However, different patterns of oxidative enzyme activity were identified in both type I and type II fibers. Overall, the caudal NMC and the SIL in the IPC had high NADH-TR and SDH activities. The caudal NMC of the IPC shared histochemical characteristics with the cricopharyngeus muscle, in that it contained predominantly slow oxidative fibers. In contrast, the type II fibers in the FOL accounted for 46% for the caudal NMC and 67% and 74% for the lower and upper portions of the rostral NMC, respectively (P < 0.01). In the SIL the type I fibers accounted for 84% for the caudal NMC and 69% and 44% for the lower and upper portions of the rostral NMC. Specifically, the ratio of the thickness of the SIL to FOL was approximately 2:1 for the caudal NMC and approximately 1:2 for the rostral NMC, respectively. However, the dimensions of both fiber layers and proportions of the muscle fiber types varied with the NMCs. In addition, two histochemically-delineated fiber layers were identified in the human IPC: a slow inner layer (SIL) with predominantly type I fibers (66%), and a fast outer layer (FOL) with predominantly type II fibers (62%) (P < 0.01). The rostral NMC is faster (39% type I, 61% type II) than the caudal NMC (70% type I, 30% type II). Each of the NMCs was innervated by a separate nerve branch derived from the pharyngeal branch of the vagus nerve. The results showed that the human IPC consists of at least two neuromuscular compartments (NMCs): rostral and caudal. In this study, human IPC muscles obtained from autopsy were studied using Sihler's stain to examine innervation patterns, and using myofibrillar ATPase, NADH tetrazolium reductase (NADH-TR), and succinic dehydrogenase (SDH) techniques to investigate the distribution and oxidative capacity of the slow- (type I) and fast- (type II) twitch fibers in the muscle. We hypothesized that the caudal fibers of the human IPC may have enzyme-histochemical characteristics similar to those of the cricopharyngeus muscle, a major component of the UES. The most-caudal portion of the IPC is believed to be part of the functional upper esophageal sphincter (UES). Abstract: The inferior pharyngeal constrictor (IPC) muscle functions during swallowing, respiration, and vocalization.

Inferior pharyngeal constrictor