Neuromuscular organization of the human upper esophageal sphincter

The upper esophageal sphincter (UES) is a key component of swallowing, and yet, its anatomy and function are still incompletely understood. The UES is a functional entity that is composed of three muscles: the cricopharyngeal (CP) muscle, the inferior pharyngeal constrictor (IPC) muscle, and the upper esophageal (UE) muscle. This study compared the anatomy of the three muscles of the UES in nine human autopsy specimens. The variables examined included the pattern of motor end plates (acetylcholinesterase stain), the proportion of fast- and slow-twitch muscle fibers (myofibrillar adenosinetriphosphatase), and the details of their nerve supply (Sihler's stain). The results demonstrated that each variable is different in the three muscles. For example, the IPC muscle is innervated by the pharyngeal plexus, the CP muscle by both the pharyngeal plexus and the recurrent laryngeal nerve (RLN), and the UE muscle by the RLN. The IPC and CP muscles showed distinct motor end plate bands, while the horizontal part of the CP muscle also contained small and randomly scattered end plates. This latter pattern was present throughout the UE muscle. Analysis of the muscle fiber types of the UES revealed a type I (slow) predominance (89%) in the CP and UE muscles and a type II (fast) predominance (62%) in the IPC muscle. However, the IPC muscle is composed of two layers: a fast, thick, outer layer (90% type II) and a slow, thin, inner layer (85% type I). The implications of these findings for the diagnosis and treatment of UES dysfunction will be discussed.