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

Click evoked middle ear muscle reflex: Spectral and temporal aspects

This study describes a time series-based method of middle ear muscle reflex (MEMR) detection using bilateral clicks. Although many methods can detect changes in the otoacoustic emissions evoking stimulus to monitor the MEMR, they do not discriminate between true MEMR-mediated vs artifactual changes in the stimulus. We measured MEMR in 20 young clinically normal hearing individuals using 1-s-long click trains presented at six levels (65 to 95 dB peak-to-peak sound pressure level in 6 dB steps). Changes in the stimulus levels over the 1 s period were well-approximated by two-term exponential functions. The magnitude of ear canal pressure changes due to MEMR increased monotonically as a function of click level but non-monotonically with frequency when separated into 1/3 octave wide bands between 1 and 3.2 kHz. MEMR thresholds estimated using this method were lower than that obtained from a clinical tympanometer in ∼94% of the participants. A time series-based method, along with statistical tests, may provide additional confidence in detecting the MEMR. MEMR effects were smallest at 2 kHz, between 1 and 3.2 kHz, which may provide avenues for minimizing the MEMR influence while measuring other responses (e.g., the medial olivocochlear reflex).

Quantification of the stapedial reflex reveals delayed responses in autism

Autism is a developmental disorder characterized, in part, by sensory abnormalities. It is well established that most if not all patients with autism have problems with auditory processing, ranging from deafness to hyperacusis, and physiological testing of auditory function (i.e. auditory brain stem responses) implicates brain stem dysfunction in autism. Additionally, previous research from this lab has revealed significantly fewer auditory brain stem neurons in autistic subjects as young as 2 years of age. These observations have led us to hypothesize that objective, noninvasive measures of auditory function can be used as an early screening tool to identify neonates with an elevated risk of carrying a diagnosis of autism. Here, we provide a detailed quantitative investigation of the acoustic stapedial reflex (ASR), a three- or four-neuron brain stem circuit, in young autistic subjects and normal developing controls. Indeed, we find significantly lower thresholds, responses occurring at significantly longer latency and right-left asymmetry in autistic subjects. The results from this investigation support deficits in auditory function as a cardinal feature of autism and suggest that individuals with autism can be identified by their ASR responses.

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.

[Is the middle ear the first filter of frequency selectivity?]

INTRODUCTION AND OBJECTIVES

The cochlea has traditionally been considered as the first frequency selection filter in the auditory pathway due to the contraction of its external ciliated cells. Yet, much evidence has emerged from work carried out during experiments with animals, some of which is anatomical (connections between the auditory pathway and motor nuclei of the middle ear muscles) and other physiological, which indicates that the middle ear might be the first filter through which specific sounds from noisy environments may initially be isolated.

METHODS

In cooperation with the Department of Mechanical Engineering of the Technical School of Industrial Engineering at the University of Valladolid (UVa) we have developed and refined a new admittance meter capable of evaluating changes in impedance that occur in the human middle ear depending on frequency. Using this device we have measured variation in impedance in 7 otologically healthy volunteers submitted to a varied range of sound environments.

RESULTS

We have found that hearing impedance is not constant but rather that the attention offered by the examined subjects when following a conversation in a noisy environment leads to variations in hearing impedance at high frequencies.

CONCLUSIONS

In the light of these findings we feel that the middle ear does not play a merely passive role in hearing but rather that the contraction of the endotympanic muscles makes possible variations in impedance such that the resonance frequency of the ear shifts towards higher frequencies, thus enhancing sound discrimination in noisy environments.

