Exploring the middle ear function in patients with a cluster of symptoms including tinnitus, hyperacusis, ear fullness and/or pain

Middle ear muscle (MEM) abnormalities have been proposed to be involved in the development of ear-related symptoms such as tinnitus, hyperacusis, ear fullness, dizziness and/or otalgia. This cluster of symptoms have been called the Tonic Tensor Tympani Syndrome (TTTS) because of the supposed involvement of the tensor tympani muscle (TTM). However, the putative link between MEM dysfunction and the symptoms has not been proven yet and the detailed mechanisms (the causal chain) of TTTS are still elusive. It has been speculated that sudden loud sound (acoustic shock) may impair the functioning of the MEM, specifically the TTM, after an excessive contraction. This would result in inflammatory processes, activation of the trigeminal nerve and a change of the MEMs state into a hypersensitive one, that may be associated to the cluster of symptoms listed above. The goal of this study is to provide further insights into the mechanisms of TTTS. The middle ear function of 11 patients who reported TTTS symptoms has been investigated using either admittancemetry and/or measurement of air pressure in the sealed external auditory canal. While the former method measured the middle ear stiffness the latter provides an estimate of the tympanic membrane displacement. Most patients displayed results consistent with phasic contractions of the TTM (n = 9) and/or Eustachian Tube (ET) dysfunction (n = 6). The MEM contraction or ET dysfunction could be evoked by acoustic stimulation (n = 3), somatic maneuvers (n = 3), or pressure changes in the ear canal (n = 3). Spontaneous TTM contraction (n = 1) or ET opening (n = 1) could also be observed. Finally, voluntary contraction of MEM was also reported (n = 5). On the other hand, tonic contraction of the TTM could not be observed in any patient. The implications of these results for the mechanisms of TTTS are discussed.

Experimental measurement and modeling analysis on mechanical properties of tensor tympani tendon

In this paper, we report mechanical properties of the tensor tympani tendon of human ear measured from uniaxial tensile, stress relaxation and failure tests. The hyperelastic Ogden model and digital image correlation method were employed to analyze experimental data. The constitutive equation of the tendon was derived through data iteration processes, and Young's modulus was presented as a function of stress. The viscoelastic property of the tendon was described by stress relaxation function and hysteresis. Furthermore, three-dimensional finite element analysis was carried out on five tendon models to investigate relationship between the structure and properties. The dimensions of the tendon were also measured by image processing techniques and presented with statistic significance. The structure and properties of the tensor tympani tendon reported in this study add new data into the study of ear tissue biomechanics.

Neurons in the cochlear nuclei controlling the tensor tympani muscle in the rat: a study using pseudorabies virus

The middle ear muscle reflex has been implicated in modulation of auditory input and protection of the inner ear from acoustic trauma. However, the identification of neurons in the cochlear nuclei participating in this reflex has not been fully elucidated. In the present study, we injected the retrograde transynaptic tracer pseudorabies virus into single tensor tympani (TT) muscles, and identified transynaptically labeled cochlear nucleus neurons at multiple survival times. Motoneurons controlling TT were located ventral to the ipsilateral motor trigeminal nucleus and extended rostrally towards the medial aspect of the lateral lemniscus. Transynaptically labeled neurons were observed bilaterally in the dorsal and dorso-medial parts of ventral cochlear nuclei as early as 48 h after virus injection, and had morphological features of radiate multipolar cells. After >or=69 h, labeled cells of different types were observed in all cochlear nuclei. At those times, labeling was also detected bilaterally in the medial nucleus of the trapezoid body and periolivary cell groups in the superior olivary complex. Based on the temporal course of viral replication, our data strongly suggest the presence of a direct projection of neurons from the ventral cochlear nuclei bilaterally to the TT motoneuron pool in rats. The influence of neurons in the cochlear nuclei upon TT activity through direct and indirect pathways may account for multifunctional roles of this muscle in auditory functions.

Human efferent adaptation of DPOAEs in the L1,L2 space

The adaptive properties of distortion product otoacoustic emissions (DPOAEs) at 2f(1)-f2 were investigated in 12 ears of normally hearing adults aged 18-30 years using long-lasting 1-s primary-tone on-times. In this manner, DPOAE adaptation at a single f2 of 1.55 kHz (f2/f1=1.21) was evaluated as a function of the levels of the primary tones in a matrix of L1, L2 settings, which varied from 45 to 80 dB SPL, in 5-dB steps. DPOAEs were elicited under both monaural and binaural stimulus-presentation conditions. Adaptation was defined as the difference in DPOAE levels between the initial 92-ms baseline measure using a standard protocol and one obtained during the final 92 ms of the prolonged 1-s primary-tones. These differences were averaged across subjects to create contour plots of mean adaptation in the L1,L2 space. The 2f(1)-f2 DPOAE revealed consistent regions of suppression (-0.5 dB difference) or enhancement (+0.5 dB difference) with respect to baseline measures within the L(1),L(2) matrix for both acoustic-stimulation conditions. Specifically, 2f(1)-f2 DPOAE suppressions of 1-2 dB occurred for both monaural and binaural presentations, typically at level combinations in which L1>L2. In contrast, larger 2f(1)-f2 DPOAE enhancements of 3-4 dB occurred for only the binaural condition, at primary-tone level combinations where L1<L2. Although adaptation activity was also evaluated for the DPOAEs at f(2)-f1, 2f(2)-f1, and 3f(1)-2f2, these emissions were either immeasurable (e.g., f(2)-f1) or only present in a subset of subjects over a narrow range of primary-tone frequencies and levels that did not support a systematic analysis. In summary, the 2f(1)-f2 results suggest that a potentially important area for adaptation measures exists in the L1,L2 space, when L1 is lower than L2. This combination of primary-tone levels can lead to large DPOAE adaptation effects that may be related to a notch in the DPOAE response/growth or input/output (I/O) function.

