Vestibular Systems Flashcards
functional anatomy, hair cells and head movement, vertigo and Meniere's disease
what is the vestibular systems designed to do ?
answer:
- which way is up/down
- which direction am i moving
wat do the 5 organs of the vestibular system do ?
independently measure linear and angular acceleration
where do the output from the vestibular organs of the inner ear flow to ?
vestibular nuclei in the brain stem
what do vestibular nuclei do?
help control posture and balance
what do we have in each inner ear?
a mirror-symmetric vestibular system
what is the cochlea for ?
processing sound
how many chambers are used for the utricle and saccule ?
2
how many chambers are used for the semi-circular canals ?
3
how many chambers total in the inner ear ?
5
what do the utricle and saccule do?
transduce linear accelerations of the head
e.g. jumping up e.g. sitting in a car that accelerates
what do semi-circular canals do ?
transduce angular accelerations
e.g. moving head to the right e.g. dipping head
what are the 3 semi-circular canals ?
- posterior
- anterior
- horizontal
what is the vestibular labyrinth ?
- simple organisation
- each organ lined with a continous sheet of epithelial cells
what are the 5 organs in the vestibular system ?
- saccule
- utricle
- 3 semicircular canals
what do cells in the epithelium of the vestibular organs do ?
produce endolymph vy the action of ion pumps
so chambers are fluid filled
what is the composition of endolymph ?
rich in potassium ions and low in sodium ions
same as cochlea
what is endolymph ?
extracellular fluid that washes over apical surfaces
where are hair cells found ?
in five clusters within vestibular labyrinth (one cluster for each organ)
also in cochlea
what do we find at the tip of the hair cells ?
sterocilia at apical ends
what are kinocilium ?
at the end of the array of stereocilia
- longest
how are sereocilia linked ?
a filamentous process - TIP LINK - connects one end of stereocilium to the sde of the longest adjacent one
what do tip links do ?
- attach to one end of ion channel and swaying of stereocilia causes ion channel to open or close
depends on direction
when are ion channels in tip link opened ?
when stereocilia move towards kinocilium
(to longer stereocilia)(pulls open ion channel)(potassium influx into stereocilia)
hair cells as mechanoelectrical transducers
- HCs depolarize even at rest
- stereocilia deflected towards kinocillium = ion channels open = depolarizing cell = enhances neurotransmitter released
- HCs depolarization sends glutamate to nerve at end of hair cell, which generates APs /increased firing
HCs themselves do not send APs, they just change MP
why do we get APs from HCs even at rest?
leakage of potassium into hair cell causes slight depolarisation
when are hair cells hyperpolarised ?
when bending away from kinocilium
HCs reduce neurotransmitter released
decreased firing from nerve
output of vestibular system ?
8th cranial nerve
- myelinated axons
- sends information into vestibular nuclei in the brain stem
what is the pattern of hair cells firing ?
- most are tonically and phasically
- some adapt to continuous stimulation (habituate)
what do hair cells code information about ?
abrupt and sustained accelerations and translations of the head
what are the inputs to the vestibular system ?
-brain stem sends inout to hair cells of the vestibular system
- can effect sensitivity of hair cells
how can te brainstem modulate signal sensitivity in the vestibular system ?
can increase or decrease excitability of some HCs making them more or less sensitive to pertubations (bending)
what accounts for the different sensitivities of the utricle and saccule from the three semi-circular canals
how the hair cells are moved
what do the utricle and saccule consist of and where are they ?
- ovoidal sac (3mm)
- sit at base of semi-circular canals
what is the macula?
- organ that contains all the hair cells in the utricle and saccule
what are stereocilia attached to ?
- gelatinous sheet called the otolithic membrane
doesn’t move, why haircells are bending
what does the otolithic membrane cover ?
entire macula
what are otolithic organs ?
- dense particles of calcium carbonate
- called otoconia
otoconia are embedded within and lie on otolithic membrane
why does the otolithic membrane lag behind in the movement of the head ?
due to inertia even though it is free to move in utricle and saccule
and because its attched to hair cells it causes them to bend
what do otolithic membrane shift relative to ?
underlying epithelium
where is the macula of each utricle oriented to ?
- lie in the horizontal plane
what does utricle work out ?
if we are accelerating forwards or backwards as its in orientation with membrane on top (horizontal plane)
what does saccule work out ?
lies in vertical plane
- is head going up or down
utricle cell actiation
- horizontal acceleration will maximally activate (depolarising) one group of cells while suppressing (hyperpolarising) an oppositely oriented group of hair cells
opposite excitation and inhibition
how are the maculas of saccules orientated ?
vertically
same opposite inhibition and excitation as utricle
examples of vertical acceleration
changes in gravity
- swimming, bungee jumping, rolle coasters
what can SOME saccule cells do ?
detect accelerations in horizontal plane
- forwards or backwards / anterior-posterior axis
how does the brian determine whether the utricle has been activated either by linear acceleration or by head tilt?
