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.

Prevalence of hearing loss in patients with late-onset Pompe disease: Audiological and otological consequences.

OBJECTIVE: The metabolic disorder Pompe disease mainly affects the skeletal muscle in adults. The hearing impairment due to stapedius muscle involvement in adult patients is not known. DESIGN: The frequency, severity, and type of hearing impairment was characterized prospectively using pure-tone audiometry, tympanometry, stapedial reflexes, otoacoustic emissions, and brainstem-evoked response audiometry in adult patients on enzyme replacement therapy for genetically confirmed Pompe disease. STUDY SAMPLE: 11 adult patients (median age: 47 years, range: 22-71). RESULTS: Four patients complained about subjective hearing disturbances. Using World Health Organization definition of hearing impairment, abnormal hearing thresholds resulting in mild hearing loss were found in 36% of patients. Compared to normative data (ISO 7029), the hearing threshold was below the median in all but three ears. Stapedial reflexes could not be elicited ipsilateral in 18% and contralateral in 36%. Auditory brainstem responses showed no retrocochlear pathology. CONCLUSIONS: The prevalence of hearing loss slightly exceeded the normative data of the general population. Consistent with previous studies the hearing impairment was usually mild. The percentage of pathological stapedial reflexes exceeded that of matched control subjects and suggests a selective involvement of the stapedius muscle, potentially as a sequela of Pompe disease.

Stapedius muscle fiber characterization during postnatal development in the rat.

The stapedius muscle (SM) is reported to prevent cochlear damage by noise. Functional demands are then the ability of fast contraction with long endurance. At the end of the third postnatal week, the middle ear of the rat is completely pneumatized and according to electrophysiological data, the auditory function starts to match the adult. We investigated the developmental changes in myosin composition of SM fibres using consecutive complete SM cross-sections (taken from rats on post natal day (PND) 7, 14, 16, 21, 28, 42 and 84) which were processed by enzymehistochemistry to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) and by immunohistochemistry using myosin heavy chain (MHC) antibodies (mAb). Fibres were assigned to mATPase type I, IIA, IIB, IIX or 'Miscellaneous' categories. Per mATPase category, the fibres were attributed to groups with specific MHC isoform compositions. Neonatal MHC expression could not be documented with the mAb used. However, embryonal (Emb) MHC was expressed at PND 7, very little at PND 14; at later PND fibres did not show Emb MHC. In general, the mATPase-based classification did not show large alterations after PND 21. Expression of MHC IIB, which was present in almost 50% of the fibres at PND 7 and 14, diminished to 3% at PND 84. A decrease in number of fibres expressing more than one MHC isoform was found. These results show that the SM is a precociously developing muscle compared to limb muscles and even to the diaphragm. Moreover, it is shown that the expression of the adult MHC isoform phenotype coincides with the onset of auditory function in the third postnatal week.

Myosin heavy chain composition of rat middle ear muscles.

OBJECTIVE: To quantitatively analyze myosin heavy chain (MHC) mRNA composition in two rat middle ear muscles (the tensor tympani and stapedius) using competitive polymerase chain reaction (PCR). MATERIAL AND METHODS: An exogenous template, including oligonucleotide sequences specific for the seven rat MHCs (2A, 2B, 2X, 2L/EOM, embryonic, neonatal and beta-cardiac) as well as beta-actin, was constructed and used as the competitor. RESULTS: The tensor tympani and stapedius contained all MHC isoforms except 2L. The tensor tympani contained approximately equal proportions of 2X (40.4% +/- 6.5%) and 2A (34.0% +/- 1.3%) MHCs, with a smaller percentage of 2B (16.6% +/- 1.5%) and neonatal (7.5% +/- 0.6%) MHCs, while beta-cardiac and embryonic MHCs were minimally expressed. The stapedius contained predominantly 2X (58.0% +/- 4.2%) and 2A (32.3% +/- 6.7%) MHCs, with a smaller percentage of 2B (7.4% +/- 0.2%) and beta-cardiac (1.9% +/- 0.1%) MHCs. Neonatal and embryonic MHCs were detected at very low levels. CONCLUSION: These results suggest that two middle ear muscles, which are mainly composed of two fast-twitching myosins (2X and 2A MHCs), contract fast and are fatigue-resistant.

