Audiometric findings with voluntary tensor tympani contraction

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.

Human middle-ear muscle pulls change tympanic-membrane shape and low-frequency middle-ear transmission magnitudes and delays

The three-bone flexible ossicular chain in mammals may allow independent alterations of middle-ear (ME) sound transmission via its two attached muscles, for both acoustic and non-acoustic stimuli. The tensor tympani (TT) muscle, which has its insertion on the malleus neck, is thought to increase tension of the tympanic membrane (TM). The stapedius (St) muscle, which has its insertion on the stapes posterior crus, is known to stiffen the stapes annular ligament. We produced ME changes in human cadaveric temporal bones by statically pulling on the TT and St muscles. The 3D static TM shape and sound-induced umbo motions from 20 Hz to 10 kHz were measured with optical coherence tomography (OCT); stapes motion was measured using laser-Doppler vibrometry (LDV). TT pulls made the TM shape more conical and moved the umbo medially, while St pulls moved the umbo laterally. In response to sound below about 1 kHz, stapes-velocity magnitudes generally decreased by about 10 dB due to TT pulls and 5 dB due to St pulls. In the 250 to 500 Hz region, the group delay calculated from stapes-velocity phase showed a decrease in transmission delay of about 150 µs by TT pulls and 60 µs by St pulls. Our interpretation of these results is that ME-muscle activity may provide a way of mechanically changing interaural time- and level-difference cues. These effects could help the brain align head-centered auditory and ocular-centered visual representations of the environment.

The function of the tensor tympani muscle: a comprehensive review of the literature

The tensor tympani muscle is structurally important in the middle ear, specifically through its involvement in the impedance of sound in response to intense auditory and non-auditory stimuli. Despite numerous studies, its true function has been debated for many years; questions still remain about its role in auditory and non-auditory reflexes and in sound damping. Some studies suggest that the tensor tympani muscle contracts as a result of non-auditory stimulation such as facial or head movements; others suggest that it contracts due to input from the cochlear nucleus, therefore by way of auditory stimulation. Whatever the cause, contraction of the tensor tympani muscle results in low frequency mixed hearing loss, either to protect the inner ear from loud sounds or to desensitize the ear to self-generated sounds. A review of these studies indicated that the tensor tympani muscle has a wide range of functions, yet the mechanisms of some of them have not been clearly demonstrated. One major question is whether the tensor tympani muscle contributes to sound damping; and if it does, what specific role it serves. The primary purpose of this review article is to explore the functions of the tensor tympani muscle in light of recent research advances.

A Non-invasive Method to Assess Tensor Tympani Muscle Contraction in Response to Tactile Stimulation

BACKGROUND

The function of the tensor tympani muscle and its effects, as well as a universal testing procedure, is not yet known. The purpose of this study is to investigate the effect of tactile stimulation on the tensor tympani muscle activity in an attempt to evaluate the functional status of the muscle.

METHODS

In this study, 20 healthy adults aged between 18 and 30 years were included. All participants had their history taken, as well as the evaluation of otoscopy, audiometry, and tympanometry. A modified reflex decay paradigm was used to evaluate the tensor tympani muscle activity, in both humans and a fresh-frozen cadaver. The tactile stimulation was applied by thumb tappings on different forehead areas.

RESULTS

Reflex decay responses were in the form of the absence of a response or either a positive or negative waveform. Tactile stimulation had no impact on reflex decay recordings obtained from the cadaver. There was no significant difference between the waveforms obtained from both ears, at different stimulation frequencies, tactile stimulation areas, or the 3 successive stimuli for 1 individual in 1 ear (P > .05). Changes observed in the reflex decay test upon tactile stimulation seem to be due to tensor tympani muscle activity as supported by the previous findings in the literature.

CONCLUSION

The application of a tactile stimulus on the forehead is a non-invasive test method to assess tensor tympani muscle contraction. Understanding the function of the tensor tympani muscle using a non-invasive method will be helpful during the decision-making process in the practice of otology.

Acoustic reflex preshoot: A sign of contraction of the tensor tympani muscle?

Two tiny muscles are attached to the ossicles of the middle ear: the stapedius, whose role as a protector of the inner ear against loud noise has been widely accepted, and the tensor tympani, which remains a mystery since no valid method of detecting its contraction has yet been developed. However, tonic contraction of the tensor tympani has been associated with a number of otological conditions including fullness, tinnitus, slight hearing loss, and Menière-like findings. Our hypothesis is that an easily recognizable sign in the acoustic immittance testing that is part of almost every basic audiological evaluation indicates the contraction of the tensor tympani. During acoustic reflex assessment, a loud stimulus causes a bilateral contraction of the stapedial muscles and an increase in impedance which appears as a deflection in the tracing of the immittance test results. In the case of a contracted tensor tympani, when an intense sound is sent to provoke the acoustic reflex, the stapedius must overcome its resistance which would otherwise have the opposite effect on the footplate. This effort can be seen as a small peak or "preshoot" preceding the greater negative deflection of the impedance. We report the cases of ten patients with unilateral aural symptoms such as tinnitus, impaired noise tolerance, fullness, ear pressure and/or ear discomfort during chewing; the preshoot was present only in the symptomatic ear. If further studies confirm our hypothesis, this could give rise to new therapeutical strategies aimed at reducing contraction of the tensor tympani and balancing its function.

