General Principles of Reception and Response in Animals

Lessons

Senior 6 · Biology

General Principles of Reception and Response in Animals

5 lessons

Lesson 4 of 5

The ear: hearing and balance

30 min

Two senses in one small space

An intore dancer leaps, stamps and stops dead without falling, while following every beat of the drums. Both feats depend on the same organ. Deep in the temporal bone of the skull, in a space smaller than a bean, the inner ear holds receptors for hearing and for balance. Both use the same kind of receptor, the hair cell, a mechanoreceptor that responds when a bundle of tiny projections on its surface is bent. This lesson follows sound from the air to the auditory nerve, and explains how the same hair cells report the turning and tilting of the head.

Definition

Hair cell: a secondary mechanoreceptor of the inner ear, with a bundle of stiff projections, the stereocilia, on its upper surface. Bending the stereocilia opens ion channels.

Cochlea: the coiled, fluid-filled tube of the inner ear that contains the receptors for hearing.

Organ of Corti: the sensory structure along the cochlea, made of hair cells resting on the basilar membrane, with their stereocilia touching the tectorial membrane above.

Frequency of a sound: the number of vibrations per second, measured in hertz (Hz); we hear it as pitch. Young people hear from about 20 Hz to 20 000 Hz.

Structure of the ear

How we hear

Collection. The pinna funnels sound waves into the ear canal. They make the eardrum vibrate at the same frequency.
Amplification. The three ossicles (malleus, incus and stapes) pass the vibrations across the air-filled middle ear to the oval window. They act as levers, and they concentrate the force collected by the large eardrum onto the much smaller oval window, so the pressure increases about twenty times. This is needed because it takes far more pressure to move fluid than air.
Pressure waves in the cochlea. The stapes pushes the oval window in and out, setting up pressure waves in the fluid (perilymph) of the cochlea. Because fluid cannot be compressed, the round window bulges out each time the oval window moves in.
The basilar membrane vibrates. The waves make the basilar membrane vibrate. It is narrow and stiff near the base of the cochlea and wide and flexible near the apex, so each frequency makes one particular region vibrate most (Figure 2).
Transduction. As the basilar membrane moves, the stereocilia of the hair cells are bent against the tectorial membrane. Bending opens ion channels; positive ions (mainly K⁺, which is very concentrated in the fluid around the stereocilia) flow in and depolarise the hair cell, which releases neurotransmitter onto a sensory neurone.
To the brain. Impulses travel along the auditory nerve to the auditory cortex in the temporal lobe, where they are perceived as sound.

The Eustachian tube joins the middle ear to the throat. Opening it, for example by swallowing, keeps the air pressure on both sides of the eardrum equal, so the eardrum can vibrate freely; that is why your ears "pop" on a fast drive down from the hills.

Pitch and loudness: the place theory

Pitch is coded by place: the brain knows which frequency was heard from which hair cells, and so which nerve fibres, are stimulated. High notes stimulate hair cells near the base; low notes stimulate those near the apex.
Loudness is coded by the amplitude of vibration: a louder sound moves the basilar membrane further, bends the stereocilia more and gives a higher frequency of impulses, from more hair cells.

Worked example 1: how much do the ossicles amplify?

The eardrum has an area of about $55\,\mathrm{mm^2}$ and the footplate of the stapes about $3.2\,\mathrm{mm^2}$ . The ossicles also act as a lever that increases the force about 1.3 times. Estimate the gain in pressure from the eardrum to the oval window.

Pressure is force divided by area: $p = \frac{F}{A}$ . The same force on a smaller area gives a larger pressure.
Area effect: $\frac{55}{3.2} = 17.2$ .
Lever effect: $17.2 \times 1.3 = 22.3$ .

Answer: the pressure at the oval window is about 22 times that at the eardrum. Without this gain, most of the energy of sound in air would be reflected at the surface of the cochlear fluid.

Balance: the vestibular apparatus

	Semicircular canals	Utricle and saccule (vestibule)
Detects	Rotation of the head: turning, nodding, tilting sideways	Position of the head relative to gravity, and straight-line acceleration
Structure	Three fluid-filled canals at right angles to each other, one for each plane; each has a swelling, the ampulla, containing hair cells whose stereocilia are embedded in a jelly cap, the cupula	Hair cells whose stereocilia are embedded in a jelly carrying crystals of calcium carbonate, the otoliths
How it works	When the head turns, the fluid (endolymph) lags behind because of its inertia, pushes the cupula over and bends the stereocilia	When the head tilts or the body speeds up, the heavy otoliths pull the jelly, bending the stereocilia

Impulses from the vestibular apparatus go mainly to the cerebellum, which adjusts the contraction of muscles to keep balance. After you spin and stop suddenly, the endolymph keeps moving for a few seconds, so the brain is told you are still turning: this is the dizziness that the dancer learns to control by fixing the eyes on one point.

