Waxholm Space atlas of the rat brain auditory system: Three-dimensional delineations based on structural and diffusion tensor magnetic resonance imaging.

The mammalian auditory system comprises a complex network of brain regions. Interpretations and comparisons of experimental results from this system depend on appropriate anatomical identification of auditory structures. The Waxholm Space (WHS) atlas of the Sprague Dawley rat brain (Papp et al., Neuroimage 97:374-86, 2014) is an open access, three-dimensional reference atlas defined in an ex-vivo magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) volume. Version 2.0 of the atlas (Kjonigsen et al., Neuroimage 108:441-9, 2015) includes detailed delineations of the hippocampus and several major subcortical regions, but only few auditory structures. To amend this, we have delineated the complete ascending auditory system from the cochlea to the cerebral cortex. 40 new brain structure delineations have been added, and the delineations of 10 regions have been revised based on the interpretation of image features in the WHS rat brain MRI/DTI volumes. We here describe and validate the new delineations in relation to corresponding cell- and myelin-stained histological sections and previous literature. We found it possible to delineate all main regions and the majority of subregions and fibre tracts of the ascending auditory pathway, apart from the auditory cortex, for which delineations were extrapolated from a conventional two-dimensional atlas. By contrast, only parts of the descending pathways were discernible in the template. Version 3.0 of the atlas, with altogether 118 anatomical delineations, is shared via the NeuroImaging Tools and Resources Collaboratory (www.nitrc.org).

The Cochlear Spiral Ganglion Neurons: The Auditory Portion of the VIII Nerve.

The VIII nerve is formed by sensory neurons that innervate the inner ear, i.e., the vestibular and the auditory receptors. Neurons of the auditory portion, the cochlear afferent fibers that innervate the sensory hair cells of the organ of Corti, have their somas in the cochlear spiral ganglion where two types of neurons can be distinguished. Afferent Type-I neurons are the 95% of the total population. Bipolar and myelinated fibers, each one innervates only one cochlear inner hair cell (IHC). In contrast, afferent Type-II neurons are only the 5% of the spiral ganglion population. They are pseudounipolar and unmyelinated fibers and innervate the cochlear outer hair cells (OHC) so that one afferent Type-II fiber contacts with multiple OHCs, but each OHC only receives one contact from one Type-II neuron. Both types of VIII nerve fibers are glutamatergic, but these asymmetric innervations of the cochlear sensory cells could suggest that the IHC codifies the truly auditory message but the OHC only informs about mechanical aspects of the state of the organ of Corti. In fact, the central nervous system (CNS) has control over the information transmitted by the Type-I neuron by means of axons from the superior olivary complex that innervate them to modulate, filter and/or inhibit the entry of auditory message to CNS. The aim of this paper is to review the current knowledge about the anatomy and physiology of the auditory portion of the VIII nerve. Anat Rec, 302:463-471, 2019. © 2018 Wiley Periodicals, Inc.

Electrophysiology of Cranial Nerve Testing: Auditory Nerve.

The electrocochleogram and brainstem auditory evoked potentials (BAEPs) are electrophysiologic signals used to assess the auditory nerve. The electrocohleogram includes the cochlear microphonic, the cochlear summating potential, and the eighth nerve compound action potential. It is used predominantly for hearing assessment and for diagnosis of Ménière disease and auditory neuropathy. Brainstem auditory evoked potentials are used for hearing assessment, diagnosis of dysfunction within the cochlea, the auditory nerve, and the brainstem auditory pathways up to the level of the mesencephalon, and intraoperative monitoring of these structures. The earliest BAEP component, wave I, and the eighth nerve compound action potential reflect the same process-the initial depolarization in the distal auditory nerve. Brainstem auditory evoked potential wave II receives contributions from the region of the cochlear nucleus and from the second depolarization in the distal auditory nerve. Wave III and later components are entirely generated rostral to the auditory nerve. Interpretation of BAEP studies is based on waves I, III, and V; auditory nerve dysfunction is manifested as prolongation of the I-III interpeak interval or absence of waves III and V. Eighth nerve tumors can cause a variety of BAEP abnormalities depending on which structures they affect. Adverse intraoperative BAEP changes can have many etiologies, including direct mechanical or thermal injury of tissue, ischemia (including cochlear ischemia or infarction due to compromise of the internal auditory artery), eighth nerve stretch, systemic or localized hypothermia, and artifactual BAEP changes due to technical factors.

