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Auditory System Notes

Outer Ear and Sound Entry

  • External ear funnels sound waves through the external auditory canal to the tympanic membrane (eardrum).
  • Human hearing is sensitive across a broad frequency range:
    • Speech frequency band is roughly f \,\in\;[1500,\;5000]\;\text{Hz}.
    • General human hearing spans from about f \,\in\;[20,\;20000]\;\text{Hz} (20 Hz to 20 kHz).
  • These frequency ranges influence how sound energy is delivered through the ear and processed by the inner ear.

Middle Ear: Mechanical Transmission

  • The tympanic membrane vibrates in response to incoming sound waves.
  • Three connected auditory ossicles transmit vibrations from the tympanic membrane to the inner ear:
    • Malleus (hammer)
    • Incus (anvil)
    • Stapes (stirrup)
  • Movement of the stapes causes vibrations of the oval window (membranous boundary of the inner ear).
  • Oval window vibrations generate fluid waves in the two perilymph-filled chambers of the cochlea, namely the scala vestibuli and scala tympani, which end at the round window where the waves are damped.
  • These fluid waves in the perilymph also push on the cochlear duct, the third fluid-filled chamber.

Inner Ear: Cochlear Fluids and Sensory Receptors

  • Inside the cochlear duct (scala media), sensory receptor cells called hair cells line one wall of the duct at the basilar membrane.
  • The tectorial membrane covers the hair cells.
  • Collectively, the hair cells, basilar membrane, tectorial membrane, and associated structures constitute the organ of Corti.
  • When fluid pressure in the perilymph pushes on the basilar membrane, the attached hair cells bend as they are deflected against the stationary tectorial membrane.

Hair-Cell Transduction and Neural Pathway

  • Deflection of the hair cells converts the mechanical pressure wave into a neural (nerve) impulse.
  • The nerve impulses from the hair cells are carried by nerve fibers that, throughout the cochlea, aggregate to form the cochlear (spiral) branch of the vestibulocochlear nerve (CN VIII).
  • The CN VIII transmits the sound nerve impulses to the brainstem and higher auditory centers.

Types of Hearing Loss and Neural Pathways

  • Sensorineural (nerve) hearing loss is caused by disease or damage to the neural pathway from the organ of Corti to the vestibulocochlear nerve and the central auditory system in the brain stem.
  • Causes of damage to the auditory nerve pathway include:
    • Aging
    • Chronic viral infections
    • Physical trauma to the temporal bone
  • Menière's disease is another common cause of hearing disorder and is characterized by a buildup of endolymph (the fluid within the membranous labyrinth).
    • If untreated, endolymphatic hydrops can lead to rupture of the membranous labyrinth, with symptoms that commonly include vertigo, fluctuating hearing loss, and tinnitus.

Diagnostic Tests and Treatments

  • Diagnostic tests are used to assess auditory nerve function and the extent of hearing impairment (specific tests are not enumerated in the provided transcript).
  • Depending on the cause and severity of the auditory nerve damage, sensorineural hearing loss is treated with devices such as:
    • Hearing aids
    • Cochlear implants

Key Concepts and Connections

  • Frequency-place (tonotopic) organization: different frequencies preferentially displace different regions of the basilar membrane; low frequencies peak far from the oval window (toward the apex), while high frequencies peak closer to the oval window (near the base).
  • Traveling waves: the pressure waves in the perilymph travel along the cochlea, causing localized bending of hair cells at the point where basilar membrane resonance best matches the stimulus frequency.
  • Transduction sequence: mechanical vibration (outer/middle ear) → oval window movement → fluid wave in scala vestibuli/scala tympani → displacement of basilar membrane → hair cell deflection against tectorial membrane → receptor potential in hair cells → neurotransmitter release → auditory nerve firing → brain processing.
  • The outer ear’s role in shaping the spectrum of sounds reaching the tympanic membrane influences overall audibility and speech perception.

Quick Reference: Terms and Anatomy

  • External ear: auricle (pinna) + external auditory canal
  • Tympanic membrane: eardrum
  • Ossicles: malleus, incus, stapes
  • Oval window: boundary between middle and inner ear; movement causes fluid motion in cochlea
  • Round window: dampens pressure waves within the cochlea
  • Cochlea contains:
    • Scala vestibuli (perilymph)
    • Scala tympani (perilymph)
    • Cochlear duct (scala media; endolymph; houses organ of Corti)
  • Organ of Corti: hair cells on basilar membrane + tectorial membrane overlay
  • Hair cells: mechanoreceptors converting mechanical stimuli into neural signals
  • Nerve pathway: cochlear branch of the vestibulocochlear nerve (CN VIII) → brainstem → auditory centers
  • Pathologies: sensorineural hearing loss; Menière's disease (endolymphatic hydrops)
  • Treatments: hearing aids, cochlear implants