Do the tensor tympani and tensor veli palatini muscles of man form a functional unit? A histochemical investigation of their putative connections

The discussion among anatomists and otolaryngologists about the muscles originating from the Eustachian tube and the connections between the tensor tympani and tensor veli palatini muscles started in the 1860s. From then on, a considerable number of contradictory hypotheses and data have been presented. However, before discussing whether or not these two muscles form a functional unit, interest should focus on the question of whether it is even possible. The cartilaginous portion of the Eustachian tube with all muscles originating from it, including the whole tensor tympani muscle, was dissected from five perfusion-fixed cadavers and removed in toto. Complete longitudinal serial sections of 10 microm were made in the axis of the tensor tympani muscle. Sections were alternatingly stained according to Cason's and Maskar's techniques. The macroscopic aspect (under the operating microscope) of a tendinous connection between the two muscles under consideration could be proven by the histochemical methods used in all cases. Based on our findings and the literature reviewed we are convinced that the tensor tympani and tensor veli palatini muscles of man constitute a functional unit. This represents an important step forward towards the understanding of the possible functions the tensor tympani muscle serve in man.

Observations on the number, distribution and morphological peculiarities of muscle spindles in the tensor tympani and stapedius muscle of man

Although the middle ear muscles have been described for the first time more than four hundred years ago their role in modulation and transmission of sound is not yet fully understood. Surprisingly very little is known about proprioceptors in these muscles, especially in man, although this seems to be the key to the understanding of their various functions. Therefore, the question for proprioceptive sensory organs in these muscles is still relevant. The tensor tympani and stapedius muscles of four women who had donated their bodies to our institute were taken. Complete serial sections of these muscles were made which were either impregnated with silver, stained with ferric oxide for acidic polysaccharides or incubated with antibodies against S-100 protein. Thereby four to eight (mean five) muscle spindles distributed along the whole muscle could be detected in the tensor tympani muscles. These spindles contain one to three intrafusal muscle fibres and their length ranges from 140 to 4270 microm (mean 1492.8 microm). Furthermore, in three stapedius muscles one to two (mean 1.7) muscle spindles were found. They were from 350 to 500 microm (mean 482 microm) long and contained only one intrafusal muscle fiber. Regarding the diameter of intrafusal muscle fibers in both, the tensor tympani as well as the stapedius muscle, no difference to extrafusal muscle fibers of these muscles could be detected. The structure of these spindles differs considerably from those found in skeletal muscles. The morphological findings presented strongly suggest that muscle spindles occur regularly in both middle ear muscles. The results presented herein are consistent with clinical findings obtained from electromyographic studies and may help to elucidate all functions the middle ear muscles might serve in man.

Observations on simultaneous perilymphatic motions and cochlear microphonics suppression

Very low frequencies interfere in the intact cochlea with higher frequencies and suppress these depending on the vibration phase of the low-frequency sound. Physiological functions of the body, mediated, for example, by the eardrum or perilymph coupling with the cerebrospinal fluid, cause a low-frequency pressure modulation of the perilymph, which generates a synchronous perilymphatic motion resulting from the unevenly distributed compliances in the cochlea. This slow streaming causes a displacement of the entire basilar membrane, with as a consequence a postponement of the operating point of the mechanoelectrical transducer as a result of the pressure drop in the helicotrema and the narrow apical cochlear turn. In this contribution, interference phenomena are described, which are caused by spontaneous contractions of the tensor tympani muscle and by respiration-synchronous perilymphatic flow. These two test signals have trapezoidal and triangular impulse functions. In both cases, as suppression pattern of the cochlear microphonics level-time function, the second derivative of the pressure-time function was observed. The suppression is found to lie between 1 and 2 dB. It depends on the level of the suppressed sound and shows a compressive nonlinearity.