Stapedius muscle fibre composition in the rat.

The stapedius muscle (SM) is supposed to prevent cochlear damage by noise. Consequently functional demands are the ability of fast contraction with long endurance. This implies the presence of a large fraction of myosin type II fibres with an appreciable oxidative capacity. We determined the myosin composition of SM fibres using consecutive complete SM cross-sections (6 week old rats) which were processed by enzyme histochemistry (EHC) to determine acid/alkali lability of myofibrillar adenosine triphosphatase (mATPase) or by immunohistochemistry (IHC) using myosin heavy chain (MyHC) antibodies. Method accuracy was determined in co-processed extensor digitorum longus (EDL). Four hundred SM and 200 EDL fibres were assigned to mATPase type I, IIA, IIB, IIX or 'miscellaneous' ('Misc') categories. Per mATPase category the fibres were attributed to groups with specific MyHC composition. In the EDL, mATPase type I and IIB fibres expressed only MyHC I and IIB respectively, whereas about 10% of the type IIA and 40% of the type IIX fibres expressed more than one MyHC. Thus IHC detects amounts of myosin isoforms which are not detected by EHC. The mATPase IIX category criterion leaves the possibility that this category contains fibres with myosin type IIA and/or IIB in larger amounts. The criteria of the mATPase categories type I, IIA or IIB preclude assignment to these categories of fibres which also contain other myosin isoforms in larger amounts. Such fibres were classified in one of the mATPase 'Misc' categories. Thus in the EDL the capability of the EHC criteria to select 'pure' fibres in terms of myosin differs per mATPase category. None of the SM fibres were assigned to the mATPase type I or IIB categories, about 25% to the type IIA, 60% to type IIX and 15% (including most fibres which expressed MyHC I) to a 'Misc' category. All SM fibres expressed two or more MyHC isoforms, MyHC IIB occurring in all fibres and substantial amounts of MyHC IIA and/or IIX in most. These findings confirm the hypothesis that such fibres have the capacity to contract fast and have the better fatigue resistance.

A histochemical characterization of muscle fiber types in the middle ear muscles of the cat. 1. The stapedius muscle.

The muscle fiber content of the stapedius muscle of the cat was determined histochemically using a combination of oxidative enzymes and glycolytic markers. The major fiber type present was determined to be the fast oxidative glycolytic type (FOG; 77%). Two other types of fiber were found that could not be placed into any of the classical muscle fiber categories. One of these fibers had little staining for actomyosin ATPase (1', 13%) while the other stained densely for this enzyme (2', 10%). These fibers could also be separated using fiber diameter as a criterion. The mean diameters of these different fibers were 22.8 Mm+/-6.3 (FOG fiber type), 14.8 micrometers+/-3.7 (1' fiber) and 14.9 micrometers+/-5.5 (2' fiber). Since the predominant fiber type (FOG) is adapted for fast contraction and fatigue resistance, the stapedius muscle of the cat is probably capable of fast repetitive contractions, a conclusion that fits well with much of the physiological data. Due to the fact that the 2' fibers were always paired with the 1' fibers, it is conceivable that these pairs may represent some specialized sensory structures (i.e. unencapsulated muscle spindles).

Ultrastructure and elemental composition of the stapedius muscle in guinea pig.

Muscules stapedius in guinea pig was investigated with regard to morphology and elemental composition. The muscle fibers are multinucleated, contain a high amount of ribosomes, and a centriole is often found adjacent to the cell nucleus. Intramitochondrial inclusion bodies occur adjacent to cristae mitochondriales. Both type I and type II fibres occur, the latter with all three subtypes. Elemental analysis from the myofibrillar area revealed unexpected low concentration of K and high concentrations of NA, P, and Ca.