Vestibular system Flashcards
What is the main function of the vestibular system? What two sensory organs are responsible for it - and which aspects do they process?
- Designed to sense the motions that arise from head movements and gravity (pulling on our body)
- Static head position and linear rotation = Otolith organs
- Rotational acceleration = Semicircular canals
How does our head move? Give names to the specific rotations.
We move through linear planes as well as rotations around such axes.
- Roll = kind of like a plane
- Pitch = nodding
- Yaw = side to side
Additionally we may also move e.g. up or down
Where do vestibular systems from inner ear go and why (provide 2 locations)?
- Vestibular signals (VIII cranial nerve) are relaed to brainstem and cerebellum
- Adjusting postural reflexes and eye movement
- Also reach parts of parietal cortex
- Construction of spatial awareness, orientation (abnormalities may lead to dizziness)
Why would vestibular and auditory be this close together from emryological stand of point?
The common precurser of both semicircular cannals and cochlea is Otic placode
What do we find inside and around the vestibular cannals? Why is that important?
(plus look at the picture)
Semicircullar cannals are filled with endolymph - enriched in K+ (similar to within cells rather than outer)
- surrounding these membranes - perilymph (more like extracellular space)
What structures do we find in/next to the vestibular labirinth?
The vestibular labyrinth has 2 kinds of structures
1. Utricle and Saccule = Otolith organs
2. Semicircular cannals - superior, posterior, horizontal (sensitivity to different rotation of the head)
- Scarpa’s ganglion - contain nerve cells innervating hair cells in the vestibular labyrinth
- separate devisions supply the aformentioned structures
=> Cranial nerve VIII axons
How does the transduction itself work?
Similar as in audition
- Stereocilia are sensitive to deflection towards or away from their longest parts (opening x closing K+ channels)
- depolarization or hyperpolarization of hair cells
- Opens voltage-gated Ca2+ channels
- release of neurotransmitters (glutamite)
=> AP could develop
What is the organization inside otolith organs? How do we call that part? Where are the longest stereocilia -> what type of motion are they sensitive to?
In each otolith organ there is a characteristic organization of sensory epithelium = macula (“spot”)
- arrows = axis of symmetry of hair cells (points to the longest stereocilia)
- saccular macula = sensitive to pitch (both forward and backward tilt) - vertical plane
- utrical macula = sensitive to linear accelaration to the horizontal plane, roll (mostly horizontal but has a tilt)
How is it with hair cells in semicircular cannals? Where? Direction?
Semicircular cannals have their hair cells in ampilla (pl. ampulli)
- houses sensory epithelium = Crista
- In each crista hair cells are arranged in the same direction (unlike in otolith organs)
What is interesting about the structure of otolith organ epithelium? What happens if we tilt our head? Why does it assimilate processes in retina?
On top of the sensory epithelium - gelatinous membrane -> Otoconia (“ear stones”) = calcium carbonate crystals
- If we tilt our head => shift in the distribution of otolithic membrane -> hair cells bend in downward fashion
=> stereocilia on one side of the axis = depolarization
=> the oppsite side = hyperpolarization
=> half activated and half inhibited by a tilt (kind of like on and off ganglions)
What happens when we accelerate or decelerate? Also as an electrical response?
Tilt and acceleration will have similar effects.
Electrical response:
- Acceleration, sustained tilt forward eould produce TONIC firing as long as the motion, position is going (deflection of sterecilia maintained) -> when head retored =
- Deceleration, backward tilt (we’re recording the same neuron again) -> TONIC hyperpolarization
Which directions do otolith organs respond to? Is it always just one of them?
Vertical axis (+elevetor up and down) = saccule (due to orientation of hair cells)
Horizontal axis = utricle
There is some overlap, but one membrane likely activated more than the other
Describe the anatomy of sensory part within semi-cercular cannals.
Inside Ampulla we have Crista (sensory epithelium) -> on top gelatinous layer with hair cells = Cupula
- One axis of depolarization
What happens if we tilt our head?
