Auditory and Vestibular System

Hair cells- motion detecting mechanoreceptors

A hair cell is a specialised mechanoreceptor that detects mechanical forces. Different groups of hair cells detect:

• Movement of surrounding water

• Self-movement in water

• Sound waves of different frequencies

• Lateral (side-to-side) movement of the head

• Rotational movement of the head

• The direction of gravity

Hair cells convert physical displacement of stereocilia into electrical signals (mechanotransduction). These signals then activate afferent nerves that carry information to the brain.

What is the structure of a generic hair cell?

• Cell body sitting on supporting cells

• Stereocilia bundle arranged in rows of increasing height

• Tip links connecting stereocilia

• Synapse with an afferent dendrite at the base

The apical surface faces the endolymph and contains mechanosensitive ion channels.

Hair cell and Stereocilia mechanism

• Endolymph is the fluid that surrounds the stereocilia of hair cells in the inner ear

• It is found in the cochlear duct, semicircular canals, utricle, and saccule

• It has an unusual ionic composition:
  

  • Very high K⁺

  • Very low Na⁺

• It is electrically positive relative to the inside of the hair cell

This environment is what makes hair-cell mechanotransduction possible.

Where the ion channels are

• Mechanically gated ion channels are located at the tips of stereocilia

• These channels are physically attached to tip links

• Tip links connect shorter stereocilia to taller ones

So movement of the bundle directly pulls on the channels.

What happens at rest

• Tip links are under slight tension

• Some ion channels are partially open

• K⁺ enters from the endolymph

• Hair cell sits at about −40 mV

• There is continuous low-level glutamate release

• The afferent neuron fires at a baseline rate

This baseline allows signals to increase or decrease.

Movement toward the tallest stereocilia (depolarisation)

• Stereocilia bend toward the tallest row

• Tip links are stretched

• More mechanosensitive channels open

• K⁺ flows from the endolymph into the stereocilia

• K⁺ then travels down the stereocilia bundle into the hair cell body

• Hair cell depolarises

• Voltage-gated Ca²⁺ channels open at the base

• More glutamate is released

• Afferent firing rate increases

Movement toward the shortest stereocilia (hyperpolarisation)

• Stereocilia bend away from the tallest row

• Tip links become slack

• Ion channels close

• Less K⁺ enters from endolymph

• Hair cell hyperpolarises

• Less Ca²⁺ entry

• Less glutamate released

• Afferent firing rate decreases

• Endolymph surrounds only the stereocilia (apical end) of the hair cell

• The cell body and synaptic base are bathed in perilymph (normal extracellular fluid, low K⁺)

So functionally:

• Apical side (stereocilia) → endolymph → K⁺ influx

• Basal side → perilymph → Ca²⁺ entry + neurotransmitter release

What are the three main parts of the ear and their roles?

• Outer ear: pinna + external auditory canal → captures sound.

• Middle ear: tympanic membrane + ossicles → amplifies sound.

• Inner ear: cochlea + vestibular system → sound detection and balance.

Inner ear- auditory

Cochlea

• Spiral-shaped part of the inner ear

• Does hearing

• Turns sound vibrations into nerve signals

Oval window

• Where sound enters the cochlea

• Vibrates when the stapes pushes on it

• Starts fluid movement inside the cochlea

Round window

• Pressure release

• Moves so the inner-ear fluid can vibrate properly

Perilymph

• Fluid in the outer chambers of the cochlea

• Low K⁺, high Na⁺

• Carries sound vibrations through the cochlea

Cochlear duct (scala media)

• Middle chamber of the cochlea

• Contains endolymph

• Holds the organ of Corti

Endolymph

• Fluid around the stereocilia

• High K⁺

• Allows K⁺ to enter hair cells when they move

Organ of Corti

• Where sound is detected

• Sits on the basilar membrane

• Contains hair cells

Hair cells

• Sensory cells for hearing

• Movement → electrical signal

• Release glutamate to the auditory

Stereo-cilia

• Hair-like projections on hair cells

• Bend with sound-induced movement

• Control K⁺ channel opening

Basilar membrane

• Vibrates with sound

• Different areas respond to different frequencies

Auditory (cochlear) nerve

• Carries signals to the brain

• Brain interprets them as sound

Flowchart

• Sound waves enter the ear canal

• Sound hits the tympanic membrane (eardrum) → it vibrates

• Vibrations pass through the ossicles(3 small bones) (malleus → incus → stapes)