An anatomic study of the tensor veli palatini and dilatator tubae muscles in relation to eustachian tube and velar function

In a gross anatomic study of 20 sides in 16 human head specimens, the tensor veli palatini, the dilatator tubae, and the tensor tympani muscles were studied. The tensor veli palatini was observed to insert onto the anterior one-third of the pterygoid hamulus, whereas the dilatator tubae rounded the middle one-third of the pterygoid hamulus without an insertion. Thus, the dilatator tubae, not the tensor veli palatini, could serve to tense the anterior velum. An insertion from the superior pharyngeal constrictor muscle onto the posterior one-third of the hamulus could provide a curbing function for the dilatator tubae muscle. Adipose tissue, located at the hamulus, could provide lubrication for the tendinous fibers of the dilatator tubae as they round the hamulus. The dilatator tubae was observed to attach to the hook of the eustachian tube and is accepted as the tubal dilator. Observed on 13 of 20 sides in 11 specimens, the bulk of the dilatator tubae remained distinct from the tensor veli palatini despite a connective tissue alliance and intermingling of some muscle fibers. On 5 of 20 sides in 5 specimens, fibers of the dilatator tubae intermingled extensively with the tensor veli palatini. Of the 20 dilatator tubae muscles dissected, 2 were observed to be deficient. The tensor veli palatini was observed to be continuous with the tensor tympani. Full color versions of the figures are available at the following website: http://www.shc.uiowa.edu/papers/tensor/.

Frequency summation observed in the human acoustic reflex

It is known that the threshold of an acoustically induced middle-ear-muscle (MEM) reflex can be lowered by the simultaneous presentation of a second tone (facilitator), which is presented to the ipsilateral or contralateral ear at a level below the acoustic reflex threshold (ART) of the facilitator itself (Sesterhenn and Breuninger, 1976; Blood and Greenberg, 1981). In the present study, a primary elicitor and a facilitator were presented to the ear contralateral to that used for measurement of the acoustic reflex (AR), and the effects of changing frequencies and sound levels of the facilitator were investigated in human subjects with normal ears. The sound levels of facilitators, which caused a significant reduction of ART for the primary elicitors (facilitation thresholds), showed an asymmetrical pattern as a function of frequency of the facilitators. The facilitation thresholds tended to be lower when a facilitator with a frequency lower than the frequency of the elicitor (1 kHz) was used. In addition, effects of the elicitor on the masked thresholds of the facilitator were examined to observe the possible interaction between elicitor and facilitator from the viewpoint of 'spread of excitation'. The underlying mechanism of summation effects of two tones are discussed based on the possible input mechanism involved in the acoustically induced MEM reflex are.

[The mechanics and function of the middle ear. Part 1: The ossicular chain and middle ear muscles]

The revelation of a sophisticated micro-mechanism within the ossicular chain--the gliding of the joints results in a characteristic change of the ossicular movement--extended our conception of the function of the ossicular chain. Contrary to our former ideas of the transmission of sound during hearing, the ossicles do not rotate around an axis, which runs through the center of mass of the malleus and anvil. The imaginary rotational axis is located outside the ossicles, resulting in a more piston-like vibration of the complete chain. The ossicular joints are rigid. This acoustic function of the ossicular chain must rigorously be separated from its behaviour at changes of static, ambient air pressure. The middle ear, working as a sensitive pressure receptor also reacts to these pressure changes with comparatively huge displacements of the drum membrane (visible in pneumatic otoscopy). Only now the malleus rotates around its axial ligaments. These movements cause a gliding motion in the malleus-incus joint, which results in a predominant up- and downward movement of the anvil; the incudo-stapedial joint glides, too, and the stapes (and the inner ear) are decoupled. This micro-mechanism explains several anatomical features of the middle ear construction, as there are the vertical alignment of the incudo-stapedial joint, the suspension of the anvil, the design of the malleus joint, etc. It also accounts for the peculiar anatomical arrangement of the middle ear muscles. Their function can be interpreted as preserving the intact cartilage of the ossicular joints.(ABSTRACT TRUNCATED AT 250 WORDS)

Myogenic influences on the electrical auditory brainstem response (EABR) in humans

Two cases demonstrating the effects of myogenic artifact on the electrical auditory brainstem response (EABR) when using a promontory stimulation site are presented. Intensity-response functions were obtained in the unparalyzed condition, then repeated after infusion of a neuromuscular paralyzing agent. In both cases, the myogenic response was observed at lower stimulus intensities than the EABR components. As intensity increased, the myogenic responses grew at extremely rapid rates and made any subsequent identification of auditory responses virtually impossible. To alleviate the adverse influence of myogenic components, general anesthesia and a paralyzing agent must be incorporated into the test protocol when acquiring the EABR using a promontory site of stimulation.

