Source: Module 4, Chapter 15 | Course: A Brain-Mind Odyssey (UC Berkeley)
Tags: sound, frequency, amplitude, pitch, loudness, timbre, Fourier analysis, cochlea, basilar membrane, hair cells, prestin, auditory nerve, cranial nerve 8, auditory pathway, primary auditory cortex, A1, hearing loss, vestibular system, semicircular canals, otolith
Sound waves are pressure variations in air characterised by frequency (perceived as pitch), amplitude (perceived as loudness), and waveform complexity (perceived as timbre). The cochlea performs a mechanical Fourier analysis: its basilar membrane varies in thickness so that different regions resonate to different frequencies. Inner hair cells transduce these vibrations into neural signals via mechanically-gated ion channels, while outer hair cells actively tune basilar membrane sensitivity using the motor protein prestin. Auditory signals travel via cranial nerve 8 through a chain of brainstem nuclei to the primary auditory cortex (A1). The vestibular system (semicircular canals, utricle, saccule, otoliths) detects orientation and acceleration for balance.
Sound wave
A pressure variation that propagates through air at approximately 1,100 feet per second. Characterised by frequency, amplitude, and waveform shape.
Frequency
The number of pressure oscillations per second, measured in hertz (Hz). Higher frequency corresponds to higher pitch. Wave velocity = frequency x wavelength, so higher frequency means shorter wavelength.
Amplitude
The magnitude of pressure variation in a sound wave. Greater amplitude is perceived as greater loudness.
Pitch / tone
The perceptual experience of a sound's frequency. Higher-frequency sounds are perceived as higher-pitched.
Loudness
The perceptual experience of a sound's amplitude.
Timbre
The perceived richness and complexity of a sound beyond pure tone. Related to the complexity of the waveform. A pure sinusoidal wave produces a pure tone; more complex waveforms (with multiple frequency components) produce richer timbre.
Fourier analysis
A mathematical principle (Joseph Fourier, 1768–1830) stating that any complex waveform can be decomposed into a set of simple sinusoidal components at different frequencies. The basilar membrane performs this analysis mechanically.
Human hearing range
Approximately 20 to 20,000 Hz.
Cochlea
The spiral-shaped structure of the inner ear, filled with fluid. Vibration of the oval window (boundary between middle ear and inner ear) sets the cochlear fluid into vibration. Contains the basilar membrane and hair cells.
Basilar membrane
A thin tissue running the length of the cochlea's spiral interior. Varies in thickness (thickest near the oval window, thinnest at the far end). Different regions resonate to different frequencies, creating a spatial map of the sound's frequency components, essentially a physical Fourier analysis.
Inner hair cells
Cells lining the basilar membrane that are primarily responsible for hearing. Characterised by a bundle of cilia (hairs) at one end. As the basilar membrane vibrates, cilia bend, tiny molecular cables tug open mechanically-gated ion channels, K+ flows in, the cell depolarises, voltage-gated Ca++ channels open, and neurotransmitter is released at the synapse with the auditory nerve. There are only about 3,500 inner hair cells per ear, and they are not replaced when damaged.
Outer hair cells
Hair cells that are thought to be minimally involved in direct auditory signal detection. Instead, they actively modulate basilar membrane sensitivity to improve detection of low-volume sounds. They contain prestin, a motor protein that elongates and contracts with membrane potential changes, physically pushing against the basilar membrane to alter its stiffness.
Prestin
A motor protein in outer hair cells, named after the musical notation "presto." Elongates and contracts in response to membrane potential changes, allowing outer hair cells to actively tune basilar membrane sensitivity.
Auditory nerve (cranial nerve 8)
Carries auditory signals from the cochlea to the brainstem. Cell bodies are located in the spiral ganglion (one per ear) and are bipolar neurons: one myelinated dendrite receives the signal from a hair cell, one myelinated axon carries it into the brainstem.
Primary auditory cortex (A1)
The cortical region in the temporal lobe that receives auditory input (via the medial geniculate nucleus of the thalamus). The end point of the main auditory pathway.
Vestibular system
Three semicircular canals plus the utricle and saccule. Detects orientation relative to gravity and acceleration during movement. Enables balance and coordinated movement.
Semicircular canals
Three tiny, fluid-filled tubes in the inner ear, oriented orthogonally to one another, detecting rotational movement.
Utricle and saccule
Bulbous cavities in the inner ear containing receptor cells that detect movement of fluid in the attached semicircular canals.
Otolith
Tiny crystals of calcium carbonate suspended in fluid above hair cells in the utricle and saccule. As the body accelerates or changes orientation relative to gravity, the inertia of the otoliths bends the hairs of sensory cells, amplifying signals for balance.