• Stapes pushes on the oval window

• This creates fluid waves in the cochlea (perilymph)

• Fluid movement causes the basilar membrane to vibrate

• This bends stereocilia on hair cells in the organ of Corti(in cochlear duct)

• Bending opens mechanically gated K⁺ channels

• K⁺ enters from endolymph → hair cell depolarises

• Depolarisation opens Ca²⁺ channels at the base

• Glutamate is released onto the auditory nerve fibre

• Auditory (cochlear) nerve carries the signal to the brain

Inner ear: Vestibucular

• The vestibular system detects head movement and head position

• It tells the brain:

  • Are you rotating?

  • Are you moving in a straight line?

  • Which way is gravity (tilt)?

• It uses hair cells, just like hearing, but for movement, not sound

Fluids (same idea as hearing)

• Perilymph (blue)

  • Surrounds the membranous structures

  • Normal extracellular fluid

• Endolymph (purple)

  • Inside the vestibular ducts

  • High K⁺

  • Bathes the stereocilia

Hair-cell depolarisation still depends on K⁺ entering from endolymph.

Two vestibular subsystems

1) Semicircular canals → rotation

What they detect

• Rotational (angular) movement of the head

• Turning your head left/right, nodding, tilting sideways

Structure

• Three canals (horizontal, anterior, posterior)

• Each canal ends in an ampulla

Ampulla

• Enlarged region at the base of each canal

• Contains the crista ampullaris (sensory organ)

How it works

• Head rotates

• Endolymph lags behind due to inertia

• This bends the cupula in the ampulla

• Stereocilia bend

• Hair cells change firing rate

• Direction of rotation is encoded by increase vs decrease from baseline

Key idea

• Semicircular canals = rotation

2) Otolith system (utricle + saccule) → linear movement & gravity

What they detect

• Linear acceleration (forward/back, up/down)

• Head tilt relative to gravity

Structures

• Utricle

• Saccule

Together called the otolith organs.

Sensory region

• Macula

• Contains hair cells

Otoliths

• Tiny calcium carbonate crystals

• Sit on a gelatinous layer above stereocilia

How it works

• Head tilts or moves linearly

• Otoliths shift due to gravity or acceleration

• This bends stereocilia

• Hair cells depolarise or hyperpolarise depending on direction

Key idea

• Utricle & saccule = straight-line movement + gravity

What the yellow stars mean on the slides

• They mark where the sensory receptors (hair cells) are

• In the:

  • Ampullae of semicircular canals

  • Maculae of utricle and saccule

Compare hearing vs vestibular hair cells

• Same basic mechanism:

  • Stereocilia bend

  • Tip links open K⁺ channels

  • K⁺ enters from endolymph

  • Glutamate released

• Different stimulus:

  • Hearing → sound vibration

  • Vestibular → head movement

Hair cell: Receptor Systems

1. Cochlear system: sound frequency, loudness, timing.

2. Otolith system: linear movement and gravity.

3. Semi-circular canal system: angular acceleration.(Rotation)

How are sound waves transmitted from the outer ear to the inner ear?

1. Sound enters the pinna and travels through the external auditory canal.

2. It vibrates the tympanic membrane.

3. Vibrations are amplified by the ossicles (malleus, incus, stapes).

4. The stapes footplate pushes on the oval window, transmitting vibrations into the cochlea.

Ossicles provide impedance matching: without them, sound waves would reflect off the fluid-filled cochlea.

What causes conductive hearing loss?

