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Auditory system is responsible for:
Detection of sound
Transmission of sound
Conversion of sound energy into electrical impulses
Interpretation of sound in cerebral cortex
Components:
External ear
Middle ear
Inner ear
Auditory nerve
Central auditory pathways
Vestibular system maintains:
Balance
Equilibrium
Spatial orientation
Coordination of eye movements with head movement
Components:
Semicircular canals
Utricle
Saccule
Vestibular nerve
Vestibular nuclei
Cerebellar connections
Communication
Speech understanding
Warning system for environment
Social interaction
Learning and education
Maintenance of posture
Stable vision during movement
Coordination of body movements
Orientation in space
Basis for:
Hearing tests
Vestibular tests
Interpretation of deafness
Diagnosis of vertigo
Audiology
Neuro-otology
Sound is a form of mechanical energy produced by vibration of matter.
It travels in the form of waves through a medium.
Requires medium for transmission.
Cannot travel in vacuum.
Mediums:
Air
Liquid
Solid
Sound waves are longitudinal waves.
Particles vibrate parallel to direction of wave propagation.
Compression → area of high pressure.
Rarefaction → area of low pressure.
Alternate compression and rarefaction form sound wave.
Number of cycles per second.
Unit = Hertz (Hz)
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Determines pitch.
Human hearing range:
20 Hz to 20,000 Hz
| Type | Frequency |
|---|---|
| Infrasonic | <20 Hz |
| Audible | 20–20,000 Hz |
| Ultrasonic | >20,000 Hz |
Psychological interpretation of frequency.
Higher frequency → higher pitch.
Energy flow per unit area.
I\propto A^2
Depends on amplitude.
Subjective perception of intensity.
Measured in phons or sones.
Maximum displacement from resting position.
Determines loudness.
Speed of sound transmission.
Approximate velocity in air:
340 m/s
Velocity is greater in:
Solids > liquids > gases
\lambda=\frac{v}{f}
Where:
λ = wavelength
v = velocity
f = frequency
Relative timing of sound waves.
Important in:
Sound localization
Interference
Quality of sound that differentiates sounds of same pitch and intensity.
Depends on:
Harmonics
Overtones
Logarithmic scale for sound intensity.
dB=10\log_{10}\left(\frac{I}{I_0}\right)
Where:
I = measured intensity
I₀ = reference intensity
| Sound | dB |
|---|---|
| Whisper | 20 dB |
| Normal speech | 60 dB |
| Traffic | 90 dB |
| Jet engine | 120–140 dB |
Measured in decibels.
Related to pressure variation.
Minimum sound intensity heard by normal ear.
Approximately:
0 dB at 1000 Hz
Around 120–140 dB
Standard reference level used in audiometry.
Unit of loudness level.
Unit of subjective loudness.
Single frequency sound.
Example:
Tuning fork
Multiple frequencies.
Example:
Speech
Random non-periodic sound.
Frequencies that are integral multiples of fundamental frequency.
Increase in amplitude when frequency matches natural frequency.
External auditory canal resonance:
Around 3000 Hz
Frequency bands processed together by cochlea.
Funnels sound into external auditory canal.
Helps determine direction of sound.
Especially:
Vertical localization
Modifies sound entering ear depending on direction.
Amplifies sound by resonance.
Maximum amplification:
10–15 dB near 3000 Hz
Clinical importance:
Speech frequencies enhanced.
Due to:
Canal length
Canal shape
Cerumen
Hair follicles
S-shaped canal
Acidic pH
Functions:
Prevent infection
Trap foreign particles
Tympanic membrane vibrates in response to sound waves.
Entire membrane vibrates at low frequency.
Segmental vibration at high frequency.
Transfers sound energy to ossicles.
Different parts vibrate at slightly different times.
Normal pathway of hearing.
Sequence:
Pinna → EAC → TM → ossicles → oval window → cochlea
Skull vibrations directly stimulate cochlea.
Clinical importance:
Basis of tuning fork tests.
Converts low-pressure air vibrations into high-pressure fluid vibrations.
Without this:
99.9% sound energy reflected at air-fluid interface.
Tympanic membrane area larger than oval window.
Area ratio:
About 17:1
Pressure amplification occurs.
Malleus handle longer than incus long process.
Lever ratio:
1.3:1
Prevents simultaneous movement of oval and round windows.
Receives TM vibrations.
Transmits vibration.
Footplate transmits energy into cochlea.
Supplied by facial nerve.
Function:
Dampens excessive stapes movement.
Supplied by mandibular nerve.
Function:
Tenses tympanic membrane.
Bilateral reflex contraction to loud sound.
Protects cochlea from:
Loud low-frequency sounds
Reflex arc:
Cochlea → VIII nerve → brainstem → VII nerve → stapedius
Reduces sound transmission by:
30–40 dB
Suppresses self-generated sounds:
Chewing
Talking
Maintains aeration of middle ear.
Equalizes middle ear pressure with atmosphere.
Drains secretions into nasopharynx.
Prevents nasopharyngeal reflux.
Skull compression causes cochlear fluid displacement.
Important at:
High frequencies
Ossicles lag behind skull vibration.
Produces relative movement.
Important at:
Low frequencies
Vibration of ear canal air stimulates TM.
Found in scala media.
| Ion | Concentration |
|---|---|
| Potassium | High |
| Sodium | Low |
Produced by:
Stria vascularis
Electrical potential:
+80 mV
Found in scala vestibuli and scala tympani.
| Ion | Concentration |
|---|---|
| Sodium | High |
| Potassium | Low |
Similar to extracellular fluid.
Essential for hair cell transduction.
High potassium endolymph enters hair cells during stimulation.
Highly vascular epithelium of lateral cochlear wall.
Produces endolymph.
Maintains ionic balance.
Generates endocochlear potential.
Approximately +80 mV.
Essential for:
Cochlear sensitivity
Sound creates traveling wave on basilar membrane.
High-frequency sounds:
Base of cochlea
Low-frequency sounds:
Apex of cochlea
y=A\sin(2\pi ft)
Georg von Bekesy described traveling wave mechanism.
Different frequencies stimulate different places on basilar membrane.
| Frequency | Cochlear Region |
|---|---|
| High | Base |
| Low | Apex |
Narrow and stiff at base.
Wide and flexible at apex.
This explains tonotopic organization.
Organ of Corti is the sensory organ of hearing.
Located on:
Basilar membrane of scala media (cochlear duct)
Converts:
Mechanical sound vibrations → Electrical nerve impulses
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Scala Vestibuli
│
Reissner Membrane
│
Scala Media
└── Organ of Corti
│
Basilar Membrane
│
Scala Tympani
Main components:
Hair cells
Supporting cells
Pillar cells
Tunnel of Corti
Tectorial membrane
Reticular lamina
Nerve endings
Triangular tunnel formed between:
Inner pillar cell
Outer pillar cell
Cortilymph fluid
Nerve fibers
Structural support
Facilitates sound transmission
| Type | Position |
|---|---|
| Inner pillar cell | Medial |
| Outer pillar cell | Lateral |
Rigid supporting cells.
Rich in microtubules.
Form tunnel of Corti.
Provide mechanical stability.
| Cell Type | Function |
|---|---|
| Deiters cells | Support outer hair cells |
| Hensen cells | Structural support |
| Claudius cells | Lateral support |
| Boettcher cells | Supporting role |
| Inner phalangeal cells | Support inner hair cells |
Specialized sensory receptor cells.
| Feature | Inner Hair Cells | Outer Hair Cells |
|---|---|---|
| Rows | Single row | Three rows |
| Number | ~3500 | ~12000 |
| Function | Sensory transduction | Cochlear amplification |
| Shape | Flask-shaped | Cylindrical |
Primary auditory receptors.
Responsible for:
Sound perception
Mainly afferent fibers.
Electromotile cells.
Modify basilar membrane movement.
Cochlear amplifier
Frequency tuning
Hair-like projections on apical surface of hair cells.
Arranged in staircase pattern.
Mechanical deflection opens ion channels.
