Tympanometric and Acoustic Stapedius Reflex Measures in Older Adults: The Blue Mountains Hearing Study

Tympanometric peak pressure, peak compensated static acoustic admittance (peak Ytm) and acoustic stapedius reflex (ASR) thresholds were obtained for a representative sample of 1565 older Australians who were participants in the Blue Mountains Hearing Study (BMHS). No significant age or gender effects were found for tympanometric peak pressure. Peak Ytm measures, however, decreased with age in the left ear only across all age groups and were consistently higher for men than for women. After allowing for hearing loss, the effect of age on ASR thresholds was inconsistent. An increase in ASR thresholds with age was observed at selected frequencies but only when measured contralaterally, and these changes were not clinically significant. Overall, our findings suggest that current normative data for peak Ytm is too restricted for application in the older population, but there is insufficient evidence to warrant alternative normative data for the ASR threshold range in this same population. Se obtuvo la presión timpanométrica pico, el pico de admitancia acústica estática compensada (pico Ytm) y los umbrales de reflejo acústico estapedial (ASR), para una muestra representativa de 1565 australianos viejos que participaban en el Estudio Auditivo de las Montañas Azules (BMHS). No se encontraron efectos significativos de la edad o género en los picos de presión timpanométrica. Las mediciones de pico Ytm, sin embargo, disminuyeron con la edad sólo en el oído izquierdo, en todos los grupos, y fue consistentemente mayor en hombres que en mujeres. Luego de corregir de acuerdo a la pérdida auditiva, el efecto de la edad sobre los umbrales de los ASR fue inconsistente. Se observó un aumento con la edad en los umbrales de los ASR en frecuencias seleccionadas, aunque sólo cuando se medía contralateralmente, y estos cambios no fueron clínicamente significativos. Globalmente, nuestros hallazgos sugieren que los datos normativos actuales para el pico Ytm son muy restrictivos para aplicar a las poblaciones más viejas, aunque existe evidencia insuficiente para garantizar datos normativos alternativos para el rango de umbrales de los ASR en esta misma población.

Adversarial relationship between combined medial olivocochlear (MOC) and middle-ear-muscle (MEM) reflexes and alarm-in-noise detection thresholds under negative signal-to-noise ratios (SNRs)

The role of auditory efferent feedback from the medial olivocochlear system (MOCS) and the middle-ear-muscle (MEM) reflex in tonal detection tasks for humans in the presence of noise is not clearly understood. Past studies have yielded inconsistent results on the relationship between efferent feedback and tonal detection thresholds. This study attempts to address this inconsistency. Fifteen human subjects with normal hearing participated in an experiment where they were asked to identify an alarm signal in the presence of 80 dBA background (pink) noise. Masked detection thresholds were estimated using the method of two-interval forced choice (2IFC). Contralateral suppression of transient-evoked otoacoustic emissions (TEOAEs) was measured to estimate the strength of auditory efferent feedback. Subsequent correlation analysis revealed that the contralateral suppression of TEOAEs was significantly negatively correlated (r = -0.526, n = 15, p = 0.0438) with alarm-in-noise (AIN) detection thresholds under negative signal-to-noise conditions. The result implies that the stronger the auditory efferent feedback, the worse the detection thresholds and thus the poorer the tonal detection performance in the presence of loud noise.

Auditory brainstem circuits that mediate the middle ear muscle reflex

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

["Irritative" pathology of the external auditory canal and decreased sound sensation]

We have not found in the literature an explanation for the intermittent and transient decreased lesser hearing sensation in patients with dysesthesia of the external auditory canal (EAC). In this paper we offer a possible explanation for it. Our hypothesis is that the stimulation of the sensory fibers of the trigeminal and facial nerves in the EAC is able to increase the stiffness of the ossicular chain by means of a reflex stimulation of malleus and stapes muscles. This intermittent and transient increase of the ossicular stiffness could explain the intermittent and transient decrease of hearing sensation in these patients.

