Source: MCB C61, UC Berkeley
Tags: sound, frequency, amplitude, timbre, Fourier analysis, ossicles, cochlea, basilar membrane, hair cells, auditory pathway, sound localisation, hearing loss, cochlear implant, vestibular system, semicircular canals, otoliths, vertigo, somatosensory system, skin receptors, Wilder Penfield, somatosensory cortex, neuroplasticity, phantom limb, whisker barrels, motor cortex, M1, supplementary motor areas, apraxia
Sound is a pressure wave characterised by frequency (pitch), amplitude (loudness), and complexity (timbre). The ear performs a physical Fourier analysis on the basilar membrane, with hair cells converting mechanical vibration into neural signals. The auditory pathway runs from the cochlea through multiple brainstem nuclei to the auditory cortex. The vestibular system uses the same inner ear structures to detect orientation and acceleration. Touch and body sensation are mapped onto the somatosensory cortex (with disproportionately large regions for fingers and lips), and the motor cortex in the frontal lobe controls voluntary movement. Neuroplasticity is demonstrated by phantom limb phenomena and cortical reorganisation after sensory loss.
Sound
A mechanical pressure wave travelling through a medium. Characterised by alternating regions of compression (higher density, above normal pressure) and rarefaction (lower density, below normal pressure).
Frequency / pitch
The number of cycles of vibration per second (measured in Hertz). Higher frequency = higher pitch = shorter wavelength. Human hearing range: 20–20,000 Hz.
Amplitude / loudness
The magnitude (height) of the sound wave. Taller waves = louder sound.
Complexity / timbre
Most sounds are mixtures of many different frequencies. Timbre is what distinguishes the same note played on different instruments.
Fourier analysis (frequency decomposition)
The mathematical decomposition of a complex waveform into a sum of simpler sinusoidal (sine wave) components, each with a specific frequency and amplitude. Named after Joseph Fourier, French mathematician. The basilar membrane performs a physical version of this.
Ossicles
Three small interconnected bones in the middle ear: hammer (malleus), anvil (incus), and stirrup (stapes). Eardrum vibration is transmitted through the ossicles to the oval window of the inner ear.
Labyrinth
The outer wall of the inner ear, containing the cochlea and semicircular canals, all filled with fluid.
Cochlea
A coiled (snail-shaped) structure in the inner ear containing the basilar membrane and hair cells. Where mechanical sound energy is converted to neural signals.
Basilar membrane
A thin tissue in the cochlea that vibrates as surrounding fluid vibrates. Varies in thickness along its length: the thicker end resonates with lower frequencies, the thinner end with higher frequencies. This spatial frequency mapping is a physical Fourier analysis.
Hair cells
Mechanoreceptor cells along the basilar membrane. When the membrane vibrates, cilia (a bundle of hairs) on one end of each cell bend, opening potassium channels. K+ influx depolarises the cell → calcium channels open → Ca2+ triggers NT release at chemical synapses with auditory nerve fibres at the opposite end. Only ~3,500 inner hair cells exist, and they cannot be replaced or repaired.
Auditory neural pathway
Hair cells → cranial nerve 8 (via spiral ganglion) → cochlear nucleus (medulla) → superior olive and lateral lemniscus (pons) → inferior colliculus (midbrain) → medial geniculate nucleus/MGN (thalamus) → auditory cortex/A1 (temporal lobe).
Sound localisation
Bilateral connectivity between brainstem auditory nuclei compares arrival times and qualities of sound between the two ears to determine the spatial location of sound sources.
Hearing loss (genetic)
Genetic anomalies causing cochlear malfunction. Example: a mutation in the cochlear ion channel connexin 26 produces abnormal ion balances, resulting in dysfunctional hair cells and deafness.
Hearing loss (noise-induced)
Exposure to sounds above 100 dB for long periods can kill hair cells. Since there are only ~3,500 inner hair cells and they cannot be replaced, cumulative damage leads to permanent hearing loss.