Sound is a pressure variation that moves through air at ~1,100 feet per second
Wave velocity = frequency x wavelength
Higher frequency = shorter wavelength = higher perceived pitch
Greater amplitude = greater perceived loudness
Timbre reflects waveform complexity: a pure sine wave gives a pure tone, while complex waveforms produce richer sounds
Fourier analysis decomposes any complex waveform into simple sinusoidal frequency components
The oval window transmits middle-ear vibrations into the fluid-filled cochlea
The basilar membrane runs the length of the cochlea's spiral and varies in thickness
Thickest near the oval window (responds to higher frequencies)
Thinnest at the far end (responds to lower frequencies)
Different regions of the basilar membrane resonate to different sound frequencies
This creates a spatial frequency map along the membrane: a mechanical Fourier analysis
Inner hair cells (the primary hearing transducers):
Cilia are interconnected by molecular cables only billionths of a metre in diameter
Basilar membrane vibration bends the cilia
Bending tugs on cables that open mechanically-gated ion channels
K+ (more concentrated outside the cell in cochlear fluid) flows in, depolarising the cell
Depolarisation opens voltage-gated Ca++ channels
Ca++ influx triggers neurotransmitter release into the synaptic cleft
Neurotransmitter activates postsynaptic receptors on auditory nerve fibres (cranial nerve 8)
If stimulation is sufficient, an action potential travels to the brain
Outer hair cells (sensitivity modulators):
Contain prestin, which changes shape with membrane potential
Shape changes push against the basilar membrane, altering its stiffness and sensitivity
This active process enhances detection of quiet sounds
The frequency composition of your own voice as you hear it differs from what others hear, because bone conduction and internal resonance add low-frequency components
The chain from cochlea to cortex:
Hair cells release neurotransmitter onto cranial nerve 8 dendrites (spiral ganglion, bipolar neurons)
Auditory nerve axons synapse in the cochlear nucleus (brainstem medulla)
Cochlear nucleus projects to the superior olive and lateral lemniscus (pons)
Various brainstem auditory centres project to the inferior colliculus (midbrain)
Inferior colliculus projects to the medial geniculate nucleus (MGN) of the thalamus
MGN sends axons to primary auditory cortex (A1) in the temporal lobe
Three main causes:
Infection: inner ear infections can irreversibly damage hair cells
Genetic: mutations affecting cochlear function, e.g. a mutation in the gene for connexin 26 (a connexon ion channel protein) produces abnormal ion balance in the cochlea, preventing hair cell function and causing deafness
Noise-induced (acoustic trauma): brief exposure to very loud sounds (gunshots, explosions) or chronic exposure to moderately loud sounds can kill hair cells by excitotoxic overstimulation. Each ear has only ~3,500 inner hair cells, and damaged ones are not repaired or replaced.
Three orthogonal semicircular canals detect rotational movement
Utricle and saccule contain receptor cells that respond to fluid movement
Otoliths (tiny calcium carbonate crystals) sit above hair cells; their inertia during acceleration or orientation changes bends the hairs, generating balance signals
The system enables us to maintain balance and coordinate movement
⚠️ The basilar membrane as a mechanical Fourier analyser is a central concept. Know how thickness variation maps frequency to position along the cochlea.
⚠️ Hair cell transduction: be able to trace the sequence from cilia bending to neurotransmitter release. Mechanically-gated ion channels are the key mechanism.
⚠️ Inner hair cells vs. outer hair cells: inner = primary transducers, outer = sensitivity modulators (prestin). This distinction is frequently tested.
⚠️ Only ~3,500 inner hair cells per ear, and they do not regenerate. This is why noise-induced hearing loss is permanent.
⚠️ Know the auditory pathway: cochlea → auditory nerve (CN 8) → cochlear nucleus → superior olive/lateral lemniscus → inferior colliculus → MGN (thalamus) → A1 (temporal lobe).
⚠️ Connexin 26 mutation as a genetic cause of deafness: abnormal ion balance in the cochlea, hair cells cannot function.
⚠️ Otoliths: calcium carbonate crystals whose inertia bends hair cells during acceleration or gravity-orientation changes.
Q: How does the basilar membrane perform a Fourier analysis of incoming sound?
A: The basilar membrane varies in thickness along the length of the cochlea (thickest near the oval window, thinnest at the far end). Different regions resonate to different frequencies, so a complex sound is decomposed into its component frequencies spatially along the membrane.
Q: Describe the transduction process in an inner hair cell, from cilia bending to neural signal.
A: Basilar membrane vibration bends the cilia. Tiny molecular cables linking the cilia tug open mechanically-gated ion channels. K+ flows in and depolarises the cell. Depolarisation opens voltage-gated Ca++ channels, and Ca++ influx triggers neurotransmitter release into the synaptic cleft. The neurotransmitter activates receptors on auditory nerve fibres, and if stimulation is sufficient, an action potential is sent to the brain.
Q: What is the role of outer hair cells and the protein prestin?
A: Outer hair cells are primarily sensitivity modulators rather than signal transducers. They contain prestin, a motor protein that changes shape with membrane potential changes. This pushes against the basilar membrane, altering its stiffness and making it more sensitive to quiet sounds.
Q: Why is noise-induced hearing loss permanent?
A: The inner hair cells (the primary hearing transducers) number only about 3,500 per ear and are not repaired or replaced when damaged. Loud sounds can kill hair cells through excitotoxic overstimulation, and the loss is irreversible.
Q: Trace the auditory neural pathway from cochlea to cortex.
A: Hair cells synapse with auditory nerve fibres (cranial nerve 8, spiral ganglion). Axons synapse in the cochlear nucleus (medulla). Projections go to the superior olive and lateral lemniscus (pons), then to the inferior colliculus (midbrain), then to the medial geniculate nucleus (MGN, thalamus), and finally to the primary auditory cortex (A1) in the temporal lobe.
Q: What are otoliths and how do they contribute to balance?
A: Otoliths are tiny calcium carbonate crystals suspended in fluid above hair cells in the utricle and saccule. When the body accelerates or changes orientation relative to gravity, the inertia of the otoliths bends the underlying hair cells, generating signals that help maintain balance.
audition, hearing, sound, frequency, amplitude, pitch, loudness, timbre, Fourier analysis, Joseph Fourier, cochlea, oval window, basilar membrane, resonance, hair cells, inner hair cells, outer hair cells, cilia, mechanically-gated ion channels, prestin, auditory nerve, cranial nerve 8, spiral ganglion, cochlear nucleus, superior olive, lateral lemniscus, inferior colliculus, medial geniculate nucleus, MGN, primary auditory cortex, A1, hearing loss, connexin 26, acoustic trauma, noise-induced hearing loss, vestibular system, semicircular canals, utricle, saccule, otolith, calcium carbonate, balance, acceleration