Human Physiology I · Sensory Physiology

Hearing and Equilibrium

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Study tools

In 30 seconds

Sound waves are pressure oscillations in air characterized by (cycles per second, perceived as ) and (perceived as ). The outer ear funnels sound to the eardrum; the middle ear's (malleus, incus, stapes) perform , efficiently transferring vibrations from air to the fluid-filled inner ear. In the , sound produces a traveling wave along the that peaks at a frequency-specific location (), where in the deflect, open mechanically gated channels, and allow from endolymph to depolarize and transduce sound. Equilibrium is sensed by the semicircular canals (rotational/angular acceleration) and the utricle and saccule (linear acceleration and head tilt).

Why this matters

Audiometry plots hearing thresholds against frequency and directly reflects tonotopy and the health of the outer, middle, and inner ear — a conductive deficit (middle ear) is distinguished from a sensorineural deficit (cochlea/nerve). The Rinne and Weber tuning-fork tests localize the site of hearing loss using bone versus air conduction. Because the basilar membrane is tonotopic, high-frequency hearing loss (common with aging and noise exposure) maps onto damage near the cochlear base. The vestibulo-ocular reflex and caloric testing (irrigating the ear canal with warm/cool water to induce endolymph convection) are used to assess vestibular function. Understanding endolymph's unique potassium composition underlies concepts in Ménière's disease and why hair-cell damage is often irreversible. Clinical values, diagnostic criteria, and testing protocols vary by institution and jurisdiction; these notes support education but do not replace clinical instruction or supervision.

The college version

1. Sound and the Outer/Middle Ear

A sound wave is a traveling oscillation of air pressure. Frequency (hertz, Hz) is the number of cycles per second and determines pitch; amplitude is the magnitude of the pressure change and determines loudness. The outer ear (pinna and external auditory canal) collects sound and directs it to the tympanic membrane (eardrum). The middle ear is an air-filled cavity containing the three ossicles — malleus, incus, and stapes — which transmit eardrum vibrations to the oval window of the inner ear. Because the cochlea is filled with fluid, which has much higher acoustic impedance than air, direct air-to-fluid transfer would reflect most sound energy. The ossicles perform impedance matching: the area advantage of the large eardrum against the small oval window, plus the lever action of the ossicles, amplifies pressure so vibrations are transmitted efficiently into the fluid.

2. The Cochlea and Auditory Transduction

The inner ear contains the cochlea, a fluid-filled spiral. Inside it, the organ of Corti sits on the basilar membrane and houses the sensory hair cells. The basilar membrane is narrow and stiff at the base (near the oval window) and wide and floppy at the apex, so a traveling wave peaks at a location determined by frequency — high frequencies near the base, low frequencies near the apex. This spatial mapping of frequency is tonotopy. Hair cells have stereocilia projecting into the endolymph, an unusual fluid rich in potassium. When the basilar membrane vibrates, stereocilia bend; tip links pull open mechanically gated cation channels, and because endolymph is potassium-rich, potassium influx (not sodium) depolarizes the hair cell. Depolarization opens voltage-gated calcium channels, triggering neurotransmitter release onto the auditory (cochlear) nerve — auditory transduction. Note that hair cells are unusual in that they use potassium, entering down its electrochemical gradient from high-potassium endolymph, as the depolarizing current.

3. Equilibrium

The vestibular apparatus detects head position and motion. The semicircular canals (three, oriented in perpendicular planes) sense dynamic equilibrium — rotational/angular acceleration. Each canal has a swelling, the ampulla, containing the crista with hair cells whose stereocilia are embedded in a gelatinous cupula; head rotation moves endolymph, deflecting the cupula and stimulating hair cells. The utricle and saccule sense static equilibrium and linear acceleration. Each contains a macula, a patch of hair cells with stereocilia embedded in an otolithic membrane weighted by calcium-carbonate crystals (otoliths). Gravity or linear acceleration shifts the otoliths, bending the hairs and signaling head tilt or straight-line motion. The vestibulo-ocular reflex (VOR) uses vestibular input to move the eyes in the opposite direction of head movement, keeping gaze stable on a fixed target.

