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The Journal of Neuroscience : the... Feb 2014Exposure to loud sound (acoustic overexposure; AOE) induces hearing loss and damages cellular structures at multiple locations in the auditory pathway. Whether AOE can...
Exposure to loud sound (acoustic overexposure; AOE) induces hearing loss and damages cellular structures at multiple locations in the auditory pathway. Whether AOE can also induce changes in myelin sheaths of the auditory nerve (AN) is an important issue particularly because these changes can be responsible for impaired action potential propagation along the AN. Here we investigate the effects of AOE on morphological and electrophysiological features of the centrally directed part of the rat AN projecting from the cochlear spiral ganglion to brainstem cochlear nuclei. Using electron microscopy and immunocytochemistry, we show that AOE elongates the AN nodes of Ranvier and triggers notable perinodal morphological changes. Compound action potential recordings of the AN coupled to biophysical modeling demonstrated that these nodal and perinodal structural changes were associated with decreased conduction velocity and conduction block. Furthermore, AOE decreased the number of release sites in the cochlear nuclei associated with the reduced amplitudes of EPSCs evoked by AN stimulation. In conclusion, AN dysmyelination may be of fundamental importance in auditory impairment following exposure to loud sound.
Topics: Action Potentials; Animals; Cochlear Nerve; Disease Models, Animal; Evoked Potentials, Auditory; Excitatory Postsynaptic Potentials; Female; Hearing Loss, Noise-Induced; Male; Myelin Sheath; Patch-Clamp Techniques; Ranvier's Nodes; Rats; Rats, Wistar
PubMed: 24523557
DOI: 10.1523/JNEUROSCI.3977-13.2014 -
The Journal of International Advanced... Dec 2017Cochlear implantation is a clinical and cost-effective treatment for severe hearing loss. Cochlear nerve size assessment by magnetic resonance imaging (MRI) has been... (Comparative Study)
Comparative Study
OBJECTIVE
Cochlear implantation is a clinical and cost-effective treatment for severe hearing loss. Cochlear nerve size assessment by magnetic resonance imaging (MRI) has been investigated for use as a prognostic indicator following cochlear implantation. This study aimed to further that research by assessing nerve size in normal-hearing adults for symmetry.
MATERIALS AND METHODS
Patients with tinnitus presenting to our center retrospectively had their nerve size assessed by MRI.
RESULTS
The study found no significant differences between right and left cochlear nerves in normal-hearing adults, supporting our hypothesis of symmetry in these individuals. This was a previously unproven and uninvestigated hypothesis.
CONCLUSION
Nerve size assessment should remain an active area of research in otological disease.
Topics: Adult; Ambulatory Care; Cochlear Implantation; Cochlear Implants; Cochlear Nerve; Female; Humans; Magnetic Resonance Imaging; Male; Retrospective Studies; Tinnitus
PubMed: 29283094
DOI: 10.5152/iao.2017.4170 -
Journal of Neurophysiology Aug 2020Hearing loss caused by noise exposure, ototoxic drugs, or aging results from the loss of sensory cells, as reflected in audiometric threshold elevation. Animal studies...
Hearing loss caused by noise exposure, ototoxic drugs, or aging results from the loss of sensory cells, as reflected in audiometric threshold elevation. Animal studies show that loss of hair cells can be preceded by loss of auditory-nerve peripheral synapses, which likely degrades auditory processing. While this condition, known as cochlear synaptopathy, can be diagnosed in mice by a reduction of suprathreshold cochlear neural responses, its diagnosis in humans remains challenging. To look for evidence of cochlear nerve damage in normal hearing subjects, we measured their word recognition performance in difficult listening environments and compared it to cochlear function as assessed by otoacoustic emissions and click-evoked electrocochleography. Several electrocochleographic markers were correlated with word scores, whereas distortion product otoacoustic emissions were not. Specifically, the summating potential (SP) was larger and the cochlear nerve action potential (AP) was smaller in those with the worst word scores. Adding a forward masker or increasing stimulus rate reduced SP in the worst performers, suggesting that this potential includes postsynaptic components as well as hair cell receptor potentials. Results suggests that some of the variance in word scores among listeners with normal audiometric threshold arises from cochlear neural damage. Recent animal studies suggest that millions of people may be at risk of permanent impairment from cochlear synaptopathy, the age-related and noise-induced degeneration of neural connections in the inner ear that "hides" behind a normal audiogram. This study examines electrophysiological responses to clicks in a large cohort of subjects with normal hearing sensitivity. The resultant correlations with word recognition performance are consistent with an important contribution cochlear neural damage to deficits in hearing in noise abilities.
