Describe the physiology of hearing.

Conduction of Sound to the Cochlea

• Hearing is the conversion of airborne sound into nerve impulses.
• Sound is conducted mechanically through the external and middle ear, transduced in the cochlea, and analysed in the central auditory pathway.
• The central problem the middle ear exists to solve is impedance mismatch. Sound arrives in air but must be transmitted into the fluid of the cochlea. Fluid has a far higher acoustic impedance, so 99.9% of sound energy striking a water surface is reflected, a loss of some 30 dB

Cochlear Transduction

• 1.
• Stapes footplate moves at the oval window, displacing perilymph in the scala vestibuli → 2.
• A travelling wave is set up along the basilar membrane (von Bekesy), moving from base to apex → 3.
• Tonotopic localisation — the wave peaks at a position determined by frequency.
• The base is narrow and stiff and responds to high frequencies; the apex is wide and floppy and responds to low frequencies.
• This place principle is preserved all the way to the cortex, and is what a cochlear implant electrode array exploits → 4.
• Shearing between the basilar and tectorial membranes deflects the stereocilia → 5.
• Deflection toward the tallest stereocilium opens mechanically gated channels through the tip links → 6.
• Potassium floods IN from the high-potassium endolymph, driven by the 140 mV gradient, depolarising the hair cell → 7.
• Depolarisation opens calcium channels and releases glutamate at the base of the inner hair cell → 8.
• The cochlear nerve fires, and the signal passes to the spiral ganglion and centrally → 9.
• Outer hair cells simultaneously amplify the travelling wave through prestin-mediated electromotility, sharpening the peak and adding 40 to 60 dB of gain
• The central auditory pathway — cochlear nerve to the cochlear nuclei in the pons; then to the superior olivary complex (largely crossed, but bilateral from this point onward); the lateral lemniscus; the inferior colliculus; the medial geniculate body; and the auditory cortex in the superior temporal gyrus (areas 41 and 42)
• Because the pathway is bilateral above the cochlear nuclei, a unilateral central lesion does not cause significant deafness — which is why unilateral deafness always indicates a lesion of the cochlea or the cochlear nerve, and never of the cortex
• The superior olive is where the two ears are first compared, and is therefore the basis of sound localisation, using interaural time differences for low frequencies and interaural intensity differences for high frequencies
• The audible range is 20 to 20,000 Hz, with greatest sensitivity between 500 and 4,000 Hz — the speech frequencies, which is what audiometry concentrates on
• The decibel is a logarithmic ratio, not an absolute unit, which is why a 10 dB change represents a tenfold change in intensity and why hearing loss cannot be averaged naively
• The acoustic (stapedial) reflex contracts stapedius bilaterally to sound above about 70 to 100 dB, stiffening the chain and protecting the cochlea from sustained loud sound. Its latency is too long to protect against impulse noise such as a blast or gunshot — which is precisely why such injuries are so damaging
• The efferent olivocochlear bundle modulates outer hair cell activity, improving the detection of signals in noise