Expansion
Outer ear collects, middle ear amplifies, cochlea transduces by place coding
Expansion
- Outer ear collects and funnels sound
- Middle ear performs impedance matching between air and fluid, amplifying about twentyfold through the area ratio of tympanic membrane to oval window and the ossicular lever. Without it, most sound energy would be reflected
- Cochlea: the travelling wave peaks at a position determined by frequency (tonotopic place coding): high frequencies at the base, low at the apex
- Hair cells in the organ of Corti are deflected against the tectorial membrane, opening mechanically gated channels. Inner hair cells transduce; outer hair cells amplify and sharpen tuning
- Cochlear nerve to cochlear nuclei, superior olive, inferior colliculus, medial geniculate, then auditory cortex, with bilateral representation
Two consequences: because representation is bilateral above the cochlear nuclei, unilateral deafness implies a lesion at or peripheral to the eighth nerve, not in the cortex. And presbyacusis and noise damage affect the basal turn first, causing high-frequency loss with difficulty in background noise.