True cilium present during embryonic life.
Guides stereocilia orientation.
Protein filaments connecting adjacent stereocilia.
Open mechanically gated ion channels during stereocilia movement.
Gelatinous acellular membrane overlying organ of Corti.
Attached medially to spiral limbus.
Covers stereocilia of outer hair cells.
| Function | Importance |
|---|---|
| Shearing movement | Stimulates stereocilia |
| Frequency analysis | Tonotopic tuning |
| Sound amplification | Enhances cochlear sensitivity |
Sound Wave
↓
Basilar Membrane Movement
↓
Stereocilia Deflection
↓
Ion Channel Opening
↓
Hair Cell Depolarization
↓
Neurotransmitter Release
↓
Auditory Nerve Stimulation
Conversion of sound energy into electrical signals.
Basilar membrane moves upward.
Stereocilia bend toward tallest stereocilia.
Tip links stretch.
Potassium channels open.
Potassium enters from endolymph.
Hair cell depolarizes.
Calcium channels open.
Neurotransmitter released.
Endolymph contains:
High potassium concentration
Potassium influx causes receptor potential.
Mainly:
Glutamate
Active mechanism enhancing basilar membrane motion.
Mediated by:
Outer hair cells
Outer hair cells change length in response to stimulation.
Motor protein in outer hair cells.
Causes electromotility of outer hair cells.
Sounds generated by outer hair cells.
Active outer hair cell movement creates reverse sound waves.
| Type | Features |
|---|---|
| Spontaneous OAE | Occurs naturally |
| Evoked OAE | Produced after sound stimulus |
Newborn hearing screening
Detect cochlear dysfunction
Evaluate outer hair cell integrity
Positive electrical potential within scala media.
Approximately:
+80 mV
Stria vascularis
Alternating current potential mimicking sound stimulus.
Mainly outer hair cells.
Direct current potential during sustained stimulation.
Synchronous firing of auditory nerve fibers.
Carry encoded sound information to CNS.
| Potential | Origin | Nature |
|---|---|---|
| Endocochlear potential | Stria vascularis | Resting DC |
| Cochlear microphonics | Outer hair cells | AC |
| Summating potential | Hair cells | DC |
| Action potential | Auditory nerve | Neural |
Different frequencies stimulate different basilar membrane regions.
| Frequency | Location |
|---|---|
| High frequency | Basal cochlea |
| Low frequency | Apical cochlea |
Frequency coded by timing of nerve impulses.
Groups of neurons fire alternately to encode high frequencies.
Nerve firing synchronized with sound wave phase.
Louder sounds → Increased firing rate.
Multiple neurons fire together with increasing intensity.
Bipolar neurons within modiolus.
Central processes form cochlear nerve.
Located at pontomedullary junction.
First site receiving bilateral auditory input.
Ascending auditory tract in brainstem.
Midbrain auditory relay center.
Thalamic auditory relay nucleus.
Located in:
Superior temporal gyrus
Heschl gyrus
Ordered arrangement of neurons according to sound frequency.
Hair Cells
↓
Spiral Ganglion
↓
Cochlear Nerve
↓
Cochlear Nuclei
↓
Superior Olivary Complex
↓
Lateral Lemniscus
↓
Inferior Colliculus
↓
Medial Geniculate Body
↓
Auditory Cortex
Ability to identify source of sound.
Sound reaches nearer ear earlier.
Sound louder in ear nearer source.
Better localization
Improved speech understanding
Better hearing in noise
Ability to differentiate speech sounds.
Interpretation of sound by cerebral cortex.
Storage and recall of auditory information.
CNS processing of auditory input.
Depends on:
Sound intensity
Frequency
One sound reduces perception of another sound.
Smallest detectable difference between two sounds.
Decrease in response during continuous stimulus.
Reduced awareness of repetitive non-significant sound.
Temporary decrease in hearing after loud sound exposure.
Efferent fibers arising from:
Superior olivary complex
Terminate mainly on:
Outer hair cells
| Function | Role |
|---|---|
| Noise suppression | Improves signal detection |
| Protective role | Protects cochlea from loud sound |
| Frequency selectivity | Sharpens tuning |
| Auditory attention | Selective hearing |
Reduces background noise effects.
Reduces acoustic trauma.
| Feature | Inner Hair Cells | Outer Hair Cells |
|---|---|---|
| Number | ~3500 | ~12000 |
| Function | Sound perception | Amplification |
| Innervation | Mainly afferent | Mainly efferent |
| Arrangement | Single row | Three rows |
| Coding Type | Mechanism |
|---|---|
| Place coding | Cochlear location |
| Temporal coding | Timing of impulses |
| Rate coding | Firing frequency |
| Volley principle | Alternating neuron firing |
Organ of Corti
Tunnel of Corti
Hair cell ultrastructure
Tectorial membrane
Cochlear transduction
Auditory pathway
Tonotopic organization
Otoacoustic emission mechanism
Cochlear amplifier
Olivocochlear bundle
Organ of Corti lies on basilar membrane
Inner hair cells = true sensory receptors
Outer hair cells = cochlear amplifier
Prestin responsible for outer hair cell motility
Endocochlear potential = +80 mV
OAE assesses outer hair cell function
Superior olive = first bilateral auditory center
High frequencies detected at cochlear base
Low frequencies detected at apex
Tip links open mechanoelectrical channels
Stria vascularis produces endolymph
Glutamate is major auditory neurotransmitter
Tuning fork tests are bedside tests used to assess:
Type of hearing loss
Relative bone and air conduction
Cochlear reserve
Most commonly used tuning fork:
512 Hz
Maximum persistence
Minimal overtones
Lies within speech frequency range
Compares:
Air conduction (AC)
Bone conduction (BC)
Normal:
AC > BC
Sound transmitted through:
Air conduction pathway
Bone conduction pathway
Pinna → EAC → TM → ossicles → oval window → cochlea
Skull vibration → cochlear fluid movement
Middle ear transformer mechanism amplifies sound.
Provides:
Hydraulic effect
Lever action
Impedance matching
Result:
AC becomes more efficient than BC.
| Result | Interpretation |
|---|---|
| AC > BC | Normal or SNHL |
| BC > AC | Conductive hearing loss |
Seen in severe unilateral SNHL.
Bone conduction from diseased ear transmitted to opposite normal cochlea.
Clinical prevention:
Mask opposite ear.
Assesses lateralization of sound.
Tuning fork placed on:
Forehead
Vertex
Sound heard equally in both ears.
Sound lateralizes to affected ear.
Reasons:
Reduced environmental masking
Increased cochlear sensitivity to bone conduction
Occlusion effect
Sound lateralizes to better ear.
Reason:
Diseased cochlea or auditory nerve cannot perceive sound effectively.
Occluded external canal increases loudness of bone-conducted sound.
Important in:
Conductive deafness
Compares patient’s bone conduction with examiner’s bone conduction.
Bone conduction duration reflects cochlear function.
| Finding | Interpretation |
|---|---|
| Prolonged Schwabach | Conductive deafness |
| Diminished Schwabach | Sensorineural deafness |
| Normal Schwabach | Normal hearing |
Ambient noise reduced.
Bone conduction perceived for longer duration.
Cochlear damage reduces perception duration.
Assesses mobility of ossicular chain, especially stapes footplate.
Air pressure applied in external auditory canal while testing bone conduction.
Increased canal pressure reduces stapes mobility.
Bone conduction sound decreases.
Positive Gelle:
Normal ossicular mobility.
Fixed stapes footplate.
Pressure change does not affect hearing.
Negative Gelle:
Stapes fixation present.
| Test | Principle | Positive Finding |
|---|---|---|
| Rinne | AC vs BC | AC > BC |
| Weber | Lateralization | Midline in normal |
| Schwabach | BC duration | Equal to examiner |
| Gelle | Stapes mobility | Reduced BC with pressure |
Abnormally rapid increase in loudness perception with increasing sound intensity.
Seen in:
Cochlear lesions
Damaged outer hair cells reduce sensitivity.
Remaining hair cells respond excessively at higher intensity.
