There are tiny thread-like connections from the tip of each cilium to a non-specific cation channel on the side of the taller neighboring cilium. These cilia increase in length along a consistent axis. Cilia emerge from the apical surface of hair cells. Tight junctions between hair cells and the nearby supporting cells form a barrier between endolymph and perilymph that maintains the ionic imbalance.įigure 12.1 illustrates the process of mechanical transduction at the tips of the hair cell cilia. This actively maintained ionic imbalance provides an energy store, which is used to trigger neural action potentials when the hair cells are moved. The fluid, termed endolymph, which surrounds the hair cells is rich in potassium. (See the chapter on Vestibular System: Structure and Function). Hair cells in the maculae of the saccule and the utricle respond to linear acceleration (gravity). Hair cells in the cristae ampullares in the semicircular ducts respond to angular acceleration (rotation of the head).
Hair cells in the Organ of Corti in the cochlea of the ear respond to sound.
Several specializations make human hair cells responsive to various forms of mechanical stimulation. In higher vertebrates the internal fluid of the inner ear (not external fluid as in fish) bathes the hair cells, but these cells still sense movements in the surrounding fluid. The hair cell first appeared in fish as part of a long, thin array along the side of the body, sensing movements in the water.
The key structure in the vertebrate auditory and vestibular systems is the hair cell.
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