Hearing aid (amplifier)
An acoustic amplifier that increases the volume of sound available to the ear. Evolved from ear trumpets to electrical amplifiers and microphones.
Cochlear implant
A surgically inserted array of electrodes in the inner ear that electrically stimulate the auditory nerve at spatial locations corresponding to specific frequency regions. A microphone picks up sound → frequency analyser performs Fourier analysis → radio transmission to receiver → electrical stimulation at appropriate locations.
Vestibular system
Sensory structures in the inner ear for detecting orientation relative to gravity and acceleration during movement. Comprises three orthogonal semicircular canals and two bulbous cavities (utricle and saccule).
Semicircular canals
Three fluid-filled canals oriented at right angles to each other. Changes in movement and orientation cause the fluid to shift, bending hair cells (receptor cells) that send signals via cranial nerve 8. The same nerve carries both vestibular and auditory information.
Otoliths
Tiny calcium carbonate stones suspended above vestibular hair cells. When fluid moves, otoliths shift and bend the hair cells, transducing gravitational and acceleration information.
Vertigo
A condition of dizziness or sensation of motion while standing still, caused by a defect or damage to the vestibular system.
Somatosensory system
The sensory system for touch, pressure, and temperature across the body surface.
Skin sensory receptors
Somatosensory neurons in the skin that terminate either as free nerve endings at the top layer of skin or at specialised structures (Merkel's disc, Pacinian corpuscle) for pressure and touch. Touch and pressure receptors use mechanically gated ion channels. Temperature receptors are TRP receptors. Cell bodies are located in dorsal root ganglia (DRG) near the spinal cord.
Wilder Penfield
Canadian neurosurgeon who first described the somatosensory (and motor) map of the body by electrically stimulating cortical regions in patients undergoing brain surgery.
Primary somatosensory cortex
Located in the anterior parietal lobe. Contains a map of the body surface. The map is not to scale: fingers and lips have disproportionately large cortical representations because they have the highest density of sensory dendrites and the smallest receptive fields, producing the highest touch acuity.
Somatosensory pathway
Sensory receptor activated → action potential propagates toward DRG → bypasses cell body → enters CNS → DRG → spinal cord and medulla → thalamus → anterior parietal lobe (primary somatosensory cortex).
Mouse whisker barrels
Mice depend heavily on their whiskers for environmental information. Each whisker has a dedicated cortical representation ("barrel") in the somatosensory cortex, analogous to the large finger/lip areas in humans.
Whisker amputation and neuroplasticity
When a whisker is removed, the cortical cells that previously received its input develop connections with neurons for adjacent whiskers. The adjacent whiskers become more sensitive through new axon and dendrite growth and strengthening of existing but underused connections. A clear example of neuroplasticity.
Phantom limb
Continued sensation of an amputated limb. After amputation, cortical neurons that previously served the missing limb develop connections with neurons for adjacent body regions (face, shoulder). Vague stimulation of those areas can produce sensations referred to the missing limb.
Mirror box therapy
A rehabilitation technique for phantom limb pain. A mirror placed so the intact limb's reflection appears in the position of the missing limb. Watching the reflected limb flex and relax can, over about 20 days, relieve phantom pain through neuroplastic reorganisation.
Primary motor cortex (M1)
Located anterior to the central sulcus in the frontal lobe. Contains a body map of neurons that send signals to initiate contraction of skeletal muscles. Signals travel from M1 → spinal cord → neuromuscular junction, where acetylcholine triggers muscle fibre contraction. Discovered by Penfield.
M1 lesions
Produce inability to move muscles corresponding to the affected part of the body map, resulting in partial paralysis. Example: damage to the right posterior frontal lobe → left-sided paralysis (contralateral).
Supplementary motor areas / premotor areas
Located anterior to M1 in the frontal lobes. Active before M1 during movement planning and sequencing. Lesions here do not cause paralysis but instead produce disorganised movements.
Apraxia
A disorder in the organisation of movement, caused by lesions in supplementary motor or premotor areas. Movement is possible but disorganised.