How it works

The pinna collects sound and the canal funnels it to the tympanic membrane.
The ossicles transfer and amplify the vibration to the oval window (impedance matching).
A traveling wave moves through cochlear fluid and peaks at a frequency-specific site on the basilar membrane (tonotopy).
Shearing forces bend hair-cell stereocilia in the organ of Corti.
Tip links open channels; potassium from endolymph enters and depolarizes the hair cell.
Depolarization opens calcium channels, releasing transmitter onto the cochlear nerve.
The auditory pathway relays frequency (place) and intensity (rate) information to the cortex.
For balance, canal and otolith hair cells respond to rotational and linear acceleration, driving the vestibulo-ocular reflex to stabilize vision.

Common confusions

Do not confuse	With	Difference
Frequency	Amplitude	Frequency = pitch (Hz); amplitude = loudness
Base of basilar membrane	Apex of basilar membrane	Base = high frequency; apex = low frequency
Endolymph	Perilymph	Endolymph is potassium-rich (bathes stereocilia); perilymph resembles extracellular fluid
Semicircular canals	Utricle/saccule	Canals = rotational (dynamic); otolith organs = linear/tilt (static)
Ampulla	Macula	Ampulla (crista/cupula) senses rotation; macula (otoliths) senses tilt/linear motion
Hair-cell depolarization	Typical neuron depolarization	Hair cells use potassium influx; most neurons use sodium influx

Memory aids

For tonotopy, "Bass is Big (apex is wide/floppy); Treble is Tight (base is narrow/stiff)." For the ossicles, "MIS" — Malleus, Incus, Stapes (in order, from eardrum to oval window). For equilibrium, "Canals Count Curves (rotation); Otoliths Orient (tilt/linear)." For the unusual current, "Hair cells are High on K⁺" — potassium influx depolarizes them.

Quick review

Topic Recap

Hearing converts air-pressure waves into neural signals through the outer, middle, and inner ear. The ossicles match impedance between air and cochlear fluid, and the basilar membrane's tonotopy codes frequency by location. Hair cells transduce vibration by potassium influx from endolymph, depolarizing and releasing transmitter onto the cochlear nerve. Equilibrium is divided between the semicircular canals (dynamic/rotational) and the utricle and saccule (static/linear), with the vestibulo-ocular reflex stabilizing gaze during movement.

Knowledge Check

Which ion depolarizes cochlear hair cells, and from which fluid does it enter?
Where on the basilar membrane do high-frequency sounds produce their maximum displacement?
Which structures sense rotational (angular) acceleration?
What is the function of the middle-ear ossicles?
Which reflex keeps the eyes fixed on a target while the head turns?

Answers and Rationales

Potassium, from endolymph. Endolymph is potassium-rich, so K⁺ flows inward down its gradient and depolarizes the hair cell.
The base (near the oval window). The basilar membrane is narrow and stiff at the base, tuned to high frequencies.
The semicircular canals. Their ampullae (crista/cupula) detect angular acceleration.
Impedance matching. They amplify pressure from the eardrum to efficiently drive the fluid-filled cochlea.
The vestibulo-ocular reflex. Vestibular input drives compensatory eye movements opposite to head rotation.

Eli explains

The same idea, in plain words

Explain it like I’m 10

Your ear is a three-room relay station for sound, plus a built-in carpenter's level for balance. Room one (the outer ear) is a funnel that catches sound waves and passes them to a thin drum. Room two (the middle ear) holds three tiny bones that work like a lever-and-piston set, taking the gentle wiggle of the air drum and turning it into a strong push against the fluid in room three (the inner ear) — because pushing liquid is much harder than pushing air. In room three, the fluid ripple runs along a ribbon (the basilar membrane) that is stiff at one end and floppy at the other; high notes make the stiff end dance, low notes make the floppy end dance, so the brain knows the pitch by where the ribbon moved. Balance lives in the same room: three little spinning-sensors measure turns, and two little pebble-pads measure tilting and going straight.

Where it stops being exact: the "ribbon" is covered in thousands of hair cells whose job is not to move but to sense bending, and "the brain knows the pitch" really means a precise place-code and timing-code of nerve firing, not just a location on a ribbon.

