NEUROMUSCULAR SPECIALIZATIONS OF THE PHARYNGEAL DILATOR MUSCLES - I -COMPARTMENTS OF THE CANINE GENIOHYOID MUSCLE

Background: Little is known about the structure and innervation of the geniohyoid muscle (GH), which is an important pharyngeal dilator musc le activated in swallowing and respiration. Methods: The neuromuscular specializations of the canine GH were studied in detail by using a co mbination of histological, histochemical, and anatomical techniques. F irst, hematoxylin and eosin staining, Gomori's rapid one-step trichrom e stain, and silver impregnation were used to determine the terminatio ns of muscle fibers and existence of fibrous septa within the muscle ( n=8), Second, myofibrillar ATPase staining was employed to document th e muscle fiber type distribution (n=8), Finally, Sihler's stain (n=10) and wholemount acetylcholinesterase staining (n=8) were used to exami ne the distribution of the nerve supply within the muscle, Results: Th e canine GH is divided into rostral and caudal compartments, which are arranged in series and separated by a transverse fibrous septum, Each compartment receives its own primary nerve branch, which supplies a s eparate motor endplate zone, The rostral compartment is innervated bil aterally, whereas the caudal compartment is innervated ipsilaterally. The rostral compartment was composed of significantly more type I (slo w twitch) muscle fibers (56%) than the caudal compartment (25%). Concl usions: The canine GH is composed of two in-series neuromuscular compa rtments rather than a single muscle as traditionally believed, This an atomical finding suggests that these two compartments may function ind ependently under different physiological conditions. (C) 1998 Wiley-Li ss, Inc.