Any blockage that prevents sound from reaching the oval window, e.g.:

• Ear canal obstruction

• Perforated tympanic membrane

• Otitis media

• Fixation or damage of ossicles

Cochlea: 3 Components and fluid types

• Scala vestibuli – perilymph

• Scala media (cochlear duct) – endolymph

• Scala tympani – perilymph

These compartments are separated by the vestibular membrane (top) and basilar membrane (bottom).

Sound enters at the base and travels toward the apex

Fluids (colour-coded in the diagrams)

• Perilymph (blue)

• Fills the outer chambers

• Normal extracellular fluid (high Na⁺, low K⁺)

• Carries pressure waves

• Endolymph (purple)

• Fills the cochlear duct

• High K⁺

• Surrounds hair-cell stereocilia

The three chambers of the cochlea (cross-section)

Each “slice” of the cochlea has three tubes:

Scala vestibuli

• Top chamber

• Filled with perilymph

• Receives vibrations from the oval window

Cochlear duct (scala media)

• Middle chamber

• Filled with endolymph

• Contains the organ of Corti

• Where hearing actually happens

Scala tympani

• Bottom chamber

• Filled with perilymph

• Ends at the round window (pressure release)

Organ of Corti (inside the cochlear duct)

• The sensory organ of hearing

• Contains hair cells

• Hair-cell stereocilia project into endolymph

• Base of hair cells releases glutamate onto the auditory nerve

Oval window and round window

• Oval window

• Where the stapes pushes

• Starts fluid movement in perilymph

• Round window

• Moves to allow fluid displacement

• Prevents pressure build-up

Base vs apex (important)

Base

• Near oval window

• Detects high-frequency sounds

Apex

• Tip of the spiral

• Detects low-frequency sounds

Modiolus and nerve

• Modiolus

• Central core of the cochlea

• Cochlear branch of CN VIII

• Carries signals from hair cells to the brain

Cochlear duct: Structures

Basilar membrane

• A flexible membrane forming the floor of the cochlear duct

• Vibrates in response to sound-induced fluid movement

• Different parts vibrate best at different frequencies:

  • Base → stiff → high frequency

  • Apex → floppy → low frequency

• Movement of this membrane is what drives hair-cell stimulation

Spiral organ (organ of Corti)

• The sensory organ of hearing

• Sits on top of the basilar membrane

• Located inside the cochlear duct

• Contains:

  • Inner hair cells

  • Outer hair cells

  • Supporting cells

• Its job is to convert basilar membrane movement into neural signals

Tectorial membrane

• A gelatinous membrane that lies above the hair cells

• Outer hair-cell stereocilia are embedded in or contact it

• When the basilar membrane moves:

  • The tectorial membrane moves slightly differently

  • This creates a shearing force

  • That force bends stereocilia

Key role: turns vibration into stereocilia bending

Hair cells

• Mechanoreceptors that detect movement

• Two types:
  

  • Inner hair cells

    • Main sensory receptors

    • Send most signals to the brain

• Outer hair cells

  • Act as amplifiers

  • Increase sensitivity and frequency tuning

• Stereocilia bending:

  • Opens mechanically gated K⁺ channels

  • K⁺ enters from endolymph

  • Hair cell depolarises

  • Ca²⁺ enters at the base

  • Glutamate is released onto auditory nerve fibres

Hair cells themselves do not fire action potentials

How they work together (simple flow)

• Sound → fluid movement

• Fluid movement → basilar membrane vibration

• Basilar membrane movement → shearing against tectorial membrane

• Shearing → stereocilia bend

• Bending → hair-cell depolarisation

• Depolarisation → nerve signal to brain

How does the auditory system encode sound frequency?

Through a place code:

• Base of cochlea: narrow, stiff → responds to high frequencies (20 kHz).

• Apex: wide, flexible → responds to low frequencies (20 Hz).

The brain determines pitch by which neurons fire.

How does the auditory system encode loudness and timing?