Topics: Action Potentials; Adolescent; Adult; Audiometry, Evoked Response; Cochlear Nerve; Hair Cells, Auditory; Hearing Loss; Humans; Middle Aged; Noise; Perceptual Masking; Recognition, Psychology; Speech Perception; Young Adult
PubMed: 32639924
DOI: 10.1152/jn.00016.2020 -
Journal of Anatomy Aug 1988The medial portion of the cochlear nerve of the rat contains astrocytes, oligodendrocytes and neurons. These neurons form what has been called the 'acoustic nerve...
The medial portion of the cochlear nerve of the rat contains astrocytes, oligodendrocytes and neurons. These neurons form what has been called the 'acoustic nerve nucleus'. This nucleus has been studied here at the electron microscopic level. Its neurons are large and round, showing an eccentric nucleus, fibrillary bodies and rough endoplasmic reticulum which is not arranged in stacks. The somata and dendrites receive synaptic endfeet which can be classified into three groups according to vesicle size and shape. In general, the ultrastructural characteristics of these cells are similar to those of bushy cells as reported by other authors. The 'acoustic nerve nucleus' can be considered to be the most peripheral part of the anterior ventral cochlear nucleus.
Topics: Animals; Astrocytes; Cochlear Nerve; Dendrites; Endoplasmic Reticulum; Female; Male; Microscopy, Electron; Neurons; Oligodendroglia; Rats; Rats, Inbred Strains; Synapses; Vacuoles; Vestibular Nuclei
PubMed: 3248967
DOI: No ID Found -
The Journal of International Advanced... Aug 2018The objective of this research was to investigate the possible relationship between tinnitus and certain bony inner ear structures using computed tomography (CT). (Comparative Study)
Comparative Study
OBJECTIVE
The objective of this research was to investigate the possible relationship between tinnitus and certain bony inner ear structures using computed tomography (CT).
MATERIALS AND METHODS
This was a prospective, controlled, double-blind study. The subjects of the study were divided into the following three groups: group 1 (G1), patients with unilateral sensorineural hearing loss (SNHL) and unilateral non-pulsatile tinnitus in the same ear; group 2 (G2), patients with normal hearing and unilateral non-pulsatile tinnitus; and group 3 (G3), healthy volunteers with neither tinnitus nor hearing loss. The basal turn length, internal acoustic canal (IAC) width and length, bony cochlear nerve canal (BCNC) width, and IAC diameter at the porus acousticus internus (PAI) were measured.
RESULTS
The mean BCNC width was significantly narrower in G1 and G2 than in the control group (G3) (p<0.001). For patients in G2, BCNC width was significantly narrower in ears with tinnitus (p<0.001) than in ears without tinnitus. The mean IAC diameter at PAI was also narrower in the G1 patients (p=0.007) compared with the other groups.
CONCLUSION
The results of this study suggest that CT evaluation of the inner ear structures is important in patients with tinnitus. According to the results, a narrow BCNC may cause phantom sensations and be related to cochlear nerve dysfunction. Therefore, it is recommended that clinicians evaluate BCNC carefully while assessing such patients.
Topics: Cochlear Nerve; Constriction, Pathologic; Double-Blind Method; Ear, Inner; Female; Hearing Loss, Unilateral; Humans; Male; Prospective Studies; Temporal Bone; Tinnitus; Tomography, X-Ray Computed; Vestibule, Labyrinth
PubMed: 29283099
DOI: 10.5152/iao.2017.3727 -
Journal of the Association For Research... Feb 2021Animal models of noise-induced hearing loss (NIHL) show a dramatic mismatch between cochlear characteristic frequency (CF, based on place of innervation) and the...
