Hair cells

Hair cells are the sensory receptors of both the auditory system and the vestibular system in all vertebrates. In mammals, the auditory hair cells are located within the organ of Corti on a thin basilar membrane in the cochlea of the inner ear. They derive their name from the tufts of stereocilia that protrude from the apical surface of the cell, a structure known as the hair bundle, into the scala media, a fluid-filled tube within the cochlea. Mammalian cochlear hair cells come in two anatomically and functionally distinct types: the outer and inner hair cells. Damage to these hair cells results in decreased hearing sensitivity, i.e. sensorineural hearing loss.

Hair bundles as sound detectors
Research of the past decades has shown that outer hair cells do not send neural signals to the brain, but that they mechanically amplify low-level sound that enters the cochlea. The amplification may be powered by movement of their hair bundles, or by an electrically-driven motility of their cell bodies. The inner hair cells transform the sound vibrations in the fluids of the cochlea into electrical signals that are then relayed via the auditory nerve to the auditory brainstem and to the auditory cortex.

Inner hair cells – from sound to nerve signal
The deflection of the hair-cell stereocilia opens mechanically gated ion channels that allow any small, positively charged ions (primarily potassium and calcium) to enter the cell. Unlike many other electrically active cells, the hair cell itself does not fire an action potential. Instead, the influx of positive ions from the endolymph in Scala media depolarizes the cell, resulting in a receptor potential. This receptor potential opens voltage gated calcium channels; calcium ions then enter the cell and trigger the release of neurotransmitters, mainly glutamate, at the basal end of the cell. The neurotransmitters diffuse across the narrow space between the hair cell and a nerve terminal, where they then bind to receptors and thus trigger action potentials in the nerve. In this way, the mechanical sound signal is converted into an electrical nerve signal. The repolarization in the hair cell is done in a special manner. The perilymph in Scala tympani has a very low concentration of positive ions. The electrochemical gradient makes the positive ions to flow through channels to the perilymph. Nerve fiber innervation is much more dense for inner hair cells than for outer hair cells. A single inner hair cell is innervated by numerous nerve fibers, whereas a single nerve fiber innervates many outer hair cells. Inner hair cell nerve fibers are also very heavily myelinated, which is contrast to the unmyelinated outer hair cell nerve fibers.

Outer hair cells – acoustical pre-amplifiers
In mammalian outer hair cells, the receptor potential triggers active vibrations of the cell body. This so-called somatic electromotility consists of oscillations of the cell’s length, which occur at the frequency of the incoming sound and in a stable phase relation. Outer hair cells have evolved only in mammals. They have not improved hearing sensitivity, which reaches similarly exquisite values also in other classes of vertebrates. But they have extended the hearing range from ca 11 kHz (maximum in some birds) to ca 200 kHz (maximum in some marine mammals). They have also improved frequency selectivity (frequency discrimination), which is of particular benefit for humans, because it enabled sophisticated speech and music.

The molecular biology of hair cells has seen considerable progress in recent years, with the identification of the motor protein (prestin) that underlies somatic electromotility in the outer hair cells.

Hair-bundle motors
Results in recent years further indicate that mammals apparently also have conserved an evolutionarily earlier type of hair-cell motility. This so-called hair-bundle motility amplifies sound in all non-mammalian land vertebrates. It is effected by the closing mechanism of the mechanical sensory ion channels at the tips of the hair bundles. Thus, the same hair-bundle mechanism that detects sound vibrations also actively “vibrates back” and thereby mechanically amplifies weak incoming sound.