Physiology of hearing

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Physiology of Hearing

Hearing is the complex process of converting acoustic pressure waves in the air into electrical nerve impulses that the brain interprets as sound. This involves a specialized transduction pathway.

The Auditory Pathway

  • External Ear: Collects and funnels sound waves toward the tympanic membrane (TM).
  • Middle Ear: The ossicles (malleus, incus, stapes) amplify sound via mechanical advantage, overcoming the impedance mismatch between air and fluid in the cochlea.
  • Inner Ear (Cochlea): Stapes movement at the oval window creates fluid waves in the perilymph (scala vestibuli/tympani), causing displacement of the Basilar Membrane.

Transduction Mechanism

Step Action
Shearing Organ of Corti hair cell stereocilia bend against the tectorial membrane.
Depolarization Bending towards the kinocilium opens K^+ channels, causing depolarization and Ca^{2+} influx.
Transmission Release of neurotransmitters onto the vestibulocochlear nerve (CN VIII).
High-Yield Exam Pearl:

  • Tonotopic Organization: The base of the basilar membrane responds to high-frequency sounds, while the apex responds to low-frequency sounds.
  • Endolymph: Unlike most extracellular fluid, it is high in K^+ and low in Na^+, which is essential for the electrical gradient required for hair cell depolarization.