Hair cells

Cochlea A snail-shaped fluid-filled organ of the inner ear, lined on its inner surface with specialized hair cells that convert sound pressure vibrations into nerve impulses. 

In the inner ear, ENaC is expressed in supporting cells surrounding hair cells and is postulated to play a role in the low sodium concentration of endolymph, critical for proper mechanotransduction and hearing. 

Non-neuronal cells (including astrocytes, mechan-osensory hair cells, macrophages, keratinocytes, endothelial cells of the vascular system, muscle cells, lymphocytes, intestinal epithelial cells and various cell-types of the lungs)... 

Deafness KCNQ4 is expressed in vestibular system, brain, and cochlea sensory hair cells. KCNQ4 has been... 

Eatock RA, Rusch A. 1997. Developmental changes in the physiology of hair cells. Sem Cell Devel Biol 8 265-275. 

Auditory Sensorineural hearing loss due to sickling in cochlear vasculature with hair cell damage... 

Goedhart, J, Hink, M. A, Visser, A. J, Bisseling, T. and Gadella, T. W, Jr. (2000). In vivo fluorescence correlation microscopy (FCM) reveals accumulation and immobilization of Nod factors in root hair cell walls. Plant J. 21, 109-19. 

Elgoyhen, A., Vetter, D., Katz, E. etal. Alpha 10 a determinant of nicotinic cholinergic receptor function in mammalian vestibular and cochlear mechanosensory hair cells. Proc. Natl Acad. Sci. U.S.A. 98,3501-3506, 2001. 

Two other myosin types have been implicated in hearing and vestibular function [62]. The defect in the Snell s waltzer mouse was found to be a mutation in a myosin VI gene that produces degeneration of the cochlea and vestibular apparatus. Myosin VI is localized to the cuticular plate of the hair cell under stereocilia. Similarly, mutations in a myosin VII gene are responsible for the shaker-1 mouse and several human genetic deafness disorders. This myosin, myosin Vila, is found in a band near the base of the stereocilia distinct from distributions of myosin ip and myosin VI. 

Insect mechanoreceptors and hair cells share evolutionary relationships 835... 

Hair cells are the sensory cells of the auditory and vestibular systems 835 Hair cells are exposed to unusual extracellular fluids and potentials 836 Mechanical transduction depends on activation of ion channels linked to extracellular and intracellular structures 836 Some of the molecules responsible for hair-cell transduction have been identified 838... 

Hair cells are specialized mechanoreceptors located in the inner ear these cells transduce mechanical forces transmitted by sound and head movement, and permit an organism to sense features of the external world. Well-characterized biophysically, a molecular description of hair-cell transduction has finally begun to emerge. 

Although many key molecules remain to be identified, striking molecular and functional correspondences between vertebrate hair cells and invertebrate mechanoreceptors have indicated that some types of mechanoreceptors probably share a common ancestor. The continued application of genetic, molecular, biological and biophysical approaches should lead to a more thorough understanding of this critical sensory cell. 

Is this system relevant for hair cells It does not appear so. Indeed, there appear to be at least two broad classes of mechanoreceptors, one like those of C. elegans touch neurons that rely on DEG/ENaC channels, and another class that apparently relies on transient receptor potential (TRP) channels. As we will see later, circumstantial evidence suggests that hair cells use TRP channels. Other mechanoreceptors in vertebrates may be related to C. elegans touch cells. 

The most important molecules so far identified from this screen include a likely transduction channel, an extracellular molecule that could gate channels, and several molecules known to be important for axonemal structure and function. Although the set of molecules is less complete than that identified for C. elegans touch receptors, the diversity of mechanotransduction in Drosophila and the apparent similarity of these receptors to those in vertebrates, including hair cells (see below), demonstrates the significance of this model system. 

The protein NompC is a TRP channel and appears to be the major transduction channel in fly bristles [6], Bristle mechanotransduction resembles hair-cell transduction remarkably in its speed, polarity and adaptation [6], suggesting the possibility of a close evolutionary relationship between these mechanoreceptors. NompC is not the only... 

Insect mechanoreceptors and hair cells share evolutionary relationships. In flies, the proneural gene atonal controls development of mechanoreceptors Mathl plays a similar role in mice. Indeed, atonal can fully substitute for Mathl in mice and Mathl can fully substitute for atonal in flies [11], suggesting that these two disparate systems might share a common genetic program used for development. As another example, as we will discuss later, Drosophila bristle mechanoreceptors and zebrafish hair cells each rely on the TRP channel NompC for mechanotransduction. 

Hair cells are the sensory cells of the auditory and vestibular systems. Hair cells are the sensory cells of the internal ear, essential for the senses of sound and balance. The hair cell s transduction apparatus, the molecular machinery that converts forces and displacements into electrical responses, can respond to mechanical stimuli of less than 1 nm in amplitude, and of tens or even hundreds of kilohertz in frequency. Indeed, our hearing is ultimately limited by Brownian motion of water molecules impinging on the transduction apparatus. 

Even though well-characterized at a biophysical level, the mechanical transduction mechanism of hair cells is still not fully understood in molecular terms. This discrepancy is in part due to the extreme scarcity of hair cells instead of the millions or even hundreds of millions of receptor cells that the olfactory and visual systems possess, only a few tens of thousands of hair cells are found in the internal ears of most vertebrate species. The small number of hair cells and the direct transduction mechanism has greatly impeded molecular biological and... 

Hair cells are neuroepithelial cells their large basolat-eral surface includes synaptic contacts with afferent and efferent nerve fibers, while the mechanically sensitive hair bundle is located on their apical surface. The hair bundle is an ensemble of 30-300 actin-filled stereocilia and a single axonemal kinocilium (Fig. 51-2). The kinocilium,... 

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