[Movement of the ear ossicles by middle ear muscle contraction]

Up to now, the function of the middle ear muscles has mainly been investigated from an acoustical point of view. However, the primary function of the middle-ear muscles, namely the induction of ossicular movements, has never been investigated systematically. For this purpose, the displacements of the ossicles, as induced by simulated muscle contractions, were measured microscopically in 13 fresh temporal bone preparations. Both muscles move all ossicles. The tensor tympani muscle pulls the umbo inwards about 100 microns. Due to the gliding motion in the malleus-incus joint, the stapes is thus pushed inwards by at the most 10 microns and, additionally, displaced anteriorly, antagonistic to the pull of the stapedius muscle. This muscle pulls the stapes backwards, lifting the anterior crus outwards and pushing the posterior crus inwards. This reduces the pressure on the cochlear fluids significantly as compared to our former concepts of the movement of the footplate, tilting outwards as a whole around an axis at the posterior pole. Furthermore, this outward displacement of the stapes is not prerequisite for the outward movement of the malleus-drumhead complex, which typically appears at the contraction of the stapedius muscle. The basic motion of the stapes is the movement backwards, which is 5 times greater and which matches the anatomic direction of the pull of the stapedius muscle. This explains the otherwise unlogical position of the stapedius muscle parallel to the footplate. Due to the gliding movement in the malleus-incus joint, this motion changes at the umbo into outward rotation, counteracting the tensor tympani muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

[The functional significance of the suspending ligaments of the ear ossicle chain]

The contraction of the tensor tympani muscle has not enough force to stabilise the drum membrane-malleus complex against a negative pressure in the external ear nor against a hyperpressure in the middle-ear, as in sneezing, Valsalva's manoeuvre, etc. The ossicular ligaments must therefore perform this task. It was shown in experiments with fresh temporal bone preparations that the superior malleolar ligament diminishes the pressure-induced displacements of the malleus. This proves that this often neglected ligament has true functional significance. The most important ligament, which retains the malleus against an outward displacement, however, is the sturdy connective tissue between the cochleariform process and the malleus' handle, enveloping the tendon of the tensor tympani muscle and crossing the middle-ear cleft together with the tendon. These connective tissue strands are much too strong for a simple sheath of tendon, as can be seen histologically. In consideration of the fact that the functional significance surpasses the function of a pure tendon sheath, this ligamentous structure can be called the malleo-cochleariform ligament.

Concomitant changes in the acoustic impedance and the cochlear microphonic potentials during twitch contractions of the middle ear muscles in cats

The effect of twitch contractions of the middle ear muscles in cats on sound transmission through the middle ear (as measured by the cochlear microphonic potential of the inner ear) was compared with the simultaneous changes in the acoustic input impedance of the middle ear at the same frequency. It was found that decreases in impedance were related to an increase in the amplitude of the cochlear microphonics and vice versa. This may imply that decreases in impedance measured during the initial phase of the acoustic reflex in man are true decreases and are not due to transient decoupling of the ossicular chain at any point.

[The fixation theory of middle ear muscle function]

The still unidentified function of the middle ear muscles might be explained by the fixation theory. However, this idea, which is being favoured nowadays, namely, that the action of the muscles controls the position of the ossicles for optimal transmission, has never been investigated experimentally except for a few studies 50 and 100 years ago. In 25 temporal bone preparations, the air pressure-induced movements of the ossicles were microscopically measured, at first without and then with a 10 g. load on the tensor tympani muscle and a 5 g. load on the m. stapedius. The fixation hypothesis could not be confirmed, since with increasing pressure the movement-reducing effect of the middle ear muscle pull decreased. This inability of the muscles to compensate higher static air pressures is also demonstrated theoretically. A 10 g. pull of the tensor tympani muscle can only withstand a suction of 30 mm H2O in the ear canal. Hence, further evaluations are discussed that seem to enforce our hypothesis of the joint-preserving function of the middle ear muscles. The required antagonism of the pull of the muscle is accomplished by the change of the direction of movement in the gliding incudo-malleal joint.

Effects of tenotomy of the tensor tympani muscle on the acoustic reflex in dogs

The acoustic reflex (AR) was recorded from 12 healthy mixed-breed dogs. Latency and amplitude were measured from ipsilateral and contralateral AR at stimulus frequencies of 1 and 2 kHz and intensities of 70 to 110 dB sound pressure level for ipsilateral AR and 70 to 120 dB hearing level for contralateral AR. Mean latencies for ipsilateral and contralateral AR were between 33.46 and 206.10 ms and between 45.26 and 180.89 ms, respectively, and amplitudes were between 0.14 and 1.79 cm3 and between 0.31 and 1.86 cm3 of air, respectively. Stimulus frequencies and intensities had significant effects (P less than 0.05) on ipsilateral and contralateral AR latencies and amplitudes. Ipsilateral and contralateral AR decays were determined by measuring compliance change during a 10-s pure-tone stimulation at frequencies of 1 and 2 kHz at an intensity of 10 dB above AR threshold. Reflex decays for 1 kHz and 2 kHz frequencies averaged 5.74% and 9.71%, respectively, for ipsilateral AR and 5.08% and 5.40%, respectively, for contralateral AR. Bilateral tympanograms and brain stem auditory-evoked responses were performed on each dog. Mean normal static compliance of the middle ear, as determined by tympanometry, was 0.15 cm3. Unilateral tenotomy of the tensor tympani muscle was done on 6 of the 12 dogs, and each of the preceding procedures were repeated within 1 week after surgical operation. Transection of the tensor tympani tendon did not alter (P greater than 0.05) the latencies or amplitudes of 1 kHz- or 2 kHz-evoked contralateral AR, the latency or amplitude of 1 kHz-evoked ipsilateral AR, or the amplitude of 2 kHz-evoked ipsilateral AR. However, the latency of 2 kHz-evoked ipsilateral AR was significantly (P less than 0.05) increased. Reflex decay increased significantly (P less than or equal to 0.001) for the contralateral reflex elicited by the 2 kHz stimulus. Neither compliance of the middle ear system nor amplitude and latency of the brain stem auditory-evoked response were affected (P greater than 0.05) by tenotomy. Since tenotomy eliminates participation of the tensor tympani in the AR, these data indicate that contraction of this muscle is not primarily responsible for the compliance changes recorded during an acoustic reflex in dogs.