Cortical lesions and somatosensory effects
Lesions in primary somatosensory cortex produce loss of sensation in the corresponding body region. Lesions in secondary somatosensory cortex produce somatosensory agnosias (touch and sensations feel strange or confusing).
Sound waves are alternating zones of compression and rarefaction in a medium. Three key properties:
Frequency (pitch): measured in Hz. Higher frequency = higher pitch = shorter wavelength
Amplitude (loudness): the height of the wave. Greater amplitude = louder
Complexity (timbre): most real-world sounds are mixtures of frequencies. Fourier analysis decomposes any complex waveform into component sine waves with specific frequencies and amplitudes
Outer and middle ear:
Sound waves vibrate the eardrum
Eardrum motion is transmitted through the ossicles (hammer → anvil → stirrup) to the oval window of the cochlea
Inner ear (cochlea):
The cochlea is a fluid-filled, coiled structure containing the basilar membrane
The basilar membrane varies in thickness: thicker portions resonate with low frequencies, thinner portions with high frequencies
This creates a spatial representation of frequency along the membrane, a physical Fourier analysis
Hair cells sit along the basilar membrane and convert mechanical vibration into neural signals
Basilar membrane vibrates → hair cell cilia bend → potassium channels open (mechanically gated)
K+ influx depolarises the cell → voltage-gated calcium channels open → Ca2+ influx → NT release
NT activates postsynaptic receptors on auditory nerve fibres (cranial nerve 8)
There are only ~3,500 inner hair cells; they cannot be replaced or repaired
The full sequence:
Hair cells → spiral ganglion (cell bodies of CN 8 bipolar neurons) → cochlear nucleus (medulla) → superior olive and lateral lemniscus (pons) → inferior colliculus (midbrain) → medial geniculate nucleus (thalamus) → A1 auditory cortex (temporal lobe)
Sound localisation:
Bilateral brainstem connectivity compares arrival time and sound quality between the two ears
Genetic hearing loss:
Cochlear malfunction from genetic anomalies
Connexin 26 mutation: abnormal ion balances → dysfunctional hair cells → deafness
Noise-induced hearing loss:
Sounds above 100 dB over long periods kill irreplaceable hair cells
Amplifiers increase volume available to the ear. Cochlear implants bypass damaged hair cells by electrically stimulating the auditory nerve at frequency-specific locations, using an external microphone and Fourier analyser.
Three semicircular canals (oriented at right angles) and two cavities (utricle, saccule) in the inner ear
Fluid in these structures shifts with changes in movement and orientation
Otoliths (tiny stones) suspended above vestibular hair cells bend the cells as fluid moves
Hair cells send signals via cranial nerve 8 (which carries both auditory and vestibular information)
Vestibular damage or defects cause vertigo
Receptor types in the skin:
Free nerve endings (top skin layer)
Merkel's disc and Pacinian corpuscle (detect pressure and touch, mechanically gated ion channels)
TRP receptors (temperature)
Pathway:
Receptor activation → action potential → dorsal root ganglia (near spinal cord, bypassing cell body) → spinal cord/medulla → thalamus → anterior parietal lobe (primary somatosensory cortex)
The somatosensory body map (Penfield):
Not to scale: fingers and lips have the largest cortical area because they have the highest sensory neuron density and smallest receptive fields
Lesions in primary somatosensory cortex → loss of sensation in corresponding body region
Lesions in secondary somatosensory cortex → somatosensory agnosias
Primary motor cortex (M1):
Anterior to the central sulcus in the frontal lobe
Contains a body map for initiating skeletal muscle contraction
Pathway: M1 → spinal cord → neuromuscular junction (acetylcholine release) → muscle contraction
M1 lesion → partial paralysis (contralateral)
Supplementary motor and premotor areas:
Anterior to M1 in the frontal lobe
Active during planning and sequencing of movements, before M1 fires
Lesion → disorganised movements (apraxia), not paralysis
Mouse whisker barrels:
Each whisker is represented by a dedicated "barrel" in the mouse somatosensory cortex
Removing a whisker causes its cortical neurons to develop connections with adjacent whisker neurons
Adjacent whiskers become more sensitive
Phantom limbs:
After amputation, cortical neurons previously serving the limb rewire to serve adjacent body regions
Stimulation of face or shoulder can produce referred sensations in the missing limb
Mirror box therapy:
The reflected image of the intact limb substitutes for the missing one
20 days of mirror-assisted flexing and relaxing can relieve chronic phantom limb pain
A powerful demonstration of how visual feedback can drive cortical reorganisation
⚠️ The basilar membrane performs a physical Fourier analysis. Thick end = low frequency, thin end = high frequency. Know this spatial-to-frequency mapping.
