How Does the Auditory System Work?

The peripheral auditory system allows sound to be transmitted from the auricle of the outer ear to the first neurons of the auditory nerve. Each part of the system plays a role in sound transmission.

The outer ear is composed of the auricle, which is the only visible part of the ear, and the external auditory canal. Its primary role is to capture sound waves and send them to the eardrum.

The middle ear is composed of the eardrum, three ossicles (malleus, incus and stapes) and the Eustachian tube. It is linked to the outer ear through the eardrum and the inner ear through the round and oval windows.

When a sound wave is sent through the external auditory canal, it vibrates the eardrum. The eardrum then sends the vibrations through the ossicles through the “handle” of the malleus. The malleus then strikes the incus, which moves the stapes. The stapes sends the vibrations to the inner ear through the oval window.

In addition to transmitting sound vibrations, the middle ear helps protect the inner ear. If the ear is exposed to too much noise, the muscle attached to the stapes (stapedius muscle) contracts to reduce the vibration of the stapes, protecting the inner ear. This is called the acoustic or stapedius reflex. Finally, the middle ear also contains the Eustachian tube, which balances the pressure on either side of the eardrum.

The inner ear is composed of the vestibule and the cochlea. The vestibule is responsible for balance, while the cochlea is responsible for hearing. The cochlea is a spiral structure that houses the organ of Corti. This organ contains cilia, which sit in a liquid called perilymph. When the ossicles of the middle ear transmit vibrations to this liquid, it causes the cilia to move. This activates a nerve impulse that is sent to the brain via the auditory nerve.

The central auditory system allows sound to be transmitted from the first neurons of the auditory nerve to the brain. This is the system responsible for interpreting auditory information.

The peripheral auditory system communicates with the central auditory system through afferent nerve fibres, which run from the organ of Corti to the auditory cortex, and efferent nerve fibres, which run the opposite way.

The cells in the organ of Corti are divided into two categories: external cilia and internal cilia. These two types of cells are linked to nerve fibres, which form the two auditory nerves (8^th paired cranial nerve). The information from the auditory nerve is then sent to the brain via several relays in the brain stem (cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus and medial geniculate body). These relays allow for additional processing of the auditory message.

In addition to transmitting sound information, the auditory pathways provide information about sound frequency, volume and location in space.