Locations of the motor endplate band and motoneurons innervating the sternomastoid muscle in the rat

Abstract

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.

Fig. 1

A: A wholemount AChE stained rat sternocleidomastoid muscle, showing that this muscle is composed of two bellies, sternomastoid (SM) and cleidomastoid (CM). Note that either SM or CM has a single motor endplate band (arrows) which is located in the middle portion of the muscle. C, clavicle; S, sternum. B: High-power view of the SM in A.

Fig. 2

Schematic drawings illustrating the distribution of the HRP-labeled motoneurons innervating the SM in rats. Left: Graphs showing the locations of the SM motoneurons in the medulla oblongata (MO) and C1-C3. CEN, central; DM, dorsomedial; VL, ventrolateral; VM, ventromedial. The shaded regions (black and grey) indicate the regions that contained labeled SM motoneurons. Specifically, the black and grey regions indicate large and small concentrations of labeled cells, respectively. Dots indicate the occurrence of rare labeled cells. Middle: Drawing illustrating the topographic arrangement of the MO and C1-C3. Right: Bar showing the longitudinal extension of the majority of the SM motoneurons from MO to C2 in the rat.

Fig. 3

Photomicrograph of a transverse section from lower medulla oblongata (MO). Note that the HRP-labeled motoneurons supplying the SM were located in the dorsomedial region of the MO. Arrow indicates the central canal.

Fig 4

Photomicrographs of transverse sections from C1. Note that the SM motoneurons were located in the dorsomedial (A) or dorsomedial and ventrolateral (B) regions in C1. Arrow indicates the central canal.

Fig. 5

Photomicrographs of transverse sections from C2. Note that the SM motoneurons were organized into either a single column (dorsomedial) (A), two columns (dorsomedial and ventrolateral) (B), or three columns (dorsomedial, ventrolateral, and ventromedial) (C). Arrow indicates the central canal.

Fig. 5

Photomicrographs of transverse sections from C2. Note that the SM motoneurons were organized into either a single column (dorsomedial) (A), two columns (dorsomedial and ventrolateral) (B), or three columns (dorsomedial, ventrolateral, and ventromedial) (C). Arrow indicates the central canal.