The louder the sound:

• Larger basilar membrane vibration

• Larger receptor potentials

• More transmitter released

• Higher action potential firing rate

Timing:

Preserved by fast axons and powerful synapses

Primary auditory pathway

• This shows how sound information travels from the ear to the brain

• It is the primary auditory pathway used for discriminative hearing (pitch, loudness, sound identity)

Step 1: Cochlear nerve (CN VIII)

• Hair cells in the organ of Corti release glutamate

• Signals travel along the cochlear nerve

• This nerve is part of the vestibulocochlear nerve (cranial nerve VIII)

Step 2: Cochlear nuclei (brainstem)

• First synapse in the auditory pathway

• Located in the medulla

• From this point, auditory information is sent to both sides of the brain

• This bilateral projection explains why damage on one side rarely causes total deafness

Step 3: Superior olivary nuclei (brainstem)

• First site of binaural comparison

• Important for sound localisation

• Compares:
  

  • Timing differences (low frequencies)

  • Loudness differences (high frequencies)

Step 4: Inferior colliculus (midbrain)

• Major integration centre for sound

• Combines information about:
  

  • Frequency

  • Intensity

  • Timing

• Involved in sound localisation and auditory reflexes

Step 5: Medial geniculate nucleus (thalamus)

• The auditory relay nucleus of the thalamus

• Filters and organises auditory input

• Sends processed signals to the auditory cortex

Step 6: Primary auditory cortex (A1)

• Located in the temporal lobe

• Tonotopically organised:
  

  • Low frequencies map to one region

  • High frequencies map to another

• This is where sound becomes consciously perceived

Higher auditory cortical areas

• Surround the primary auditory cortex

• Process complex sounds, especially speech

• Include Wernicke’s area:
  

  • Responsible for language comprehension

  • Damage → fluent but meaningless speech (poor understanding)

Key exam points to remember

• Pathway is mostly bilateral after the cochlear nuclei

• Thalamic relay = medial geniculate nucleus

• Primary auditory cortex is tonotopic

• Wernicke’s area = understanding speech, not producing it

Auditory System: Origin of sound

• The brain compares what each ear hears

• This first happens in the superior olivary complex (brainstem)

• Two different nuclei do two different comparisons:

  • Timing differences

  • Loudness differences

Medial Superior Olive (MSO) — timing

• Compares when sound arrives at the left vs right ear

• Uses interaural time difference (ITD)

• Best for low-frequency sounds

Why only low frequency:

• Low-frequency waves are long

• The phase of the wave is clear and comparable between ears

• The brain can tell which ear was stimulated first

What it tells you:

• Sound arriving earlier at the left ear → sound is on the left

• Sound arriving earlier at the right ear → sound is on the right

Key phrase to remember:

• MSO = timing = low frequency

Lateral Superior Olive (LSO) — loudness

• Compares how loud the sound is in each ear

• Uses interaural level difference (ILD)

• Best for high-frequency sounds

Why only high frequency:

• High-frequency sounds are short wavelength

• The head blocks them → head shadow effect

• One ear gets a quieter signal than the other

What it tells you:

• Louder in left ear → sound is on the left

• Louder in right ear → sound is on the right

Key phrase to remember:

• LSO = loudness = high frequency

What is the primary auditory pathway?

1. Cochlear nerve

2. Cochlear nuclei

3. Superior olivary nuclei

4. Inferior colliculus

5. Medial geniculate nucleus (thalamus)

6. Primary auditory cortex (A1)

A1 is arranged tonotopically, preserving the frequency map.

Outer hair cells

OHCs actively amplify vibrations:

• They contract/elongate in response to sound (electromotility).

• This boosts basilar membrane motion.

• It increases sensitivity and frequency resolution.

Why is the auditory system vulnerable to damage?

• Only ~3,500 inner hair cells per ear.

• Loud sounds cause excessive vibration → mechanical destruction.

• Excess glutamate at synapses can destroy afferents.

• Genetic conditions can eliminate hair cells.

How do cochlear implants help?

They bypass lost hair cells by directly stimulating the auditory nerve with electrical signals, preserving tonotopic coding along the cochlea.