Animal models of noise-induced hearing loss (NIHL) show a dramatic mismatch between cochlear characteristic frequency (CF, based on place of innervation) and the dominant response frequency in single auditory-nerve-fiber responses to broadband sounds (i.e., distorted tonotopy, DT). This noise trauma effect is associated with decreased frequency-tuning-curve (FTC) tip-to-tail ratio, which results from decreased tip sensitivity and enhanced tail sensitivity. Notably, DT is more severe for noise trauma than for metabolic (e.g., age-related) losses of comparable degree, suggesting that individual differences in DT may contribute to speech intelligibility differences in patients with similar audiograms. Although DT has implications for many neural-coding theories for real-world sounds, it has primarily been explored in single-neuron studies that are not viable with humans. Thus, there are no noninvasive measures to detect DT. Here, frequency following responses (FFRs) to a conversational speech sentence were recorded in anesthetized male chinchillas with either normal hearing or NIHL. Tonotopic sources of FFR envelope and temporal fine structure (TFS) were evaluated in normal-hearing chinchillas. Results suggest that FFR envelope primarily reflects activity from high-frequency neurons, whereas FFR-TFS receives broad tonotopic contributions. Representation of low- and high-frequency speech power in FFRs was also assessed. FFRs in hearing-impaired animals were dominated by low-frequency stimulus power, consistent with oversensitivity of high-frequency neurons to low-frequency power. These results suggest that DT can be diagnosed noninvasively. A normalized DT metric computed from speech FFRs provides a potential diagnostic tool to test for DT in humans. A sensitive noninvasive DT metric could be used to evaluate perceptual consequences of DT and to optimize hearing-aid amplification strategies to improve tonotopic coding for hearing-impaired listeners.
Topics: Acoustic Stimulation; Animals; Chinchilla; Cochlear Nerve; Hearing Loss, Noise-Induced; Humans; Male; Neural Conduction; Noise; Speech; Speech Perception
PubMed: 33188506
DOI: 10.1007/s10162-020-00755-2 -
Brain Research Apr 2015Perceptual performance in persons with hearing loss, especially those using devices to restore hearing, is not fully predicted by traditional audiometric measurements...
Perceptual performance in persons with hearing loss, especially those using devices to restore hearing, is not fully predicted by traditional audiometric measurements designed to evaluate the status of peripheral function. The integrity of auditory brainstem synapses may vary with different forms of hearing loss, and differential effects on the auditory nerve-brain interface may have particularly profound consequences for the transfer of sound from ear to brain. Loss of auditory nerve synapses in ventral cochlear nucleus (VCN) has been reported after acoustic trauma, ablation of the organ of Corti, and administration of ototoxic compounds. The effects of gradually acquired forms deafness on these synapses are less well understood. We investigated VCN gross morphology and auditory nerve synapse integrity in DBA/2J mice with early-onset progressive sensorineural hearing loss. Hearing status was confirmed using auditory brainstem response audiometry and acoustic startle responses. We found no change in VCN volume, number of macroneurons, or number of VGLUT1-positive auditory nerve terminals between young adult and older, deaf DBA/2J. Cell-type specific analysis revealed no difference in the number of VGLUT1 puncta contacting bushy and multipolar cell body profiles, but the terminals were smaller in deaf DBA/2J mice. Transmission electron microscopy confirmed the presence of numerous healthy, vesicle-filled auditory nerve synapses in older, deaf DBA/2J mice. The present results suggest that synapses can be preserved over a relatively long time-course in gradually acquired deafness. Elucidating the mechanisms supporting survival of central auditory nerve synapses in models of acquired deafness may reveal new opportunities for therapeutic intervention.
Topics: Animals; Cochlear Nerve; Cochlear Nucleus; Deafness; Evoked Potentials, Auditory, Brain Stem; Female; Male; Mice; Mice, Inbred CBA; Mice, Inbred DBA; Reflex, Startle; Synapses; Synaptic Transmission; Vestibulocochlear Nerve Diseases
PubMed: 25686750
DOI: 10.1016/j.brainres.2015.02.012 -
Journal of the Association For Research... Apr 2022Auditory function declines with age, as evidenced by communication difficulties in challenging listening environments for older adults. Declining auditory function may...
Auditory function declines with age, as evidenced by communication difficulties in challenging listening environments for older adults. Declining auditory function may arise, in part, from an age-related loss and/or inactivity of low-spontaneous-rate (SR) auditory nerve (AN) fibers, a subgroup of neurons important for suprathreshold processing. Compared to high-SR fibers, low-SR fibers take longer to recover from prior stimulation. Taking advantage of this difference, the forward-masked recovery function paradigm estimates the relative proportions of low- and high-SR fibers in the AN by quantifying the time needed for AN responses to recover from prior stimulation (ΔT). Due to the slower recovery of low-SR fibers, ANs that need more time to fully recover (longer ΔT) are estimated to have a larger proportion of low-SR fibers than ANs that need less time (shorter ΔT). To test the hypothesis that low-SR fiber activity is reduced in older humans, the current study assessed recovery functions in 32 older and 16 younger adults using the compound action potential. Results show that ΔT is shorter for older adults than for younger adults, consistent with a theorized age-related loss and/or inactivity of low-SR fibers. ΔT did not differ between individuals with and without a prior history of noise exposure as assessed by self-report. This study is the first to successfully assess forward-masked recovery functions in both younger and older adults and provides important insights into the structural and functional changes occurring in the AN with increasing age.
