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HYPOTHESIS: The aim of this study was to investigate the anatomy and pathology of the anterior epitympanum and of the tensor fold. BACKGROUND: Early studies reported data that are primarily still relevant, but contemporary reports present conflicting data, including several erroneous concepts. METHODS: Fifty-one temporal bones were dissected, and the anatomic details were photographed in 42 normal and nine infected bones. Histology was documented from seven serially sectioned bones, five normal and two infected. RESULTS: The tensor fold formed the frontal wall of the anterior epitympanum between tensor tendon and attic bony wall, the anterior insertion consisting of composite connective and fatty tissue with some bone trabeculae. The transverse crest was posterior to it and extended from the anterior tympanic spine to the facial canal. The tensor fold angle in 78% of the specimens was between 45 degrees and 80 degrees, seldom horizontal, and the size of the supratubal recess (or space) increased as the fold angle increased. In 14 ears (27%) the fold had a membrane defect connecting the two spaces. Blockade of the tympanic isthmus caused inflammatory obliteration of the anterior epitympanum when the tensor fold was intact. CONCLUSIONS: The anterior epitympanum, a closed space around the anterior half of the head of the malleus, is normally closed by an intact tensor fold, but about one fourth of ears may show membrane defects. Aeration occurs via the tympanic isthmus through a constriction formed by the head of the malleus with the medial attic wall. In surgery for ears with epitympanal pathology, incus transposition should be combined with resection of the thin portion of the tensor fold for safeguarding permanent attic aeration.

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The brain stem position, organization and number of motoneurons innervating the rabbit tensor tympani muscle (TTM) were determined by retrograde axonal transport of cholera toxin B/horseradish peroxidase conjugate (CTB-HRP) and wheat germ agglutinin HRP conjugate (WGA-HRP) tracers. The synaptic input to the TTM motoneurons was examined with WGA-HRP. The results show the motoneurons of the TTM to be localized in a cluster ventro-lateral to the outer margin of the ipsilateral trigeminal motor nucleus (VMN) and dorso-lateral to the superior olive. The number of labeled cells was greater in the combined CTB-HRP/WGA-HRP injected cases. The TTM motoneurons were triangular and elongated in shape and smaller than those of the VMN. An extensive network of dendritic branches was present ventro-laterally in the vicinity of the superior olive. Similar, but less extensive collections of dendritic processes were observed to course dorso-medially, rostrally and caudally. Axons were observed to project first dorsally or laterally, towards the trigeminal motor root, then after a sharp turn coursed ventrally within the trigeminal motor root (VMR). Transneuronal transport of the WGA-HRP was not accomplished in any preparation, suggesting among other things, system or species differences in the effectiveness of the WGA-HRP conjugate as a transynaptic tracer. It is concluded that the TTM acoustic reflex in rabbits and other mammals, its threshold, prolonged contraction capacity, and its influence on middle ear sound transmission may be related to its demonstrated extensive synaptic field in the reflex chain, particularly in the area of the superior olive, while its many other physiological functions may be made possible by the number, location, and multi-dimensional orientation of its motoneurons and dendrites.

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The innervation of the tensor tympani muscle and the stapedius muscle in the rat was studied. This was done by acetylcholinesterase in toto staining of the tympanic bullae and of muscles dissected separately, acetylcholinesterase staining of serial cross-sections of the muscles, silver impregnation of serial sections of complete tympanic bullae, serial semithin sections stained according to Laczko & Levai and electron microscopy of both muscles. The gross innervation of the muscles and the relation to other nerves in the bulla are described. It is shown that both muscles are innervated by very thin nerve fibres which form a well-organised elaborate network in the muscles, with very short branches that connect with motor endplates. Electron microscopically there are indications that the endplates in the stapedius muscle seem to enable faster activation of the muscle fibres than those of tensor tympani muscle. No morphological evidence for any sensory innervation of the muscles could be detected in the muscles themselves, in the connective tissue related to the muscles, or in the contents of the bulla tympanica. It is postulated that the afferent input of the acoustic middle ear muscle reflex is sound alone and that sensory information from the muscles themselves or from other structures in the tympanic bulla do not contribute to the reflex.

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The gross anatomy, microscopical anatomy, morphometry, enzyme histochemistry, immunohistochemistry and electron microscopy of the tensor tympani muscle of the rat was studied. The aim of the study was to create an integrated insight into the morphology of the muscle and to discuss functional implications. The tensor tympani muscle of the rat is an atypical muscle. It is a small muscle composed of very small muscle fibres with a complex microscopical and submicroscopical architecture. Histochemically the muscle consists mainly of fast oxidative glycolytic fibres, but discrepancies are found when anti-heavy chain myosin antibodies are used for fibre typing. Different adult heavy chain myosin isotypes coexist in one single muscle fibre. The electron microscopical study shows that bundles of myofilaments branch and interconnect with other bundles of myofilaments. The findings suggest that the muscle is able to contract fast and is fatigue-resistant. Both features seem to suggest that the muscle has a function in protecting the inner ear against noise damage.

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Measurement of the middle ear muscle reflex is an important tool in audiologic examination; however, the precise function of the tensor tympani and stapedius muscle is not fully understood. The function of the middle ear muscles in speech discrimination and noise-induced hearing loss is the main object of our study. Morphologic and enzyme-histochemical properties of the middle ear muscles of the rat indicate a complex, fine-tuned function of the middle ear muscles. Both middle ear muscles are merely composed of relatively small fast-twitch fibers. Almost all fibers possess an enzymatic profile that allows aerobic as well as anaerobic metabolism. The innervation of the muscles is extensive, and motor end plates are well developed. In a parallel study, middle ear muscle contraction (to be analyzed by electromyography) will be correlated with a change of the sound transmission characteristics of the middle ear (measured by the electrocochlear and brainstem auditory evoked responses). Preliminary results of the electrophysiologic measurements (electrocochleogram and brainstem auditory evoked response) are presented.

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The histological characteristics of the middle ear muscles, Musculus tensor tympani and M. Stapedius, of the guinea pig were studied using freeze-fracturing technique. No gap junction was observed but the tight junction was found in the tensor tympani muscle, possibly between muscle cells. No distinct difference was found between the size distributions of the apertures and caveolae on the fracture face of the sarcolemma of the two kinds of muscles.

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