The Auditory System (original) (raw)
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Two classes of outer hair cells along the tonotopic axis of the cochlea
Neuroscience, 2006
The molecular basis of high versus low frequency hearing loss and the differences in the sensitivity of outer hair cells depending on their cochlear localization are currently not understood. Here we demonstrate the existence of two different outer hair cell phenotypes along the cochlear axis. Outer hair cells in low frequency regions exhibit early sensitivity for loss of Ca v 1.3 (␣1 subunit 1.3 forming the class D L-type voltage-gated Ca 2؉ channel), while high frequency regions display a progressive susceptibility for loss of the Ca 2؉ -activated large conductance K ؉ (BK) channel. Despite deafness, young Ca v 1.3-deficient mice displayed distortion-product otoacoustic emissions (DPOAEs), indicating functional outer hair cells in the higher frequency range of the cochlea. Considering that DPOAEs are also found in the human deafness syndrome DFNB9 caused by mutations in the synaptic vesicle protein otoferlin, we tested the expression of otoferlin in outer hair cells. Surprisingly, otoferlin showed a distinct tonotopic expression pattern at both the mRNA and protein level. Otoferlin-expressing, Ca v 1.3 deletion-sensitive outer hair cells in the low frequency range could be clearly separated from otoferlin-negative, BK deletion-sensitive outer hair cells in the high frequency range.
Cochlear anatomy and central auditory pathways
This chapter provides a brief review of the anatomy of the cochlea and its central auditory pathways. Since there is great similarity between the basic features of the human cochlea and those in a variety of animals, we have chosen the chinchilla as a representative animal model.
Cochlear efferents in developing adult and pathological conditions
Cell and tissue research, 2015
Cochlear activity is regulated by the olivo-cochlear bundle, which originates from the brainstem and projects onto the hair cells and auditory nerve fibers. Two efferent components can be distinguished: the medial and lateral olivo-cochlear efferent originating from the medial, and the lateral nuclei of the superior olivary complex. The input of the efferent systems on hair cells occurs during development and persists in the adult cochlea. Recent studies have shown that the efferent innervations are required to set the activity pattern in developing hair cells and auditory nerve fibers and to protect the synaptic structures in adult cochlea. In addition, efferent innervations undergo plasticity during pathological conditions such as noise-trauma or aging. This review discusses the mechanisms underlying the control of the hair cells and afferent fibers excitability by efferent neurons and their putative role in developing adult and pathological conditions.
Frontiers in neuroscience, 2016
There are three times as many outer hair cells (OHC) as inner hair cells (IHC), yet IHC transmit virtually all acoustic information to the brain as they synapse with 90-95% of type I auditory nerve fibers. Here we review a comprehensive series of experiments aimed at determining how loss of the IHC/type I system affects hearing by selectively destroying these cells in chinchillas using the ototoxic anti-cancer agent carboplatin. Eliminating IHC/type I neurons has no effect on distortion product otoacoustic emission or the cochlear microphonic potential generated by OHC; however, it greatly reduces the summating potential produced by IHC and the compound action potential (CAP) generated by type I neurons. Remarkably, responses from remaining auditory nerve fibers maintain sharp tuning and low thresholds despite innervating regions of the cochlea with ~80% IHC loss. Moreover, chinchillas with large IHC lesions have surprisingly normal thresholds in quiet until IHC losses exceeded 80%,...
Age-Related Hearing Loss: Sensory and Neural Etiology and Their Interdependence
Frontiers in Aging Neuroscience, 2022
Age-related hearing loss (ARHL) is a common, increasing problem for older adults, affecting about 1 billion people by 2050. We aim to correlate the different reductions of hearing from cochlear hair cells (HCs), spiral ganglion neurons (SGNs), cochlear nuclei (CN), and superior olivary complex (SOC) with the analysis of various reasons for each one on the sensory deficit profiles. Outer HCs show a progressive loss in a basal-to-apical gradient, and inner HCs show a loss in a apex-to-base progression that results in ARHL at high frequencies after 70 years of age. In early neonates, SGNs innervation of cochlear HCs is maintained. Loss of SGNs results in a considerable decrease (~50% or more) of cochlear nuclei in neonates, though the loss is milder in older mice and humans. The dorsal cochlear nuclei (fusiform neurons) project directly to the inferior colliculi while most anterior cochlear nuclei reach the SOC. Reducing the number of neurons in the medial nucleus of the trapezoid body...
Projections from the dorsal and ventral cochlear nuclei to the medial geniculate body
Frontiers in Neuroanatomy, 2014
Direct projections from the cochlear nucleus (CN) to the medial geniculate body (MG) mediate a high-speed transfer of acoustic information to the auditory thalamus. used anterograde tracers to label the projection from the dorsal CN (DCN) to the MG in guinea pigs. We examined this pathway with retrograde tracers. The results confirm a pathway from the DCN, originating primarily from the deep layers. Labeled cells included a few giant cells and a larger number of small cells of unknown type. Many more labeled cells were present in the ventral CN (VCN). These cells, identifiable as multipolar (stellate) or small cells, were found throughout much of the VCN. Most of the labeled cells were located contralateral to the injection site. The CN to MG pathway bypasses the inferior colliculus (IC), where most ascending auditory information is processed. hypothesized that CN-MG axons are collaterals of axons that reach the IC. We tested this hypothesis by injecting different fluorescent tracers into the MG and IC and examining the CN for double-labeled cells. After injections on the same side of the brain, double-labeled cells were found in the contralateral VCN and DCN. Most double-labeled cells were in the VCN, where they accounted for up to 37% of the cells labeled by the MG injection. We conclude that projections from the CN to the MG originate from the VCN and, less so, from the DCN. A significant proportion of the cells send a collateral projection to the IC. Presumably, the collateral projections send the same information to both the MG and the IC. The results suggest that T-stellate cells of the VCN are a major source of direct projections to the auditory thalamus. nucleus of IC; ICd, dorsal cortex of IC; IClc, lateral cortex of IC; icp, inferior cerebellar peduncle; m, molecular layer of DCN; MG, medial geniculate body; MGd, dorsal division of MG; MGm, medial division of MG; MGsg, suprageniculate division of MG; MGv, ventral division of MG; MVe, medial vestibular nucleus; NBIC, nucleus of the brachium of the IC; py, pyramid; RB, red beads; sp5, spinal trigeminal tract; SpVe, spinal vestibular nucleus; VCN, ventral cochlear nucleus Frontiers in Neuroanatomy www.frontiersin.org
Structure and innervation of the cochlea
Brain Research Bulletin, 2003
The role of the cochlea is to transduce complex sound waves into electrical neural activity in the auditory nerve. Hair cells of the organ of Corti are the sensory cells of hearing. The inner hair cells perform the transduction and initiate the depolarization of the spiral ganglion neurons. The outer hair cells are accessory sensory cells that enhance the sensitivity and selectivity of the cochlea. Neural feedback loops that bring efferent signals to the outer hair cells assist in sharpening and amplifying the signals. The stria vascularis generates the endocochlear potential and maintains the ionic composition of the endolymph, the fluid in which the apical surface of the hair cells is bathed. The mechanical characteristics of the basilar membrane and its related structures further enhance the frequency selectivity of the auditory transduction mechanism. The tectorial membrane is an extracellular matrix, which provides mass loading on top of the organ of Corti, facilitating deflection of the stereocilia. This review deals with the structure of the normal mature mammalian cochlea and includes recent data on the molecular organization of the main cell types within the cochlea.