Characteristic of cochlear SNHL.
Helps differentiate:
Cochlear lesions
Retrocochlear lesions
Soft sounds not heard.
Loud sounds suddenly become intolerable.
Same sound perceived differently in two ears.
| Type | Description |
|---|---|
| Diplacusis binauralis | Different pitch in two ears |
| Diplacusis monauralis | Double hearing in same ear |
Unequal cochlear frequency perception.
Disturbed tonotopic organization.
Ménière disease
Cochlear disorders
Better hearing in noisy environment.
Classically seen in:
Otosclerosis
Normal individuals raise voice in noise.
Conductive deaf patient hears raised speech better.
Increased sensitivity to ordinary sounds.
Reduced stapedius reflex
Facial nerve palsy
Central auditory dysfunction
Bell palsy
Migraine
Autism
Cochlear disorders
Difficulty hearing in noisy environments despite normal audiogram.
Synaptic damage between:
Inner hair cells
Auditory nerve fibers
Known as:
Cochlear synaptopathy
Poor speech discrimination
Difficulty in background noise
Areas where inner hair cells or neurons are nonfunctional.
Sound detected by adjacent healthy regions instead.
Poor hearing aid benefit.
Distorted frequency perception.
Defect in sound transmission through external or middle ear.
Reduced sound energy reaching cochlea.
Wax
TM perforation
Otitis media
Otosclerosis
Bone conduction relatively preserved.
Air-bone gap present.
Defect in cochlea, auditory nerve, or central pathways.
Impaired mechanoelectrical transduction.
Hair cell loss.
Neural degeneration.
Reduced AC and BC
No air-bone gap
Poor speech discrimination
Recruitment may occur
Age-related progressive bilateral SNHL.
Hair cell degeneration.
Spiral ganglion degeneration.
Stria vascularis atrophy.
Basilar membrane stiffness.
High-frequency hearing loss
Poor speech discrimination
Hearing loss due to prolonged noise exposure.
Outer hair cell damage
Free radical injury
Mechanical trauma
Basal turn of cochlea
4 kHz notch
| Type | Description |
|---|---|
| Acoustic trauma | Single intense exposure |
| Chronic NIHL | Repeated exposure |
Damage to cochlea or vestibular apparatus by drugs.
| Drug | Target |
|---|---|
| Aminoglycosides | Cochlea + vestibule |
| Cisplatin | Cochlea |
| Loop diuretics | Stria vascularis |
| Salicylates | Reversible cochlear dysfunction |
Hair cell degeneration
Oxidative stress
Mitochondrial injury
High-frequency SNHL
Tinnitus
Vertigo
Perception of sound without external stimulus.
| Type | Description |
|---|---|
| Subjective | Heard only by patient |
| Objective | Heard by examiner |
Abnormal cochlear hair cell activity.
Increased spontaneous neural firing.
Central auditory pathway hyperactivity after sensory deprivation.
Presbycusis
Noise exposure
Ménière disease
Ototoxic drugs
Acoustic neuroma
| Feature | Conductive | Sensorineural |
|---|---|---|
| Site | External/middle ear | Cochlea/nerve |
| Air conduction | Reduced | Reduced |
| Bone conduction | Normal | Reduced |
| Air-bone gap | Present | Absent |
| Speech discrimination | Usually good | Poor |
| Recruitment | Absent | Present |
| Rinne test | Negative | Positive |
| Weber test | To affected ear | To normal ear |
AC > BC normally due to middle ear transformer action.
Weber lateralizes to affected ear in conductive deafness.
Recruitment is characteristic of cochlear pathology.
Paracusis Willisii classically seen in otosclerosis.
Presbycusis causes bilateral high-frequency SNHL.
NIHL characteristically produces 4 kHz notch.
Hyperacusis occurs in facial nerve palsy due to stapedius paralysis.
Hidden hearing loss = cochlear synaptopathy.
Tinnitus commonly results from cochlear hair cell dysfunction.
Vestibular system is responsible for:
Maintenance of equilibrium
Posture
Coordination of eye movements
Spatial orientation
Works in coordination with:
Visual system
Proprioceptive system
Cerebellum
Vestibular labyrinth consists of:
Bony labyrinth
Membranous labyrinth
System of cavities within petrous temporal bone.
Contains:
Perilymph
Vestibule
Semicircular canals
Cochlea
Central part of bony labyrinth.
Connects:
Cochlea anteriorly
Semicircular canals posteriorly
Contains:
Utricle
Saccule
Three canals arranged approximately at right angles:
Superior (anterior)
Posterior
Lateral (horizontal)
Function:
Detect angular acceleration.
Membranous system suspended within bony labyrinth.
Contains:
Endolymph
Utricle
Saccule
Semicircular ducts
Cochlear duct
Rich in potassium
Poor in sodium
Produced by:
Stria vascularis
Rich in sodium
Poor in potassium
Similar to extracellular fluid.
| Feature | Bony Labyrinth | Membranous Labyrinth |
|---|---|---|
| Nature | Osseous cavity | Membranous sac |
| Fluid | Perilymph | Endolymph |
| Location | Temporal bone | Inside bony labyrinth |
| Function | Protection | Sensory transduction |
Three semicircular ducts:
Superior
Posterior
Lateral
Each canal has:
Ampulla at one end
| Canal | Plane |
|---|---|
| Superior | Vertical |
| Posterior | Vertical |
| Lateral | Horizontal |
Detect angular/rotational acceleration.
Larger otolith organ in vestibule.
Detects:
Horizontal linear acceleration
Head tilt
Smaller sac in vestibule.
Detects:
Vertical linear acceleration
| Feature | Utricle | Saccule |
|---|---|---|
| Size | Larger | Smaller |
| Position | Superior | Inferior |
| Macula orientation | Horizontal | Vertical |
| Detects | Horizontal acceleration | Vertical acceleration |
Sensory receptor located in ampulla of semicircular canal.
Consists of:
Hair cells
Supporting cells
Cupula
Detect angular acceleration.
Sensory receptor in utricle and saccule.
Contains:
Hair cells
Supporting cells
Otolithic membrane
Detect:
Linear acceleration
Gravity
Specialized sensory receptor cells.
Convert:
Mechanical energy → electrical impulses
Cell body
Stereocilia
Kinocilium
Structural support
Metabolic support
Ionic balance maintenance
Gelatinous structure over crista ampullaris.
Same density as endolymph.
Moves with endolymph flow.
Deflection stimulates hair cells.
Gelatinous membrane over macula.
Contains:
Otoconia
Calcium carbonate crystals.
Function:
Increase inertia.
Flask-shaped.
Surrounded by chalice-shaped afferent endings.
Rapid dynamic response.
Cylindrical.
Multiple bouton nerve endings.
Sustained tonic response.
| Feature | Type I | Type II |
|---|---|---|
| Shape | Flask-shaped | Cylindrical |
| Innervation | Chalice ending | Bouton ending |
| Response | Dynamic | Sustained |
| Number | Fewer | More |
Largest cilium on hair cell.
Determines polarity of hair cell response.
Deflection:
Towards kinocilium → excitation
Away from kinocilium → inhibition
Hair-like projections arranged in rows.
Mechanical receptors for movement detection.
Vestibular sensory ganglion.
Contains:
Bipolar neurons
Located:
Internal auditory canal
Utricle
Superior semicircular canal
Lateral semicircular canal
Saccule
Posterior semicircular canal
| Structure | Nerve Supply |
|---|---|
| Utricle | Superior vestibular nerve |
| Lateral canal | Superior vestibular nerve |
| Superior canal | Superior vestibular nerve |
| Saccule | Inferior vestibular nerve |
| Posterior canal | Inferior vestibular nerve |
Usually branch of:
Anterior inferior cerebellar artery (AICA)
End artery.
No significant collateral circulation.
Clinical importance:
Ischemia causes sudden vestibular/cochlear dysfunction.
Semicircular canals detect:
Rotational movement of head
Do NOT detect:
Linear acceleration
Based on inertia of endolymph.