The tensor tympani muscle reflex in the mouse

Click evoked electromyographic (EMG) recordings were made from the contralateral tensor tympani muscle of anaesthetised mice. The mean threshold of the EMG response was around 50 dB SPL (peak equivalent) with a mean latency close to 4 ms. The mean amplitude of the response increased over a range of 70 dB to reach a level of around 150 microV with a mean latency around 3.5 ms. The tensor tympani muscle activity was investigated also in profoundly hearing-impaired mutant mice with either cochlear dysfunction (deafness) or brainstem dysfunction (quivering). No evoked EMG activity was detected in either group of hearing-impaired mutants. The data suggest that EMG activity in the mouse can provide a sensitive monitor of auditory function. The study of reflex activity in further mouse mutants is likely to provide information on the vulnerability of the reflex to different types of naturally occurring cochlear and brainstem pathology.

Are two muscles needed for the normal functioning of the mammalian middle ear?

Why do two muscles occur in the middle ear of most mammals? In this article selected explanations as to why two muscles occur are critically evaluated. It is argued that these explanations are not compelling. Data are then reviewed that indicate that the tensor tympani has no function to fulfil in the normal operation of the middle ear.

Effect of the acoustic reflex on inner ear damage induced by industrial noise

Shipyard noise is a variable noise which induces permanent threshold shift (PTS) in exposed workers. In humans, the stapedius reflex has been found to be very stable in this type of exposure. Temporary threshold shift (TTS) in the absence of stapedius reflex has been found to extend downwards through the speech-frequencies instead of showing a high frequency dip as when the stapedius reflex is normal. The features of PTS produced by the same type of noise was investigated in rabbits with and without functioning middle ear muscles. The auditory sensitivity was measured by auditory brain stem response (ABR) and by the stapedius reflex response. Middle ear muscle function was blocked by denervation of the stapedius muscle or by general anesthesia. With normal middle ear reflex very little PTS was found. When the muscles were inactivated during the noise exposure the PTS was very extensive and covered the mid frequency range. On the basis of previous findings in humans and the present animal study it is suggested that the features of the stapedius reflex should be considered both in assessment of individual susceptibility and design of optimal acoustic environments.

The pressure equilibrating function of pars flaccida in middle ear mechanics

In the adult male rat the precise volumes of the middle ear cavity, the lateral attic compartment and a maximally retracted pars flaccida were calculated to 550 microliter, 21 microliter and 3 microliter respectively, using Woods metal. Small intratympanal pressure or volume changes caused the pars flaccida to move in a medial-lateral direction. As the volume of a maximally retracted or bulging pars flaccida only amounted to about 0.5% of the total middle ear volume and the pars flaccida reacted even at volume changes of 0.1% of the middle ear, it would seem that the pressure equilibrating capacity of the elastic pars flaccida is negligible. On the other hand the pars flaccida is extremely sensitive to pressure and volume changes in the middle ear and may take part in the pressure equilibrating system as a detector for minute pressure and/or volume changes in the middle ear.

Role of the tensor tympani muscle in eustachian tube function

In order to determine the role of the tensor tympani muscle in Eustachian tube function, pressure changes in the external and middle ear of 13 cats were measured under four experimental conditions. It was revealed that contraction of the tensor tympani muscle during swallowing did not result in any tympanic pressure rise which might assist in tubal ventilation. Acoustic stimulation was then used to measure consistent contraction of the tensor tympani muscle. Combined contraction of the tensor veli palatini and tensor tympani muscle under the condition of positive tympanic pressure failed to open the tube. It was concluded that the tensor tympani muscle might not play any part in tubal function.

Evaluation of tensor tympani muscle dominance in the biphasic acoustic reflex

The Acoustic reflex frequently causes a biphasic change in impedance at onset. Understanding the cause of the biphasic response is important for establishing a physiological basis for the clinical measurement of reflex latency. The decrease in impedance at onset may be due to uncoupling of impedance contributed by the cochlea. Subsequent increases in impedance predominantly reflect stapedius muscle activity. The clinical implications of this physiologic model are discussed.

