Source: A Brain-Mind Odyssey, Ch. 15
Tags: hearing, auditory system, sound, frequency, pitch, timbre, tympanic membrane, ossicles, cochlea, basilar membrane, hair cells, tonotopic, Fourier analysis, auditory cortex A1, decibel, hearing loss, cochlear implant, vestibular system, semicircular canals, otolith, echolocation, acoustic shadow
Hearing begins when sound pressure waves vibrate the eardrum, are amplified by three tiny bones in the middle ear, and transmitted to the fluid-filled cochlea. The basilar membrane performs a physical Fourier analysis, with different positions resonating at different frequencies. Hair cells along the membrane transduce vibration into neural signals that travel via cranial nerve VIII through the brainstem and thalamus to primary auditory cortex (A1). The vestibular system, attached to the cochlea, detects gravity and acceleration using hair cells and otoliths.
Tympanic membrane (eardrum)
A thin, drum-like tissue that vibrates in response to air pressure changes. Forms the boundary between the outer and middle ear.
Ossicles
Three small interconnected bones in the middle ear (hammer/malleus, anvil/incus, stirrup/stapes) that amplify and transmit vibrations from the eardrum to the cochlea.
Cochlea
A coiled, bony structure in the inner ear filled with fluid. Found only in mammals and marsupials (more linear in other vertebrates). Houses the basilar membrane and hair cells.
Basilar membrane
A thin tissue running the length of the cochlea that varies in thickness. Thickest near the oval window (responds to high frequencies), thinnest at the far end (responds to low frequencies). Performs a spatial Fourier analysis of incoming sound.
Hair cell
Mechanoreceptor cells on the basilar membrane with cilia that bend as the membrane vibrates, initiating neural signals. Inner hair cells (~3,500 per cochlea) carry the main auditory signal. Outer hair cells (~12,000) modulate basilar membrane sensitivity.
Tonotopic organisation
The spatial mapping of sound frequency along the basilar membrane and throughout the auditory pathway. High frequencies at the base, low frequencies at the apex.
Otolith
Tiny calcium carbonate crystals in the vestibular system, suspended above hair cells. Their inertia bends the hairs as the body accelerates or changes orientation, contributing to balance.
Decibel (dB)
A logarithmic scale of sound intensity. 0 dB is the approximate threshold of human hearing; each 10 dB increase represents a tenfold increase in loudness.
The classic "tree falling in the forest" question highlights two definitions:
Physical: variations in air pressure produced by the event
Perceptual: a mental experience requiring a being with an auditory system
Loudness: related to the amplitude (magnitude) of pressure variation
Pitch: related to the frequency of pressure variation
Timbre: related to the complexity of the sound waveform
A pure tone is a simple sinusoidal wave at a single frequency
Most natural sounds are complex waveforms (mixtures of frequencies)
Timbre is what makes different instruments sound distinct
Human hearing: 20 to 20,000 Hz
Unit: Hertz (Hz) = cycles per second
Speed of sound: ~335 m/s (~1,100 ft/s, ~750 mph)
Much slower than light (186,000 miles/second), which is why you see lightning before hearing thunder
Joseph Fourier (French mathematician, 1768-1830)
Any complex waveform can be represented as a sum of simple sinusoidal waves of various frequencies and amplitudes
The basilar membrane physically performs this analysis on incoming sound
Fundamental frequency: the lowest frequency of a periodic waveform
Overtones: any frequencies higher than the fundamental
Harmonics are overtones whose frequencies are integer multiples of the fundamental
The fundamental plus its overtones are collectively called partials
Timbre: the quality that distinguishes sounds of the same pitch and loudness
Determined largely by the relative amplitude of each harmonic
Why a violin and a trumpet playing the same note sound different
Tympanic membrane (eardrum): vibrates in response to sound waves
Ossicles: hammer (malleus), anvil (incus), stirrup (stapes)
Amplify and transmit vibration from the eardrum to the oval window of the cochlea
A coiled, hard, bony structure filled with fluid
Vibration of the oval window sets the cochlear fluid into vibration
Basilar membrane: runs the length of the cochlea
Varies in thickness: thickest at the base (near the oval window), thinnest at the apex
Different regions resonate at different frequencies due to this thickness gradient
Performs a spatial Fourier analysis: incoming sound is decomposed into its component frequencies, each represented at a specific location along the membrane
Tonotopic organisation: high frequencies are transduced at the base of the cochlea, low frequencies at the apex (like a piano laid out along the membrane)
Sit on the basilar membrane with a bundle of cilia at one end
As the basilar membrane vibrates, cilia are bent by the surrounding fluid
Mechanism of transduction:
Cilia are connected by tiny cables (tip links) to mechanically gated ion channels
When the cilia bend, the cables pull ion channels open
K⁺ flows into the cell (unusual: K⁺ is the depolarising ion here)
Depolarisation causes voltage-gated Ca²⁺ channels to open
Ca²⁺ influx triggers fusion of synaptic vesicles with the cell membrane
Neurotransmitter is released into the synaptic cleft
Postsynaptic receptors of the auditory nerve (cranial nerve VIII) are activated
If sufficient, an action potential propagates toward the brain
Inner hair cells: ~3,500 per cochlea
Carry the vast majority of auditory signals to the spiral ganglion and cochlear nucleus
Outer hair cells: ~12,000 per cochlea
Fewer connections to the spiral ganglion; receive more input from the brainstem
Contain the protein prestin, which elongates and contracts with membrane potential changes
This changes the shape of the cell, pushing against the basilar membrane and modifying its stiffness and sensitivity
Act as a cochlear amplifier, fine-tuning the membrane's response
