In biology, hair cells are part of the ear. It is because of their hairy appearance at the microscopic level that scientists call them hair cells. The function of the cells is to sense noise, which are actually disturbances of the air called sound waves. When sound passes into the ear, the hairy cells wiggle in response to the air movement, and pass on electric signals from their movement to nerves that transmit the sound onto the brain to interpret.
An ear has three major parts. The outer ear encompasses the external ear which helps sound waves to pass into the middle ear. The middle ear contains the eardrum, which receives the sound waves, and passes on a vibration to the middle ear bones, which in turn vibrate. This vibration represents the environmental sound waves to the inner ear.
Inside the inner ear, which is located on the internal side of the eardrum, are more tiny pieces of hearing equipment. The cochlea is the major component, and part of the cochlea is the Organ of Conti. The Organ of Conti is inside the cochlea, and is made up of many hair cells. The cells on the side nearest the outer ear are called outer hair cells, and the cells further inside are inner hair cells.
All of these cells have what appear to be hairs on the upper end of the cell. The bottom of the cells are stuck onto a membrane called the basilar membrane. The hairlike protuberances sticking out from the top of the cells are actually extensions of the cell, and are more technically known as stereocilia. Above the line of hair cells in the Organ of Conti is a membrane called the tectorial membrane, and the hairy parts of the cells touch this membrane.
Sound waves enter the cochlea in the inner ear and cause a vibration in the basilar membrane. This wiggles the stereocilia on the cells and the cells themselves also wiggle. The wiggling motion provokes the cell to produce an electrical impulse which relates to the sound, and as nerve cells are located under the cells, this impulse moves into the nerve cells. The nerve cells then pass on the message of sound to the brain, and the brain interprets the type and intensity of the sound.
Hair cells can become less responsive to sounds if they are damaged. Loud noises are capable of damaging the cells, because the hairy parts of the cell bend too much and cannot restore their original ability to move in response to sounds, in much the same way as a strong wind can irredeemable bend a sapling tree so it loses its flexibility. Research has shown that some animals, like birds, can grow new hair cells and thus improve damaged hearing, but humans do not naturally fix these cells if damaged, and hearing loss is irreparable until experimental treatments prove otherwise.