Intrinsic Properties of the Adult Human Mylohyoid Muscle: Neural Organization, Fiber-Type Distribution, and Myosin Heavy Chain Expression

Abstract

The mylohyoid (MH) muscle plays a critical role in chewing, swallowing, respiration, and phonation. The present study was designed to test the hypothesis that the functional properties of the MH are reflected by its intrinsic specializations, including the neural organization, fiber-type distribution, and myosin heavy chain (MHC) expression. Adult human MH muscles were investigated to determine the nerve supply pattern using Sihler's stain, banding pattern and types of motor endplates using acetylcholinesterase (AChE) staining and silver impregnation, and muscle fiber type and MHC composition using immunocytochemical and immunoblotting techniques. The adult human MH was found to have the following neuromuscular specializations. First, the muscle was innervated by several branches of the MH nerve derived from the mandibular division of the trigeminal nerve. Each of the nerve branches supplied a distinct region of the muscle, forming a segmental innervation pattern. Second, the MH had a single motor endplate band which was located in the middle of the muscle length. Both en plaque and en grappe types of motor endplates were identified on the MH muscle fibers. Finally, the adult human MH fibers expressed unusual MHC isoforms (i.e., slow-tonic, alpha-cardiac, embryonic, and neonatal) which coexisted with the major MHC isoforms (i.e., slow type I, fast type IIa, and fast type IIx), thus forming various major/unusual (or m/u) MHC hybrid fiber types. The m/u hybrid fibers (84% of the total fiber population) were the predominant fiber types in the adult MH muscle. Determination of the neuromuscular specializations of the MH is helpful for better understanding of the muscle functions and for development of strategies to treat MH-related upper airway disorders.

Newly Revealed Cricothyropharyngeus Muscle in the Human Laryngopharynx

“…Details regarding staining procedures have been given in our previous publications (Sanders et al, 1994;Mu and Sanders, 1998a,b, 2000, 2001, 2007, 2008Ren and Mu, 2005). In brief, the specimens were (1) fixed for 4 weeks in 10% unneutralized formalin; (2) macerated and depigmented for 3 weeks in 3% potassium hydroxide (KOH) solution with several changes; (3) decalcified for 2 weeks in Sihler's solution I (one part glacial acetic acid, one part glycerin, and six parts 1% aqueous chloral hydrate) with several changes; (4) stained for 4 weeks in Sihler's solution II (one part stock Ehrlich's hematoxylin, one part glycerin, and six parts 1% aqueous chloral hydrate); (5) destained for 4 hr in Sihler's solution I; (6) immersed for 1 hr in 0.05% lithium carbonate solution to darken the nerves; (7) cleared for 3 days in 50% glycerin; and (8) preserved for 4 to 8 weeks before microdissection in 100% glycerin with a few thymol crystals for transparency.…”

“…The three major MHCs in the muscle fibers were detected using ABC method as described (Mu et al, , 2007aWang et al, 2004;Ren and Mu, 2005;Mu and Sanders, 2007). Briefly, the serial cross-sections were (1) fixed in 4% paraformaldehyde for 10 min; (2) blocked in 2% bovine serum albumin (BSA) with 0.1% Triton X-100 for 20 min; (3) incubated with primary mAbs for 1 hr at room temperature (mAb NOQ7-5-4D) or overnight at 48C (mAbs MY-32 and SC-71); (4) incubated with a Vectastain anti-mouse IgG (ATCC, Rockville, MD) for 1 hr and reacted in ABC reagent for 1 hr; (5) reacted for 10 min with a solution containing 3,3 0 -diaminobenzidine (DAB) as chromogen to localize peroxidase for primary antibodies according to DAB substrate kit (SK-4100; Vector Laboratories, Burlingame, CA); and (6) dehydrated in graded concentrations of ethanol, cleared in xylene, and mounted with Permount.…”

“…The unusual MHC isoforms in the muscle fibers were detected using indirect immunofluorescent staining as described (Mu et al, , 2007aWang et al, 2004;Ren and Mu, 2005;Mu and Sanders, 2007). Briefly, the sections adjacent to those stained with the ABC method were (1) fixed in 4% fresh paraformaldehyde for 10 min; (2) blocked in 2% BSA with 0.1% Triton X-100 for 20 min; (3) incubated with primary mAbs at 48C for either 12 hr (mAbs N2.261 and F1.652) or 24 hr (mAbs ALD-58 and BA-G5); (4) blocked again with 1% BSA for 5 min; (5) incubated in a fluorescein isothiocyanate-labeled goat anti-mouse IgG (FITC-GAM; 1:50 dilution; Sigma, St Louis, MO) for 1 hr; and (6) mounted with Vectashield mounting medium (Vector Laboratories) and kept in the dark at 48C.…”

“…Unfortunately, traditional anatomical methods usually fail to precisely determine the intramuscular neural patterns. To this end, Sihler's stain has been proved to be a reliable technique that clears muscle tissue, while counterstaining the nerves which can be traced from main trunk to the terminals (Sanders et al, 1994;Mu and Sanders, 1998a,b, 2000, 2001, 2007, 2008Ren and Mu, 2005).…”

Unusual Communication Between the Mylohyoid Nerve and Lingual Nerve in a North Indian Male Cadaver: Its Clinical Significance

“…The mylohyoid muscle plays an important role in chewing, swallowing, respiration and phonation [1]. It has been analyzed that the MHN may carry a sensory innervation of the chin and the lower incisor teeth through an alternate pathway.…”