Histochemical Properties of Myofiber Types and Lack of Muscle Spindles in the Caudal Pharyngeal Constrictor Muscles of Sheep

Abstract

The purpose of the present study was to examine histochemical properties of myofiber types and presence or absence of muscle spindles in the caudal pharyngeal constrictor (thyropharyngeus and cricopharyngeus) muscles of sheep. Unfixed muscle sections were stained with myosin ATPase (preincubation pH4.3 and 10.5), reduced nicotinamido adenine dinucleotide dehydrogenase, and 3-hydroxybutyrate dehydrogenase (3-HBD). Myofiber types were classified by the differences of the histochemical properties. The thyropharyngeus muscle had 12% type I, 88% type IIA, and no type IIB myofibers. The cricopharyngeus muscle possessed 53% type I, 47% type IIA, and no type IIB myofibers. The myofiber types of the thyropharyngeus and cricopharyngeus muscles were small in diameter. Part of type I myofibers showed a strong 3-HBD activity, whereas all type IIA myofibers showed no 3-HBD activity in the two muscles. No muscle spindle was found in the two muscles. The thyropharyngeus and cricopharyngeus muscles composed of types I and IIA myofibers seem to be adapted to close the pharyngeal cavity repeatedly for the rumination.