What are the two major vestibular receptor systems and what do they detect?

1. Otolith system (utricle & saccule): detects linear acceleration and gravity.

2. Semi-circular canals: detect angular acceleration (rotational movement).

Otolith Macula: Structure

What the macula is

• The macula is the sensory epithelium of the otolith organs (utricle and saccule)

• Its job is to detect linear acceleration and head tilt (gravity)

Hair cells

• Sensory mechanoreceptors

• Each hair cell has a bundle of stereocilia (and one kinocilium)

• The direction the bundle bends determines whether the cell depolarises or hyperpolarises

• The apical ends of hair cells face endolymph

• The basal ends synapse with afferent vestibular nerve fibres

Otolithic (gelatinous) membrane

• A gelatinous layer sitting on top of the hair-cell stereocilia

• Stereocilia are embedded in this membrane

• It moves relative to the hair cells during head movement or tilt

Otoconia (otoliths)

• Tiny calcium carbonate crystals

• Sit on top of the otolithic membrane

• Add mass and inertia

• This extra weight makes the membrane shift when:
  

  • You accelerate

  • You tilt your head relative to gravity

This shift is what bends the stereocilia.

Fluids around the macula

• Endolymph

  • Surrounds the stereocilia

  • High K⁺

  • Enables depolarisation when channels open

• Perilymph

  • Surrounds the macula outside the membranous labyrinth

  • Normal extracellular fluid

Afferent nerve fibres

• Carry signals from hair cells to the brain

• Firing rate changes depending on stereocilia deflection

How the structure works together

• Head tilt or linear movement → otoconia shift

• Otolithic membrane moves

• Stereocilia bend

• K⁺ channels open or close

• Hair-cell transmitter release changes

• Vestibular nerve firing changes

How do otolith organs detect linear acceleration?

• This is the otolith system (utricle + saccule)

• It shows how the ear detects linear acceleration (moving in a straight line) and head tilt

Key structures involved

• Hair cells with stereocilia

• Otolithic membrane (gel-like layer)

• Otoconia (otoliths) on top of the membrane

• Endolymph around the stereocilia

• Afferent vestibular nerve fibres at the base

What happens when the head starts moving

• The head moves (arrow in the diagram)

• The otolithic membrane + otoconia lag behind

• This lag happens because otoconia have mass and inertia

• As a result, the otolithic membrane shifts relative to the hair cells

How hair cells are activated

• Movement of the otolithic membrane bends the stereocilia

• If stereocilia bend towards the tallest cilium:
  

  • Mechanically gated K⁺ channels open

  • K⁺ enters from endolymph

  • Hair cell depolarises

  • More neurotransmitter released

  • Afferent firing increases

• If stereocilia bend away from the tallest cilium:

  • Channels close

  • Hair cell hyperpolarises

  • Less neurotransmitter released

  • Afferent firing decreases

What the + signs and spikes mean

• + signs = depolarisation of the hair cell

• More spikes = increased firing in the vestibular nerve

• Fewer spikes would indicate inhibition

Why “lag” is essential

• If the otolithic membrane moved exactly with the head, nothing would bend

• The lag is what converts motion into a signal

• This allows detection of:
  

  • Starting to move

  • Stopping

  • Speed changes

  • Direction of movement

Otolith Organs: Hair Cells form an orderly pattern

• Otolith organs detect head tilt and gravity by using many hair cells arranged in different directions

• The brain works out direction from which hair cells are activated and which are inhibited

Orderly pattern of hair cells

• Hair cells in the macula are not all aligned the same way

• Each hair cell has a preferred direction (the direction that depolarises it most)

• Different groups of hair cells are oriented at different angles

This creates a direction map.

What gravity does

• Gravity pulls the otoconia in a constant downward direction

• This shifts the otolithic membrane

• The shift bends stereocilia, but:
  

  • Some hair cells bend towards their preferred direction → depolarise

  • Others bend away → hyperpolarise

What happens when you tilt your head

• Tilting changes the direction of otoconia movement

• A different set of hair cells is now excited

• Another set is inhibited

So head tilt is encoded by a pattern of activity, not a single cell.