During head movement:
Canal moves with skull
Endolymph lags behind due to inertia
This creates:
Relative endolymph movement
Cupula gets deflected.
Hair cells stimulated.
Excitation
Increased vestibular nerve firing
Inhibition
Reduced firing
Semicircular canals work in pairs.
Examples:
Right horizontal canal ↔ left horizontal canal
Right superior ↔ left posterior
Example:
Head turns right:
Right lateral canal excited
Left lateral canal inhibited
This improves:
Sensitivity
Accuracy
Maintenance of balance during movement.
Primarily mediated by:
Semicircular canals
Eye movements occur in plane of stimulated canal.
In horizontal canals:
Ampullopetal flow causes greater response.
In vertical canals:
Ampullofugal flow causes greater response.
| Law | Description |
|---|---|
| First | Eye movements occur in plane of stimulated canal |
| Second | Horizontal canals respond more to ampullopetal flow |
| Third | Vertical canals respond more to ampullofugal flow |
Initial movement stimulates canals.
Endolymph catches up.
Cupula returns to neutral.
Sensation decreases.
Endolymph continues moving after stopping.
Cupula deflects opposite direction.
Produces:
Opposite sensation of movement
Post-rotatory nystagmus
Head movement occurs.
Endolymph lags behind.
Cupula deflects.
Hair bundles bend.
Ion channels open.
Potassium influx occurs.
Hair cell depolarization.
Neurotransmitter released.
Vestibular nerve stimulated.
Head rotation
↓
Endolymph lag
↓
Cupular displacement
↓
Hair cell stimulation
↓
Vestibular nerve activation
↓
Vestibular nuclei
↓
Eye/postural reflexes
False sensation of movement due to vestibular dysfunction.
Due to sensory mismatch between:
Vestibular
Visual
Proprioceptive inputs
Otoconia displaced into semicircular canal.
Most commonly:
Posterior canal
Inflammation of vestibular nerve.
Endolymphatic hydrops causing:
Vertigo
Hearing loss
Tinnitus
Semicircular canals detect angular acceleration.
Utricle detects horizontal acceleration.
Saccule detects vertical acceleration.
Cupula has same density as endolymph.
Deflection toward kinocilium causes excitation.
Vestibular system works on push-pull mechanism.
Labyrinthine artery is an end artery.
Constant rotation reduces vestibular sensation due to endolymph equilibration.
Ewald’s laws are extremely important for viva and PG entrance exams.
Otolith organs are specialized vestibular receptors responsible for:
Static equilibrium
Detection of linear acceleration
Detection of gravity
Components:
Utricle
Saccule
Maintenance of body position when head is stationary.
Utricle
Saccule
Detects:
Head tilt
Position relative to gravity
Movement in straight line.
| Type | Example |
|---|---|
| Horizontal acceleration | Car moving forward |
| Vertical acceleration | Elevator movement |
Maculae of utricle and saccule.
Vestibular receptors sensitive to gravity.
Hair cells
Otolithic membrane
Otoconia
Gravity causes displacement of otoconia.
Hair cells bend.
Vestibular nerve stimulated.
Detects:
Horizontal linear acceleration
Head tilt in horizontal plane
Horizontal
Forward movement
Side-to-side motion
Detects:
Vertical linear acceleration
Vertical
Elevator movement
Jumping
Calcium carbonate crystals embedded in otolithic membrane.
Tiny crystalline particles.
Located over gelatinous otolithic membrane.
Add weight and inertia to otolithic membrane.
Enhance sensitivity to gravity and acceleration.
Head movement → Otoconia shift due to inertia.
Otolithic membrane displaced.
Hair cell stereocilia bend.
Receptor potential generated.
Head Tilt / Linear Movement
↓
Otoconia Displacement
↓
Hair Cell Bending
↓
Vestibular Nerve Activation
↓
Brainstem & Cerebellum
↓
Postural Adjustment
Caused by:
Dislodged otoconia entering semicircular canals
Positional vertigo
Nystagmus
Automatic reflexes maintaining:
Balance
Posture
Stable vision
| Reflex | Function |
|---|---|
| Vestibulo-ocular reflex | Stabilizes gaze |
| Vestibulospinal reflex | Maintains posture |
| Vestibulocollic reflex | Stabilizes head |
| Vestibulocerebellar reflex | Coordinates balance |
| Righting reflexes | Maintain upright position |
Reflex causing eye movement opposite to head movement.
Maintains stable retinal image during head motion.
Head rotation stimulates semicircular canals.
Vestibular nuclei activate ocular motor nuclei.
Eyes move opposite to head movement.
Head Rotation
↓
Semicircular Canal Activation
↓
Vestibular Nerve
↓
Vestibular Nuclei
↓
III, IV, VI Cranial Nuclei
↓
Compensatory Eye Movement
Reflex controlling posture and muscle tone.
| Tract | Function |
|---|---|
| Lateral vestibulospinal tract | Extensor tone |
| Medial vestibulospinal tract | Head-neck posture |
Prevents falling.
Maintains upright posture.
Reflex stabilizing head position.
Activates neck muscles in response to head movement.
Maintains stable head posture.
Vestibular nuclei ↔ Cerebellum
Fine balance coordination
Smooth postural adjustments
Motor coordination
Maintain body posture against gravity.
Vestibular input
Visual input
Proprioceptive input
Reflexes restoring normal body position.
Maintain upright orientation.
Preventing fall during imbalance.
Semicircular Canals
↓
Vestibular Nerve
↓
Vestibular Nuclei
↓
Medial Longitudinal Fasciculus
↓
III, IV, VI Cranial Nerve Nuclei
↓
Extraocular Muscles
Ratio of eye movement velocity to head movement velocity.
Approximately:
1
Ensures stable vision during movement.
Ability of visual fixation to suppress vestibular nystagmus.
Cerebellar modulation inhibits vestibular responses.
Impaired in cerebellar lesions.
CNS adaptation after vestibular injury.
Brain recalibrates vestibular input using:
Visual pathways
Proprioceptive pathways
Contralateral vestibular system
Reduction in vertigo
Improved balance
Decreased spontaneous nystagmus
| Feature | Utricle | Saccule |
|---|---|---|
| Position | Superior | Inferior |
| Macula orientation | Horizontal | Vertical |
| Detects | Horizontal acceleration | Vertical acceleration |
| Reflex | Main Function |
|---|---|
| Vestibulo-ocular | Stable vision |
| Vestibulospinal | Posture |
| Vestibulocollic | Head stabilization |
| Vestibulocerebellar | Coordination |
| Righting reflex | Upright posture |
| Feature | Utricle | Saccule |
|---|---|---|
| Receptor | Macula | Macula |
| Detects | Horizontal motion | Vertical motion |
| Static equilibrium | Yes | Yes |
Utricle and saccule
Macula structure
Otoconia displacement
Vestibular pathways
Vestibulo-ocular reflex
Vestibulospinal tract
Righting reflexes
Vestibular ocular pathway
Utricle detects horizontal acceleration
Saccule detects vertical acceleration
Otoconia composed of calcium carbonate
Macula responsible for static equilibrium
VOR stabilizes retinal image
Gain of normal VOR ≈ 1
Vestibulospinal tract maintains posture
Fixation suppression mediated by cerebellum
Vestibular compensation occurs after vestibular injury
BPPV caused by displaced otoconia
Nystagmus is rhythmic, involuntary oscillatory movement of eyeballs.
Usually occurs due to:
Vestibular imbalance
Cerebellar disorders
Brainstem lesions
Visual pathway abnormalities
Vestibular system maintains stable gaze during head movement via:
Vestibulo-ocular reflex (VOR)
Imbalance between right and left vestibular systems causes:
Rhythmic eye movements
Stabilizes retinal image during head movement.
Head movement occurs.
Semicircular canal stimulated.
Vestibular nuclei activated.
Ocular motor nuclei stimulated.
Eyes move opposite to head movement.
Slow drift of eyes away from target.
Produced by vestibular system.
Due to tonic imbalance between vestibular nuclei.