⚠️ Hair cell transduction sequence: cilia bend → K+ channels open → depolarisation → Ca2+ channels open → NT release. Only ~3,500 inner hair cells exist, and they are irreplaceable.
⚠️ Know the full auditory pathway: hair cells → CN 8/spiral ganglion → cochlear nucleus (medulla) → superior olive/lateral lemniscus (pons) → inferior colliculus (midbrain) → MGN (thalamus) → A1 (temporal lobe).
⚠️ CN 8 carries both auditory and vestibular information.
⚠️ The somatosensory map is not to scale. Fingers and lips are disproportionately represented because of high sensory neuron density and small receptive fields.
⚠️ M1 lesion = paralysis (contralateral). Supplementary motor area lesion = apraxia (disorganised movement, not paralysis). Do not confuse these.
⚠️ Phantom limb phenomena and whisker barrel reorganisation are exam-favourite examples of neuroplasticity.
Q: How does the basilar membrane perform frequency analysis?
A: The basilar membrane varies in thickness along its length. Different regions resonate at different frequencies: the thicker end vibrates with low frequencies and the thinner end with high frequencies. This creates a spatial map of the component frequencies of a sound, essentially a physical Fourier analysis.
Q: Describe the transduction mechanism of cochlear hair cells.
A: When the basilar membrane vibrates, hair cell cilia bend. This mechanically opens potassium channels. K+ flows in, depolarising the cell. Depolarisation opens voltage-gated calcium channels. Ca2+ influx triggers release of neurotransmitter at synapses with auditory nerve fibres (cranial nerve 8).
Q: Why is noise-induced hearing loss permanent?
A: There are only about 3,500 inner hair cells in the cochlea, and they cannot be replaced or repaired. Sounds above 100 dB for extended periods can kill these cells, resulting in irreversible hearing loss.
Q: What structures make up the vestibular system, and what do they detect?
A: Three orthogonal semicircular canals, the utricle, and the saccule. They detect orientation relative to gravity and acceleration during movement, using fluid shifts that bend vestibular hair cells.
Q: Why are fingers and lips disproportionately large on the somatosensory cortex map?
A: These body parts have the highest density of sensory neuron dendrites and the smallest receptive fields, producing the highest acuity for touch. More cortical space is devoted to processing their signals.
Q: What is the difference between a lesion in M1 and a lesion in the supplementary motor area?
A: An M1 lesion produces paralysis (inability to move) in the corresponding contralateral body region. A supplementary motor area lesion produces apraxia (disorganised movement) without paralysis.
Q: How does mirror box therapy relieve phantom limb pain?
A: A mirror creates a visual illusion of the missing limb by reflecting the intact limb. Watching the "missing" limb flex and relax provides visual feedback that drives cortical reorganisation, and after about 20 days of practice, can relieve chronic phantom limb pain.
Q: What is the full auditory pathway from hair cell to cortex?
A: Hair cells → spiral ganglion (CN 8 cell bodies) → cochlear nucleus (medulla) → superior olive and lateral lemniscus (pons) → inferior colliculus (midbrain) → medial geniculate nucleus (thalamus) → A1 auditory cortex (temporal lobe).
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