Diverse synaptic terminals on rat stapedius motoneurons

Stapedius motoneurons (SMN) mediate the contraction of the stapedius muscle, which protects the inner ear from injury and reduces the masking effects of background noise. A variety of inputs to SMNs are known to exist, but their terminal ultrastructure has not been investigated. We characterized the synaptic terminals on retrogradely labeled SMNs found just ventromedial to the facial motor nucleus. About 80% of the terminals contained round synaptic vesicles. One type (Sm Rnd) had small, round vesicles filling the terminal with occasional dense core vesicles and formed an asymmetric synapse. Sm Rnd terminals were small with lengths of apposition to the SMN less than 3 microm. Partial reconstructions from serial sections demonstrated that these terminals formed up to three synapses per terminal. Another terminal type (Lg Rnd) had large, round vesicles and asymmetric synapses. Most Lg Rnd terminals were small but some were extensive, e.g., abutting the SMN for up to 10 microm. One of these terminals formed at least seven synapses. Another terminal type (Pleo) had pleomorphic vesicles and symmetric active zones that, in some cases, were invaginated by spines from the SMN. A fourth uncommon terminal type (Het Rnd) had round vesicles of heterogeneous sizes and asymmetric synapses. A fifth rare terminal type (Cist) had large, round vesicles and an accompanying subsurface cistern in the SMN. These were generally the same kinds of terminals found on other motoneurons, but the high proportion of round vesicle synapses indicate that SMNs receive mostly excitatory inputs.

Motoneurons of the stapedius muscle in the guinea pig middle ear: afferent and efferent transmitters

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the stapedius muscle of the middle ear in order to gain more insight into the neuronal regulation of the muscle. To identify motoneurons, we injected the fluorescent neuronal tracer Fluorogold (FG) into the muscle after preparation of the middle ear in adult guinea pigs. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located medial and ventral to the nucleus of the facial nerve ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that the majority contains calcitonin gene-related peptide. Our data further demonstrate close spatial relationships of motoneurons to structures immunoreactive to either serotonin, substance P or neuronal nitric oxide and reveal that these neurons are under neuropeptidergic and nitrergic influence.

Central auditory pathways mediating the rat middle ear muscle reflexes

The middle ear muscle (MEM) reflexes function to protect the inner ear from intense acoustic stimuli and to reduce acoustic masking. Sound presented to the same side or to the opposite side activates the MEM reflex on both sides. The ascending limbs of these pathways must be the auditory nerve fibers originating in the cochlea and terminating in the cochlear nucleus, the first relay station for all ascending auditory information. The descending limbs project from the motoneurons in the brainstem to the MEMs on both sides, causing their contraction. Although the ascending and descending pathways are well described, the cochlear nucleus interneurons that mediate these reflex pathways have not been identified. In order to localize the MEM reflex interneurons, we developed a physiologically based reflex assay in the rat that can be used to determine the integrity of the reflex pathways after experimental manipulations. This assay monitored the change in tone levels and distortion product otoacoustic emissions within the ear canal in one ear during the presentation of a reflex-eliciting sound stimulus in the contralateral ear. Preliminary findings using surgical transection and focal lesioning of the auditory brainstem to interrupt the MEM reflexes suggest that MEM reflex interneurons are located in the ventral cochlear nucleus.

The development of vestibulocochlear efferents and cochlear afferents in mice

We have reinvestigated the embryonic development of the vestibulocochlear system in mice using anterograde and retrograde tracing techniques. Our studies reveal that rhombomeres 4 and 5 include five motor neuron populations. One of these, the abducens nucleus, will not be dealt with here. Rhombomere 4 gives rise to three of the remaining populations: the facial branchial motor neurons; the vestibular efferents; and the cochlear efferents. The migration of the facial branchial motor neurons away from the otic efferents is completed by 13.5 days post coitum (dpc). Subsequently the otic efferents separate into the vestibular and cochlear efferents, and complete their migration by 14.5 dpc. In addition to their common origin, all three populations have perikarya that migrate via translocation through secondary processes, form a continuous column upon completion of their migrations, and form axonal tracts that run in the internal facial genu. Some otic efferent axons travel with the facial branchial motor nerve from the internal facial genu and exit the brain with that nerve. These data suggest that facial branchial motor neurons and otic efferents are derived from a common precursor population and use similar cues for pathway recognition within the brain. In contrast, rhombomere 5 gives rise to the fourth population to be considered here, the superior salivatory nucleus, a visceral motor neuron group. Other differences between this group and those derived from rhombomere 4 include perikaryal migration as a result of translocation first through primary processes and only then through secondary processes, a final location lateral to the branchial motor/otic efferent column, and axonal tracts that are completely segregated from those of the facial branchial and otic efferents throughout their course inside the brain. Analysis of the peripheral distribution of the cochlear efferents and afferents show that efferents reach the spiral ganglion at 12.5 dpc when postmitotic ganglion cells are migrating away from the cochlear anlage. The efferents begin to form the intraganglionic spiral bundle by 14.5 dpc and the inner spiral bundle by 16.5 dpc in the basal turn. They have extensive collaterals among supporting cells of the greater epithelial ridge from 16.5 dpc onwards. Afferents and efferents in the basal turn of the cochlea extend through all three rows of outer hair cells by 18.5 dpc. Selective labeling of afferent fibers at 20.5 dpc (postnatal day 1) shows that although some afferents are still in early developmental stages, some type II spiral ganglion cells already extend for long distances along the outer hair cells, and some type I spiral ganglion cells end on a single inner hair cell. These data support previous evidence that in mice the early outgrowth of afferent and efferent fibers is essentially achieved by birth.