- Cupula blocks the flow of endolympth -> it would stop it in motion and become slightly displaced
=> deflection of hair cells towards longest stereocilia = depolarization - Each crista has its functional pair on the other side of the head
- Tilt -> one gets activated -> the other deactivated
Is there again something similar to on and off mechanism of retinal ganglion cells? What happens with the firing of neurons if we start rotating our head -> maintain rotation -> change direction?
- Each crista has its functional pair on the other side of the head
- Tilt -> one gets activated -> the other deactivated
Picture: First few seconds of motion = increase in firing -> BUT if we maintain the same speed, motion = cupula will relax ton neutral position -> Opposite motion = decrease in firing (deflecting hair cells in the opposite direction)
What’s the difference between otolith organs and semicircular cannals when it comes to functional pairs?
How does it work when we tilt our head forward right? How about forward middle?
Otolith organs operate on the same axis on both sides of the head
BUT semicircular cannals NOT
- horizontal will stay horizontal on both sides
- BUT superior on one side has a function pair of inferior on the other
- BUT vice versa
-> This mainly applies if we tilt our head also to one side (e.g. forward right = right superior, left inferior)
- If we’re tilting just forward = both superior will be activated (slightly less then to their preferred side)
What about right direction in horizontal axis?
=> Right horizontal cannal activates (endolympth moves towards side of motion)
=> Left horizontal cannal DEactivates (the other side)
How does information get from the sensory neurons to the cerebrum or cerebellum?
Vestibular organs send information to Scarpa’s ganglion cells -> along 8th cranial nerve -> brainstem -> innervate vestibular nuclear complex
=> Vestibulocerebellum
Where exactly could we find vestibular nuclear complex? What is its primary function?
Lateral tegmentum of upper pons
Helps us to stabilize/fixate eye gaze despite head motion
Describe the muscles of eye movement? (how do we call the motions they enable?)
Latin origin:
- ductio = “lead”
- ab = “away”, ad = “to/towards”
- supra = “up”, infero = “down”
1. Lateral rectus = abduction, it abducts the eye (movement temporally)
2. Medial rectus = adduction (movement nasally)
3. Superior r = elevation, supraduction
4. Inferior r. = depression, infraduction
5. Superior oblique
6. Inferior oblique
What cranial nerves move our eyes? Add which muscles.
- Abducens nerve (from Abducens nucleus)
- Lateral rectus
- Trochlear nerve (Trochlear nucleus)
- exits DORSAL aspect and crosses over!
- CONTRALATERAL Superior oblique
- Oculomotor nerve (Oculomotor nucleus)
- remaining muscles
Why do we need Vestibulo-ocular reflex? What would be expect if turning our head to the right?
It allows us to match our eyes on velocity and position with the head motion
- If I turn my head to the right -> expecting to activate my right semicircular cannals -> my eyes have to move to the left (left lateral rectus, right medial rectus - while relaxing the opposites)
So how does the Vestibulo-cochlear reflex work?
- Activation of left side Scarpa’s ganglio cells
- Innervating neurons in Medial vestibular nucleus
- One neuron will release EXCITATORY neurotrans. in contralateral abducens nucleus
- One n = Contraction of contralateral lateral rectus (right)
- Second n. = internuclear interneuron growing from a.n. to oculomotor nucleus (crossing back to the side of activation - left) => contraction of ipselateral medial rectus
- Second n. supresses firing of motor n. in ipselateral abducens nucleus
4. Same as the excitatory path
What are the 2 tracts concerned with postural control?
- Medial vestibulospinal tract
- starts in medial vestibular nucleus
- runs bilaterally -> motor n. in cervical cord (neck, proximal upper extremity muscles)
- Lateral vestibulospinal tract - more lateral position but still Medial decending system
- start in lateral vestibular nucleus
- runs on the ipselateral aspect of the spine
-> up to lambo-sacral enlargement
What relay station could be found in the cerebrum?
What information do we integrate? Which part of the brain contributes to e.g. sea sickness?
- Vestibular nuclear complex
- Projects to ventral posterior nucleus of thalamus
- Parietal lobe
- face representation region, proprioception, vestibular processing => all integrated (also with other e.g. vision)
Plus connected to reticular formation
- controls visceral functions -> may make us feel sick (even in the absence of motion) e.g. sea sickness
