Physiology of hearing

Physiology of hearing

 The physiology of hearing involves the process by which sound waves are converted into electrical signals that the brain interprets as sound. Here’s a step-by-step breakdown of how hearing occurs:

1. Sound Wave Transmission

• Sound waves travel through the external ear (pinna and ear canal) and strike the tympanic membrane (eardrum), causing it to vibrate.

2. Middle Ear (Ossicular Chain)

• The vibrations are transmitted through the three ossicles (tiny bones) in the middle ear:

  • Malleus (hammer)

  • Incus (anvil)

  • Stapes (stirrup)

• The ossicles amplify the sound and transfer it to the oval window of the cochlea.

3. Cochlear Function (Inner Ear)

• The cochlea is a fluid-filled, spiral-shaped organ that contains the organ of Corti, the sensory receptor for hearing.

• The stapes' movement at the oval window creates pressure waves in the cochlear fluid (perilymph).

• These waves cause the basilar membrane to vibrate, stimulating hair cells (sensory receptors) in the organ of Corti.

4. Hair Cell Activation & Transduction

• The bending of stereocilia (hair-like projections) on hair cells opens mechanically gated ion channels, leading to depolarization.

• This triggers the release of neurotransmitters (e.g., glutamate) that activate the auditory nerve (CN VIII).

5. Neural Pathway to the Brain

• Electrical signals travel via the auditory nerve to the cochlear nuclei in the brainstem.

• From there, signals pass through:

  • Superior olivary complex (sound localization)

  • Lateral lemniscus

  • Inferior colliculus (midbrain, for reflexive responses to sound)

  • Medial geniculate nucleus (thalamus)

• Finally, signals reach the auditory cortex (temporal lobe) for sound interpretation.

6. Frequency & Loudness Perception

• Frequency (pitch): Determined by which part of the basilar membrane vibrates (high frequencies near the base, low frequencies near the apex).

• Loudness: Determined by the amplitude of vibrations and the number of activated hair cells.

Key Structures Involved:

• Outer Ear: Pinna, ear canal, tympanic membrane.

• Middle Ear: Ossicles (malleus, incus, stapes), Eustachian tube.

• Inner Ear: Cochlea, organ of Corti, hair cells, auditory nerve.

• Central Pathway: Cochlear nuclei → superior olivary complex → inferior colliculus → thalamus → auditory cortex.

Disruptions in Hearing Physiology Can Lead To:

• Conductive hearing loss (e.g., earwax blockage, otosclerosis).

• Sensorineural hearing loss (e.g., hair cell damage, noise-induced hearing loss).

• Central hearing disorders (e.g., brainstem or cortical lesions).

This process allows us to perceive and interpret sounds in our environment efficiently.