Hearing

Hearing (also called audition) involves converting sound waves into neural signals, enabled by the anatomical features of the ear. A fleshy outer structure, the auricle, directs sound waves toward the auditory canal. The canal leads through the external auditory meatus of the temporal bone to the tympanic membrane (eardrum), which vibrates when struck by sound waves.

External ear

• Auricle (sometimes called pinna if it’s movable, as in certain animals)
• Auditory canal
• Tympanic membrane

Middle ear

• The middle ear contains three small bones—malleus, incus, and stapes—collectively called the ossicles.
  The malleus attaches to the tympanic membrane and articulates with the incus, which in turn articulates with the stapes. Vibrations pass through these bones to the inner ear via the oval window.
• The Eustachian tube connects the middle ear to the pharynx, allowing pressure balance across the tympanic membrane.

Inner ear

• Often described as a bony labyrinth within the temporal bone, the inner ear comprises two main regions:
  1. Cochlea – for hearing
  2. Vestibule – for balance
• Both regions send signals along separate fiber bundles that merge into the vestibulocochlear nerve. Within the cochlea, the spiral ganglia house the neurons that transduce sound into neural impulses. The stapes connects to the oval window, where fluid waves begin.

Scala and fluid motion

• Inside the cochlea, the scala vestibuli extends from the oval window above the cochlear duct, while the scala tympani extends below and ends at the round window…
  Vibrations transferred by the ossicles move the fluid in these scalae.
• High-frequency sound waves affect areas near the oval window, while low-frequency waves travel farther toward the tip of the cochlea.
• The round window’s membrane bulges in or out with fluid displacement.

Organ of Corti

• A cross-section of the cochlea reveals the cochlear duct between the scala vestibuli and scala tympani. The basilar membrane, beneath the organ of Corti, moves in response to fluid waves.
• Each organ of Corti contains hair cells, whose stereocilia (microvilli-like extensions) project into the tectorial membrane. When fluid waves move the basilar membrane, the tectorial membrane slides over the stereocilia, bending them:
  • Bending toward the tallest stereocilium opens ion channels, causing depolarization and nerve impulses.
  • Bending away from the tallest stereocilium reduces tension, closing channels and hyperpolarizing the cell.

Frequency tuning

• Because a specific segment of the basilar membrane resonates at a particular frequency, hair cells at that segment fire only for corresponding sound waves.
• Humans detect frequencies roughly from 20 Hz (low pitch) to 20 kHz (high pitch). Lower pitches cause vibrations nearer the cochlear apex, and higher pitches affect regions near the round and oval windows.