Unit
- Outer ear function: The pinna collects sound waves and channels them through the ear canal to the tympanic membrane. This process amplifies frequencies important for speech, providing natural directional hearing. Any obstruction or malformation can reduce sound transmission by up to 30 decibels.
- Middle ear mechanics: Three ossicles (malleus, incus, stapes) transmit vibrations from the eardrum to the oval window. The lever action and area ratio provide impedance matching, converting air vibrations to fluid waves in the inner ear with approximately 22 times amplification.
- Cochlear structure: The cochlea is a spiral-shaped organ with three fluid-filled chambers (scala vestibuli, scala media, scala tympani). The organ of Corti sits on the basilar membrane and contains hair cells that convert mechanical vibrations into electrical signals.
- Hair cell transduction: Inner hair cells (approximately 3,500) are the primary sensory receptors. Stereocilia bend in response to fluid movement, opening ion channels that depolarize the cell and trigger neurotransmitter release at the auditory nerve synapse.
- Tonotopic organization: The basilar membrane has a frequency gradient: high frequencies (up to 20,000 Hz) at the base, low frequencies (down to 20 Hz) at the apex. This tonotopy is preserved throughout the auditory pathway to the cortex.
- Auditory nerve and brainstem: The spiral ganglion cells (type I and type II) send axons via the cochlear nerve to the cochlear nucleus in the brainstem. From there, pathways cross and ascend to the superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex.
- Sensorineural hearing loss: Damage to hair cells or the auditory nerve reduces the ability to convert sound into neural signals. Common causes include aging, noise exposure, ototoxic drugs, and genetic factors. This type of loss is typically permanent and may benefit from cochlear implants.
- Conductive hearing loss: Problems in the outer or middle ear prevent sound from reaching the cochlea. Causes include cerumen impaction, otitis media, otosclerosis, and tympanic membrane perforation. These are often treatable medically or surgically and are not implant candidates.
- Outer hair cell function: Outer hair cells (approximately 12,000) act as cochlear amplifiers by changing their length in response to electrical stimulation. This electromotility enhances basilar membrane vibration at low sound levels, providing up to 50 dB of gain and sharp frequency tuning crucial for speech discrimination.
- Efferent auditory system: The olivocochlear bundle sends fibers from the brainstem to the cochlea, modulating outer hair cell activity. This reflex reduces cochlear amplification in noisy environments, improving signal to noise ratio and protecting against acoustic injury. Dysfunction may contribute to hyperacusis or tinnitus.
Scope of Practice
Role: Audiologist in Cochlear Implant Clinic
Within scope:
- Performing diagnostic audiometry and interpreting results for implant candidacy.
- Counseling patients on hearing loss types and treatment options.
- Recommending appropriate hearing devices based on audiologic findings.
Outside scope:
- Diagnosing medical causes of hearing loss (refer to otologist).
- Performing surgical implantation or prescribing medications.
- Interpreting radiologic imaging for cochlear anatomy.
Quick Revision
- Hearing involves outer, middle, and inner ear structures that convert sound waves to neural signals.
- The cochlea's tonotopic organization preserves frequency information along the basilar membrane.
- Sensorineural hearing loss results from hair cell or nerve damage and may be treated with cochlear implants.
- Conductive losses are typically managed medically and do not require implantable devices.