Middle-ear influence on otoacoustic emissions. II: contributions of posture and intracranial pressure

Although it seems likely that body tilt or surgically provoked variations in intracranial pressure (ICP) can result in variations of intralabyrinthine pressure, the channels for pressure transmission remain controversial and the reasons why evoked otoacoustic emissions (EOAEs) exhibit attendant modifications are unclear. The theoretical framework implemented in the companion paper [Avan et al. part I, 2000] provides sensitive and non-invasive means to identify the middle-ear mechanism(s) entailed in EOAE changes. It was thus applied to analyze the influence of posture on EOAE phases and magnitudes as a function of frequency, in a series of experiments involving body tilt from sitting to supine (0 degrees or -30 degrees ). Controlled ICP variations were surgically carried out in a series of hydrocephalic patients and the resulting EOAE changes were compared to posture data and model predictions. In all cases, the EOAE changes closely resembled those due to an increase in the stiffness of the stapes' annular ligament, in keeping with the assumption that ICP gets transmitted to intralabyrinthine spaces and modifies the hydrostatic load on the stapes, thereby influencing EOAE features. A small additional contribution of middle-ear pressure to EOAE changes was identified in addition to the main stapes component. Dynamical EOAE measurements showed that sudden ICP changes were transmitted to the inner ear within 8-30 s. The high sensitivity of EOAE phases below 2 kHz to ICP changes, together with the absence of any significant confounding middle-ear effect, favors EOAEs for a reliable non-invasive monitoring of ICP and intralabyrinthine pressures.

Middle ear influence on otoacoustic emissions. I: noninvasive investigation of the human transmission apparatus and comparison with model results

Evoked otoacoustic emissions (EOAEs) are generated within the cochlea in response to external sounds, and they can be acoustically detected in the external auditory meatus after backward propagation through the middle ear. In addition to being used to probe the cochlear mechanisms, they are expected to be sensitive to minute changes in middle ear impedance. Systematic measurements of the changes in the vectorial components of EOAEs were carried out after various manipulations of the human middle ear in order to characterize the influence of stiffness and inertia of the stapes and tympanic-membrane systems. For this purpose, stapedius muscle contractions were elicited by high-level contralateral sound, controlled changes in middle ear pressure (range +/-100 daPa) were produced and the tympanic membrane was loaded with water droplets. A computer model of the middle ear network was implemented using a standard lumped-element electric analog of the middle ear (Zwislocki's model). Forward and backward transmission changes were simulated and model predictions were compared to experimental data. Apart from the case of positive middle ear pressures, a close qualitative correspondence was found between model and real-ear results. Each of the effects was characterized by a unique pattern of phase and magnitude changes as a function of frequency, in relation to the mechanical characteristics of the involved subsystem (i.e. stapes stiffness, tympanic-membrane stiffness or mass) and its resonance properties. Owing to their high sensitivity, EOAEs could be helpful for an accurate individual multifrequency analysis of middle ear impedance by comparisons under rest and test conditions.

The stapedial reflex as a topographical marker of proximal involvement of the facial nerve in leprosy. A pilot study

This study aimed to determine the parameters necessary for a study of stapedial reflexes in leprosy patients to ascertain if the facial nerve is involved more proximally than the stylomastoid foramen. It involved leprosy patients with and without facial nerve involvement and non-leprosy controls. Clinical examination of the patients' ears, a tympanogram and audiogram to exclude conductive and sensorineural deafness, followed by the measurement of a stapedial reflex and the acoustic reflex threshold, were carried out. The number of absent reflexes and the acoustic reflex thresholds did not differ between the three groups of subjects. A definitive study would be logistically impossible. Suggestions are made as to more exact patient selection in order to demonstrate any stapedial reflex changes due to leprosy. The findings of this study do not suggest that facial nerve pathology extends proximally to the stylomastoid foramen, unless such proximal involvement is subclinical to the detection methods used.

