The hair cells are sensory cells located on the basilar membrane in the inner ear (in the organ of Corti). There are two types of sensory cells, termed the outer hair cells and the inner hair cells.
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There are about 23,000 hair cells on the 30 mm long basilar membrane in the organ of Corti. The inner and outer hair cells each have their function.
The two types of hair cells in combination are essential to the function of hearing. Damaged hair cells are not regenerated.
The inner hair cells are the main sensory cells of the function of hearing because they are the link between the inner ear and the auditory nerve in the brain.
When a sound is perceived, the basilar membrane, on which the hair cells are located, vibrates. Different frequencies in the sound signal give rise to vibrations at different places along the basilar membrane.
Frequencies of about 1,000 Hz, for example, stimulate the inner hair cells in the middle turn of the cochlea. When the nerve cell observes this, the information is transmitted via neural impulses to the brain, which knows exactly which impulse corresponds to the sound impact of which frequency and intensity.
In short, the inner hair cells convert a “physical sound” to an “electrical sound”, which can be perceived by the brain.
The outer hair cells contain muscle tissue and allow the basilar membrane to vibrate sufficiently even at very weak sounds.
The outer hair cells act like a servo mechanism in a car, in the sense that slight vibrations of weak sounds are mechanically amplified, whereby the inner hair cells are stimulated.
The outer hair cells are easily damaged by noise, such as gun shots and heavy industrial noise.
A large number of nerve fibres are connected to the hair cells. Some of these transmit signals from the hair cells to the brain. These are called afferent nerves, and they are typically attached to the inner hair cells. The afferent nerves transmit control signals from the brain to nerve cells closer to the ear.
The nerve fibres from each ear enter the brainstem. From there they enter the brain and cross to the opposite side of the head and terminate on the surface of the brain just above the opposite ear behind the temporal bone. This is where the main brain centre for hearing is located – the auditory cortex.
SEE ALSO:
Organ of CortiInner ear
HearingHearing, a tour of the earAcoustic traumaSteeply sloping hearing losses