The tensor tympani muscle of cat and dog contains IIM and slow-tonic fibres: an unusual combination of fibre types

Using recently developed highly specific antisera to the full range of known adult mammalian skeletal muscle myosins, an immunohistochemical and histochemical examination was made of the middle ear muscle tensor tympani in the dog and cat. Approximately half the fibres were of the IIM type and there was a substantial population of apparently slow-tonic fibres, both these types being rare in mammals. In addition, some type I but no IIA nor IIB fibres were detected. Moreover, as no multiple end-plate innervation, thought to be typical of slow-tonic fibres, could be demonstrated in this muscle by acetylcholinesterase staining or by Ruffini gold impregnation, it is suggested that in tensor tympani the slow-tonic fibres are focally innervated. The very short length of the fibres, only 1-2 mm, is probably sufficient to permit adequate depolarization of a whole fibre by a single centrally situated end-plate. The functional implications of this combination of very rare fibre types in tensor tympani are unclear at present.

Effects of prolonged noise exposure in chinchillas with severed middle ear muscles

Using the round window-recorded cochlear microphonic as the index of acoustic reflex activity, we noted a decay in the strength of middle ear muscle contraction in the chinchilla following an eight hour exposure to octave band noise (500 Hz. center frequency, 95 dB. sound pressure level). On the basis of this observation it was concluded that the prolonged exposure reduced the effectiveness of the acoustic reflex in protecting the cochlea. This reduction, however, may have been underestimated if the exposure was also sufficient to reduce cochlear output. The present investigation examined this possibility by comparing the effects of a similar exposure in chinchillas with intact (normal) and severed middle ear muscles. Following exposure, the cochlear microphonic magnitude increased slightly in the normal group. Decreases in the cochlear microphonic were observed in the animals with severed muscles even when the overall exposure level was reduced to simulate the effects of middle ear muscle contraction. These findings indicate that although the muscles did afford some degree of protection to the cochlea during the exposure, the protective effects of the acoustic reflex may have been reduced even beyond our original observations.

The acoustic reflex pattern studied by the averaging technique

The acoustic reflex threshold and its pattern in response to stimuli of different intensities, were investigated by means of the signal-averaging technique in 20 normal, 20 sensorineural-impaired and 10 successfully stapedectomized ears. Trains of tone bursts between 110 and 0 dB HL were used. The frequencies tested were 500, 1 000, 2 000 and 4 000 Hz. In all normal subjects, the pattern of the acoustic reflex for stimuli between 110 and 100 dB HL was biphasic with an initial positive plateau followed by a longer negative one. For stimuli below 80 dB HL, the pattern of the reflex was monophasic with a single positive peak. In the sensorineural-impaired ears, the same double pattern of the waveform was obtained. In the stapedectomized ears, no compliance changes were observed for any sound stimuli. By the averaging technique, the difference between the acoustic reflex threshold and the auditory threshold was found to be between 7.5 and 32.5 dB, in normal and sensorineural-impaired ears.

Dynamic tympanometry

Tympanometry was performed under the contraction of the middle ear muscles (dynamic tympanometry). A tympanogram of lesser compliance was observed under a stapedius reflex which was produced by contralateral acoustic stimulation. In patients with reversed (downward) stapedius reflex, the dynamic tympanogram showed either a higher peak amplitude or a shift of conventional tympanogram to the negative pressure side. Voluntary contraction of the tensor tympani produced a lesser compliance with positive pressure, whereas there was an apparent increase of compliance with negative pressure in the external auditory meatus. A shift of the dynamic tympanogram to the negative pressure side was interpreted as being due to contraction of the tensor tympani muscle. Dynamic tympanometry may also be utilized as a recording of reflex decay.

Electromyogram of the tensor tympani muscle in man during swallowing

Experiments were carried out on 2 patients who underwent an operation for chronic otitis media, whereupon the tensor tympani muscle was visualized. A unipolar platinum electrode was inserted into the muscle belly. EMG recordings were made during swallowing and other motor activity. Distinct, pronounced EMG activity was recorded from both patients every time they swallowed. It was concluded that the tensor tympani muscle participates in the act of swallowing and thereby probably contributes to the ventilation of the middle ear.

The tensor tympani, stapedius, and tensor veli palatini muscles--an electromyographic study

Electromyographic recordings of the activity of the tensor veli palatini, tensor tympani, and stapedius muscles were obtained from several adult human subjects. Muscle responses were recorded under four stimulus conditions, ie, contralateral intense wide-band noise, air jet to the eye, swallow, and electrical stimulation of the tongue. The results indicated that the two tensor muscles responded to the same stimuli in similar patterns. The latter muscle differed from the response of the stapedius.

