Neuromuscular compartments and fiber-type regionalization in the human inferior pharyngeal constrictor muscle

The inferior pharyngeal constrictor (IPC) muscle functions during swallowin g, respiration, and vocalization. The most-caudal portion of the IPC is bel ieved to be part of the functional upper esophageal sphincter (UES). We hyp othesized that the caudal fibers of the human IPC may have enzyme-histochem ical characteristics similar to those of the cricopharyngeus muscle, a majo r component of the UES. In this study, human IPC muscles obtained from auto psy were studied using Sihler's stain to examine innervation patterns, and using myofibrillar ATPase, NADH tetrazolium reductase (NADH-TR), and succin ic dehydrogenase (SDH) techniques to investigate the distribution and oxida tive capacity of the slow- (type I) and fast- (type II) twitch fibers in th e muscle. The results showed that the human IPC consists of at least two ne uromuscular compartments (NMCs): rostral and caudal. Each of the NMCs was i nnervated by a separate nerve branch derived from the pharyngeal branch of the vagus nerve. The rostral NMC is faster (39% type 1, 61% type II) than t he caudal NMC (70% type I, 30% type II). In addition, two histochemically-d elineated 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). However, the dimension s of both fiber layers and proportions of the muscle fiber types varied wit h the NMCs. Specifically, the ratio of the thickness of the SIL to FOL was <similar to>2:1 for the caudal NMC and similar to1:2 for the rostral NMC, r espectively. 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. In contrast, the type II fibers in the FOL accounted for 46% for the caudal N MC and 67% and 74% for the lower and upper portions of the rostral NMC, res pectively (P < 0.01). The caudal NMC of the IPC shared histochemical charac teristics with the cricopharyngeus muscle, in that it contained predominant ly slow oxidative fibers. Overall, the caudal NMC and the SIL in the IPC ha d high NADH-TR and SDH activities. However, different patterns of oxidative enzyme activity were identified in both type I and type II fibers. This st udy provided histochemical evidence for the concept that the caudal NMC wit hin the IPC contributes to the functional UES. In addition, the two histoch emically-defined fiber layers in the IPC may be a specialized adaptation in humans to enable different upper-airway functions during respiration, swal lowing, and speech. Anat Rec 264:367-377, 2001. (C) 2001 Wiley-Liss, Inc.