Three tiny middle-ear bones bridge air to cochlear fluid
Hearing detects pressure oscillations in air, water, or solids and ranks among the classic five senses. In vertebrates the outer ear gathers waves, the middle ear’s ossicles lever them, and the spiral cochlea turns motion into nerve spikes bound for the cortex.
Hearing is a form of mechanosensation, like touch: it depends on sensing molecules jostling outside the body. In people the pinna funnels sound down the canal to the eardrum, an airtight membrane that vibrates in step with the incoming wave. Because the outer ear of most mammals is lopsided, sounds from above and below are filtered differently, which lets the animal judge height. Wax from glands in the canal skin guards the eardrum against damage and microbes.
Air and fluid resist motion very differently, so the middle ear acts as an impedance matcher. In a small air-filled chamber the hammer, anvil and stirrup, the body's three smallest bones, pass vibrations to the oval window of the inner ear, while the round window bulges to let the fluid move. Two tiny muscles, the stapedius and tensor tympani, stiffen the chain reflexively to protect it. Inside the spiral cochlea, the basilar membrane is tuned by position: high frequencies resonate near the entrance, low ones near the tip. Its motion depolarises hair cells, which do not fire themselves but release transmitter onto auditory-nerve fibres that do.
Signals climb from the cochlear nucleus in the brainstem to the inferior colliculus, which also drives reflexes such as the startle response, then through the medial geniculate nucleus of the thalamus to the primary auditory cortex in the temporal lobe, where sound is thought to become conscious. Nearby Wernicke's area is needed to understand speech. Strokes or injuries along this route, especially on both sides, can cause hearing problems or even auditory hallucinations.
Audiometers test hearing behaviourally, while brainstem evoked potentials, otoacoustic emissions and electrocochleography work even on unconscious people and have made infant screening routine. Loss is conductive, sensorineural or mixed, and graded by the quietest sound the better ear catches: 25 to 40 decibels for mild, 40 to 70 for moderate, 70 to 95 for severe. Some now prefer the gentler term aural diversity.
Source: Hearing