Represents pathological vestibular drive.
Rapid corrective eye movement back toward fixation.
Generated by cerebral cortex and brainstem gaze centers.
Direction of nystagmus named after fast phase.
Example:
Right-beating nystagmus → fast phase toward right.
Vestibular imbalance
↓
Slow eye deviation
↓
Retinal fixation error
↓
Corrective fast movement
↓
Repetitive oscillation = nystagmus
Normal reflex eye movement.
Occurs at extreme lateral gaze.
Produced by moving visual field.
Example:
Watching passing trees from moving train.
Produced by rotational or caloric stimulation.
Usually jerk type.
Horizontal or rotatory.
Peripheral vestibular stimulation
Vestibular disease
Suppressed by visual fixation
Associated with vertigo
Equal speed in both directions.
No fast or slow component.
Congenital visual disorders
Multiple sclerosis
Brainstem lesions
Unequal eye movements with:
Slow phase
Fast corrective phase
Most vestibular nystagmus is jerk type.
Triggered by change in head position.
BPPV
Central vestibular lesions
Latent onset
Fatigable
Brief duration
Rhythmic eye movement induced by moving visual stimuli.
Combination of:
Smooth pursuit
Saccadic correction
Indicates intact visual and brainstem pathways.
Torsional rotation of eyeballs.
Posterior semicircular canal lesions
BPPV
Present without external stimulation.
Vestibular neuritis
Ménière disease
Brainstem lesions
| Type | Characteristics | Causes |
|---|---|---|
| Pendular | Equal movements both sides | Congenital/CNS |
| Jerk | Slow + fast phase | Vestibular lesions |
| Positional | Triggered by position | BPPV |
| Rotatory | Torsional movement | Posterior canal lesion |
| Optokinetic | Visual stimulus induced | Physiological |
| Spontaneous | Present at rest | Vestibular pathology |
| Feature | Peripheral | Central |
|---|---|---|
| Vertigo | Severe | Mild |
| Hearing symptoms | Common | Rare |
| Fatigability | Present | Absent |
| Visual suppression | Present | Absent |
| Direction | Unidirectional | Direction-changing |
| Neurological signs | Absent | Present |
| Type | Horizontal/rotatory | Vertical/multidirectional |
Caloric test evaluates function of:
Horizontal semicircular canal
Superior vestibular nerve
Temperature change in external auditory canal causes:
Endolymph movement
Vestibular stimulation
Horizontal canal positioned vertically.
Achieved by:
Elevating head 30°
Warm or cold water changes endolymph density.
Creates convection currents.
This stimulates:
Horizontal semicircular canal
Warm water:
Endolymph rises
Mimics head turning toward same side
Result:
Increased vestibular firing
Fast phase toward same side.
Example:
Warm water right ear → right-beating nystagmus.
Cold water:
Endolymph falls
Mimics head turning opposite side
Result:
Reduced vestibular firing
Fast phase toward opposite side.
Example:
Cold water right ear → left-beating nystagmus.
Cold → Opposite
Warm → Same
Refers to:
Direction of fast phase of nystagmus.
Uses:
Warm water → 44°C
Cold water → 30°C
Standardized vestibular stimulation.
Thermal stimulus applied.
Endolymph convection current generated.
Cupula deflected.
Hair cells stimulated/inhibited.
Vestibular nuclei activated.
Nystagmus produced.
Warm/cold stimulus
↓
Endolymph movement
↓
Cupular displacement
↓
Vestibular stimulation
↓
Vestibulo-ocular reflex
↓
Nystagmus
Reduced response from one labyrinth.
Unilateral vestibular weakness.
Vestibular neuritis
Ménière disease
Labyrinthitis
Nystagmus stronger in one direction regardless of stimulated ear.
Baseline vestibular asymmetry.
Calculating:
Canal paresis
Directional preponderance
CP=\frac{(RW+RC)-(LW+LC)}{RW+RC+LW+LC}\times100
Where:
RW = Right warm
RC = Right cold
LW = Left warm
LC = Left cold
DP=\frac{(RW+LC)-(LW+RC)}{RW+RC+LW+LC}\times100
| Finding | Meaning |
|---|---|
| Canal paresis | Peripheral vestibular weakness |
| Directional preponderance | Vestibular asymmetry |
Tympanic membrane perforation
Acute otitis media
External ear infection
Recent ear surgery
Severe cervical spine disease
Uncooperative patient
TM perforation present.
Warm air → 50°C
Cold air → 24°C
Vestibular neuritis
Ménière disease
Acoustic neuroma
Brainstem lesions
Unilateral vestibular hypofunction
Direction of nystagmus named by fast phase.
Slow phase is vestibular in origin.
Fast phase is cortical/brainstem corrective movement.
Peripheral vestibular nystagmus is usually horizontal or rotatory.
COWS:
Cold → Opposite
Warm → Same
Caloric test mainly tests horizontal semicircular canal.
Head elevated 30° during caloric testing.
Canal paresis indicates unilateral labyrinthine weakness.
Positional nystagmus is characteristic of BPPV.
Peripheral nystagmus suppressed by visual fixation; central nystagmus is not.
Vestibular pathways are neural connections carrying balance-related impulses from vestibular receptors to:
Brainstem
Cerebellum
Spinal cord
Ocular motor nuclei
Cerebral cortex
Vestibular Receptors
↓
Vestibular Nerve
↓
Vestibular Nuclei
├── Cerebellum
├── Ocular Motor Nuclei
├── Spinal Cord
└── Cerebral Cortex
Located in:
Floor of fourth ventricle
Pons and medulla
| Nucleus | Main Function |
|---|---|
| Superior vestibular nucleus | Eye movement control |
| Medial vestibular nucleus | Vestibulo-ocular reflex |
| Lateral vestibular nucleus (Deiters nucleus) | Posture and extensor tone |
| Inferior vestibular nucleus | Cerebellar integration |
Receive vestibular sensory impulses.
Coordinate:
Eye movements
Posture
Muscle tone
Balance
Vestibular nuclei connected to:
Flocculonodular lobe of cerebellum
Through:
Inferior cerebellar peduncle
Coordination of equilibrium and posture.
| Cranial Nerve Nucleus | Function |
|---|---|
| Oculomotor nucleus (III) | Eye movement |
| Trochlear nucleus (IV) | Superior oblique movement |
| Abducens nucleus (VI) | Lateral rectus movement |
Via:
Medial longitudinal fasciculus (MLF)
Produces vestibulo-ocular reflex (VOR).
| Tract | Function |
|---|---|
| Lateral vestibulospinal tract | Extensor muscle tone |
| Medial vestibulospinal tract | Head-neck posture |
Maintain posture
Maintain equilibrium
Prevent falling
Vestibular cortical areas located in:
Parietal lobe
Insular cortex
Temporoparietal junction
Conscious perception of balance
Spatial orientation
Motion perception
Lateral vestibular nucleus.
Facilitates extensor muscles.
Maintains upright posture.
Medial vestibular nucleus.
Coordinates head and neck movements.
Vestibular Apparatus
↓
Vestibular Nerve
↓
Vestibular Nuclei
↓
Vestibulospinal Tracts
↓
Spinal Motor Neurons
↓
Postural Muscles
Vestibular part of cerebellum.
| Component | Function |
|---|---|
| Flocculus | Eye movement coordination |
| Nodulus | Balance regulation |
Maintenance of equilibrium
Coordination of eye movement
Vestibular compensation
Vestibular signals integrated with:
Visual input
Proprioceptive input
Smooth postural adjustments
Coordinated balance
Gaze stabilization
| System | Function |
|---|---|
| Vestibular system | Detects movement and gravity |
| Vision | Provides orientation |
| Proprioception | Detects body position |
| Cerebellum | Coordinates responses |
Detects:
Head movement
Gravity
Acceleration
Initiates:
Vestibular reflexes
Postural adjustments
Provides spatial orientation.
Stabilizes posture.
Helps fixation suppression.
Loss of visual input worsens vestibular imbalance.