Olivocochlear efferents: Their action, effects, measurement and uses, and the impact of the new conception of cochlear mechanical responses.

The anatomy and physiology of olivocochlear (OC) efferents are reviewed. To help interpret these, recent advances in cochlear mechanics are also reviewed. Lateral OC (LOC) efferents innervate primary auditory-nerve (AN) fiber dendrites. The most important LOC function may be to reduce auditory neuropathy. Medial OC (MOC) efferents innervate the outer hair cells (OHCs) and act to turn down the gain of cochlear amplification. Cochlear amplification had been thought to act only through basilar membrane (BM) motion, but recent reports show that motion near the reticular lamina (RL) is amplified more than BM motion, and that RL-motion amplification extends to several octaves below the local characteristic frequency. Data on efferent effects on AN-fiber responses, otoacoustic emissions (OAEs) and human psychophysics are reviewed and reinterpreted in the light of the new cochlear-mechanical data. The possible origin of OAEs in RL motion is considered. MOC-effect measuring methods and MOC-induced changes in human responses are also reviewed, including that ipsilateral and contralateral sound can produce MOC effects with different patterns across frequency. MOC efferents help to reduce damage due to acoustic trauma. Many, but not all, reports show that subjects with stronger contralaterally-evoked MOC effects have better ability to detect signals (e.g. speech) in noise, and that MOC effects can be modulated by attention.

Three-Dimensional Analysis of the Fundus of the Human Internal Acoustic Canal.

OBJECTIVES: Documentation of the nerve components in the internal acoustic canal is essential before cochlea implantation surgery. Interpretations may be challenged by wide anatomical variations of the VIIIth nerve and their ramifications. Malformations may further defy proper nerve identification. DESIGN: Using microcomputed tomography, we analyzed the fundus bone channels in an archival collection of 113 macerated human temporal bones and 325 plastic inner molds. Data were subsequently processed by volume-rendering software using a bony tissue algorithm. Three-dimensional reconstructions were made, and through orthogonal sections, the topographic anatomy was established. RESULTS: The technique provided additional information regarding the anatomy of the nerve foramina/channels of the human fundus region, including variations and destinations. Channel anastomosis were found beyond the level of the fundus. A foramen of the transverse crest was identified. CONCLUSIONS: Three-dimensional reconstructions and cropping outlined the bone canals and demonstrated the highly variable VIIIth nerve anatomy at the fundus of the human inner acoustic canal. Myriad channel interconnections suggested an intricate system of neural interactive pathways in humans. Particularly striking was the variable anatomy of the saccule nerve channels. The results may assist in the preoperative interpretation of the VIIIth nerve anatomy.

A Comparison of Cochlear Nerve Size in Normal-Hearing Adults Using Magnetic Resonance Imaging.

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.

Classification and Current Management of Inner Ear Malformations.

Morphologically congenital sensorineural hearing loss can be investigated under two categories. The majority of congenital hearing loss causes (80%) are membranous malformations. Here, the pathology involves inner ear hair cells. There is no gross bony abnormality and, therefore, in these cases high-resolution computerized tomography and magnetic resonance imaging of the temporal bone reveal normal findings. The remaining 20% have various malformations involving the bony labyrinth and, therefore, can be radiologically demonstrated by computerized tomography and magnetic resonance imaging. The latter group involves surgical challenges as well as problems in decision-making. Some cases may be managed by a hearing aid, others need cochlear implantation, and some cases are candidates for an auditory brainstem implantation (ABI). During cochlear implantation, there may be facial nerve abnormalities, cerebrospinal fluid leakage, electrode misplacement or difficulty in finding the cochlea itself. During surgery for inner ear malformations, the surgeon must be ready to modify the surgical approach or choose special electrodes for surgery. In the present review article, inner ear malformations are classified according to the differences observed in the cochlea. Hearing and language outcomes after various implantation methods are closely related to the status of the cochlear nerve, and a practical classification of the cochlear nerve deficiency is also provided.

Sound localization in the alligator.

