Vestibular system (wk 8) Flashcards
What are the 3 main contributors to orientation?
Vision (visual space), vestibular system (inertial space), proprioception (internal space)
What is vision?
Is ambiguous (depends on expectation), cannot tell us anything in isolation. Requires validation by the vestibular system. Is very good for detecting motion – optic flow. Problems of detecting motion/ self motion?, circular/linear/ roll vection. Tends to dominate (fairground illusions) cinema versions.
What does the inner ear do?
It is the organs of balance
What do the otolith organs and semicircular canals do?
Detect movements in any direction. Otoliths – linear acceleration/ tilt. Canals – angular velocity
What do hair cells (cilia) do?
Otoliths and canals contain sensory hair cells (stereocilia). Movement causes deflection of hairs. Motion towards stereocilium depolarises receptor and increases firing rate of afferent.
Describe the pull-push action of the left-right signals of the vestibular system:
Rotation/ acceleration is signalled by difference in firing rate between the two ears. Lose one ear – strong sense of rotation cases vertigo and dizziness.
Describe the structure of utricle and saccule:
-> Contain endolymph. Receptors in the macula. Utricle – approx horizontal. Saccule – approx horizontal. Respond to acceleration (and gravity). Systematic variation in direction of polarisation. Slow adaptation. Main role postural stabilisation.
Describe the vestibular system with an upright head and tilt/ acceleration ambiguity:
With heard upright: Utricle roughly horizontal, saccule vertical. Detect tilt/ linear acceleration. Cells oriented in all directions to pick up any movement.
-Tilt/ acceleration ambiguity -> Somato-gravic illusion – produces ‘false climb’ illusion in aircraft. False climb illusion dangerous for pilots – intuitive response is to pitch the aircraft downwards causing a crash into the sea/ land. Pilots must ignore sensations and rely on instruments.
What are semicircular canals?
Canals are perpendicular to each other. Therefore detect rotation in any direction
Describe the cupula as being neutrally buoyant:
Normally the cupula is the same density as the surrounding endolymph fluid e.g. it is neutrally buoyant. Therefore does not respond to changes in orientation e.g. when lying down
Describe the 5 stages of positional alcohol nystagmus:
- Normally, the cupula should be neutrally buoyant with respect to surrounding fluid
- After alcohol ingestion, the cupula becomes lighter that the surrounding fluid. Therefore rises when lying down
- Produces a left-beating nystagmus when lying on left ear
- After heavy water (D2O) ingestion, the cupula becomes heavier than the surrounding fluid. Therefore sinks when lying down.
- Produces a right-beating nystagmus when lying on left ear
What is the vestibular ocular reflex?
Stabilisation of gaze is achieved via vestibular input from both sides of head. Canals operate in a ‘push-pull’ manner. Head rotation causes excitation on one side, inhibition on the other. Asymmetry of firing rates from both ears is the key signal of rotation
What is caloric vestibular stimulation?
Method of modulating the firing rate of the primary vestibular afferents, by irrigation of the ear canal with warm or cold water
What are comatose patients?
-> Reflex eye movements evoked by Caloric Vestibular Stimulation indicate integrity of the brainstem and can be used for outcome prediction. 92% of patients with abolished reflex eye movements died. 67% of patients with normal response had good recovery.
What is galvanic vestibular stimulation?
Non-invasive method for altering vestibular nerve activity. Constant current (1-5mA) over mastoid (seconds/ minutes). Typically bipolar binaural. Generally well tolerated (inc. elderly, stroke patients). Motion sickness usual.