Role of utricle and saccule

• Utricle: mainly covers horizontal directions

• Saccule: mainly covers vertical directions

• Together, they cover all possible head tilt directions

How the brain reads this

• The brain compares:
  

  • Which afferent fibres increase firing

  • Which decrease firing

• From this population pattern, it determines:
  

  • Direction of gravity

  • Direction of linear movement

Main outputs of the Vestibular System

Otolith afferents project via the vestibulospinal tract.

Targets:

Anti-gravity muscles of legs and trunk

Functions:

• Maintaining upright posture

• Preventing falls

• Compensation for linear disturbances

Semi Circular Canal System: Structure

Contains:

• Ampullary crest with hair cells

• Gelatinous cupula

• Endolymph inside the canal

• Afferent nerve fibres

Semi Circular Canal: eg, left horizontal canal

What semicircular canals do

• They detect head rotation (angular acceleration)

• They tell you when you start, change, or stop turning your head

How it works

• When the head starts to turn:
  

  • The canal moves with the head

  • The fluid (endolymph) lags behind

  • This bends the cupula

  • Hair cells bend → signal sent to the brain

• When the head keeps turning:
  

  • Fluid catches up

  • Bending stops

  • Signal reduces

• When the head stops:
  

  • Fluid keeps moving briefly

  • Cupula bends the opposite way

  • Brain senses deceleration

Why do we have three semi-circular canals?

They are arranged at right angles so that together they detect rotation in all three axes.

Each canal has a partner on the opposite side of the head.

• Semicircular canals work in mirror-image pairs

• When one canal is excited, its partner is inhibited

• This push–pull system makes head rotation signals precise and fast

How the canals are paired

• Each canal on one side of the head is paired with a canal on the opposite side that lies in the same plane

• Examples:
  

  • Left horizontal ↔ Right horizontal

  • Left anterior ↔ Right posterior

  • Right anterior ↔ Left posterior

These pairs detect rotation in opposite directions.

What happens when you turn your head

• Head turns to one side

• Endolymph movement:
  

  • Excites one canal → firing rate increases

  • Inhibits the partner canal → firing rate decreases

• The brain compares increase vs decrease, not absolute firing

This contrast tells the brain:

• Direction of rotation

• Speed of rotation

Neck and Shoulder muscles

Ampullary afferents → vestibulospinal tract → neck and shoulder muscles.

Role:

• Stabilising head position during movement

• Counteracting unwanted rotational disturbances

What is the vestibulo-ocular reflex (VOR)?

What the vestibulo-ocular reflex (VOR) is

• A brainstem reflex that keeps vision stable during head movement

• It moves the eyes in the opposite direction to the head

• It works without needing the cortex, so it is extremely fast

Example:

• Head turns left → eyes move right → image stays on the fovea

Where the signal starts

• Semicircular canals detect head rotation

• Hair cells change firing rate

• Signal travels to the vestibular nuclei in the brainstem

Core pathway (horizontal canal example)

• Horizontal semicircular canal activates

• → Vestibular nuclei

• → Abducens nucleus (CN VI)

  • Activates lateral rectus of one eye

• → via medial longitudinal fasciculus (MLF)

• → Oculomotor nucleus (CN III)

  • Activates medial rectus of the opposite eye

Result:

• Both eyes move together in the opposite direction to head movement

Why the MLF is important

• The medial longitudinal fasciculus links eye movement nuclei

• It ensures both eyes move together

• It must conduct signals very fast

Why the VOR must be extremely fast

• Head movements are rapid

• Visual feedback would be too slow

• Delay would cause:
  

  • Blurred vision

  • Loss of fixation

So the pathway is:

• Short

• Heavily myelinated

• Brainstem-based

What are conjugate eye movements and their cortical control?

Smooth pursuit movements that keep a moving object in focus.

Controlled primarily by the visual cortex projecting to brainstem eye movement centres.