Topographic mapping of motor pools onto skeletal muscles

We have studied the segmental innervation of 2 rat skeletal muscles, the diaphragm and the serratus anterior. Both muscles are thin, flat, and composed of several sectors that form a clear rostrocaudal progression. Each is innervated through a single nerve, which is in turn supplied by motor neurons from several cervical spinal segments. Using intracellular recording, we found that in both cases, the rostrocaudal axis of the motor pool is systematically mapped onto the rostrocaudal axis of the muscle's surface. For the diaphragm, electrophysiological results were confirmed by immunohistochemical identification of denervated fibers following section of single ventral roots and by retrograde labeling of motoneurons following localized application of fluorescent dyes. In addition, an immunohistochemical method was used to study the arrangement of motor axons in the phrenic nerve, which supplies the diaphragm, and to show that contributions from individual ventral roots are compartmentalized within this nerve. We suggest that segmental ordering of axons in the nerve, axonal guidance at branch points in the nerve, and positional labels within the muscle may all contribute to the rostrocaudal mapping of motor pools onto muscle.