Locations of the Motor Endplate Band and Motoneurons Innervating the Sternomastoid Muscle in the Rat

Sternocleidomastoid (SCM) is a long muscle with two bellies, sternomastoid (SM) and cleidomastoid (CM) in the lateral side of the neck. It has been widely used as muscle and myocutaneous flap for reconstruction of oral cavity and facial defects and as a candidate for reinnervation studies. Therefore, exact neuroanatomy of the SCM is critical for guiding reinnervation procedures. In this study, SM in rats were investigated to document banding pattern of motor endplates (MEPs) using whole-mount acetylcholinesterase (AChE) staining and to determine locations of the motoneurons innervating the muscle using retrograde horseradish peroxidase (HRP) tracing technique. The results showed that the MEPs in the SM and CM were organized into a single band which was located in the middle portion of the muscle. After HRP injections into the MEP band of the SM, ipsilaterally labeled motoneurons were identified in the caudal medulla oblongata (MO), C1, and C2. The SM motoneurons were found to form a single column in lower MO and dorsomedial (DM) nucleus in C1. In contrast, the labeled SM motoneurons in C2 formed either one (DM nucleus), two [DM and ventrolateral (VL) nuclei], or three [DM, VL, and ventromedial (VM)] columns. These findings are important not only for understanding the neural control of the muscle but also for evaluating the success rate of a given reinnervation procedure when the SM is chosen as a target muscle.