proximal muscle from a laterally situated point. A Spearman correlation analysis showed that there was no correlation between cortical position and where the peak response of a given muscle occurred. These quantitative results strongly support the view that in the forelimb area of the cat MCx there exists widespread 'colonies' of corticospinal neurons with common spinal cord targets.