Unique fiber phenotype composition and metabolic properties of the stapedius and tensor tympani muscles in the human middle ear.

The middle ear muscles have vital roles, yet their precise function in hearing and protection remains unclear. To better understand the function of these muscles in humans, the morphology, fiber composition, and metabolic properties of nine tensor tympani and eight stapedius muscles were analyzed with immunohistochemical, enzyme-histochemical, biochemical, and morphometric techniques. Human orofacial, jaw, extraocular, and limb muscles were used as references. The immunohistochemical analysis showed that the stapedius and tensor tympani muscles were markedly dominated by fibers expressing fast contracting myosin heavy chain MyHC-2A and MyHC-2X (79 ± 6% vs. 86 ± 9%, respectively, p = 0.04). In fact, the middle ear muscles had one of the highest proportions of MyHC-2 fibers ever reported for human muscles. Interestingly, the biochemical analysis revealed a MyHC isoform of unknown identity in both the stapedius and tensor tympani muscles. Muscle fibers containing two or more MyHC isoforms were relatively frequently observed in both muscles. A proportion of these hybrid fibers expressed a developmental MyHC isoform that is normally absent in adult human limb muscles. The middle ear muscles differed from orofacial, jaw, and limb muscles by having significantly smaller fibers (220 vs. 360 µm2 , respectively) and significantly higher variability in fiber size, capillarization per fiber area, mitochondrial oxidative activity, and density of nerve fascicles. Muscle spindles were observed in the tensor tympani muscle but not in the stapedius muscle. We conclude that the middle ear muscles have a highly specialized muscle morphology, fiber composition, and metabolic properties that generally showed more similarities to orofacial than jaw and limb muscles. Although the muscle fiber characteristics in the tensor tympani and stapedius muscles suggest a capacity for fast, fine-tuned, and sustainable contractions, their difference in proprioceptive control reflects different functions in hearing and protection of the inner ear.

Fine structure of the tendon of the human stapedius muscle: comparison of normals and clinically otosclerotic patients.

The tendon of the human stapedius muscle was studied in normal post mortem material and in clinical otosclerotic patients, using light and electron microscopy. Cross section profiles of collagen fibrils were measured in various regions of the tendon and the amount of elastin was estimated. The normal stapedius tendon consisted of three concentrically arranged portions: A cylindrical central part, a tube-like mid-portion, and a cortical layer. The central part was made up of collagen fibrils with only a few elastic fibers, the mid-portion contained collagen fibrils together with significantly more elastic material, while the cortical layer, again, showed a smaller amount of elastic fibers. Mean diameters of collagen fibrils in the central part of the tendon were 65.12 +/- 11.89 nm, in the intermediate layer 41.00 +/- 9.63 nm, and in the cortical layer 70.28 +/- 19.58 nm. Stapedius tendons from clinically otosclerotic patients, though showing the same construction, were characterized by significantly altered collagen fibrillar diameters (Mann-Whitney U-test). In the central part, mean diameters were reduced to 61.05 +/- 14.70 nm, in the mid-portion increased to 50.90 +/- 10.08 nm, and in the cortical layer reduced to 61.09 +/- 8.49 nm. The changes of collagen cross section profiles estimated for the entire tendon were significant as well: 59.68 +/- 18.74 nm in controls versus 57.82 +/- 12.53 nm in otosclerotic patients. Elastin content in the mid-portion of control stapedius tendons increased with age (13% at 35 years of age to 35% at 70 years of age).(ABSTRACT TRUNCATED AT 250 WORDS)