Background:
Little is known about the structure and innervation of the geniohyoid muscle (gh), which is an important pharyngeal dilator muscle activated in swallowing and respiration.
Methods:
The neuromuscular specializations of the canine gh were studied in detail by using a combination of histological, histochemical, and anatomical techniques. first, hematoxylin and eosin staining, gomori's rapid one-step trichrome stain, and silver impregnation were used to determine the terminations of muscle fibers and existence of fibrous septa within the muscle (n = 8). second, myofibrillar atpase staining was employed to document the muscle fiber type distribution (n = 8). finally, sihler's stain (n = 10) and wholemount acetylcholinesterase staining (n = 8) were used to examine the distribution of the nerve supply within the muscle.
Results:
The 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 separate motor endplate zone. the rostral compartment is innervated bilaterally, whereas the caudal compartment is innervated ipsilaterally. the rostral compartment was composed of significantly more type i (slow twitch) muscle fibers (56%) than the caudal compartment (25%).
Conclusions:
The canine gh is composed of two in-series neuromuscular compartments rather than a single muscle as traditionally believed. this anatomical finding suggests that these two compartments may function independently under different physiological conditions.