Hair cells are sensory receptors which are located in the inner ear. These microscopic hair like extensions are crucial for a healthy auditory system and a healthy vestibular system. The outer hair cells amplify the low level sounds that enter the cochlea, thereby playing an important role in the auditory system. The amplification is either powered by the movement of the hair bundles or by an electrically driven motility of the cell bodies.
The inner hair cells transform the sound vibrations into electrical signals. These signals are relayed via the auditory nerve to the auditory brain stream and to the auditory cortex. The hair cell stereocilia deflects, thereby opening mechanically gated ion channels. This deflection allows positively charged ions (calcium and potassium) into the cell.
It is important to note that the hair cell itself does not fire an action potential. The influx of positive ions from the endolymph in the cochlear duct depolarizes the cell, resulting in a receptor potential (transmembrane potential difference). This opens voltage-dependent calcium channels (VDCC), thereby allowing calcium ions to enter the cell.
The calcium ions trigger the release of neurotransmitters at the basal end of the cell. These neurotransmitters diffuse across the narrow space between the hair cell and the nerve terminal. Here they bind to the receptors and trigger action potentials in the nerve. This is how the mechanical sound signal is converted into an electrical nerve signal.
Damage to the inner ear hair cells is one of the major causes of hearing loss. Prolonged exposure to loud noise, disease, injury and ageing are the main causes of hair cell damage. Once damaged, these delicate hair cells of the inner ear do not grow back. Hearing loss due to damage to the hair cells is called sensorineural hearing loss.
Extensive research is going on in this field, in an endeavor to help patients affected by loss of hearing due to damage to the hair cells. Scientists are working on ways to regrow hair cells. They have succeeded in doing so in mice. Transfer of genes responsible for hair cell genesis and replacement of missing cells via transfer of stem cells are two strategies which scientists are working on.
Scientists are also gaining a better understanding of the cellular and molecular mechanisms the body uses to protect auditory hair cells from damage. This knowledge will help them develop methods to enhance the chances of survival of hair cells in the event of a trauma of a disease.