In early tetrapods, it is assumed that the tympana were acoustically coupled through the pharynx and therefore inherently directional, acting as pressure difference receivers. The later closure of the middle ear cavity in turtles, archosaurs, and mammals is a derived condition, and would have changed the ear by decoupling the tympana. Isolation of the middle ears would then have led to selection for structural and neural strategies to compute sound source localization in both archosaurs and mammalian ancestors. In the archosaurs (birds and crocodilians) the presence of air spaces in the skull provided connections between the ears that have been exploited to improve directional hearing, while neural circuits mediating sound localization are well developed. In this review, we will focus primarily on directional hearing in crocodilians, where vocalization and sound localization are thought to be ecologically important, and indicate important issues still awaiting resolution.

Identification of cranial nerves near large vestibular schwannomas using superselective diffusion tensor tractography: experience with 23 cases.

BACKGROUND: The preservation of the facial nerve (FN) and acoustic function in large vestibular schwannoma (VS) surgery is challenging because of nerve course uncertainties and morphological deviations. Preoperative diffusion tensor tractography (DTT) has been proposed to predict the FN location. This study was conducted to evaluate the effectiveness of this technique for identifying the FN, cochlear nerve (CN) and trigeminal nerve (TN) in large VSs. METHODS: The study included 23 consecutive patients with VS of Hannover classification T3b to T4b from November 2013 through May 2014. Diffusion tensor images and anatomical images were acquired. The DTT images of the cranial nerves were extracted before surgery for each patient to determine the relationships of these nerves with the tumor. The results were then validated during the tumorectomy. RESULTS: In 21 (91.30%) patients, the location of the FN on the DTT images agreed with the intraoperative findings, including in 2 patients in whom the FN passed through the interface between the parenchyma and the cystic changes and in 3 patients with a membranoid FN. The CN or fibers of unclear function were observed on DTT images in four patients with functional hearing. One penetrating fiber of unknown function was effectively constructed. The TN was accurately detected on the DTT images for all patients. CONCLUSIONS: DTT effectively revealed the location of the FN, including cases in which the FN was membranoid or passed through the interface between an area exhibiting cystic changes and the tumor nodule. Fibers aside from the FN and the TN were revealed by DTT in patients who retained functional hearing. Penetrating fibers were also found using DTT. This technique can be useful during VS resection.

Cochlear nerve diameters on multipoint measurements and effects of aging in normal-hearing children using 3.0-T magnetic resonance imaging.

OBJECTIVE: The aim of this study was to determine differences in diameter of cochlear nerves (CN) among three measurement points at the midpoint of the internal auditory canal (IAC), IAC fundus and cochlear aperture, and to evaluate whether nerve size varies with age. METHODS: A total of 336 normal-hearing ears of 201 children were assessed, who were underwent 3D-Fiesta sequence scanning of inner ear. All subjects were divided into 12 groups at one year interval. The diameter measurements of CN were obtained in the midpoint of the IAC, IAC fundus and cochlear aperture respectively on the axial and oblique sagittal images of 3.0-T MRI. SPSS 18.0 statistics software was applied for data analysis, and all of the data showed a normal distribution and expressed in x ± s. RESULTS: The diameters of normal-hearing children's CN at the midpoint of the IAC, IAC fundus and cochlear aperture were respectively: 1.12 ± 0.08 mm, 1.05 ± 0.06 mm, 0.87 ± 0.14 mm, and there were significant differences among the three measuring points (F = 527.57, p < 0.05). The diameters of the CN had no significant difference (p > 0.05) in age groups, gender and sides (p > 0.05), and there was no correlation between the diameters of normal children's CN and age (r is -0.129, 0.128 and -0.113, respectively). CONCLUSION: The diameters of normal-hearing children's CN change with different points of the internal auditory canal, of which the maximum value is in the midpoint of the IAC, followed by the IAC fundus, and the cochlear aperture is at the minimum; moreover the normal size doesn't change with age.

Olivocochlear innervation maintains the normal modiolar-pillar and habenular-cuticular gradients in cochlear synaptic morphology.