Motoneuron axon distribution in the cat stapedius muscle

Stapedius-motoneuron cell bodies in the brainstem are spatially organized according to their acoustic response laterality, as demonstrated by intracellular labeling of physiologically identified motoneurons [Vacher et al., 1989. J. Comp. Neurol. 289, 401-415]. To determine whether a similar functional spatial segregation is present in the muscle, we traced physiologically identified, labeled axons into the stapedius muscle. Ten labeled axons were visible in the facial nerve and five could be traced to endplates within the muscle. These five axons had 39 observed branches (others may have been missed). This indicates an average innervation ratio (> or = 7.8) which is much higher than that obtained from previous estimates of the numbers of stapedius motoneurons and muscle fibers in the cat. One well-labeled stapedius motor axon innervated only a single muscle fiber. In contrast, two labeled axons had over 10 endings and innervated muscle fibers spread over wide areas in the muscle. Two of the axons branched and coursed through two primary stapedius fascicles, indicating that the muscle zones innervated by different primary fascicles are not functionally segregated. In another series of experiments, retrograde tracers were deposited in individual primary nerve fascicles. In every case, labeled stapedius-motoneuron cell bodies were found in each of the physiologically identified stapedius-motoneuron regions in the brainstem. These observations suggest there is little, if any, functional spatial segregation based on separate muscle compartments in the stapedius muscle, despite there being functional spatial segregation in the stapedius-motoneuron pool centrally.

[The latency and time of the growing reflex in the stapedius muscle]

Latency time and rise time of the stapedius muscle reflex were examined in 32 healthy young people. No significant difference between woman and man were reflected. Obtained result were compared with the previous data of the various authors. Necessity of the own norm estimation of this parameters were emphasized.

Serotoninergic innervation of stapedial and tensor tympani motoneurons

Retrograde tracing and neurotransmitter immunohistochemistry were combined to determine whether serotonin neurons innervated stapedial and tensor tympani motoneurons. With high-power light microscopy, putative axo-somatic and axo-dendritic contacts were observed between serotonin-positive endings and both stapedial and tensor tympani motoneurons, indicating that serotonin neurons terminate on brainstem motoneurons innervating the middle-ear muscles. With this connection, the serotonin system may directly modulate middle-ear muscle activity.

Effects of stapedius-muscle contractions on the masking of auditory-nerve responses

There has been little exploration of the mechanisms by which stapedius muscle contractions reduce the masking of responses to high-frequency sounds by low-frequency sounds. To fill this gap in knowledge, controlled stapedius contractions were elicited with direct shocks in anesthetized cats, and measurements were made of the effects of these contractions on the masking of single auditory-nerve fibers and on the attenuation of middle-ear transmission. The results show that the stapedius-induced reductions of masking can be much larger than the attenuations of low-frequency sound. With a 300-Hz band of masking noise centered at 500 Hz, and signal tones at 6 or 8 kHz, unmasking effects over 40 dB were observed for sounds 100 dB SPL or less. The data suggest that much larger unmasking might occur. The observed unmasking can be explained completely by a linear stapedius-induced attenuation of sound transmission through the middle ear and a nonlinear growth rate of masking for auditory-nerve fibers. No central effects are required. It is argued that the reduction of the upward spread of masking is probably one of the most important functions of the stapedius muscle.

[Contralateral modification of transitory evoked otoacoustic emissions]

BACKGROUND

In recent publications the influence of contralateral white noise on transient evoked otoacoustic emissions (TEOAE) is discussed with regard on contributions of the efferent auditory system.

METHODS

In the present study the effects have been investigated with regards to middle-ear muscles, efferents and cross hearing. TEOAE to monaural 40-80 dB SPL clicks were recorded in normal-hearing adults under simultaneous presentation of 20-60 dB SPL broadband noise to the contralateral ear. Control runs were performed before, during a short break of, and after contralateral stimulation. The control run before contralateral stimulation was used as a reference.

RESULTS

Decrease in TEOAE, and increase in accompanying noise floor, were found to follow the contralateral stimulation. In particular a 1-3 dB decrease was found for contralateral noise levels of 40 and 60 dB SPL, even though the readings at 60 dB only were statistically significant (paired-samples t test, p = 0.05). For both TEOAE and noise floor no systematic dependence on click intensity was seen. The control runs during temporary break and after contralateral noise revealed an increase in both TEOAE and noise floor. As a rule, the TEOAE adapted to the reference within 2-3 min following the cessation of contralateral stimulation, whereas the increased noise floor level was still noted after 10 min.

