Muscle fiber-type distribution pattern in the human cricopharyngeus muscle

Our previous studies described that the human cricopharyngeus (CP) is composed of two neuromuscular compartments (NMCs), horizontal and oblique. The present study was designed to explore the differences in muscle fiber-type distribution between the NMCs within the human CP and to examine the oxidative capacity of the muscle fibers. Seven adult human CP muscles obtained from autopsies were stained for myofibrillar ATPase, reduced nicotinamide adenine dinucleotide tetrazolium reductase (NADH-TR), and succinic dehydrogenase (SDH) to analyze enzyme-histochemical fiber-type characteristics. Notable findings obtained from this study are as follows: (1) Different NMCs within the human CP contained different percentages of muscle fiber types. The horizontal CP (CPh) contained more slow-twitch fibers than the oblique CP (CPo). (2) Each of the NMCs was dividable histochemically into two layers or subcompartments: a slow fiber-type inner layer and a relatively fast fiber outer layer. (3) As a whole, type I fibers had higher levels of NADH-TR and SDH than type II fibers. However, in both type I and II muscle fiber types, different patterns of oxidative enzyme activity were seen. Histochemically defined fiber layers of the CP are not seen in other mammals, suggesting that CP function is more specialized in humans.