Morphological studies of inner hair cell (IHC) synapses with cochlear nerve terminals have suggested that high- and low-threshold fibers differ in the sizes of their pre- and postsynaptic elements as well as the position of their synapses around the hair cell circumference. Here, using high-power confocal microscopy, we measured sizes and spatial positions of presynaptic ribbons, postsynaptic glutamate receptor (GluR) patches, and olivocochlear efferent terminals at eight locations along the cochlear spiral in normal and surgically de-efferented mice. Results confirm a prior report suggesting a modiolar > pillar gradient in ribbon size and a complementary pillar > modiolar gradient in GluR-patch size. We document a novel habenular < cuticular gradient in GluR patch size and a complementary cuticular < habenular gradient in olivocochlear innervation density. All spatial gradients in synaptic elements collapse after cochlear de-efferentation, suggesting a major role of olivocochlear efferents in maintaining functional heterogeneity among cochlear nerve fibers. Our spatial analysis also suggests that adjacent IHCs may contain a different synaptic mix, depending on whether their tilt in the radial plane places their synaptic pole closer to the pillar cells or to the modiolus.

Origin, course and distribution of the nerves to the posterosuperior wall of the external acoustic meatus.

Patients with Ramsay Hunt syndrome have various clinical symptoms including vesicular rash of the external acoustic meatus and auricle. In addition to facial nerve paresis, neurological disturbances of various cranial nerves such as the acoustic nerve, glossopharyngeal nerve and vagus nerve are reported in patients of Ramsay Hunt syndrome. To understand the reasons for the clinical symptoms, we observed the nerve branches of the auricle and external acoustic meatus. We used 18 halves of 11 Japanese cadavers. All cadavers were fixed in 8% formalin and preserved in 30% ethanol. Dissection was performed under a stereomicroscope and the communication among the nerve branches was analyzed. Posterosuperior wall of the acoustic meatus was innervated by nerve branches that emerged from the tympanomastoid fissure in 17 specimens (17/18). These branches always crossed the facial canal and had more than one communicating branch with the facial nerve inside the canal (17/17) or in the petrous bone (1/17). These branches originated from the superior ganglion of the vagus. In the origin from the vagus nerve, some of these branches communicated with the glossopharyngeal nerve (3/17). In addition to these branches, the facial nerve, after originating from the stylomastoid foramen, bifurcated into two nerve branches in some specimens (7/17). Nerve branches around the external acoustic meatus and the auricle have various communications before reaching the central nervous system. The variety of communications could explain the varied symptoms of Ramsay Hunt syndrome.

The interaural time difference pathway: a comparison of spectral bandwidth and correlation sensitivity at three anatomical levels.

Temporal differences between the two ears are critical for spatial hearing. They can be described along axes of interaural time difference (ITD) and interaural correlation, and their processing starts in the brainstem with the convergence of monaural pathways which are tuned in frequency and which carry temporal information. In previous studies, we examined the bandwidth (BW) of frequency tuning at two stages: the auditory nerve (AN) and inferior colliculus (IC), and showed that BW depends on characteristic frequency (CF) but that there is no difference in the mean BW of these two structures when measured in a binaural, temporal framework. This suggested that there is little frequency convergence in the ITD pathway between AN and IC and that frequency selectivity determined by the cochlear filter is preserved up to the IC. Unexpectedly, we found that AN and IC neurons can be similar in CF and BW, yet responses to changes in interaural correlation in the IC were different than expected from coincidence patterns ("pseudo-binaural" responses) in the AN. To better understand this, we here examine the responses of bushy cells, which provide monaural inputs to binaural neurons. Using broadband noise, we measured BW and correlation sensitivity in the cat trapezoid body (TB), which contains the axons of bushy cells. This allowed us to compare these two metrics at three stages in the ITD pathway. We found that BWs in the TB are similar to those in the AN and IC. However, TB neurons were found to be more sensitive to changes in stimulus correlation than AN or IC neurons. This is consistent with findings that show that TB fibers are more temporally precise than AN fibers, but is surprising because it suggests that the temporal information available monaurally is not fully exploited binaurally.

The temporal bone microdissection of miniature pigs as a useful large animal model for otologic research.

CONCLUSION: Compared with traditional animal models, the miniature pig may be a better model for biomedical research because its morphology has many similarities with that of humans. OBJECTIVE: To investigate the suitability of the miniature pig as an animal model for otological research as regards morphology. METHODS: Microdissection of the temporal bone of 10 miniature pigs was performed and recorded on photographs. RESULTS: The morphology and measurements of the external, middle, and inner ear, and the lateral recess of the miniature pigs were completed by microdissection. The temporal bone structures, including the external, middle, inner ear, and the lateral recess, were similar in the miniature pig and humans.

Establishing normal diameter range of the cochlear and facial nerves with 3D-CISS at 3T.