CONCLUSIONS

Traditionally, suppressing effects of contralateral stimulation on TEOAE have been attributed to cochlear efferents (CEs). Occasionally, the middle-ear muscle and cross hearing involvement have been considered as well. Substantially, the present results and findings of other workers are inconsistent with the basic knowledge of CE functioning: (I) The decrease in TEOAE under contralateral stimulation is in conflict with an increase in cochlear microphonics and summating potentials observed during activation of CEs: (II) contralateral suppression of TEOAE exhibited no significant dependence on the test-stimulus level while the CEs are known to be efficient in the range of the low signal intensities only, and (III) acoustic activation of the CEs can hardly be expected to reach levels sufficient to influence the TEOAE mechanism. The present findings, i.e. decrease in TEOAE and increase in noise floor level, can more reasonably be explained as being mainly attributable to activation of the middle-ear muscles.

[Electrostimulation-induced stapedius reflex. Presentation of a standardized procedure and results of clinical studies]

The electro-tactile elicitation of the stapedius muscle reflex is well known but not used in clinical practice. Despite a complex reflex pathway in the brainstem, correlations between reflex parameters and neurological impairments are still possible. We have developed a method for eliciting the electro-tactile stapedius muscle reflex and automatic analysis of different reflex parameters by using an analog-digital converter and a personal computer-integrated signal analysis system. Clinical investigations were carried out with a group of 55 healthy persons (control group), 13 patients with central facial palsies, 51 patients with multiple sclerosis (MS group) and 24 patients with absent acoustic-stapedius muscle reflexes. Findings demonstrated that the latency between stimulus and beginning of the reflex (L1) was significantly longer in the MS group compared with the control group. The best parameter for dividing the MS group into groups with different disability scores was S, which was the parameter for the increase in the L1 growth curve at different stimulus levels. Reproduction of the parameter S in repeated tests with different locations of the stimulus-electrode was excellent.

Postnatal developmental changes in facial nerve morphology. Computer-aided 3-D reconstruction and measurement

Accurate measurements of the lengths and angles of the facial nerve were obtained in eight normal human temporal bones of varying ages from 7 days to 76 years. Measurements were made on serial histological sections, using computer-aided three-dimensional (3-D) reconstruction. The most noteworthy of the findings demonstrated that both the mastoid portion of the facial nerve and the segment of the facial nerve between the second genu and the divergence of the chorda tympani nerve lengthened with age. The mastoid segment lengthened more significantly than the latter, indicating the facial canal grows more than the facial nerve in its mastoid portion. This difference in growth rates results in the site of the chorda tympani nerve divergence shifting with age relative to the stylomastoid foramen.

Motor unit size of M. stylohyoideus and M. stapedius in the mouse

To elucidate the relation between the caliber of myelinated nerve fibers (MNFs) and motor unit size (i.e., number of muscle fibers per motor unit), motor unit size was measured for the mouse m. stylohyoideus, innervated predominantly by extra large MNFs, and compared with that of the mouse m. stapedius, innervated predominantly by large MNFs. All muscle fibers and MNFs were counted morphologically, and mean motor unit size was calculated for each muscle. The results showed that the average of 12 mean motor unit sizes for the m. stylohyoideus (35.8 +/- 8.2), innervated predominantly by extra large MNFs, was greater than that for the m. stapedius (6.5 +/- 1.3), innervated predominantly by large MNFs.

Ultrastructure of nerve endings in the stapedius muscle of the squirrel monkey

The stapedius muscle of the squirrel monkey (Saimiri sciureus) was studied electron microscopically with regard to neuromuscular junctions, muscle-tendon junctions and tendons. Two types of neuromuscular junctions were identified. The first type which was predominant in number showed well-developed primary and secondary synaptic clefts. They were almost the same with those of other mammalian skeletal muscles. The second type showed a flat-shaped junction with fewer secondary synaptic clefts. Muscle-tendon junctions were characterized by many interdigitations between the muscle fibers and tendon. The tendon was composed of collagen fibers, approximately 70-100 nm in diameter. Muscle spindles were not present, while nerve endings resembling the Golgi tendon organ were found adjacent to the tendon and the muscle-tendon junctions. These findings suggest that a feedback system may exist and regulate the fine contractile function of this muscle.