Sensory input from:
Muscles
Tendons
Joints
Detects body position.
Assists posture maintenance.
Integrates:
Vestibular input
Visual input
Proprioceptive input
Coordinates motor responses.
| Mechanism | Function |
|---|---|
| Vestibulospinal reflex | Maintains posture |
| Vestibulocollic reflex | Stabilizes head |
| Righting reflex | Restores body alignment |
| Cerebellar coordination | Fine balance adjustment |
Balance maintained by:
Vision
Vestibular input
Proprioception
Patient stands with:
Feet together
Eyes open then closed
Increased swaying with eyes closed.
Proprioceptive or vestibular dysfunction.
Eyes Closed
↓
Visual Input Removed
↓
Dependence on Vestibular + Proprioception
↓
Defect Causes Swaying
Patient marches in place with eyes closed.
Normal vestibular tone is symmetrical.
Rotation toward side of vestibular weakness.
Vestibular asymmetry causes unequal postural tone.
Vestibular Weakness
↓
Asymmetrical Vestibulospinal Output
↓
Unequal Muscle Tone
↓
Body Rotation During Stepping
| Connection | Function |
|---|---|
| Cerebellar | Coordination |
| Ocular motor | Eye movement |
| Spinal cord | Posture |
| Cortical | Conscious balance perception |
| System | Main Role |
|---|---|
| Vestibular | Detects acceleration |
| Vision | Orientation |
| Proprioception | Position sense |
| Cerebellum | Coordination |
| Tract | Function |
|---|---|
| Lateral vestibulospinal | Extensor tone |
| Medial vestibulospinal | Head-neck posture |
Vestibular pathways
Vestibular nuclei
Vestibulospinal tracts
Medial longitudinal fasciculus
Cerebellar vestibular connections
Flocculonodular lobe
Maintenance of balance
Romberg physiology
Fukuda stepping mechanism
Vestibular nuclei located in floor of fourth ventricle
MLF connects vestibular nuclei to ocular motor nuclei
Lateral vestibulospinal tract maintains extensor tone
Flocculonodular lobe = vestibular cerebellum
Balance depends on vestibular system + vision + proprioception + cerebellum
Positive Romberg indicates vestibular/proprioceptive defect
Fukuda test rotates toward side of vestibular weakness
Superior vestibular nucleus involved in VOR
Vestibular cortical areas located in parietal-insular region
Spatial orientation refers to awareness of body position and movement in space.
Maintained by integration of:
Vestibular inputs
Visual inputs
Proprioceptive inputs
Central integration occurs in:
Vestibular nuclei
Cerebellum
Cerebral cortex
Syndrome produced by abnormal sensory stimulation during motion.
Sea travel
Air travel
Car travel
Virtual reality exposure
Rotatory stimulation
Mismatch between sensory inputs.
Mainly between:
Vestibular system
Visual system
Proprioceptive system
Motion sickness occurs when information from sensory systems does not match expected pattern.
Vestibular system senses motion.
Eyes may see stationary cabin.
Result:
Sensory mismatch
Motion sickness
Conflicting sensory signals generated.
Vestibular nuclei activated.
Connections to autonomic centers stimulated.
Vomiting center activated.
Histamine
Acetylcholine
Dopamine
Dizziness
Vertigo
Nausea
Vomiting
Pallor
Sweating
Salivation
Look at stable horizon
Avoid reading during travel
| Drug | Mechanism |
|---|---|
| Hyoscine | Anticholinergic |
| Dimenhydrinate | Antihistamine |
| Promethazine | Antihistamine |
Motion sickness occurring in astronauts exposed to microgravity.
Absence of gravitational reference causes vestibular mismatch.
Disorientation
Nausea
Impaired coordination
CNS gradually adapts after few days.
Ability to recognize body position relative to surroundings.
Detects:
Angular acceleration
Linear acceleration
Gravity
Provides environmental reference.
Provides body position information.
Integrate sensory information.
Coordinates posture and movement.
Conscious perception of orientation.
Vestibular input
+
Visual input
+
Proprioceptive input
↓
Vestibular nuclei & cerebellum
↓
Spatial orientation & equilibrium
Illusion of movement of self or surroundings.
Usually:
Rotatory sensation
Imbalance between vestibular systems of two sides.
Unequal vestibular input interpreted as movement by brain.
Nystagmus
Nausea
Vomiting
Postural instability
| Feature | Vertigo | Dizziness |
|---|---|---|
| Sensation | Rotation/movement | Light-headedness |
| Vestibular origin | Usually present | Usually absent |
| Nystagmus | Common | Rare |
Vertigo due to lesions of:
Labyrinth
Vestibular nerve
BPPV
Ménière disease
Vestibular neuritis
Labyrinthitis
| Feature | Peripheral Vertigo |
|---|---|
| Severity | Severe |
| Hearing symptoms | Common |
| Nystagmus | Horizontal/rotatory |
| Fatigability | Present |
| Visual suppression | Present |
Vertigo due to lesions in:
Brainstem
Cerebellum
Central vestibular pathways
Stroke
Multiple sclerosis
Cerebellar tumor
| Feature | Central Vertigo |
|---|---|
| Severity | Mild/moderate |
| Hearing symptoms | Rare |
| Nystagmus | Vertical/multidirectional |
| Neurological signs | Common |
| Visual suppression | Absent |
| Feature | Peripheral | Central |
|---|---|---|
| Site | Labyrinth/CN VIII | Brainstem/cerebellum |
| Vertigo | Severe | Mild |
| Hearing loss | Common | Rare |
| Nystagmus | Horizontal/rotatory | Vertical |
| Neurological signs | Absent | Present |
| Fatigue | Present | Absent |
Loss of vestibular function on one or both sides.
| Type | Features |
|---|---|
| Unilateral | Vertigo + imbalance |
| Bilateral | Oscillopsia + imbalance |
Loss of vestibular input to CNS.
Severe vertigo
Nystagmus
Postural imbalance
No severe vertigo
Oscillopsia
Gait instability
Illusion that surroundings are moving during head movement.
Occurs due to:
Defective vestibulo-ocular reflex
CNS adaptation following vestibular injury.
Brain rebalances vestibular tone using:
Visual input
Proprioception
Cerebellar plasticity
Resting vestibular tone restored.
Compensation during movement.
Early mobilization
Vestibular exercises
Sedative drugs
Immobility
Visual deprivation
Disorder caused by endolymphatic hydrops.
Classic triad:
Episodic vertigo
Fluctuating SNHL
Tinnitus
Excess endolymph accumulation in membranous labyrinth.
Distension of membranous labyrinth
Hair cell dysfunction
Intermittent rupture of membranes
Episodic spinning vertigo
Low-frequency SNHL
Roaring tinnitus
Aural fullness
Excessive endolymph within membranous labyrinth.
Increased production or decreased absorption of endolymph.
Cochlear duct
Saccule
Utricle
Distortion of sensory receptors
Episodic vestibular symptoms
Benign paroxysmal positional vertigo caused by displaced otoconia.
Posterior semicircular canal
Free-floating otoconia within canal.
Otoconia attached to cupula.
Head movement causes abnormal endolymph flow.
Cupula stimulated inappropriately.
Vertigo and positional nystagmus produced.
Brief vertigo
Triggered by head movement
Fatigable positional nystagmus
Otoconia displacement
↓
Entry into semicircular canal
↓
Abnormal endolymph movement
↓
Cupular stimulation
↓
Vertigo + nystagmus
Inflammation of membranous labyrinth.
| Type | Features |
|---|---|
| Serous | Toxic inflammation |
| Suppurative | Bacterial infection |
Hair cell dysfunction due to inflammation.
Vertigo
SNHL
Tinnitus
Nystagmus
Acute inflammation of vestibular nerve.
Usually viral.
Sudden unilateral vestibular loss.
Sudden severe vertigo
Nausea
Vomiting
No hearing loss
Hearing usually preserved unlike labyrinthitis.
Abnormal communication between labyrinth and middle ear.
Commonly involves:
Lateral semicircular canal
Cholesteatoma
Trauma
Surgery
Pressure changes stimulate labyrinth abnormally.
