||The defining feature of hair
cells is the hair bundle (left), the organelle of sensory transduction
formed by ordered arrays of stereocilia. Hair cells sense bundle
deflection producing a receptor potential, i.e. a time
dependent modulation of their membrane potential. This is caused by the
opening of ion channels located on the stereocilia and sensitive to
mechanical deformation (mechano-sensitive channels). It is currently
believed that mechano-sensitive channels are attached to one or
possibly both ends of thin filaments, called tip-links
| connect stereocilia of
adjacent rows. Bundle deflection stretches the tip-links, which in turn
increases the channel open probability. The modulation of the cell
membrane potential is caused by the influx of ion currents, carried
mostly by potassium ions, through open mechano-sensitive channels.
|The driving force for this
transduction current is provided by the large
potential difference across the apical membrane of the hair
cell. This is due to the stereocilia projecting into endolymph,
a fluid that fills the space between the top surface of the organ of
Corti and the Reissner's membrane (see Anatomy section). The
endolymph is a potassium-rich extracellular fluid whose
standing potential is about 80 mV more positive than perilymph,
a fluid similar, and continuous with, cerebrospinal fluid that fills
the spiral canal and surrounds the basolateral membrane of the hair
cells. The intracellular potential of inner hair cells,
referred to perilymph, is about -45 mV, whereas that of the outer
hair cells is about
-70 mV. Thus, the driving force for current is a potential difference
comprised between 125 mV and 150 mV.
|Inner hair cells (IHCs) are
contacted mostly by afferent nerve fibers that leave the
organ of Corti to reach the cochlear nuclei in the brainstem. IHCs are
thought to be the main sensory receptors of the cochlea that signal the
vibration of the organ of Corti to the
|| brain. The animation at left
visualizes the activation of an IHC by the fluid viscous drag applied
to its stereocilia by the oscillation of the tectorial
membrane. Outer hair cells are the target of abundant efferent
innervation and possess a unique type of motility. They convert
receptor potentials into cell length changes at
acoustic frequencies (see Physiology section). The animation at
visualizes the activation of the outer hair cell motor driven
by the motion of the tectorial membrane into which the tips
of the tallest stereocilia are inserted. A second class of sensory
receptors, the outer hair cells couple visco-elastically the reticular
lamina to the basilar membrane
(see Anatomy Section) through their supporting Deiters'
cells (see Capacitance/Viscosity in Index).
See what happens to your stereocilia while
you happily dance in a discotheque! (see also Gale et al. 2004).
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cells in the basal turn of the guinea-pig cochlea to
tones. J Physiol.
- Dallos P. (1985) Membrane potential and response
changes in mammalian cochlear hair cells during
J Neurosci. 5:1609-15.
- Gale JE, Piazza V, Ciubotaru CD, Mammano F (2004) A
mechanism for sensing noise damage in the inner ear. Current Biology 14: 526–529.
- Gillespie PG and Corey DP (1997) Myosin and
adaptation by hair cells. Neuron 19 (5):955-958.
- Gillespie PG, Walker RG (2001) Molecular basis of
mechanosensory transduction. Nature, 413:194-202. Review.
- Kachar B, Parakkal M, Kurc M, Zhao Y, Gillespie PG
(2000) High-resolution structure of hair-cell tip links. Proc Natl
Acad Sci U S A, 97:13336-41.
- Pickles JO, Comis SD, Osborne MP. ( 1984) Cross-links
between stereocilia in the guinea pig organ of Corti, and their
possible relation to sensory transduction. Hear Res.15:103-12.