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). The inferior pharyngeal constrictor (IPC) muscle functions during swallowing, respiration, and vocalization.
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