- combines angular acceleration signal of semi-circular canals
- absence is linear
- presence is head tilt
same hair cells still activated in both
when does angular acceleration occur ?
whenever an object alters its rate of rotation about an axis
e.g. to side / left right/
what detects angular accelerations ?
the three semi-circular canals in each ear
- send signal to mid brain
structure of semi-circular canals
- made up of closed tubes that are filled with endolymph and extend from the saccule and utricle
how do semi-circular canals detect acceleration ?
using motion of endolymph fluid
how do semi-circular canals detect the motion of enodlymph ?
- gelatinous diaphragm called the CUPULA
- at base of cupula are hair cells that only get excited in one direction
- sends information to brain stem
where is the cupula attached and not attached ?
- attached to epithelium
- except at ampullary crista
what is ampullary crista ?
where epithelium is penetrated by hair cells
how does the cupula work ?
- endolymph moves as a result of accelration
- presses against cupula
- causes it to move
- moves stereocilia of hair cells to bend and either hyperpoarises or depolarises them
- signal to brain stem
how are hair cells in cupula different to hair cells in utricle and saccule ?
all hair cells are oriented in the same direction - only excited in one direction suppressed in the other
how are the three canals organised ?
- perpendicular to one another
- three mutually orthogonal axis
how do semi-circular canals act ?
in pairs (from each ear)
e.g head movement to left or right detected by left and right horizontal canals
what happens to hair cells in semi-circular canals when head turns to the left?
- left orizontal canal hair cells activated
- right horizontal hair cells are inhibted (closes tip links and decrease firing)
always activates side you move to
how are anterior and posterior semi-circular canals organised ?
at 45 degrees
what happpens in the semi-circular canals if you move your hea 45 degrees to the right ?
- triggers anterior canal in right ear
- combines with posterior semi-circular canal in left ear
rotations combine anterior and posterior canals form each ear
vertigo
as a result of one of the vestibular labyrinths can cause disorientation
- produces conflicting and incorect information from damaged ear
better to get rid of ear that’s damaged as signals from one won’t be conflicting
CNS and vestibular activity
- learns to associate specific patterns of vestibular activity for each motor behaviour
- any unsual vestibular responses evokes a protective reflex
main protective reflex from CNS of unusual vestibular responses
throwing up
what is Meniere’s disease ?
effects the receptor cells of vestibular labyrinth
- causes sporadic relapsing vertigo lasting from 10 minutes to 10’s of hours
what is Meniere’s diseas associated with and why ?
- tinnitus
- beacuse it also affects cochlear hair cells
what is the cause of Meniere’s diease ?
- unkown
- oedema (swelling) of endolymphatic saces damaging epethelial cells
- potentially poor drainage of endolymph
where does enolymph normally exit the vestibular labyrinth ?
- endolymphatic duct
- reabsorbed into spinal fluid in endolymphatic sac
where does output output from vestibular labyrithns go ?
input to vestibular nuclei in medulla
where do vestibular nuclei project to ?
- cerebellum and basal ganglia and spinal cord
where is the vestibular complex ?
floor of fourth ventricle
what does the vestibular complex do ?
- 4 major nuclei
- combine information from vestibular system, visual system, spinal cord and cerebellum
what are the 4 vestibular nuclei ?
- medial
- lateral
- superior
- descending
where do vestibular nuclei send outputs to ?
- reticular and spinal cnetres concerned with skeletal movements
- cerebellum and thalamus and oculomotor nuclei
where do superior and medial vestibular nuclei get inout from ?
semi-circular canals
medial vs superior nuclei
- medial is mostly excitatory neurons
- superior mostly inhibitory
what do superior and medial nuclei do ?
help generate gaze reflexes
where does the lateral nucleus get input from ?
- semi-circular canals
- otolith organs
where does lateral nucleus send outputs to ?
lateral vestibulospinal tract
what is the lateral nucleus involved in ?
posture reflexes
where does the descending nucleus get input from ?
otolith organs - utricle and saccule
where does descending nucleus send outputs to ?
spinal cord, cerebellum, reticular formation, contralateral vestibular nuclei
what does the descending nucleus do ?
integrates central motor signals with vestibular signals