PURPOSE: We explored normal diameter values of the cochlear (CN) and facial (FN) nerves on 3-dimensional constructive interference in steady state (3D-CISS) using a 3-tesla magnetic resonance (MR) imaging scanner. METHODS: A total of 142 patients underwent MR imaging of the inner ear at 3T. We retrospectively analyzed data from 172 ears. All ears had normal hearing and no history of ipsilateral facial palsy. Parasagittal images of 0.5-mm thickness perpendicular to the internal auditory canal were reconstructed from 0.4-mm thick axial 3D-CISS images for all ears. Two independent radiologists measured the long diameter (LD), short diameter (SD), and cross-sectional area (CSA) of each CN and FN. For each measurement, we calculated the mean value from the 2 observers and evaluated interobserver agreement. We also evaluated the correlation between nerve diameter and sex and age. RESULTS: Both observers could perform measurements in 157 CN from 172 ears (91%) and 165 FN from 172 ears (96%). For the CN, the mean LD was 1.35±0.16 mm, the mean SD was 0.99±0.18 mm, and the mean CSA was 1.07±0.30 mm2; for the FN, the mean values were 1.18±0.17 mm (LD), 0.87±0.16 mm (SD), and 0.83±0.27 mm2 (CSA). Interobserver reliability was good for each measurement (r=0.569 to 0.691, P<0.01). The diameters of the 2 nerves tended to be larger in men, but only the CSA of the FN was significantly larger in men (P<0.05). Age did not affect nerve diameter. CONCLUSION: We present normal values of CN and FN diameters on 3D-CISS at 3T. Our results warrant further study to clarify the pathophysiological state of the CN or FN using 3D-CISS.

Dorsal location of the cochlear nerve on vestibular schwannoma: preoperative evaluation, frequency, and functional outcome.

The cochlear nerve is most commonly located on the caudoventral portion of the capsule of vestibular schwannomas and rarely on the dorsal portion. In such a condition, total removal of the tumor without cochlear nerve dysfunction is extremely difficult. The purpose of our study was to identify the frequency of this anatomical condition and the status of postoperative cochlear nerve function; we also discuss the preoperative radiological findings. The study involved 114 patients with unilateral vestibular schwannomas operated on via a retrosigmoid (lateral suboccipital) approach. Locations of the cochlear nerve on the tumor capsule were ventral, dorsal, caudal, and rostral. Ventral and dorsal locations were further subdivided into rostral, middle, and caudal third of the tumor capsule. The postoperative cochlear nerve function and preoperative magnetic resonance (MR) findings were reviewed retrospectively. In 56 patients that had useful preoperative hearing, useful hearing was retained in 50.0% (28 of 56) of patients after surgery. The cochlear nerve was located on the dorsal portion of the tumor capsule in four patients (3.5%), and useful hearing was preserved in only one of these patients (25%) in whom the tumor had been partially resected. This tumor-nerve anatomical relationship was identified in all tumors of <2 cm at preoperative MR cisternography. MR cisternography has the potential to identify the tumor-nerve anatomical relationship, especially in small-sized tumors that usually require therapeutic intervention that ensures hearing preservation. Hence, careful evaluation of the preoperative MR cisternography is important in deciding the therapeutic indications.

3D model of frequency representation in the cochlear nucleus of the CBA/J mouse.

The relationship between structure and function is an invaluable context with which to explore biological mechanisms of normal and dysfunctional hearing. The systematic and topographic representation of frequency originates at the cochlea, and is retained throughout much of the central auditory system. The cochlear nucleus (CN), which initiates all ascending auditory pathways, represents an essential link for understanding frequency organization. A model of the CN that maps frequency representation in 3D would facilitate investigations of possible frequency specializations and pathologic changes that disturb frequency organization. Toward this goal, we reconstructed in 3D the trajectories of labeled auditory nerve (AN) fibers following multiunit recordings and dye injections in the anteroventral CN of the CBA/J mouse. We observed that each injection produced a continuous sheet of labeled AN fibers. Individual cases were normalized to a template using 3D alignment procedures that revealed a systematic and tonotopic arrangement of AN fibers in each subdivision with a clear indication of isofrequency laminae. The combined dataset was used to mathematically derive a 3D quantitative map of frequency organization throughout the entire volume of the CN. This model, available online (http://3D.ryugolab.com/), can serve as a tool for quantitatively testing hypotheses concerning frequency and location in the CN.