Electromyographic correlation of tensor tympani and tensor veli palatini muscles in man

It is the purpose of this study to attempt a correlation of function, by electromyographic means, of the tensor tympani and tensor veli palatini muscles in humans. Despite the small number of patients tested, it is believed that the similarities and characteristics of the two are unmistakably equivalent. A separate theory for the combined tensor function is discussed in distinction to the stapedius mediated acoustic reflex. The concept of a single tensor muscle, with two anatomic divisions, a common nerve supply, and parallel function, is therefore submitted.

The biphasic acoustic reflex: a new perspective

The anatomic, neurologic, and physiologic characteristics of the middle ear structures have created confusion concerning the nature of the intraaural muscle reflex. After reviewing the relevant literature, we correlated this information with our studies of human temporal bones and computer analyses of the acoustic reflex responses of individuals with normal hearing. Our hypothesis is that although the stapedius is the initiator of the reflex and the primary contributor to ossicular chain fixation, the tensor tympani is responsible for the major observed response. The negative deflection present in normals tested on the impedance bridge is caused by the stapedius, whereas the large positive deflection is the result of the tensor tympani contraction. We postulate that proprioceptive feedback mechanisms located within the stapedius muscle and tendon permit and/or initiate tensor tympani contraction during acoustic stimulation.

Volume displacement of the tympanic membrane in the sitting position as a function of middle ear muscle activity. A quantitative microflow method

An open microflow meter system has been worked out for quantitative recording of the volume displacement of the tympanic membrane and its movement direction at stapedius reflex. An acoustically elicited M. stapedius contraction can be recognized by the characteristic response and latency time. The stapedius reflex contraction causes an outward or inward movement of the tympanic membrane. The magnitude of the volume displacement of the tympanic membrane is influenced by the middle ear pressure and in some ears the movement direction of the tympanic membrane changes.

Value of the isolated tensor tympani muscle reflex elicited by electric lingual stimulation

A bilateral reflex contraction of the isolated tensor tympani muscle has been obtained in man by bipolar electric stimulation of the edge of the tongue (DC generator). The avoidance of the elevated and variable body electric resistance and of the variable contact resistance from the hand to the ground plate, represents an appreciable improvement of this method in comparison with the unipolar method described in a earlier paper. A further improvement has also been achieved by applying the electric stimulus to one inferior aspect of the tongue near the floor of the mouth, nearer to the lingual nerve. An analysis has been subsequently conducted in normal subjects and in patients affected by pathologies of the tympano-ossicular system; otosclerosis, tympanosclerosis, unilateral complete suprastapedial facial paralysis, interrumption of the ossicular chain; in cases of interruption of the afferent arc: section of the unilateral lingual nerve; involvement of its central portion: cerebello-pontine angle tumour, brain stem tumour. A chiasma-like central nervous pattern is suggested.

Bipolar electric stimulation to elicit and isolated tensor tympani reflex

A bipolar electrode is described, designed to obtain improved reflex responses from the tensor tympani muscle by electric stimulation of the under surface of the tongue, d.c. generator, 7-9 V d.c. In comparison with the previous unipolar method of stimulation the responses under oscilloscopic analysis appear very constant and of greater size. The latency, in normal subjects, is 117 msec.

Tensor tympani, a 'tuner' of tensor palati muscle

The author has proved experimentally (in two dogs) that there is reflex hypertonia of the tensor palati muscle, synchronous with the 'shortening' reaction of the tensor tympani muscle in response to its "static" relaxation during the gradual passive inward displacement of the drum resulting from the negative intratympanic 'dip' due to absorption of air imprisoned within the middle ear. The author coined the term 'tuning' for the reflex hypertonia of tensor palati which is directly proportional to the degree of the slackness of its 'tuner', the muscle-tensor tympani. The degree of opening of the eustachian tube on swallowing depends upon the degree of 'tuning' of the tensor palati. The 'untuned' tensor palati fails to open the eustachian tube during swallowing. Presumably the excitation of tympanic chemoreceptors (glomus body) by the excess of CO2 during hypoxia of the tympanic cleft strengthens the 'shortening' reaction as well as the excitability of the tensor tympani muscle.

Effects of reflex middle-ear muscle contractions on cochlear responses to bone-conducted sound

The effects of contralaterally elicited middle-ear muscle (MEM) reflexes on cochlear microphonic responses to air- and bone-conducted tones were examined in decerobrate cats. Stapedius effects on bone condn air conduction were almost identical in configuration and amplitude to those on air conduction at all frequencies. However, tensor tympani effects were more complex, the configuration of the bone-conduction effects varying with the location of the transducer on the skull and with frequency. The relative contributions of the two muscles to the effects of joint contractions varied markedly between animals. It is suggested that non-reflex MEM contractions associated with activity of the facial musculature might provide protection against masking of environmental sounds by the low-frequency bone-conducted sound generated by such activity.

