Physiology of Hearing and Impedance Matching

Study Notes

Sound is a mechanical energy waveform that creates pressure in its medium of transmission.
Auditory perception is crucial for communication and language development.
The pinna acts as a sound funnel directing waves into the external auditory canal.
Sound localization occurs as sounds reach the ipsilateral ear faster and louder.
The external auditory canal amplifies sounds in the frequency range of 3-4 kHz due to its resonating properties.

Sound transitions from air in the external canal to fluid in the cochlea, where hair cells act as pressure sensors.
Air-to-liquid impedance matching is crucial since over 99% of sound energy would be reflected without an intermediary.
The tympanic membrane and ossicular chain (malleus, incus, stapes) are essential for impedance matching.
The footplate of the stapes exerts 13 times more pressure than the tympanic membrane, relying on the area ratio of 13:1.
The ossicular chain amplifies pressure at the footplate of the stapes by 18 times due to the lever system of the malleus and incus.

Equal pressure in the external auditory canal and tympanic cavity is necessary for optimal tympanic membrane sensitivity.
The Eustachian tube regulates middle ear pressure to minimize pressure differentials during sound transmission.
The ossicles are minimally frictional due to being suspended by thin ligaments, facilitating sound movement.

Sound pressure creates traveling waves in the cochlear duct, moving the basilar membrane where the organ of Corti is located.
Place Theory describes sound frequency identification via specific resonating basilar membrane locations.
High frequencies stimulate the basal turn of the cochlea, while sound intensity is detected by action potential production rates in the auditory nerve.

Hearing loss categorization: conductive and sensorineural.
Conductive hearing loss arises from issues in the external and middle ear preventing sound pressure transfer.
Sensorineural hearing loss is due to inner ear or auditory nerve conditions.

Conductive Hearing Loss:
- Otitis media with effusion (most common in children).
- Wax impaction (prevalent across all ages).
- Acute and chronic otitis media.
- Ossicular chain discontinuity due to trauma or infection.
- Otosclerosis (second most common cause in Caucasian adults).
Sensorineural Hearing Loss:
- Presbycusis (age-related, often due to ischaemia of the Organ of Corti).
- Noise-induced hearing loss.
- Ototoxicity from aminoglycosides and loop diuretics.
- Meningitis.
- Viral infections (e.g., Measles, Mumps, Herpes Zoster).