Central distribution of the stapedius motoneurons in the rat--a study of topographical anatomy and HRP transport experiments

A detailed anatomical description of the stapedius muscle area in the rat was given and on this basis the central distribution of the neurons innervating the stapedius muscle was investigated by the horseradish peroxidase transport method. The stapedius muscle showed a circum-pennate structure with a para-centrally located tendon and was composed of about 500 muscle fibers. It contained no special sensory structures and was innervated by a single branch of the facial nerve consisting of myelinated axons. Horseradish peroxidase experiments provided the following findings. The stapedius muscle received few sensory components. The stapedius motoneurons resided in the brainstem ipsilaterally in a column-like region ventromedial to the facial motor nucleus. Some of them (up to 6%) were scattered rostrally as far as the level of the facial nerve exit. Almost all the stapedius motoneurons showed fusiform morphology with significantly smaller mean diameter and lesser size variation than the facial nucleus motoneurons. The number of the stapedius motoneurons per animal on one side amounted about 150 which was about the same as that of axons in the stapedius nerve. Their axons except for those of rostrally scattered ones shared the intramedullary route with axons of the facial nucleus motoneurons.

The innervation of the middle ear muscles of the rat

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.

Brainstem facial-motor pathways from two distinct groups of stapedius motoneurons in the cat

To determine the brainstem origins and axonal routes of stapedius motoneurons, we labeled motoneurons by injecting cat stapedius muscles with horseradish peroxidase. Some injections were made in normal cats and some in cats in which the middle segment of the internal facial genu had been cut. By tracing labeled axons and by comparing the locations of labeled cell bodies in normal and lesioned cats, we divided stapedius motoneurons into two groups: "perifacial" and "accessory." Perifacial stapedius motoneurons have cell bodies located around the motor nucleus of the facial nerve and axons which follow the classical course of facial motor axons through the internal genu of the facial nerve. Accessory stapedius motoneurons have cell bodies near the descending facial motor root and axons which ascend to the rostral tip of the internal facial genu, abruptly reverse direction, and then join the descending facial motor root. The sharply hooked course of axons of accessory stapedius motoneurons is similar to the course of axons from other accessory nuclei of cranial nerves V-VII. Our present results, with those of McCue and Guinan (J. Neurophysiol. 60:1160-1180, '88), demonstrate that cats have two groups of stapedius motoneurons which can be separated anatomically by the locations of their cell bodies or by the courses of their axons, and which, on the average, have different response properties.

Aberrant reinervation of the stapedius muscle following facial palsy

This case report describes the clinical course of a patient with Ramsay Hunt Syndrome. Partial recovery of the lower motor neuron facial palsy was associated with decreased hearing and a reduction of the middle ear compliance on voluntary contraction of the facial musculature. It is suggested that this is due to misdirection of regenerating nerve fibres, normally destined for facial muscles, to stapedius muscle.

The recruitment order of stapedius motoneurons in the acoustic reflex varies with sound laterality

In many muscles, motor units are recruited in a fixed order with increasing strength of muscular contraction. We show that for the stapedius muscle of the cat, vastly different recruitment orders can be obtained, depending on which ear is acoustically stimulated. The data support the idea that the distribution of inputs to a motoneuron pool can be inhomogeneous and a significant factor in determining recruitment order.