Development of the labyrinthine efferent system

The data presented here show that labyrinthine and facial branchiomotor efferent cells in the chicken and the mouse become postmitotic overlappingly, both spatially and temporally. Differential migration of labyrinthine efferents and facial motoneurons leads to the already described distinct distribution of labyrinthine efferents and facial motoneurons in adult brains. Differences exist between the chicken and the mouse with respect to the origin of labyrinthine efferents (rhombomere 4 and 5 for the chicken; rhombomere 4 alone for the mouse) and the way contralateral labyrinthine efferents form (migration across the floor plate in the chicken; extension of an axon across the floor plate in the mouse). The different routes taken by migrating motoneurons may all be mediated by substances released from the floor plate, some of which were recently characterized. Labyrinthine efferent axons and facial motoneuron axons segregate at distinctly different areas in the chicken and mouse: outside the brain in the former and inside the brain in the latter. Examination of the possible basis for pathway selection tends to support the idea that efferents use intact afferent fibers as highways for their navigation to distinct sensory epithelia.

Stapedial reflex in amyotrophic lateral sclerosis

OBJECTIVE

To examine mechanisms controlling the stapedial reflex in patients with amyotrophic sclerosis (ALS).

METHODS

The stapedial reflex was examined using impedance audiometry in 38 patients with sporadic ALS and in 25 age matched controls.

RESULTS

All patients showed normal reflex decay test results. There were no significant differences between patients with ALS and control subjects in reflex threshold, latency, amplitude, or contraction time (C50). Although each reflex variable in the patients with classic or progressive muscular atrophy types of ALS showed no significant difference from that in control subjects, the patients with bulbar type ALS showed significantly longer latency, C50, and retraction time (D50), and significantly lower amplitude than control subjects. Three types of abnormal reflex waveforms (polyphasic, abnormally delayed retraction, and abnormally early retraction) were noted in six patients.

CONCLUSION

The subclinical involvement of the stapedius motor neurons or of the supranuclear stapedius motor system might be responsible for the abnormalities of the stapedial reflex in ALS.

The stapedial reflex in cephalic tetanus

Three patients are presented with cephalic tetanus following injuries to the face. Two were adults and one a child. All three had bilateral VIIth cranial nerve involvement and one patient also presented with involvement of the IIIrd, IVth and VIth cranial nerves. The patients initially an ipsilateral VIIth cranial nerve weakness which later in the course of the illness developed into hyperactivity of the VIIth cranial nerve. The contralateral VIIth cranial nerve demonstrated a similar pattern. The stapedial reflex was tested serially. The stapedius muscle activity preceded that of the muscles of the face thus serving as an indicator of improvement or impending deterioration. Deflections measuring more than 1 cm, on stapedial reflex threshold testing, were indicative of stapedial reflex spasm. In the stapedial reflex decay test, both ill-sustained (intermittent) and sustained spasms of the stapedius muscle were seen.

