Vestibular Function Flashcards
What is the vestibular function?
Sensory system essential for control of posture and balance
Found in inner ear
Series of fluid filled membranous tubes (labyrinths) embedded in temporal bone
What does the vestibular apparatus consist of?
3 semi-circular canals
Utricle (semi-circular canals all connect)
Saccule
What is the structure of the 3 semi-circular canals?
All at right angles
Swellings at their bases (ampulla)
Ampulla contains sensory hair cells
What are the utricle and saccule collectively known as?
Maculae (Otolith organs)
What does the utricle detect?
Back/front tilt
Horizontal acceleration
What does the saccule detect?
Vertical acceleration
What does the semi-circular canals detect?
Rotational acceleration
What are the sensory receptors in the semi-circular canals and how do they function?
Hair cells within the ampulla
Hair cells are embedded in a flexible gelatinous structure (cupula) that stretches across entire width of ampulla
Becomes distorted by movement of endolymph fluid in canals
What does the cilia of the hair cells synapse with?
Directly with sensory neurons of the vestibular nerve (CN VIII)
How does detection of rotational acceleration work?
If skull is rotated left or right from rest
Endolymph does not move at first because of inertia
Ampulla moves instantly because it is embedded in the skull
Inertia produces drag = bends cupula (+cilia) in opposite direction to movement
Endolymph catches up eventually
Sudden stop causes endolymph to continue to move due to momentum creating sense of dizziness and continued movement
What are the 2 types of cilia of the hair cells?
Single, very large kinocilium
Progressively smaller stereocilia
What is the result of distortion of the cilia in the direction of the kinocilium?
Depolarisation and increased discharge of APs in vestibular nerve
What is the result of distortion of the cilia away from the kinocilium?
Hyperpolarisation and decreased discharge of APs in the vestibular nerve
How does the body make a 3D image of body position?
Using the pattern of firing and inhibition received by result of distortion
Where does most integration of the sensory information from the vestibular system?
Cerebellum
How is the macula orientated in the utricle?
Horizontal plane
How is the macula orientated in the saccule?
Vertical plane
What does the cilia of the maculae protrude into?
Gelatinous mass = otolith membrane
What is embedded in the otolith membrane?
CaCO3 crystals (otoliths)
How is tilt of the head detected by the macula in the utricle?
otoliths = greater density than endolymph = affected more by gravity
Otoliths and membrane move when head tilted = distorted jelly = moves cilia
What does backward tilt result in?
Otolith moves towards kinocilium = depolarisation and increased discharge of APs
What vertical forces does the vertically orientated macula in the saccule respond to?
Elevation in a lift
Provides information on orientation of head when lying down
What are tonic labyrinthine reflexes?
Keep the axis of head in a constant relationship with the body
Using information from maculae and neck proprioceptors
What are dynamic righting reflexes?
Rapid postural adjustments that are made to stop you from falling when you trip
What are vestibulo-ocular reflexes?
Association between vestibular apparatus, visual apparatus and postural control
In the medullary vestibular nuclei, afferents from vestibular system synapse with afferent fibres going to extraocular nucleii = eye movement
What is static reflex?
Head tilts
Eyes intort/extort to compensation
What is dynamic vestibular nystagmus?
Series of saccadic eye movements that rotate the eye against direction of passive rotation
When eyeball comes to end of range of movement = eye rapidly flicks back to straight ahead
What occurs in post-rotatory nystagmus?
If rotated left, during acceleration = left nystagmus
End of rotation, during deceleration = right nystagmus
Due to endolymph catching up and pushing the cupula in opposite direction
What is caloric stimulation?
Test for horizontal semi-circular canal Outer ear is washed with cold/warm fluid Temperature difference gets through thin bone = convection currents = affect endolymph Warm fluid = nystagmus to affected side Cold fluid = away from affected side
When is motion sickness most likely to occur?
If visual and vestibular system inputs to the cerebellum are in conflict
Cerebellum generates ‘sickness signal’ to hypothalamus causing nausea, vomiting, decreased BP, dizziness, sweating, pallor
What is labyrinthitis?
Acute interference with normal vestibular function as a result of infection = all ANS symptoms + vertigo
What is vertigo?
Perception of movement in the absence of movement
May be nystagmus
Gross impairment of posture and balance
What is Meniere’s disease?
Vertigo, nausea, nystagmus and tinnitus
Associated with over production of endolymph causing increased pressure
Cause is unknown
What can cause nystagmus at rest?
Lesions of the brainstem
