Hearing and Equilibrium

Overview – Hearing and Equilibrium

The ear plays a dual sensory role: detecting sound (hearing) and maintaining balance (equilibrium). These functions are mediated by specialised mechanoreceptors called hair cells, located within the cochlear and vestibular apparatus of the inner ear. Hearing involves transmission and transduction of sound waves, while equilibrium monitors the position and movement of the head to coordinate balance and posture. Disorders of either system can result in deafness, vertigo, or gait disturbance—making mastery of this anatomy and physiology essential in clinical neurology and ENT.

Ear Anatomy

Outer Ear (Air-Filled)

  • Pinna (Auricle): Funnels sound into external auditory canal
  • External Auditory Canal: Conducts sound to tympanic membrane
    • Contains ceruminous glands → secretes cerumen (earwax)
  • Tympanic Membrane: Vibrates in response to sound

Middle Ear (Air-Filled)

  • Auditory Ossicles:
    • Malleus → Incus → Stapes
    • Amplify sound ≈20x before reaching inner ear
  • Stapedius & Tensor Tympani Muscles: Reflexively dampen loud sounds
  • Oval Window: Receives vibration from stapes → cochlea
  • Eustachian Tube: Equalises pressure between middle ear and nasopharynx
Ear Anatomy
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).

Inner Ear (Fluid-Filled Labyrinth)

Vestibular Apparatus
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).
  • Cochlea (Hearing): Contains 3 ducts:
    • Scala vestibuli (perilymph)
    • Scala media (endolymph)
    • Scala tympani (perilymph)
Cochlea
Simplified Anatomy of the Cochlea. (Modelled as a long tube running from oval window, to the helicotrema, and back. This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).
  • Spiral Organ of Corti (on basilar membrane):
    • Inner hair cells: Main sensory transducers
    • Outer hair cells: Amplify sound signals
    • Covered by tectorial membrane
spiral organ of corti

Audiotransduction (How We Hear)

  1. Soundwaves → funnelled by auricle → external canal
  2. Vibrate tympanic membrane
  3. Transferred via ossicles → oval window
  4. Oval window movement → perilymph wave in scala vestibuli
  5. Vibration passes through vestibular membrane → scala media
    • High pitch = near oval window
    • Low pitch = further along cochlea
  6. Basilar membrane vibrates → hair cells bend into tectorial membrane
  7. Tip links stretch → K⁺ influx → depolarisation → Ca²⁺-mediated NT release
  8. Signal travels via cochlear nerve → brain
Hearing Audiotransduction
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).

Hearing Pathway

Hair cells → Cochlear Nerve → Cochlear Nuclei (Medulla) →
→ Superior Olivary Nucleus → Lateral Lemniscus →
→ Inferior Colliculus → Medial Geniculate Body (Thalamus) →
→ Primary Auditory Cortex (Temporal Lobe)

Pathway From the Cochlea to the Brain
What-when-who, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons

Pitch & Volume

  • Pitch: Coded by where along the basilar membrane the vibration occurs
  • Volume: Coded by degree of hair cell displacement

Types of Deafness

  • Conduction Deafness:
    • Impaired transmission (e.g. wax, perforated tympanum, ossicle fusion)
  • Sensorineural Deafness:
    • Impaired transduction or nerve pathway (e.g. hair cell loss, CN VIII damage)

Equilibrium – Vestibular System

Vestibular Apparatus
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).

Vestibular Apparatus (Inner Ear)

  • Vestibule:
    • Utricle & Saccule → sense linear acceleration & gravity
    • Contain Maculae (with otolithic membrane & CaCO₃ otoliths)
Vestibule - equilibrium
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).
  • Semicircular Canals:
    • Detect rotational (angular) acceleration
    • Each ends in ampulla containing Crista Ampullaris
      • Hair cells project into cupula (gelatinous cap)
      • Movement of endolymph bends cupula → signal generated
Semicircular canals - equilibrium
This work by Cenveo; Creative Commons Attribution 3.0 US (http://creativecommons.org/licenses/by/3.0/us/).

Planes of Detection:

  • Anterior canal → ear-to-shoulder
  • Posterior canal → nodding motion
  • Lateral canal → shaking head ‘no’
2 semicircular canals
Source: Unattributable

Vestibular Pathway

  • Hair cells → Vestibular Nerve → Vestibular Nuclei (Brainstem)
    • Integrated with visual and somatic input
    • Output to cerebellum, eye movement centres, and postural reflexes

Hair Cell Function

  • Hair cells = mechanoreceptors
  • Stereocilia movement affects tip link-gated K⁺ channels:
    • Stretch = channel opens → depolarisation
    • Compress = channel closes → hyperpolarisation
  • NT release via Ca²⁺-mediated exocytosis
hearing & equilibrium - Hair Cells

Summary – Hearing and Equilibrium

Hearing and balance are mediated by hair cells in the inner ear, which detect sound vibrations and head movements, respectively. The cochlea is responsible for hearing via the organ of Corti, while the vestibular apparatus regulates equilibrium through the maculae and semicircular canals. Understanding these pathways is crucial for diagnosing causes of deafness, vertigo, and gait disorders.
For a broader context, see our Nervous System Overview page.

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