Hearing (Auditory System)

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The action of hearing sound is performed by the ear. It is divided into three sections: the outer ear, the middle ear and the inner ear. The outer ear consists of the pinna -- the fleshy part of the ear that is visible outside the head. The middle ear stretches from the tympanic membrane (or "ear drum") to the oval window. There are three bones in the middle ear: the malleus (or hammer), the incus (or anvil) and the stapes (or stirrup).

When soundwaves hit the tympanic membrane, the sound is transduced into mechanical vibrations that reverberate through the middle ear and press on the oval window.

The inner ear consists of the cochlea and semicircular canals. The cochlea contains endolymph -- a fluid high in potassium. In the center of the cochlea is the basilar membrane, the surface of which is covered with small hair-like cilia. These cilia are connected together by tip links connected to insertional plaques on the cilia.

When soundwaves cause fluctuations in the oval window, this causes waves in the endolymph which cause the cilia to tip over. The tip links pull open potassium channels allowing potassium to enter into the auditory receptors. This influx of positive ions causes the receptor potential to become more positive, or 'depolarized'. This depolarization causes vesicles to release neurotransmitter into the synapse.

Information from the cochlea travels along the auditory nerve or vestibulocochlear nerve (cranial nerve VIII) to the cochlear nucleus in the midbrain.