The vestibulo-ocular system, vertigo and vomiting
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The otolith system sits within the vestibule, which two structures make up the otolith system?
Utricle and saccule
Where do the ampullae sit and what do they contain?
Sit at the end of each semi-circular duct and house vestibular receptors
What kind of movement does the otolith system pick up? (3 things)
Linear movements of the head (side to side, front to back) + responds to acceleration/deceleration, gravity.
What is the ‘jelly’ like material in the vestibular system and how does it help to detect acceleration, decelaration and gravity?
• Otolith • As head moves, the jelly stays still so the sterocilia tilt with head • Jelly starts to move and this causes tilt of sterocilia in opposite direction - this is detection of linear acceleration/decel. • Tilting the head moves jelly and tilts sterocilia again in direction of gravity
What tract controls posture/signals from the otolith system?
Lateral vestibulospinal tract
Which system/structure responds to rotation of the head (angular movements, not linear movements)?
The semi-circular ducts and ampullae
What is the gelatinous membrane in the ampulla called?
cupula (endolymph, like the otolith)
What happens when you turn your head left (in a left horizonal canal)?
Produces excitation - fluid lags behind, therefore produces excitatory signal (depolarisation). When the head stops the fluid moves but causes the opposite to happen, stopping the afferents. The same will happen with the right horizontal canal.
Which tract controls stability of head/semicircular ducts?
Medial vestibulospinal tract
What is the pathway for the vestibulo-ocular reflex?
• AP fired from horizontal scc • to vestibular nuclei • stimulate abducens (VI) • stimulates oculomotor (III) (via medial longitudinal fassiculus) • both nerves cause opposing muscles to simultaneously relax This is the vestibulo-ocular reflex
How is the vestibulo-ocular reflex so fast?
The medial longtiduinal fasiculus is big and thickly myelinated so allows for extremely fast transmission, but it is vulnerable to deymyelination so in MS, a patient’s conjugate eye movements will be affected.
Which area of the cerebellum is important in maintaining accuracy of the vestibulo-ocular reflex?
Flocculo-nodular lobe
The accessory optic system: (i) what does it receive projections from (iii) what does it project to
(i) - big fast retinal ganglion cells (ii) - projects to pontine nuclei and olive
What is nystagmus?
Constant, involuntary, cyclical, lateral movement of the eyeball - eyes move side-to-side in a saw-tooth pattern. Sometimes due to cerebellar damage.
What is vertigo?
The sensation of moving around in space or having objects move around the person. Vestibular afferents are reporting that the head is turning or the brain the thinks the visual image should be steady but it’s drifting.
What is motion sickness?
Induction of nausea (often accompanied by pallor + cold sweating) and vomiting by motion or perceived motion.
Some are far more susceptible to motion sickness, but who in particular?
There is a genetic linkage - more likely if: • family history • asian (oriental) origin • female • migraine sufferer
Susceptibility to motion sickness increases risks for other types of nausea and vomiting such as?
Eg. • Pregnancy sickness • Anticipatory emesis • post-op emesis
What is the mechanism of motion sickness?
• Sensory conflict can account for most forms • Vestibular system is 100% involved • Visual system often involved • Cerebral cortex must be involved in nausea but the accompanying signs are brainstem mediated
What is the nausea and vomiting centre?
Nucleus tractus solitarius
What class of drug can be used to treat nausea and vomiting due to toxins in the blood/gut?
5-HT3 antagonists - they act at area postrema and vagal afferents to decrease stimulation to NTS and hence less vomiting.
What class of drugs treat nausea and vomiting caused by the vestibular/sensory conflict (motion sickness)?
• Muscarinic (mAChR) antagonists (eg. hyoscine) • histamine (H1) antagonists (eg. diphenhydramine) • mixed mAChR + H1 antagonists (eg. promethazine) • mixed mAChR, H1, dopamine antagonists (eg prochloroperazine) The further down the list, the more ‘severe’ the drug is, they have more side effects too.