Topics: Acoustic Stimulation; Action Potentials; Aged; Auditory Perception; Auditory Threshold; Cochlear Nerve; Hearing; Humans; Nerve Fibers
PubMed: 35020090
DOI: 10.1007/s10162-021-00827-x -
Hearing Research Jun 2018Cochlear synaptopathy, i.e. the loss of auditory-nerve connections with cochlear hair cells, is seen in aging, noise damage, and other types of acquired sensorineural...
Cochlear synaptopathy, i.e. the loss of auditory-nerve connections with cochlear hair cells, is seen in aging, noise damage, and other types of acquired sensorineural hearing loss. Because the subset of auditory-nerve fibers with high thresholds and low spontaneous rates (SRs) is disproportionately affected, audiometric thresholds are relatively insensitive to this primary neural degeneration. Although suprathreshold amplitudes of wave I of the auditory brainstem response (ABR) are attenuated in synaptopathic mice, there is not yet a robust diagnostic in humans. The middle-ear muscle reflex (MEMR) might be a sensitive metric (Valero et al., 2016), because low-SR fibers may be important drivers of the MEMR (Liberman and Kiang, 1984; Kobler et al., 1992). Here, to test the hypothesis that narrowband reflex elicitors can identify synaptopathic cochlear regions, we measured reflex growth functions in unanesthetized mice with varying degrees of noise-induced synaptopathy and in unexposed controls. To separate effects of the MEMR from those of the medial olivocochlear reflex, the other sound-evoked cochlear feedback loop, we used a mutant mouse strain with deletion of the acetylcholine receptor required for olivocochlear function. We demonstrate that the MEMR is normal when activated from non-synaptopathic cochlear regions, is greatly weakened in synaptopathic regions, and is a more sensitive indicator of moderate synaptopathy than the suprathreshold amplitude of ABR wave I.
Topics: Acoustic Stimulation; Animals; Auditory Threshold; Cochlea; Cochlear Diseases; Cochlear Nerve; Disease Models, Animal; Evoked Potentials, Auditory, Brain Stem; Hearing Loss, Sensorineural; Mice, Inbred CBA; Mice, Knockout; Muscle Contraction; Nerve Degeneration; Receptors, Nicotinic; Reflex, Acoustic; Stapedius; Synapses
PubMed: 29598837
DOI: 10.1016/j.heares.2018.03.012 -
Medicina (Kaunas, Lithuania) Oct 2022Background and Objectives: Many otologists face a dilemma in the decision-making process of surgical management of patients with cochlear nerve (CN) aplasia. The goal of...
Background and Objectives: Many otologists face a dilemma in the decision-making process of surgical management of patients with cochlear nerve (CN) aplasia. The goal of this study is to provide fresh evidence on cochlear implantation (CI) results in patients with CN aplasia. Materials and Methods: We scrutinized functional outcomes in 37 ears of 21 children with bilateral CN aplasia who underwent unilateral or bilateral CI based on cross-sectional and longitudinal assessments. Results: The Categories of Auditory Performance (CAP) scores gradually improved throughout the 3-year follow-up; however, variable outcomes existed between individuals. Specifically, 90% of recipients with a 1-year postoperative CAP score ≤1 could not achieve a CAP score over 1 even at 3-year postoperative evaluation, while the recipients with a 1-year postoperative CAP score >1 had improved auditory performance, and 72.7% of them were able to achieve a CAP score of 4 or higher. Meanwhile, intraoperative electrically evoked compound action potential was not correlated with postoperative CAP score. Conclusions: Our results further refine previous studies on the clinical feasibility of CI as the first treatment modality to elicit favorable auditory performance in children with CN aplasia. However, special attention should be paid to pediatric patients with an early postoperative CAP score ≤1 for identification of unsuccessful cochlear implants and switching to auditory brainstem implants.
Topics: Humans; Child; Cochlear Implantation; Cross-Sectional Studies; Cochlear Nerve; Cochlear Implants; Treatment Outcome
PubMed: 36295634
DOI: 10.3390/medicina58101474