Anastomoses of the vestibular, cochlear, and facial nerves.

The internal auditory canal (IAC) is 10 to 17 mm in length, and the facial nerve and vestibulocochlear nerve, which consist of the cochlear nerve, the superior vestibular nerve, and the inferior vestibular nerve, run together in the IAC packaged in dura mater. Oort first described the vestibulocochlear anastomoses in 1918, which is important for the understanding of the pathogenesis and pathophysiology of otologic disorders. The current study documents the existence of vestibulofacial and vestibulocochlear neural connections and topographical relationship of the nerves as part of a radiologic evaluation of 73 human temporal bones from brainstem to the lateral portion of IAC.

Specific targeting of retrocochlear auditory pathway for optimal pharmacotherapy delivery using a mouse model.

HYPOTHESIS: Optimal pharmacotherapy entails a safe delivery method that specifically targets auditory structure(s) of interest. A retrocochlear neuronal tracer may enable comparison of various pharmacotherapy delivery methods and localization of the drug along the auditory pathway. BACKGROUND: Sensorineural hearing loss (SNHL) can involve cochlear hair cell or neural cell death, which often is accompanied by secondary degeneration of central auditory neurons. Targeting the precise location of nerve degeneration is important for treatment success. To be clinically relevant, the method of drug delivery must be safe and reliable while being maximally absorbed by the relevant inner ear structures of interest. METHODS: We compared 3 methods of FluoroGold (FG) delivery, a retrograde neuronal tracer, in delineating the retrocochlear auditory pathway using a normal-hearing strain of CBA mice. FG was delivered either intratympanic (IT), intracochlear (IC), or through the round window (RW). Five days after FG injection, mice were sacrificed for cell counts in the cochlear nucleus (CN), superior olivary complex (SOC), and the lateral lemniscus (LL). RESULTS: Although neurons in the CN and SOC were abundantly labeled by FG in all 3 injection methods, the IT method was the most reproducible and specific. The average cells for the CN, SOC, and LL were 851 ± 121, 2629 ± 367, and 112 ± 30, respectively. Accurate cell counts could not be established for the IC and RW injection methods because of nonspecific cell staining. Only 1 of the 5 IC-injected mice had specific labeling along the retrocochlear auditory pathway. Cell counts for the single mouse with specific IC staining in the CN, SOC, and LL were 177, 1839, and 56, respectively. Similarly, 2 of the 5 RW-injected mice had specific labeling, whereas the rest were nonspecific. The average cell counts for the 2 mice with specific labeling in the CN, SOC, and LL was 723.5 ± 580.0, 2173.5 ± 998.0, and 131.5 ± 8.0, respectively. CONCLUSION: The IT injection method resulted in reproducible, specific staining of neuronal cells along the retrocochlear auditory pathway compared with the RW or IC route of delivery.

Inner-ear morphology of the New Zealand kiwi (Apteryx mantelli) suggests high-frequency specialization.

The sensory systems of the New Zealand kiwi appear to be uniquely adapted to occupy a nocturnal ground-dwelling niche. In addition to well-developed tactile and olfactory systems, the auditory system shows specializations of the ear, which are maintained along the central nervous system. Here, we provide a detailed description of the auditory nerve, hair cells, and stereovillar bundle orientation of the hair cells in the North Island brown kiwi. The auditory nerve of the kiwi contained about 8,000 fibers. Using the number of hair cells and innervating nerve fibers to calculate a ratio of average innervation density showed that the afferent innervation ratio in kiwi was denser than in most other birds examined. The average diameters of cochlear afferent axons in kiwi showed the typical gradient across the tonotopic axis. The kiwi basilar papilla showed a clear differentiation of tall and short hair cells. The proportion of short hair cells was higher than in the emu and likely reflects a bias towards higher frequencies represented on the kiwi basilar papilla. The orientation of the stereovillar bundles in the kiwi basilar papilla showed a pattern similar to that in most other birds but was most similar to that of the emu. Overall, many features of the auditory nerve, hair cells, and stereovilli bundle orientation in the kiwi are typical of most birds examined. Some features of the kiwi auditory system do, however, support a high-frequency specialization, specifically the innervation density and generally small size of hair-cell somata, whereas others showed the presumed ancestral condition similar to that found in the emu.