Simple Example

Put on headphones and play a low bass note, then a high whistle. The bass makes you feel vibration near the floppy, far end of the inner ear's ribbon, and the whistle excites the stiff, near end. Your brain reads those two different locations as "low pitch" and "high pitch." Now spin in a chair and stop suddenly — the fluid in your semicircular canals keeps swirling for a moment, so you briefly feel as if you are still turning; that is your angular-acceleration sensors overshooting.

Worked example

Trace sound from air to brain:

Sound enters the outer ear and strikes the tympanic membrane, converting air-pressure waves into mechanical vibration.
Ossicular chain transmits the vibration. Malleus → incus → stapes push against the oval window.
Impedance matching amplifies pressure. The eardrum-to-oval-window area ratio and ossicular lever action concentrate the force, overcoming the fluid's high impedance.
A traveling wave moves through the cochlear fluid, displacing the basilar membrane.
Tonotopic peak. The wave peaks at the frequency-matched location — base for high pitch, apex for low pitch.
Hair cells bend. The shearing motion between the basilar membrane and tectorial membrane deflects stereocilia.
Potassium influx depolarizes. Tip links open mechanically gated channels; potassium from endolymph rushes in (its electrochemical gradient is inward), depolarizing the hair cell.
Calcium entry triggers release. Depolarization opens voltage-gated calcium channels, and neurotransmitter is released onto the cochlear nerve.
Nerve signal → brainstem → cortex. Action potentials travel via the cochlear nerve to brainstem nuclei and then to auditory cortex, which decodes the tonotopic pattern as pitch and the firing rate as loudness.

Key takeaways

High yield: Hair cells depolarize via potassium influx from potassium-rich endolymph — the reverse of most neurons.
High yield: Tonotopy — high frequency at the base, low frequency at the apex of the basilar membrane.
High yield: The ossicles perform impedance matching (area ratio + lever action) to drive fluid.
High yield: Semicircular canals = rotational (dynamic) equilibrium; utricle and saccule = linear/tilt (static) equilibrium.
High yield: The ampulla (crista/cupula) is the rotation sensor; the macula (otoliths) is the tilt/linear sensor.
High yield: Frequency = pitch; amplitude = loudness.
High yield: The vestibulo-ocular reflex keeps eyes fixed on a target during head movement.
Bending stereocilia in one direction opens channels; the opposite direction closes them.
The organ of Corti's outer hair cells amplify and tune the traveling wave.

Keep learning

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Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

Describe how sound waves are characterized by frequency and amplitude and how the outer, middle, and inner ear transmit them to the cochlea.
Explain impedance matching by the ossicles, tonotopy along the basilar membrane, and the mechanism of auditory transduction in hair cells.
Describe the structures and function of the semicircular canals, utricle, and saccule in sensing dynamic and static equilibrium.
Explain the vestibulo-ocular reflex and its role in stabilizing gaze during head movement.

Key vocabulary

Sound wave: Traveling oscillation of air pressure
Frequency: Cycles per second (Hz)
Pitch: Perceptual quality of sound frequency
Amplitude: Magnitude of the pressure change
Loudness: Perceived intensity of sound
Outer/middle/inner ear: The three anatomical divisions of the ear
Ossicles: Malleus, incus, stapes in the middle ear
Impedance matching: Pressure amplification from air to cochlear fluid
Cochlea: Fluid-filled spiral of the inner ear
Organ of Corti: Sensory epithelium on the basilar membrane
Basilar membrane: Ribbon of varying stiffness supporting the organ of Corti
Tonotopy: Spatial mapping of frequency along the basilar membrane
Hair cells: Mechanosensitive receptor cells with stereocilia
Endolymph: Potassium-rich fluid bathing the stereocilia
Potassium influx: Entry of K⁺ that depolarizes hair cells
Auditory transduction: Conversion of mechanical vibration into neural signal
Semicircular canals: Three canals sensing rotational acceleration
Utricle: Otolith organ sensing horizontal linear acceleration/tilt
Saccule: Otolith organ sensing vertical linear acceleration/tilt
Macula: Sensory patch of hair cells in utricle and saccule
Ampulla: Swelling of a semicircular canal housing the crista
Vestibulo-ocular reflex: Eye movement opposing head movement
Dynamic vs static equilibrium: Rotational acceleration vs. linear/tilt sensing

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