Protecting your hearing

Hair cells that are destroyed by loud sound are not replaced in humans, so noise-induced hearing loss is permanent. The global standard for safe listening devices published by the World Health Organization and the International Telecommunication Union in 2019 sets a safe weekly limit for adults equivalent to 80 dB for 40 hours. Turning earphones down and limiting time near very loud speakers protect hearing for life.

Worked example 2: reading an audiogram

A hearing test of a 70-year-old (illustrative) shows normal thresholds at 250 Hz and 500 Hz but a large hearing loss at 4 000 Hz and 8 000 Hz. Use the place theory to suggest where in the cochlea the damage is.

High frequencies make the narrow, stiff base of the basilar membrane vibrate most.
Hearing is lost only at high frequencies, so the hair cells (or their neurones) near the base must be damaged, while those near the apex still work.
This fits what is known about age-related hearing loss: the hair cells at the base, which every sound passes on its way along the cochlea, are lost first.

Consequence: the person may hear vowels, which are lower-pitched, but miss consonants such as "s" and "f", so speech sounds loud enough yet unclear.

Warning

Common mistakes

"The ossicles make the sound louder by making it vibrate faster." They do not change the frequency; they increase the pressure of the vibrations.
"The semicircular canals detect gravity." They detect rotation; the utricle and saccule detect gravity and straight-line movement.
"The cochlea is in the middle ear." The middle ear holds the ossicles in air; the cochlea is in the fluid-filled inner ear.

CrashCourse: "Hearing & Balance: Crash Course Anatomy & Physiology #17". Watch for the path of sound through the ear and how the vestibular system senses movement.

✦

Practice: the ear

Name the three ossicles in order, from the eardrum to the oval window.
State the function of (a) the Eustachian tube, (b) the round window.
Explain why the middle ear needs to amplify the pressure of sound vibrations.
An eardrum of area $60\,\mathrm{mm^2}$ passes vibrations to an oval window of area $3.0\,\mathrm{mm^2}$ . Ignoring the lever effect, calculate the pressure gain.
Describe how a vibration of the basilar membrane leads to depolarisation of a hair cell.
Explain how the ear distinguishes a high-pitched sound from a low-pitched one.
Explain why a person feels dizzy for a few seconds after spinning round and stopping.
A worker uses a loud machine for many years without ear protection. Predict and explain the effect on her hearing.

Show answersHide answers

Malleus, incus, stapes.
(a) It equalises the air pressure on both sides of the eardrum. (b) It bulges outwards when the oval window moves in, allowing the incompressible fluid of the cochlea to move.
Fluid is much harder to set in motion than air, so without an increase in pressure most of the sound energy would be reflected at the oval window instead of entering the cochlea.
$\frac{60}{3.0} = 20$ times.
The basilar membrane moves the hair cells relative to the tectorial membrane; their stereocilia bend; ion channels open; positive ions (K⁺) flow in; the hair cell depolarises and releases neurotransmitter onto a sensory neurone.
Different frequencies make different parts of the basilar membrane vibrate most: high frequencies the narrow, stiff base, low frequencies the wide, flexible apex. The brain identifies the pitch by which hair cells and nerve fibres are stimulated.
The endolymph in the semicircular canals keeps moving after the head stops, because of its inertia. It bends the cupula and the stereocilia, so impulses tell the brain the head is still turning, conflicting with what the eyes report.
Prolonged loud noise destroys hair cells, especially at the base of the cochlea; they are not replaced, so she will have a permanent hearing loss, worst at high frequencies.

Key points

Eardrum → malleus, incus, stapes (pressure gain about ×20) → oval window → pressure waves in the cochlear fluid → round window bulges.
The basilar membrane vibrates; stereocilia of hair cells bend against the tectorial membrane; ion channels open; the hair cell depolarises; impulses pass along the auditory nerve.
Pitch is coded by place (base: high frequencies; apex: low); loudness by the amplitude of vibration and impulse frequency.
Semicircular canals detect rotation; the utricle and saccule detect gravity and linear acceleration. Lost hair cells are not replaced, so noise damage is permanent.

Sources and credits

7.4.5: Hearing and Vestibular Sensation, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structure of the ear, transduction by hair cells, the place coding of pitch and the vestibular system. Adapted by Ganzaa.
Safe listening: Safe listening devices and systems: a WHO-ITU standard, World Health Organization and International Telecommunication Union, 2019.
Video: "Hearing & Balance: Crash Course Anatomy & Physiology #17", CrashCourse, embedded from YouTube.

Figures 1 and 2, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

1Two senses in one small space

2Structure of the ear

3How we hear

4Pitch and loudness: the place theory

5Balance: the vestibular apparatus

6Protecting your hearing