Human Auditory System

Human auditory system enables humans to hear sounds and is also responsible for the perception of pitch, loudness and timbre.

The complex mechanism of hearing occurs when auditory system converts variations in the sound pressure level into neural impulses. The neural impulses resulting from information at both ears are combined by the masses of nerve cells in the brain and conveyed to the auditory cortex.

The auditory system consists of three parts – the outer ear, the middle ear and the inner ear.

Outer ear
The outer ear consists of pinna, the visible flap of skin and cartilage attached to the side of the head, and meatus, the external auditory canal. The auditory canal is about 0.5 cm in diameter extending about 3 cm into the head and ends at a stretched thin sheet of tissue called the eardrum or tympanic membrane.
The pinna modifies the spectra of incoming sound waves as a result of reflection and attenuation when they hit it.
These changes provide additional information that helps the brain to localize the sound.
The auditory canal amplifies sounds that are between 3 and 12 kHz.
The tympanic membrane resonates at about 2 kHz resulting in more efficient transmission of the signals to the middle ear in this frequency range.
Any vibrations of the tympanic membrane caused by the sound wave are transferred to the middle ear.

Middle Ear
The middle ear is an air-filled cavity made up of a series of delicate bones or ossicles: the malleus (hammer), incurs (anvil) and stapes (stirrup). These ossicles transfer the lower-pressure vibration of the tympanic membrane into higher-pressure sound vibrations at the oval (or elliptical) window, another smaller membrane-covered opening in the inner ear (cochlea). Since the inner ear is filled with fluid which is almost incompressible (very high impedance) rather than air, ossicles act as an impedance matching device and generate high pressure ensuring the easy and efficient propagation of sound energy to the inner ear fluids. The middle ear contains the sound information in wave form which is converted to neural impulses in the cochlea.

Inner Ear
Cochlea is a coiled tube filled with fluid, roughly 2 mm in diameter and 3 cm in length. It is divided into three chambers along its length. The basilar membrane separates two of the chambers called scala media or cochlear duct and scala tympani. The third chamber is scala vestibuli separated from cochlear duct by Reissner’s membrane.

The cochlear duct is filled with an extracellular fluid and also has the organ of Corti which sits above the basilar membrane and supports about 30000 hair cells with nerves connected to each of them. Hair cells are arranged in a row of inner hair cells and three rows of outer hair cells. Each hair cell contains a bundle of 100-200 specialized cilia at the top acting as the sensors for any mechanical changes in the hearing. The tectorial membrane rests above the longest cilia and moves back and forth with each cycle of sound. The movement of tectorial membrane tilts the cilia and allows electric current into the hair cell. The nerve attached with the hair cells fires and transmits the message to the auditory region of the brain.

The basilar membrane is stiffest near the oval window, and becomes more flexible toward the opposite end, allowing it to act as a frequency spectrum analyzer. Hair cells close to the oval window transmit information about high-frequency, or high-pitched, sound, while those at the far end of basilar membrane provide information about low-pitched sound.

If the hair cells in a particular region of the cochlea are destroyed, the nerves will not fire and the brain will not receive any information. If part of the hair cells are destroyed, the brain may receive a distorted message that it cannot interpret.