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.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.