The Machinery of Hair Cells

The Scientist has a fastastic illustrated feature on the workings of cochlear hair cells in their current online issue. In addition to pointing out the different cell types in the inner ear, there are a few informative blurbs about mechanotransduction and how stereocilia are organized and linked. This is part of the larger theme "Focus on Neuroscience" issue, which has lots of short articles about from channel dynamics to Alzheimer's disease.

The hair cells of the inner ear are unique in that they are sensory epithelial cells, and not neural tissue themselves (like olfactory receptors or the retina). There is a Flash animation here which shows what happens to the cell in response to sound. First the stereocilia (the "hairs" of the cell) are bent by the sound vibrations traveling through the cochlea's membranes and fluid. This physically opens mechanically-gated channels in the stereocilia themselves, which allow a momentary influx of potassium from the surrounding fluid. This slight depolarization (from the positive K+ ions) in turn causes calcium to enter the cell via voltage-gated channels (Ca++). As calcium accumulates, 2 things happen: potassium channels open and K+ leaves the cell, and the neurotransmitter glutamate is released into the synapse of the connecting afferent neuron. Therefore the cell's membrane potential closely follows the pattern of stereocilia displacement.

Below is a confocal microscopy image of a hair cell before (left) and just after (right) depolarization (right before and after the influx of calcium in response to the presence of K+). The color gradient represents the concentration of calcium, and following initial depolarization there is massive influx of Ca++ into the cell. (credit: Fettiplace lab page)

As a side note, the mechanoreceptor on the stereocilia has yet to be identified and is a major question in hearing research. A few years ago it was believed to be identified as a TRP1a channel, however subsequent studies involving knockouts of this channel in mice resulted in no hearing defects at all.