Outer hair cells have a cylindrical shape. Their diameter is about nine micrometers which is slightly larger than the diameter of a red blood cell. Their apical end is capped with a rigid flat plate into which the stereocilia are embedded and their synaptic end is a hemisphere (see Figure 7 in Outer Hair Cell Is Pressurized). Each of these three regions (flat apex, middle cylinder and hemispheric base) has a specific function. The stereocilia at the top of the cell are responsible for converting the mechanical energy of sound into electrical energy. Synaptic structures are found at the base of the hair cell and they are responsible for converting electrical energy into chemical energy by modulating the release of neurotransmitters that activate the 8th nerve fibers contacting the cell. The top and the bottom of the outer hair cell perform functions that are common to all hair cells. The elongated cylindrical portion of the outer hair cell is where electrical energy is converted into mechanical energy. This function is unique to the outer hair cell. No other hair cell (nor any other kind of cell) is able to change its length at acoustic frequencies in response to electrical stimulation. These length changes can be greater than 1 percent of the cell's original length if the electrical stimulation is large. The physical mechanism responsible for this electromotility is not known but there is recent evidence that cellular membranes can move in response to high frequency electrical stimuli. It is possible that all membranes generate mechanical force but that the outer hair cell is specialized to generate length changes. It should come as no surprise that there are structural features that are unique to the outer hair cell which facilitate its ability to undergo rapid length changes.
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