[Innervation of the stapedius muscle. A study using horseradish peroxidase in the guinea pig]

The numbers and locations of motoneurons to the stapedius muscle were determined by retrograde axonal transport of horseradish peroxidase (HRP) in the guinea pig. With the aid of a drill the bulla was opened and the stapedius muscle exposed. HRP solution (0.2-0.3 microliter) was injected into the muscle. Following a survival time of 1-2 days the animals were perfused with a fixative and the brains were removed. Frozen sections were reacted fro the demonstration of HRP using tetramethylbenzidine as chromagen. All labelled neurons were observed not within the traditionally recognised facial nucleus but ventral and medial to the latter. All parent cells (up to 64) lay ipsilateral to the injection. These neurons were remarkably smaller in shape than the neurons in the facial nucleus.

The acoustic middle ear muscle reflex in albino rats

The acoustic middle ear muscle reflex was studied in albino rats anesthetized with chloralose. The best frequency of the reflex and the threshold at this frequency were on average about 3 kHz and 57 dB SPL, respectively. The threshold increased as frequency increased above, and decreased below, the best frequency at a rate of about 20 dB/octave. Above about 12 kHz, the muscular response showed instability and habituation. Thresholds were similar between stapedius and tensor tympani reflexes and between ipsilateral and contralateral reflexes. The middle ear transmission loss due to the reflex was the greatest and nearly constant below about 1 kHz, where the loss was about 18 dB at the maximal stimulation. Above this frequency the loss decreased as frequency increased up to 20 kHz. Thus the reflex, unlike that in other animals, suppressed transmission over the whole range of reflex-eliciting frequencies. The transfer function of the reflex had a well damped low-pass characteristic with a cut-off frequency of about 20 Hz. From the above characteristics of the reflex, the role of the rat's tympanic muscles in improving ultrasonic hearing under ambient noises was suggested.

Number and distribution of stapedius motoneurons in cats

Cell bodies of stapedius motoneurons were identified by retrograde transport of horseradish peroxidase (HRP) following injections into the stapedius muscle. Large injections were made in an attempt to label all stapedius motoneurons. To control for labeling of non-stapedial neurons resulting from spread of HRP, we determined the locations of brainstem neurons labeled by HRP applied to the facial nerve, the chorda tympani nerve, the auricular branch of the vagus nerve, the tensor tympani muscle, and the cochlea. In three cats analyzed in detail, 1,133-1,178 neurons projecting to the stapedius muscle were identified. Arguments are given which suggest that in these three cats all stapedius motoneurons were labeled. The labeled stapedius neurons may all be motoneurons because they all stain positively for acetylcholinesterase and have medium-coarse Nissl bodies. Most stapedius motoneurons were located around the motor nucleus of the facial nerve. Staphedius motoneurons were also found near the descending limb of the facial-nerve root, in the peri-olivary neuropil, and in the reticular formation with the ascending fibers of the facial-nerve root.

Identification of stapedius muscle motoneurons in squirrel monkey and bush baby

The location of brainstem neurons which mediate the stapedius reflex was identified by injecting horseradish peroxidase into the stapedius muscle of squirrel monkeys and bush babies. Retrogradely labeled neurons, arranged in a one- to three-cell column, were found medial to the main facial motor nucleus in squirrel monkeys and ventral to it in bush babies. Nissl, protargol, and acetylcholinesterase stains were subsequently used to identify and describe this unique column of cells. It was found that staining characteristics, as well as shape, size, and location, distinguish stapedius muscle motoneurons from closely associated cell groups. Furthermore, stapedius muscle motoneurons are morphologically similar to periolivary cells and morphologically dissimilar to cells within the facial motor nucleus.

Intra-aural reflexes elicited by a cochlear prosthesis in monkeys

Objective audiological tests are needed for pre- and postsurgical evaluation of cochlear prosthesis patients who are unable to give reliable subjective responses. In this study we demonstrated that contralateral intra-aural reflexes were elicited by a cochlear prosthesis in the monkey. Reflex variables measured include threshold, latency and amplitude. These findings indicate that the electrically elicited intra-aural reflex response may be useful to evaluate the peripheral auditory system in subjects with sensory deafness.