Fiber pathways and branching patterns of biocytin-labeled olivocochlear neurons in the mouse brainstem

Olivocochlear neurons have somata in the superior olivary complex in the brainstem and project fibers to the cochlea. The purpose of the present study was to demonstrate the fiber pathways and branching patterns of olivocochlear fibers within the brainstem. Olivocochlear fibers were labeled by extracellular injections of biocytin into the cochlea of mice. The injections labeled two populations of olivocochlear fibers. Thin olivocochlear fibers arose from small somata of the lateral olivocochlear group located ipsilaterally in the lateral superior olive. Thick olivocochlear fibers arose from larger somata of the medial olivocochlear group located bilaterally in the periolivary nuclei. The lateral olivocochlear and medial olivocochlear fibers had similar courses but differed in their branching patterns. Branches from lateral olivocochlear fibers terminated near their somata of origin in the lateral superior olive or in the lateral vestibular nucleus. Branches from medial olivocochlear fibers terminated in the inferior vestibular nucleus or in the cochlear nuclear complex. A few branches from medial olivocochlear fibers projected to the contralateral side. Although they project primarily to the cochlea, olivocochlear neurons also give off branches to a variety of nuclei in the brainstem, thus involving auditory and non-auditory nuclei in the olivocochlear reflex system.

Location of motoneurons innervating the middle ear muscle of the chicken, (Gallus domesticus)

The motoneuron pool for the musculus columellae, the avian equivalent to the m. stapedius, was identified by retrograde labeling with WGA-HRP. It consists of a discrete group of approximately 65 neurons located along the dorsolateral border in the ventral subnucleus of the facial nuclear complex. Other facial motoneurons were only labeled when diffusion of the tracer into neighbor structures was not excluded. The dorsal subnucleus of the facial nerve innervates the m. depressor mandibulae.

Posteroventral cochlear nucleus projections to olivocochlear neurons

The presence of ascending auditory inputs from the posteroventral cochlear nucleus (PVCN) to olivocochlear neurons was examined in guinea pig by using the combination Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde and horseradish peroxidase (HRP) retrograde tract-tracing technique. By labeling the somata of olivocochlear neurons after injection of HRP into the cochlea and simultaneously labeling terminal endings of PVCN efferent neurons after injection of PHA-L into PVCN, we observed neuronal connections between these two elements within all regions of the superior olivary complex known to contain olivocochlear neurons. These regions include the superior paraolivary nucleus, medial nucleus of the trapezoid body, lateral superior olive, and periolivary regions. All possible projection patterns regarding side of input and output of both large (four combinations) and small (two combinations) olivocochlear neurons were observed. However, the most frequently observed pattern was the PVCN projection to a contralaterally located and contralaterally projecting, large olivocochlear neuron. Thus the most prevalent pattern demonstrated a feedback pathway that crossed the brainstem twice. Additional patterns demonstrated pathways that fed back to the same cochlea as well as pathways that fed forward to the opposite cochlea.

Intracellularly labeled stapedius-motoneuron cell bodies in the cat are spatially organized according to their physiologic responses

This study examines whether the locations of stapedius-motoneuron cell bodies are correlated with their responses to sound. Single-unit recordings and injections of horseradish peroxidase were made in axons of stapedius motoneurons in the fascicles which run from the facial nerve to the stapedius muscle in the cat. Single units were characterized physiologically by their responses to ipsilateral, contralateral, and binaural sounds. Labeled cell bodies (N = 28) were found in all of the brainstem regions previously identified as containing stapedius motoneurons. Motoneurons characterized as having similar response properties had cell bodies in relatively circumscribed locations. Most (eight of 12) motoneurons excited by sound in either ear had cell bodies in a narrow band around the facial nucleus. Most (seven of eight) motoneurons excited by ipsilateral but not contralateral sound had cell bodies in the cleft between the superior olivary complex and the facial nucleus. All four motoneurons excited by contralateral but not ipsilateral sound had cell bodies located ventromedial to the facial nucleus. The three motoneurons excited only by binaural sound had cell bodies located dorsal to the superior olivary complex. (Two of these were also in the cleft between the superior olivary complex and the facial nucleus.) The cell body of the one motoneuron showing activity in the absence of sound stimulation was located dorsolateral to the facial nucleus. These results show that the cell bodies of stapedius motoneurons with similar electrophysiologic properties tend to have similar locations in the brainstem. The results are consistent with the idea that the stapedius-motoneuron pool is divided into subgroups that are spatially segregated in terms of their patterns of input from the two ears.