Is there morphological separation between the spinal cord motor nuclei which innervate the heads of a multiheaded muscle?

Horseradish peroxidase was injected into the individual heads of the pectoralis muscles of the dog or applied to the nerve which supplies each of these heads. The location and numbers of labeled motoneurons in the spinal cord were studied using light microscopy. There was longitudinal overlap of the pectoral nuclei, but no separation in their mediolateral or dorsoventral positions. The cutaneous trunci muscle motor nucleus is distinctly separate from the motor nuclei of the pectoral muscles, even though they share a common nerve supply. The methods of horseradish peroxidase application to the cut nerve or injection into the muscle are compared.