Pressure in EAC produces:
Vertigo
Nystagmus
Positive fistula test without actual fistula.
Seen in:
Congenital syphilis
Ménière disease
| Feature | Labyrinthitis | Vestibular Neuronitis |
|---|---|---|
| Hearing loss | Present | Absent |
| Tinnitus | Present | Absent/minimal |
| Site | Labyrinth | Vestibular nerve |
| Vertigo | Severe | Severe |
| Cause | Infection | Usually viral |
| Feature | Ménière Disease | BPPV |
|---|---|---|
| Duration | Minutes to hours | Seconds |
| Hearing loss | Present | Absent |
| Tinnitus | Present | Absent |
| Trigger | Spontaneous | Positional |
| Pathology | Hydrops | Otoconia displacement |
Motion sickness occurs due to sensory conflict.
Vestibular system, vision, and proprioception maintain spatial orientation.
Peripheral vertigo causes severe rotatory vertigo with hearing symptoms.
Central vertigo commonly associated with neurological signs.
Bilateral vestibular failure causes oscillopsia.
Vestibular compensation depends on cerebellar plasticity.
Ménière disease caused by endolymphatic hydrops.
Posterior semicircular canal most commonly affected in BPPV.
Vestibular neuronitis causes vertigo without hearing loss.
Labyrinthitis causes vertigo with hearing loss.
Positive fistula test suggests labyrinthine fistula.
| Property | Definition | Unit | Clinical Importance |
|---|---|---|---|
| Frequency | Number of vibrations per second | Hertz (Hz) | Determines pitch |
| Intensity | Energy carried by sound wave | Decibel (dB) | Determines loudness |
| Pitch | Subjective perception of frequency | — | High vs low tone |
| Loudness | Subjective perception of intensity | — | Depends on intensity + frequency |
| Timbre | Quality of sound | — | Distinguishes sounds |
| Velocity | Speed of sound propagation | m/s | Faster in solids |
| Wavelength | Distance between two wave peaks | Meter | Inversely proportional to frequency |
| Feature | Air Conduction (AC) | Bone Conduction (BC) |
|---|---|---|
| Pathway | External → Middle → Inner ear | Skull bones → Inner ear |
| Efficiency | Better | Lesser |
| Structures bypassed | None | External & middle ear |
| Normal hearing | AC > BC | BC < AC |
| Clinical significance | Tests entire auditory pathway | Assesses cochlear reserve |
| Feature | Endolymph | Perilymph |
|---|---|---|
| Location | Membranous labyrinth | Between bony & membranous labyrinth |
| Potassium | High | Low |
| Sodium | Low | High |
| Similar to | Intracellular fluid | Extracellular fluid |
| Produced by | Stria vascularis | CSF-related |
| Function | Hair cell depolarization | Mechanical sound conduction |
| Feature | Inner Hair Cells | Outer Hair Cells |
|---|---|---|
| Number | ~3500 | ~12000 |
| Arrangement | Single row | Three rows |
| Main role | Sensory transduction | Cochlear amplifier |
| Shape | Flask-shaped | Cylindrical |
| Innervation | Mainly afferent | Mainly efferent |
| Damage causes | Hearing loss | Loss of frequency selectivity |
| Potential | Origin | Type | Clinical Significance |
|---|---|---|---|
| Endocochlear potential | Stria vascularis | DC | Maintains ionic gradient |
| Cochlear microphonics | Outer hair cells | AC | Mimics sound stimulus |
| Summating potential | Hair cells | DC | Sustained stimulation |
| Action potential | Auditory nerve | Neural | Auditory nerve firing |
| Theory | Principle | Limitation |
|---|---|---|
| Place theory | Frequency coded by cochlear site | Poor low-frequency explanation |
| Frequency theory | Frequency coded by nerve firing rate | Cannot explain high frequencies |
| Volley principle | Groups of neurons alternate firing | Limited at very high frequencies |
| Travelling wave theory | Wave travels on basilar membrane | Most accepted theory |
| Coding Type | Mechanism |
|---|---|
| Place coding | Different cochlear areas detect frequencies |
| Temporal coding | Frequency encoded by timing |
| Rate coding | Intensity encoded by firing rate |
| Volley coding | Alternating neuronal firing |
| Phase locking | Nerve impulse synchronized with sound wave |
| Feature | Conductive Hearing Loss | Sensorineural Hearing Loss |
|---|---|---|
| Site of lesion | External/middle ear | Cochlea/auditory nerve |
| Rinne test | Negative | Positive |
| Weber test | Toward affected ear | Toward normal ear |
| Bone conduction | Better preserved | Reduced |
| Speech discrimination | Usually preserved | Poor |
| Hearing aid benefit | Good | Variable |
| Feature | Recruitment | Hyperacusis |
|---|---|---|
| Cause | Cochlear lesion | Stapedius paralysis |
| Mechanism | Abnormal loudness growth | Increased sound sensitivity |
| Hearing loss | Usually present | May be absent |
| Example | Ménière disease | Facial palsy |
| Type | Features |
|---|---|
| Subjective tinnitus | Heard only by patient |
| Objective tinnitus | Heard by examiner |
| Pulsatile tinnitus | Rhythmic with pulse |
| Non-pulsatile tinnitus | Continuous sound |
| Musical tinnitus | Complex auditory hallucination |
| Test | Physiological Basis |
|---|---|
| Pure tone audiometry | Hearing threshold |
| Rinne test | AC vs BC comparison |
| Weber test | Lateralization |
| Tympanometry | Middle ear compliance |
| OAE | Outer hair cell function |
| BERA/ABR | Auditory pathway integrity |
| Speech audiometry | Speech discrimination |
| Feature | Peripheral Disorder | Central Disorder |
|---|---|---|
| Site | Cochlea/CN VIII | Brainstem/cortex |
| Hearing loss | Common | Variable |
| Speech discrimination | Mildly reduced | Markedly impaired |
| Sound localization | Usually preserved | Impaired |
| ABR | Early wave abnormality | Late wave abnormality |
| Feature | Semicircular Canals | Otolith Organs |
|---|---|---|
| Receptor | Crista ampullaris | Macula |
| Detects | Angular acceleration | Linear acceleration |
| Structures | 3 canals | Utricle & saccule |
| Main stimulus | Rotation | Gravity & linear motion |
| Feature | Utricle | Saccule |
|---|---|---|
| Position | Superior | Inferior |
| Macula orientation | Horizontal | Vertical |
| Detects | Horizontal acceleration | Vertical acceleration |
| Main stimulus | Side-to-side movement | Up-down movement |
| Feature | Type I Hair Cell | Type II Hair Cell |
|---|---|---|
| Shape | Flask-shaped | Cylindrical |
| Nerve ending | Chalice ending | Bouton ending |
| Response | Rapid | Sustained |
| Function | Dynamic response | Tonic response |
| Feature | Peripheral Vertigo | Central Vertigo |
|---|---|---|
| Severity | Severe | Mild-moderate |
| Hearing loss | Common | Rare |
| Nystagmus | Horizontal/rotatory | Vertical |
| Neurological signs | Absent | Present |
| Vomiting | Common | Less severe |
| Feature | Peripheral | Central |
|---|---|---|
| Direction | Horizontal/rotatory | Vertical/multidirectional |
| Fatigability | Present | Absent |
| Visual fixation suppression | Present | Absent |
| Vertigo severity | Severe | Mild |
| Feature | Physiological | Pathological |
|---|---|---|
| Cause | Normal vestibular stimulation | Disease |
| Duration | Temporary | Persistent |
| Symptoms | Minimal | Vertigo/imbalance |
| Example | Caloric nystagmus | Vestibular neuritis |
| Stimulus | Nystagmus Direction |
|---|---|
| Warm water | Toward stimulated ear |