Orientation of stapedial nerve fibers in the facial nerve trunk

By staining with toluidine blue and vital staining with methylene blue, the topographical orientation of the stapedial nerve fibers in the facial nerve trunk was visualized in adult guinea pigs. The stapedial nerve with its perineural sheath emerges from the facial nerve trunk in the posteromedial aspect of the upper part of the vertical portion. Toward the horizontal portion, the stapedial nerve fibers gradually ramify, lose their investments, and intermingle with other nerve fibers inside the facial nerve trunk. In the labyrinthine portion, the stapedial nerve fibers are distributed diffusely near the surface along the posterior, medial, and anterior margins of the facial nerve trunk. These findings lead to the conclusion that the stapedial nerve fibers of guinea pigs do not form a single funiculus throughout the intratemporal course of the facial nerve.

Effects of facial nerve compression on the stapedial nerve

A series of experiments on guinea pigs was conducted to determine the prognostic dependability of the stapedial reflex measurement in Bell's palsy. Comparison of the threshold of evoked electromyographic response of the stapedius muscle with that of the orbicularis oris muscle revealed that the stapedial nerve had a lower excitability than did the nerve innervating the orbicularis oris muscle. This lower excitability correlates with the histological finding that the stapedial nerve fibers have a smaller average diameter. The results indicate the resistance of the stapedial nerve to injury of the facial nerve. Functional recovery after cramping of the facial nerve tended to occur later in the stapedial nerve than in the nerve innervating the orbicularis oris muscle. The resistance of the stapedial nerve and the longer period required to recover function in this nerve were factors influencing the prognostic ambiguity of this test.

The locations of stapedius and tensor tympani motoneurons in the cat

The numbers and locations of motoneurons to the stapedius and tensor tympani muscles were determined by retrograde transport of horseradish peroxidase. Stapedius motoneurons lay outside the traditionally recognized facial nucleus, in several distinct locations: (1) in the interface between the facial nucleus and the superior olive; (2) in a thin, scattered lamina of somewhat smaller cells spread dorsal to the facial nucleus; and (3) in a cluster located ventromedial to the rostral third of the facial nucleus. Some cells also lay dorsal to the superior olive or scattered in the reticular formation, just medial to the descending loop of the facial nerve. Tensor tympani motoneurons also lay outside the traditionally recognized trigeminal motor nucleus, in an area just ventral to it. Both motoneuron pools were large, producing innervation ratios that establish stapedius and tensor tympani among the most finely innervated muscles yet studied. The degree of intermingling of large and small cells in these pools may explain, in part, why it has been easier to identify slow muscle fibers physiologically in tensor tympani than in stapedius.

Identification of the motoneurons innervating the stapedius muscle in Gallus gallus: a horseradish peroxidase study

The location of the stapedial motoneurons in Gallus gallus was investigated by means of the retrograde transport of HRP, injected into the stapedius muscle. The labeled neurons are located in both the ventral and dorsal divisions of the VII nerve nucleus, in a lateral and ventral position respectively, facing the superior olivary nucleus. The neurons are distributed in two size classes. The functional implications of these findings are discussed, in relation both to the absence of the acoustic stapedial reflex in birds and to the functional properties of the stapedius muscle.

[A contribution to the "acoustic-mechanical effect" of the ipsilateral registered stapedius-reflex (author's transl)]

The ipsilateral registered stapedius reflex is often reduced for intensity levels greater than 105 dB (SPL). Because of a reduced impedance, that is not influenced by the reflex arc, an "inverse" reflex curve can be registered. This phenomenon described by Lehnhardt et al. (1) as a "acoustic'mechanical effect" occurs without latency and interferes at levels higher than 105 dB (SPL) with the reflectory impedance change. This effect often dominates and causes a pronounced inverse ipsilateral reaction characterized by an impedance reduction. Frequently middle ears with a steep gradient of the tympanogram show an acoustic-mechanical effect. The missing reflectory component of the impedance change in deafness, in otosclerosis and in facial nerve disorders mostly causes an inverse impedance change. Investigations in temporal bone, in middle-ear models and from the middle ear of the guinea pig led to the assumption, that inverse impedance changes be related to coupling phenomenons of the tympanic-membrane and the ossicles. Excluding artifacts of the measuring device and investigating different states of the middle ear, it can be concluded, that these phenomenons are characteristic for intensity levels greater than 105 dB (SPL).