Time-domain and frequency-domain effects of tensor tympani contraction on middle ear sound transmission in gerbil

The middle ear is a high-fidelity, broadband impedance transformer that transmits acoustic stimuli at the eardrum to the inner ear. It is home to the two smallest muscles in mammalian species, which modulate middle ear transmission. Of this pair, the function of the tensor tympani muscle (TTM) has remained obscure. We investigated the acoustic effects of this muscle in young adult gerbils. We measured changes in middle ear vibration produced by pulse-train-elicited TTM contraction - in the time-domain with a click stimulus and in the frequency-domain with multitone zwuis stimuli. In our click experiments, there was generally a small reduction in the primary peak of the response and a slight increase in the subsequent ringing, but there was essentially no change in the delay of the click response at the umbo (less than 1 µs change). In our multitone experiments, there were consistent patterns of attenuation and enhancement in the velocity responses at the umbo and ossicles. TTM contraction produced a narrow band of enhancement around 6 kHz (maximally ~5 dB) that can be modeled with an increased stiffness of an overdamped spring-mass resonance. At frequencies below 2 kHz and above 35 kHz, TTM contraction attenuated middle ear vibrations by as much as fivefold.

Rehearsal Sound Exposure and Choir Singers' Hearing: A Pilot Field Study

Exposure to high- and moderate-intensity sound is inevitable for professional singers during their working day, the majority of which is spent in rehearsal, preparing for a performance. The impact of self-produced sound exposure on singers' hearing within the rehearsal setting has not been examined.

OBJECTIVES

This original pilot field study investigates the feasibility of data collection and hypothesis testing of singers' hearing within the rehearsal environment.

METHODS

18 professional choir singers are examined for hearing threshold changes following routine rehearsal sound exposure. Pure Tone Audiometry is measured before, immediately after, and approximately 24 hours after rehearsal.

RESULTS

This study does not identify definitive Temporary Threshold Shift in this population under these conditions. That said, mean temporary threshold shift was found 3.61 dB higher than the recovery threshold shift in the right ear at 3000 Hz (P = 0.06), and this may be important to look at for future studies.

CONCLUSIONS

Methodological challenges of this field study include dynamic experimental conditions intrinsic to the rehearsal process, environmental and musical influence on Pure Tone Audiometry results and estimation of sound intensity exposure.

Muscles in and around the ear as the source of "physiological noise" during auditory selective attention: A review and novel synthesis

The sensitivity of the auditory system is regulated via two major efferent pathways: the medial olivocochlear system that connects to the outer hair cells, and by the middle ear muscles-the tensor tympani and stapedius. The role of the former system in suppressing otoacoustic emissions has been extensively studied, but that of the complementary network has not. In studies of selective attention, decreases in otoacoustic emissions from contralateral stimulation have been ascribed to the medial olivocochlear system, but the acknowledged problem is that the results can be confounded by parallel muscle activity. Here, the potential role of the muscle system is examined through a wide but not exhaustive review of the selective attention literature, and the unifying hypothesis is made that the prominent "physiological noise" detected in such experiments, which is reduced during attention, is the sound produced by the muscles in proximity to the ear-including the middle ear muscles. All muscles produce low-frequency sound during contraction, but the implications for selective attention experiments-in which muscles near the ear are likely to be active-have not been adequately considered. This review and synthesis suggests that selective attention may reduce physiological noise in the ear canal by reducing the activity of muscles close to the ear. Indeed, such an experiment has already been done, but the significance of its findings have not been widely appreciated. Further sets of experiments are needed in this area.

Classification of Tinnitus: Multiple Causes with the Same Name

Tinnitus is spoken of as if it were a single thing, but there are many different causes, likely many different mechanisms, and many different subtypes. This article reviews a broad range of approaches to understand and demarcate different tinnitus subtypes, which will be critical for exploring and finding cures for different subtypes.

An Integrative Model Accounting for the Symptom Cluster Triggered After an Acoustic Shock

Acoustic shocks and traumas sometimes result in a cluster of debilitating symptoms, including tinnitus, hyperacusis, ear fullness and tension, dizziness, and pain in and outside the ear. The mechanisms underlying this large variety of symptoms remain elusive. In this article, we elaborate on the hypothesis that the tensor tympani muscle (TTM), the trigeminal nerve (TGN), and the trigeminal cervical complex (TCC) play a central role in generating these symptoms. We argue that TTM overuse (due to the acoustic shock), TTM overload (due to muscle tension), and ultimately, TTM injury (due to hypoxia and "energy crisis") lead to inflammation, thereby activating the TGN, TCC, and cortex. The TCC is a crossroad structure integrating sensory inputs coming from the head-neck complex (including the middle ear) and projecting back to it. The multimodal integration of the TCC may then account for referred pain outside the ear when the middle ear is inflamed and activates the TGN. We believe that our model proposes a synthetic and explanatory framework to explain the phenomena occurring postacoustic shock and potentially also after other nonauditory causes. Indeed, due to the bidirectional properties of the TCC, musculoskeletal disorders in the region of the head-neck complex, including neck injury due to whiplash or temporomandibular disorders, may impact the middle ear, thereby leading to otic symptoms. This previously unavailable model type is experimentally testable and must be taken as a starting point for identifying the mechanisms responsible for this particular subtype of tinnitus and its associated symptoms.