| Cold water | Away from stimulated ear |
COWS:
Cold Opposite
Warm Same
| Feature | Canal Paresis | Directional Preponderance |
|---|---|---|
| Meaning | Reduced labyrinthine response | Preference for one direction |
| Suggests | Peripheral weakness | Central/peripheral imbalance |
| Caloric test finding | Reduced response | Directional asymmetry |
| Reflex | Main Function |
|---|---|
| Vestibulo-ocular reflex | Stabilizes gaze |
| Vestibulospinal reflex | Maintains posture |
| Vestibulocollic reflex | Stabilizes head |
| Vestibulocerebellar reflex | Coordinates balance |
| Righting reflex | Restores upright posture |
| Vestibular Nucleus | Function |
|---|---|
| Superior | Eye movement coordination |
| Medial | Vestibulo-ocular reflex |
| Lateral (Deiters) | Extensor tone |
| Inferior | Cerebellar integration |
| Test | Physiological Basis |
|---|---|
| Caloric test | Horizontal canal stimulation |
| Dix-Hallpike test | Positional vertigo |
| Fukuda stepping test | Vestibulospinal asymmetry |
| Romberg test | Balance maintenance |
| ENG/VNG | Eye movement recording |
| Head impulse test | Vestibulo-ocular reflex |
| Cause | Type | Key Feature |
|---|---|---|
| BPPV | Peripheral | Positional vertigo |
| Ménière disease | Peripheral | Vertigo + SNHL |
| Vestibular neuritis | Peripheral | Acute severe vertigo |
| Acoustic neuroma | Peripheral | Progressive unilateral SNHL |
| Cerebellar stroke | Central | Neurological deficits |
| Multiple sclerosis | Central | Demyelinating features |
Conductive vs sensorineural hearing loss
Peripheral vs central vertigo
Peripheral vs central nystagmus
Warm vs cold caloric responses
Endolymph vs perilymph
Inner vs outer hair cells
Utricle vs saccule
Theories of hearing
Vestibular nuclei and functions
Auditory tests and physiological basis
Sound waves
↓
Pinna collects sound
↓
External auditory canal
↓
Tympanic membrane vibration
↓
Ossicular chain movement
↓
Stapes footplate movement at oval window
↓
Perilymph movement in scala vestibuli
↓
Basilar membrane vibration
↓
Hair cell stimulation in organ of Corti
↓
Electrical impulse generation
↓
Cochlear nerve
↓
Auditory pathway
↓
Auditory cortex
Sound stimulus
↓
Mechanical vibration of tympanic membrane
↓
Ossicular amplification
↓
Hydraulic transmission to cochlea
↓
Traveling wave on basilar membrane
↓
Deflection of stereocilia
↓
Opening of ion channels
↓
Hair cell depolarization
↓
Neurotransmitter release
↓
Auditory nerve stimulation
↓
Perception of sound in auditory cortex
Basilar membrane movement
↓
Shearing movement between basilar membrane and tectorial membrane
↓
Deflection of stereocilia
↓
Tip-link mediated ion channel opening
↓
Potassium influx from endolymph
↓
Hair cell depolarization
↓
Calcium channel opening
↓
Neurotransmitter release
↓
Spiral ganglion stimulation
↓
Auditory nerve impulse generation
Low-intensity sound
↓
Outer hair cell stimulation
↓
Prestin-mediated electromotility
↓
Outer hair cell contraction and elongation
↓
Amplification of basilar membrane vibration
↓
Enhanced inner hair cell stimulation
↓
Improved sensitivity and frequency selectivity
Hair cells of organ of Corti
↓
Spiral ganglion
↓
Cochlear nerve
↓
Dorsal and ventral cochlear nuclei
↓
Superior olivary complex
↓
Lateral lemniscus
↓
Inferior colliculus
↓
Medial geniculate body
↓
Auditory radiation
↓
Primary auditory cortex (temporal lobe)
Loud sound stimulus
↓
Cochlea
↓
Cochlear nerve
↓
Cochlear nucleus
↓
Superior olivary complex
↓
Facial nerve nucleus
↓
Facial nerve
↓
Stapedius muscle contraction
↓
Reduced stapes movement
↓
Reduced sound transmission to cochlea
Sound stimulus
↓
Outer hair cell activation
↓
Prestin-mediated electromotility
↓
Reverse traveling wave generation
↓
Transmission through ossicles and tympanic membrane
↓
Acoustic signal in external auditory canal
↓
Otoacoustic emission recording
Skull vibration
↓
Cochlear fluid movement by:
Compressional mechanism
Inertial ossicular movement
Osseotympanic mechanism
↓
Basilar membrane vibration
↓
Hair cell stimulation
↓
Auditory nerve activation
Sound reaches both ears
↓
Interaural time difference analyzed
+
Interaural intensity difference analyzed
↓
Superior olivary complex processing
↓
Auditory cortex interpretation
↓
Determination of sound direction
Speech sound enters ear
↓
Cochlear transduction
↓
Auditory pathway transmission
↓
Primary auditory cortex
↓
Wernicke area processing
↓
Language comprehension
Head movement
↓
Endolymph displacement
↓
Cupula/otolithic membrane movement
↓
Hair bundle deflection
↓
Opening of mechanosensitive ion channels
↓
Potassium influx
↓
Hair cell depolarization
↓
Neurotransmitter release
↓
Vestibular nerve stimulation
Head rotation
↓
Semicircular canal movement
↓
Endolymph lag due to inertia
↓
Cupular deflection
↓
Hair cell stimulation in crista ampullaris
↓
Vestibular nerve firing
↓
Brain interprets angular acceleration
Head movement
↓
Semicircular canal stimulation
↓
Vestibular nerve
↓
Vestibular nuclei
↓
Connections to ocular motor nuclei:
III nerve nucleus
IV nerve nucleus
VI nerve nucleus
↓
Extraocular muscle activation
↓
Compensatory eye movement opposite to head movement
Unilateral vestibular lesion
↓
Vestibular imbalance
↓
Visual and proprioceptive input enhancement
↓
Cerebellar adaptation
↓
Rebalancing of vestibular nuclei activity
↓
Reduction of vertigo and nystagmus
↓
Functional recovery
Vestibular asymmetry
↓
Slow deviation of eyes
↓
Retinal fixation error
↓
Corrective fast eye movement
↓
Repetitive oscillation
↓
Nystagmus
Warm/cold stimulus in external auditory canal
↓
Temperature-induced endolymph density change
↓
Convection current generation
↓
Cupular displacement
↓
Vestibular stimulation/inhibition
↓
Vestibulo-ocular reflex activation
↓
Nystagmus generation
Otoconia detach from utricle
↓
Migration into semicircular canal
↓
Head movement
↓
Abnormal endolymph flow
↓
Cupular stimulation
↓
False vestibular signal
↓
Brief vertigo and positional nystagmus
Conflicting vestibular, visual, and proprioceptive inputs
↓
Sensory mismatch in CNS
↓
Vestibular nuclei activation
↓
Autonomic center stimulation
↓
Vomiting center activation
↓
Nausea and motion sickness
Vestibular receptor stimulation
↓
Vestibular nuclei
↓
Integration with:
Cerebellum
Visual system
Proprioceptive system
↓
Motor output generation
↓
Maintenance of posture, gaze, and balance
Vestibular input
+
Visual input
+
Proprioceptive input
↓
Vestibular nuclei and cerebellum integration
↓
Vestibulospinal and vestibulo-ocular reflexes
↓
Postural adjustment and gaze stabilization
↓
Maintenance of equilibrium
Inner hair cells
Outer hair cells
Tunnel of Corti
Pillar cells
Tectorial membrane
Basilar membrane
Scala vestibuli
Scala media
Scala tympani
Reissner membrane
Basilar membrane
Organ of Corti
Narrow and stiff at base
Wide and flexible at apex
Tonotopic organization
Stereocilia
Kinocilium
Tip links
Afferent nerve endings
High frequency → base
Low frequency → apex
Produces endolymph
Generates endocochlear potential
Cochlear nucleus
Superior olivary complex
Inferior colliculus
Medial geniculate body
Auditory cortex
Canals arranged approximately at right angles
COWS:
Cold → Opposite
Warm → Same
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