Auditory & Vestibular Systems Flashcards
Auditory - stimulus & receptor
Stimulus = pressurized sound wave
Receptor = hair cell
3 areas of auditory system
- outer ear - sound waves are collected & travel to tympanic membrane
- middle ear - tympanic membrane to cochlea
- contains the 3 bones: malleolus, incus & stapes - stapes attaches to oval window - Inner ear - past stapes bone including cochlea & labrynth
Cochlea
- anatomy
- function
Contains organ of corti
- sits on basilar membrane
- contains 3 outer hair cells (accept sound) and connect to one inner hair cell (fine tune the sound)
The hair cells are stimulated by the tectorial membrane which moves in response to pressure variations in the fluid
Function: hearing
Stimulation - electrical potential @ hair cell during hearing
@ cilia, K rushes in, causing depolarization
Ca rushes in
release of excitatory NT
–> sound transmitted via cochlear nerve (CN VIII)
Tonotropic arrangement in the cochlea
higher frequencies stimulate hair cells @ BASE of the cochlea
lower frequencies travel further and stimulate hair cells @ APEX of cochlea
Functions of Auditory system (3)
- Activating effect of sound (alert/arousal)
- Orients head and eyes toward sound
(inferior colliculus - info from both ears, then travels to superior colliculus - responsible for turning of head) - Recognition of sound
Primary Auditory Cortex
- location
- function
Area 41 of temporal lobe
Function: conscious awareness of intensity of sound (loudness & pitch)
- contains same tonotrophic arrangement as cochlea
Secondary Auditory Cortex
- location
- function
Areas 22,42 of temporal lobe
Function: classification of sounds - dog barking? bell ringing
comparing sounds w/ memories of other sounds
Wernicke’s area
responsible for comprehension of sound
Vestibular System
- functions (3)
- perceive motion
- Maintain balance (vestibule-spinal reflex)
- Stabilize eyes (VOR)
3 systems w/n Vestibular system
- Peripheral sensory apparatus - located in inner ear - labrynth
- Central processing system - located in vestibular nuclei complex in BS
- Motor systems - ocular & skeletal muscle control
Peripheral Sensory Apparatus of Vestibular system
- function
- composed of
Function: provide sensory input about angular & linear acceleration and orient head w/ respect to gravity to maintain stable gaze
Composed of: semicircular canals, otoliths (utricle & saccule) and ampulla
Semicircular canals (3) - function
filled w/ endolymph, sensitive to yaw, pitch & roll planes and all 3 are oriented in right angles to eachother
- anterior (sup) = sagittal movement (flex/ext)
- posterior (inf) = lateral side bending of the head
- horizontal (lateral) = axial rotation
- sits at 30deg
Otoliths
- saccule & utricle
- function
Used for balance control, hair cells send signal to postural muscles
Saccule: macula oriented in horizontal plane, maximally stimulated w/ linear mvmt
Utricle: macula oriented in vertical plane
Ampulla
- what is it
- function
expansion that connects utricle w/ distal end of semicircular canal; contains crista ampullaris & hair cells
Function: convert head motion to neuronal firing (via hair cells)
Central Processing System of Vest system
- composed of
- how does it work
Composed of 4 vestibular nuclei - each which maintains a baseline firing rate that is SYMMETRICAL @ REST
- head movements cause change in firing rates of both vestibular nerve but in opposite direction….
- -> turn head to the R, R fires FASTER & vice versa
IF this is messed up it will cause a nystagmus
Motor systems of Vestibular system
- function
- tracts involved
Function: generate compensatory eye movements for gaze stability and body movements to maintain posture and body equilibrium w/ locomotion
- MLF - connects directly to eye muscles via CN III, IV and VI
- Medial vestibulospinal tract - maintains UE and head position
- Lateral vestibulospinal tract - maintains stability of trunk and LE
Physiology behind Semicircular canals
3 coplanar pairs are formed between R&L
–> this paring is associated w/ push-pull change in quantity of semicircular canal output (2 mechanisms are sending the same message to the brain)
Example:
Turn your head to the right you stimulate the R horizontal canal while the L horizontal canal is inhibited… OR
Flex head - stimulates R ant canal & inhibits L posterior
Each canal has its own signature eye movement which makes it important for testing for which canal may be contributing to problem
Functions of Semicircular canals vs. Utricle & Saccule
Semicircular canals - respond to ANGULAR ACCELERATION
Saccule & Utricle - repsond to LINEAR acceleration and head position in gravity
Why is Vestibular Contribution to Eye movement important?
- Gaze stability (VOR)
- Overriding VOR - conscious decision to turn your head
- Optokinetic reflex - slower movement, larger perception
Vestibular-ocular Reflex (VOR)
- what is it
- regulated by
- deficit
Maintain gaze stability during head motion which allows the desired object to stay on fovea, even when walking/ running or moving your head
Regulated by semicircular canals
Deficit = oscillopsia
EXAMPLE: turn your head to the right - R medial rectus (CN III) and L lateral rectus (CN VI) to maintain gaze straight
VOR Gain
head and eye movement of equal velocity which is regulated by semicircular canals
Vestibular system is responsible for maintaining gaze > 60deg/sec
CNS responsible for smooth pursuit (
How to test VOR?
Smooth pursuit - CNS - track finger
Saccades - vestibular system - look at tip of nose, then finger
VOR - tilt head down to 30 to maximally stimulate horizontal canals - perform quick movements while staring at nose
Nystagmus & VOR
- what is it
- which way does the fast beat go
differences between sides in the tonic firing rate w/n vestibular nuclei, indicating one is more active than the other
Fast beat - towards more ACTIVE side (stimulated ear) b/c it will drift to the weaker, less active side
Nystagmus & Peripheral lesion
beats AWAY from the side w/ a deficit-causing lesion
TOWARD side w/ an irritative lesion
Nystagmus & CNS
no pattern
BPPV
- what is it
- symptoms
single most common cause of dizziness
common in >65 due to otoconia falling out of place (vestibular degeneration), which displace the fluid and stimulate hair cells when they are not moving
Symptoms arise w/ change in head position & is usually unilateral
Meniere’s Disease
- what is it
- what does it cause
malabsorption of endolymph due to drainage system being blocked which increases swelling
Causes: fullness in ear, reduced hearing, rotational vertigo & nausea
Constantly tells your brain your moving causing dizziness for days (Meneire’s attack) - 48-72 hours
Increase prevalence of migranes and can lead to deterioration of auditory & vestibular systems
Oscillopsia
- what is it
- what causes it
apparent movement of a stationary visual environment
can be due to bilateral vestibular loss, dysfunction of semicircular canals leading to deficit of VOR
- caused by ototoxicity from taking gentamycin
If loss is equal bilaterally, no complaints of vertigo
Central vs. Peripheral Disorders of Vestibular system
Central - damage to vestibular nuclei, pathways or BS/cerebellum
- Symptoms: oscillopsia (impaired VOR), nausea, disquilibrium (balance), impaired smooth pursuit, diplopia
Peripheral - damage to peripheral sensory apparatus (i.e. BPPV, Menieres, neuritis, labryinthis, etc)
- Symptoms: hearing loss, vertigo, tinnitus, fast beating nystagmus, gait ataxia, impaired VOR
Contraindications to Vestibular Treatment (4)
unstable vestibular disorders (acute PLF)
recent surgeries
acute TBI
positive vertebra-basilar insufficiency test
