Vestibular-ocular system Flashcards
1
Q
Anatomy of vestibular systm
A
- bony labyrinth contains semicircular canals and vestibule
- contains the membranous labyrinth
- inside vestibule is saccule and utricle which house the otolith system
- witin semicircular canals is semicircular ducts, each of which has a swollen area called ampullae - houses receptors for canal system
- the utricle, saccule and ducts make up the vest. system - 5 patches of hair cells (1 in utricle, saccule and 1 in each ampullae)
2
Q
Hair cells
A
- increase or decrease afferent activity depending on way theyre tilted
- at rest, the MP is -40mV, just enough for a bit of glutamate to leak - causes neuron to fire some potentials
- tilt sterocilia towards longest one, you increase depolarisation and NT release to increase AP firing
- tilt them towards smallest one, you get decreased firing
3
Q
What does the Otolith system do?
A
- compromises of utricle and saccule
- it is movements of the head that causes these changes
4
Q
Macula of the otolith system
A
- picks up linear head movements - responds to acceleration and deceleration and direction of gravity
- sterocilia stick up into the endolymph filled compartment, embedded in the gelatinous material
- jelly produces the tilting of the cilia
- jelly is weighed down by otoconia crystals giving it mass and inertia
5
Q
Hair cells form an orderly pattern
A
- different groups of hair cells are orientated at different angles
- each direction of movement activates a unique group of hair cells
- between utricle and saccule, all directions are covered
- info is fed via lateral vestibulospinal tract to antigravity muscles - helps you stay upright on tube
6
Q
Semi-circular canal system
A
- responds to angular movements of the head (not linear)
- in the ampullae we have region of jelly called Cupula, sits on top of ampullary crest which contains stereocilia.
- turn head to left, produces excitation in left horizontal canal
- head turns but the fluid in the canal lags behind the movement, meaning that the cupula will at first be hanging back, causing the stereocili to be in their depolarised position. When the movement stops, the fluid continues moving for an instant after the head stops due to inertia - changes the direciton of the cilia and stops APs
- mirrored on right hand side - anything that excites the left canal will inhibit the right one and vice versa
- between all 3 canals, you have every angular movement covered
- info sent via medial vestibulospinal tract to muscles that move head and body
7
Q
Vestibulo-ocular reflex
A
- ensures stability of gaze
- turn head to left: increase in AP, sent to vestibular nuclei - increases stimualtion and is fed to abducens nerve on opposite side of head
- contains MNs that drive lateral rectus muscle on right side
- also has interneurons that project to oculomotor and causes contraction of medial rectus on left side
- head movement causes eyes to move in opposite direction to keep image fied on fovea
- medial longitudinal fasiculus rus up the back of the CN nerve nuclei involved - keeps it very fast
8
Q
Vestibulo-ocular reflex - cerebellum
A
- Flocculo-nodular lobe helps keep image fixed on fovea
- vestibular apparatus feeds into granule cells which connect to purkinje cells - inhibit vest. nuclei
- the stronger the inhibition, the weaker the efferent prat of the reflex pathway and vice versa
- Retinal ganglia project to accessory optic system - involved in controlling eye movements
- project to pontine nuclei and inferior olivary nucleus - connects to granule cells
- these can increase or decrease inhibition depending on whether there has been an error in one way or the other
- olivary nucleus also projects climbing fibres - can produce a permanent change in the gain of the loop - recalibrating the system after somthings has changed
9
Q
What goes wrong?
A
- Nystagmus
- vertigo
- nausea and vomiting
- motion sickness
10
Q
What is nystagmus?
A
- constant involuntary, lateral movements of eye
- the eyes move side to side
- imbalance between left and right vest. systems - causes eye drift
11
Q
Vertigo
A
- the sensation of moving around in space or having objects move around the person
- vestibular afferent report that head is turning (feel like spinning)
- brain thinks visual image should be steady but is drifting (world spinning round you)
12
Q
Nausea and vomiting
A
- due to conflict between vestibular and other sensory inputs (esp. visual)
- motion sickness is thought to have exactly the same mechanism
13
Q
Motion sickness
A
- induction of nausea and vomiting by motion or perceived motion
- more likely if: fam history, asian origin, female, migraine sufferer
- suscepitbility to motion sickness increases risk for: pregnancy sickness, anticipatory emesis, post-operative emesis
- due to motion - induced by travel in boats, cars, planes etc
- perceived motion - induced by moving visual scenes which give the illusion of self motion
14
Q
Motion sickness mechanism
A
- vest. system is essential
- visual system is freq. involved but not essential
- conflict between these stimulates vestibular nuclei, which stimulates nucleus tractus solitarius
- this increases ADH, distress and aversion, as well as sympathetic activity (pale and sweating) and gut dysrhythmia (prep. for vomit)
15
Q
Why does this make us fel ill?
A
- part of natural defence against toxins? - posioning produces aberrrant activity in vest. hair cells