Sound energy reaches the inner ear not only through the ear canal but also through direct vibration of the skull bones surrounding the cochlea
This is why your voice sounds different in a recording compared to when you hear yourself speak
Cochlea → cranial nerve VIII (auditory nerve) and spiral ganglion → cochlear nucleus (medulla) → other cells in medulla and pons → inferior colliculus (midbrain) → medial geniculate nucleus, MGN (thalamus) → primary auditory cortex, A1 (temporal lobe)
Summarised: cochlea → brainstem → midbrain → thalamus → cortex
Humans have symmetrical ears; the brain uses interaural differences to localise sound:
Interaural time difference (sound arrives at the closer ear first)
Interaural level difference (sound is louder in the closer ear)
Acoustic shadow: the head blocks sound, creating a "shadow" on the far side
A sound directly in front produces no shadow difference between ears
A sound to one side places the opposite ear in shadow
Less effective at low frequencies (wavelengths wider than the head)
Owls: asymmetrical ears allow vertical as well as horizontal sound localisation; can locate prey in complete darkness with extraordinary precision
Logarithmic: each 10 dB increase = 10-fold increase in loudness; 20 dB = 100-fold
Reference points:
0 dB: approximate human hearing threshold
~25 dB: quiet room
~40-50 dB: normal speech
~80 dB: noisy restaurant
Infection: inner ear infections that irreversibly damage hair cells
Genetic: mutations in genes such as connexin-26 (produces abnormal ion balance in the cochlea, preventing hair cell function; causes congenital deafness)
Noise-induced: acoustic trauma from loud sounds; hair cells die from excitotoxic overstimulation
Acoustic amplifiers: from the historical ear trumpet to modern miniature battery-powered devices (microphone + amplifier, worn behind or within the ear)
Simply increase the volume of sound available to the ear
Cochlear implant: a surgically implanted array of electrodes in the inner ear
A tiny external microphone picks up sound
A frequency analyser performs a crude Fourier analysis, extracting frequency components
Electrodes electrically stimulate the auditory nerve at spatial locations corresponding to specific frequency ranges
Produces a partial reconstruction of the incoming sound
Three semicircular canals plus two bulbous structures: the utricle and saccule
All attached to the cochlea
Detect orientation relative to gravity and acceleration during movement
Three orthogonal canals provide complete information about head orientation and acceleration in three dimensions
Receptor cells are hair cells that detect movement of fluid in the canals
Otoliths: tiny calcium carbonate crystals suspended in fluid above hair cells
As the body accelerates or changes orientation, the inertia of the otoliths bends the hair cells
These tiny stones amplify the vestibular signal
The vestibular sense operates mostly below conscious awareness; you notice it when disrupted (spinning, dizziness)
⚠️ Know the hair cell transduction sequence: cilia bend → tip links pull open ion channels → K⁺ influx → depolarisation → Ca²⁺ influx → vesicle fusion → NT release → CN VIII activation.
⚠️ The basilar membrane performs a physical Fourier analysis. Thick end (base) = high frequencies, thin end (apex) = low frequencies. This is tonotopic organisation.
⚠️ Inner hair cells (3,500) carry the main signal; outer hair cells (12,000) modulate sensitivity via prestin.
⚠️ The auditory pathway: cochlea → CN VIII → cochlear nucleus → medulla/pons → inferior colliculus → MGN → A1. More relay stations than the visual or olfactory pathway.
⚠️ Acoustic shadow is less useful at low frequencies because the wavelength exceeds the head's width.
⚠️ Connexin-26 mutation is a key genetic cause of congenital deafness.
⚠️ Cochlear implants bypass damaged hair cells entirely, electrically stimulating the auditory nerve directly.
Q: How does the basilar membrane analyse incoming sound?
A: The basilar membrane varies in thickness along its length. Different regions resonate at different frequencies (thick base for high frequencies, thin apex for low frequencies). This spatial separation of frequency components is equivalent to a Fourier analysis, representing different frequencies at different positions along the membrane.
Q: Describe the transduction process in a cochlear hair cell, from cilia bending to action potential.
A: Bending of cilia tugs on tip links connected to mechanically gated ion channels. K⁺ flows into the cell, depolarising it. This depolarisation opens voltage-gated Ca²⁺ channels. Calcium influx triggers vesicle fusion and neurotransmitter release into the synaptic cleft. The neurotransmitter activates postsynaptic receptors on the auditory nerve (CN VIII), and if the signal is sufficient, an action potential is sent to the brain.
Q: What is the difference between inner and outer hair cells?
A: Inner hair cells (~3,500 per cochlea) send the primary auditory signal to the brain via the spiral ganglion. Outer hair cells (~12,000) make fewer connections to the ganglion but receive brainstem input and contain prestin, a protein that changes cell shape. Outer hair cells push against the basilar membrane to adjust its stiffness and sensitivity, acting as a cochlear amplifier.
Q: How do otoliths contribute to balance?
A: Otoliths are tiny calcium carbonate crystals suspended in fluid above vestibular hair cells. When the body accelerates or changes orientation relative to gravity, the inertia of the otoliths causes them to lag behind, bending the hair cells and generating a neural signal that the brain uses to maintain balance.
Q: Why does your voice sound different in a recording compared to when you hear yourself speak?
A: When speaking, much of the sound energy reaches the cochlea through direct vibration of the skull bones, not just through the ear canal. A recording captures only the airborne component, so it sounds different from the bone-conducted version you are accustomed to hearing.
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