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Unique fiber phenotype composition and metabolic properties of the stapedius and tensor tympani muscles in the human middle ear
Umeå University, Faculty of Medicine, Department of Clinical Sciences, Otorhinolaryngology.
Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
Umeå University, Faculty of Medicine, Department of Integrative Medical Biology (IMB).
Umeå University, Faculty of Medicine, Department of Clinical Sciences, Otorhinolaryngology.ORCID iD: 0000-0002-8692-9384
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2023 (English)In: Journal of Anatomy, ISSN 0021-8782, E-ISSN 1469-7580, Vol. 243, no 1, p. 39-50Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
John Wiley & Sons, 2023. Vol. 243, no 1, p. 39-50
Keywords [en]
capillaries, fiber type, middle ear, mitochondria, myosin heavy chain
National Category
Physiology Otorhinolaryngology
Identifiers
URN: urn:nbn:se:umu:diva-206354DOI: 10.1111/joa.13861ISI: 000947787000001PubMedID: 36914412Scopus ID: 2-s2.0-85150637645OAI: oai:DiVA.org:umu-206354DiVA, id: diva2:1753275
Funder
Norrbotten County Council, NLL-968473Available from: 2023-04-26 Created: 2023-04-26 Last updated: 2024-04-17Bibliographically approved

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Rönnblom, AntonThornell, Lars-EricShah, FarhanTano, KristerStål, Per

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