Hearing and Balance: Frequency, Place and Head Motion
The cochlea sorts sound by frequency mechanically, so position along it stands for pitch before any neuron is involved. The same kind of hair cell, placed in fluid-filled loops instead, reports how the head is turning, which is why one organ serves two senses.
What a learner can do afterwards
- Explain the place code for frequency and what part of the cochlea does the sorting
- Describe hair cell transduction and why it is fast enough to follow sound
- Say how the semicircular canals and otolith organs report different kinds of head motion
1 · Read
Your cochlea sorts pitch mechanically, before any neuron fires, so you can follow a voice or a tune. High-frequency waves bend the basilar membrane near the base; low-frequency waves bend it near the apex. Each hair cell thus serves one frequency, like touch receptors serve one kind of touch, so the membrane reads sound the way a prism reads light.
Above the membrane sits the organ of Corti, its hair cells crowned with tilted rows of stereocilia tied together and leaning into the tectorial sheet. When the membrane moves, the sheet slides and bends the bundles: toward the tallest hairs, tethers yank ion channels open and the cell fires; toward the shortest, channels close. Direct mechanical gating is why hearing keeps up with sound.
The same kind of hair cell works in balance. The utricle and saccule weigh tilt and straight-line motion with otolith crystals sliding on gel. The three semicircular canals sense rotation: turning the head leaves fluid lagging, which bends the cupula and its hairs. Comparing canals maps any spin in three dimensions.
Together the membrane's hair cells encode 20 to 20,000 hertz, apex low and base high. Their signals leave through the vestibulocochlear nerve, and brainstem nuclei compare the ears to pin down where sounds live.
Place along the membrane picks the pitch, bent bundles open the current, and twin organs track tilt and spin.
The rewritten readings were written for this dataset.
More to read
Two senses within the inner ear
Special senses use specialised organs: the nose detects smells, the tongue detects tastes, and eyes provide vision. Hearing, or audition, and balance, or equilibrium, both involve the inner ear.
Hearing uses the spiral-shaped cochlea. Sound moves its fluid and deflects hair-cell bundles. Different locations respond best to different pitches.
Nearby balance structures include fluid-filled semicircular canals and the small chambers called otolith organs. The canals detect head rotation. Otolith organs detect linear acceleration, such as speeding up in a car, and gravity's direction.
Both systems use sensory hair cells. Their different arrangements allow the inner ear to detect sound and head motion.
Terms for detecting and locating sound
Inside the cochlea, the basilar membrane supports the sensory hair cells. Its stiffness varies along its length. High frequencies cause the strongest movement toward one end; low frequencies peak toward the other. The location of greatest movement helps encode frequency, heard as pitch. This relationship is called place coding.
Amplitude describes wave height and contributes to perceived loudness. The absolute threshold is the minimum sound energy detected on half of trials. Binaural cues use information from both ears to help locate a sound.
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