Holographic vibration analysis of the ossicular chain

The mechanics of movement of the ossicular chain has been investigated on human temporal bone preparations by means of time average holography. The malleus and incus moved like a lever around an axis, which position was somewhat dependent upon frequency. Simulation of contraction of the muscles of the middle ear gave as result a change in the pattern of movement of the ossicular chain. The position of the rotatory axis was changed, and the amplitudes of vibration were reduced. This should be a protecting mechanism.

Middle ear muscle activity during speech in stapedectomized and laryngectomized subjects

Middle ear muscle responses associated with speech production were observed in normal-hearing, stapedectomized, and laryngectomized subjects. Impedance changes associated with speech production were monitored by an electroacoustic impedance bridge simultaneously with vocal output. Results from stapedectomized subjects indicate that the tensor tympani muscle contracts prior to vocalization and is part of the neurological pattern of speech production. Data collected from laryngectomized subjects suggest that the presence of sensory fibers from the larynx is not a prerequisite for middle ear muscle activity during speech production.

Eardrum displacement following stapedius muscle contraction

Simultaneous monitoring in human subjects on the same ear of eardrum displacement by tympanomanometry, and impedance with the electroacoustic bridge, provided information concerning contraction of the stapedius muscle and its effect on eardrum displacement. Extensive control procedures were employed to elicit only the stapedius reflex; lower intensity auditory stimulation, electrocutaneous stimulation of the homolateral external ear canal, and anesthetization of nerves leading to the tensor tympani. Following these procedures the following results were obtained: (1) Extremely small biphasic and monophasic eardrum movements were seen in the stapedius--only ear to auditory and electrocutaneous stimulation; the form of the response was much less predictable to auditory stimulation. (2) At high sound intensities relatively large inward and biphasic movements of the eardrum occurred in the normal ear, unquestionably due to contraction of the tensor tympani. These results were further validated in a group of stapedectomized ears, without the stapedius but with normal tensor tympani. (3) Biphasic responses did not occur in the tensor tympani--only ear, only monophasic inward responses. (4) Upon air-jet stimulation to the orbit of the eye, these subjects had an accentuated tensor response in that large inward movements of the eardrum occurred as compared with those in normal ears, suggesting that there is an alteration of the tensor response by the presence of the stapedius muscle. Estimates of the actual eardrum displacement were calculated based on a model of the external ear canal and eardrum.

Tympanic muscle reflex elicited by electric stimulation of the tongue in normal and pathological subjects

A bilateral reflex contraction of the tensor tympani muscle has been obtained in man by electric stimulation of the tongue (1-2 mA). The stimulus is well tolerated and always effective. The advantage is stressed of eliciting a contraction of this muscle without involvement of the stapedius, as occurs with other methods. An analysis has been subsequently conducted in normal subjects and in patients affected by pathology of the tympano-ossicular system: tympanosclerosis, otosclerosis, suprastapedial facial paralysis; in cases of interruption of the afferent arch: section of the homo-lateral lingual nerve; in cases of involvement of its central portion: cerebello-pontine-angle tumours; and in cases of section of chorda tympani. A chiasm-like central nervous pattern is suggested.

Peripheral control of acoustic signals in the auditory system of echolocating bats

Many species of echolocating bats emit intense orientation sounds. If such intense sounds directly stimulated their ears, detection of faint echoes would be impaired. Therefore, possible mechanisms for the attenuation of self-stimulation were studied with Myotis lucifugus. The acoustic middle-ear-muscle reflex could perfectly and transiently regulate the amplitude of an incoming signal only at its beginning. However, its shortest latency in terms of electromyograms and of the attenuation of the cochlear microphonic was 3–4 and 4–8 msec, respectively, so that these muscles failed to attenuate orientation signals by the reflex. The muscles, however, received a message from the vocalization system when the bat vocalized, and contracted synchronously with vocalization. The duration of the contraction-relaxation was so short that the self-stimulation was attenuated, but the echoes were not. The tetanus-fusion frequency of tha stapedium muscle ranged between 260 and 320/sec. Unlike the efferent fibres in the lateral-line and vestibular systems, the olivo-cochlear bundle showed no sign of attenuation of self-stimulation.

Spontaneous middle ear muscle activity in man: a rapid eye movement sleep phenomenon

Changes in compliance of the tympanic membrane have been detected in normal human sleep, presumably due to spontaneous contraction of the stapedius and tensor tympani muscles of the middle ear. In the waking state, these muscles generally respond to loud sound (middle ear reflex). Middle ear muscle activity typically erupts before or at the onset of rapid eye movement (REM) sleep and persists throughout the REM period in a discontinuous pattern resembling that exhibited by rapid eye movements. Approximately 80 percent of all nocturnal middle ear muscle activity is contained in REM sleep. Half of the remaining 20 percent occurs in the 10-minute intervals just prior to the onset of REM sleep. Middle ear muscle activity is often associated with other phasic events such as momentary enhancement of electromyogram inhibition, apnea, and K complexes. Rapid eye movements and middle ear muscle activity, though significantly correlated in REM sleep, are not always simultaneous.