[Contralateral stapedial reflex in children, adults and high-frequency loss hearing impaired patients (author's transl)]

Contralateral stapedius reflexes were measured in 332 patients. Results were analysed and statistically checked with regard to amplitude, slope of reflex in the recovery phase and partial and total reflex area. The results seem to indicate that none of the measure parameters have any direct diagnostic value. Optimal results measuring contralateral stapedius reflex are to be expected at 1000 Hz.

[Stapedius-reflex-audiometry by measuring the threshold for the re-inforcement of a pre-existing reflex (author's transl)]

The possibilities for predicting the pure tone threshold on the basis of the acoustic stapedius reflex threshold are improved by using two stimuli, e.g., white noise and pure tone, or two pure tones according to Sesterhenn and Breuninger. The principle of the monaural loudness summation with regard to the stapedius reflex was systematically examined with the aim to obtain the best suited range for clinical use with a double tone stimulus. The following procedure proved to be the most reliable one for an objective audiometry: A distinct reflex is produced by a basic pure tone of 4 kHz, 10 dB above its reflex threshold. The tone to be examined (1 kHz) was added with a delay of 0.5 s. A 1 kHz tone 28 dB below the reflex threshold (median) produced a clear re-inforcement of the basic reflex. Objective extrapolation possibilities from stapedius reflex threshold to pure tone threshold are thereby improved.

[Study of the wave-form of the stapedial reflex in response to sonorous stimuli of different intensity]

The threshold of the acoustic reflex and its pattern in response to different intersity stimuli, was investigated by means of the signal-averaging technique in 10 normal ears. Trains of tone bursts between 100 and 0 dB HL were used. The frequencies tested were 500, 1000, 2000 and 4000 Hz. In all 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 < 80 dB HL the pattern of the reflex was monophasic, characterized by a single positive peak (latency between 120 and 170 msec).

Stapedius reflex in multiple sclerosis

An analysis was carried out of recordings of the crossed stapedial reflex response in a series of normal subjects and 30 patients with multiple sclerosis. Parameters measured included latency, contraction time, amplitude, and relaxation time. Criteria derived from the normal group and applied to the multiple sclerosis group showed clear evidence of abnormality in 10 patients. This can be attributed to lesions in the pathways of the second and third order neurones in the trapezoid body and superior olivary complex. Present evidence suggests that further refinements in the measurement technique might significantly increase the detection rate of abnormalities in multiple sclerosis.

Contraction properties and functional morphology of the avian stapedius muscle

The influence of the stapedius muscle contraction on middle ear volume and acoustic impedance was investigated in the chicken, Gallus gallus. The time course of twitch responses to electrical stimulation (measured as volume and impedance changes) was found to be largely independent of the stimulus voltage, having a contraction time of 22 ms and a half-relaxation time of 22 ms. The stapedius muscle was therefore characterized as a fast twitch muscle. Slow contraction properties were also revealed: A summation of responses to repetitive stimulation beginning at 2.5 Hz and a slow decline to baseline were seen in volume and impedance change recordings. The morphological characteristics were consonant with that of a homogeneously fast muscle: Only fibres with high ATPase activity were identified and no fibres with "en grappe" or multiple innervation were observed. The slow characteristics were suggested to be due to visco-elastic elements in the middle ear. The chicken stapedius muscle is suggested to be analogous to both the stapedius and the tensor tympani of mammals.

Quantitative studies on the nervus stapedius of the mouse with the electron microscope

A nerve fiber count analysis was performed with the electron microscope on the nerve to the stapedius in seven mice. On an average, 77 (81%) of the total nerve fibers (95) were myelinated and 18 (19%) unmyelinated. The nerve consisted mostly of large myelinated fibers, minor diameters of which measured 3-4 micrometer most frequently like the large fiber zone of the motor root of the facial nerve and the facial trunk. The nerve fibers to the stapedius come from the large fiber zone of the facial nerve trunk.