👂 Cochlea & Hearing

The basilar membrane's deflection at the peak location bends the stereocilia — tiny hair-like bundles — on the outer and inner hair cells. This bending opens mechanosensitive ion channels, causing potassium and calcium ions to flow in, depolarising the cell and triggering neurotransmitter release at the synapse with the auditory nerve. Outer hair cells also act as amplifiers, contracting and expanding to boost the mechanical response by up to 50 dB, a process called electromotility driven by the protein prestin.

Loud sounds (above ~85 dB over 8 hours, or above ~120 dB even briefly) can destroy hair cells, particularly in the basal high-frequency region. Unlike fish and birds, mammals cannot regenerate cochlear hair cells, so the loss is permanent. A single concert at 110 dB can cause temporary threshold shifts that become permanent after repeated exposure. Researchers are currently investigating gene therapies and small molecules that might stimulate hair-cell regeneration in mammals — results in mice are promising.

Tonotopy is the systematic, frequency-ordered organisation of the auditory pathway. The cochlea's frequency map is preserved in the spiral ganglion, cochlear nucleus, inferior colliculus, medial geniculate nucleus of the thalamus, and primary auditory cortex (A1), where neurons responding to similar frequencies remain spatially adjacent. This organisation facilitates efficient processing of harmonically complex sounds like speech and music, and it is why cochlear implants can restore some speech perception despite using only 22 electrodes across the full frequency range.

A cochlear implant bypasses damaged hair cells by electrically stimulating the auditory nerve directly. A microphone worn behind the ear sends sound to a speech processor, which divides it into frequency bands and sends the band amplitudes to corresponding electrode pairs along the implanted array. With 22 electrodes spanning roughly 1.5 octaves each, recipients can perceive speech at around 80% sentence accuracy in quiet but struggle in noise due to the limited spectral resolution compared to the ~3,500 inner hair cells of a healthy cochlea.

Human hearing bandwidth is partly set by the physical length (35 mm) and stiffness gradient of the basilar membrane — longer membranes extend the low-frequency limit while stiffer bases raise the high-frequency limit. Many mammals extend much higher: bats use echolocation at 150 kHz, and dolphins up to 160 kHz. Infants can sometimes hear up to 20 kHz, but the high-frequency limit declines with age, with most adults losing sensitivity above 14–16 kHz by their thirties.

Otoacoustic emissions (OAEs) are faint sounds generated by the active movements of outer hair cells and measurable with a sensitive microphone in the ear canal. They can be elicited by a click or pure tone (evoked OAEs) or occur spontaneously. Because they require intact outer hair cells, their absence at a specific frequency band confirms hair-cell damage in that region. OAE screening is now routinely performed on newborns in the UK within days of birth, detecting permanent hearing loss in roughly 1 in 1,000 babies.

Audiologists measure hearing using pure-tone audiometry, plotting hearing thresholds in decibels of hearing level (dB HL) at standard frequencies from 250 Hz to 8,000 Hz. Normal hearing is defined as 0–20 dB HL at all frequencies. Mild loss is 21–40 dB HL, moderate 41–60 dB HL, severe 61–80 dB HL, and profound above 80 dB HL. The audiogram shape reveals the type of loss: a high-frequency "ski slope" pattern is typical of noise damage or presbycusis, while a flat pattern suggests a conductive cause such as middle-ear fluid.