Vestibular function Flashcards
what is the vestibular system
sensory system essential in the control of posture and balance
what is the vestibular system and where is it found
inner ear - series of fluid filled membraneous tubes (labyrinths) that are embedded in the temporal bone
what are the components of the vestibular apparatus
3 semi-circualr canals - at right angles to each other
utricle
saccule
all contain sensory hair cells
what are the otolith organs
the utricle and saccule collectively
what do the otolith organs detect
Both:
linear accelleration
encode info about the position of the head in space
utricle - back/front head tilt
saccule - vertical movement
what do the semicircular canals (SSC)
detect rotational acceleration
what is the structure of the SSC
filled with endolymph fluid - contain sensory cells in swellings (cristae) at the base (ampulla) of the bony canals
what is contained in the cristae
flexible gelatinous structure called the cupula - stretches across entrire width of the ampulla
what does the cupula respond to
movement of endolymph fluid within the canals
what are found in the base of the cupula
cilia of hair cells that synapse with the sensory neurones of the vestibular nerve
how do the SSC canals detect rotational acceleration
- skull rotated right or left
- ampulla moves as it is embedded in the skull
- BUT endolymph does not move at first (inertia)
- inertia produces drag which bends the cupula and and ciliary in it in the opposite direction
- constant velocity - endolymph catches up and rotates at same speed - cupula and ciliary no longer bending
sudden stop:
- causes endolymph to carry on moving in same direction
- drag of cupula in opposite direction
- creates continuing sense of movement and dizziness
what are the two types of cilia of the hair cells
single large kinocilium
progressively smaller stereocilia
how does movement of the cilia allow the brain to determine its position in time and scape
movement of the cilia in different directions causes depolarisation or hyperpolarisation of the vestibular nerve
all cupula orientated slightly differently so the brain can build up a 3D image of body position form pattern of firing and inhibition
what happens when the cilia distort TOWARDS the kinocilium
DEPOLARISATION
increases discharge of APs in the vestibular nerve
what happens when the cilia distort AWAY from the kinocilium
HYPERPOLARISATION
decreased discharge of APs in the vestibular nerve
where does integration of this sensory information from the cilia and vestibular nerve take place
cerebellum
what are the sensory apparatus of the saccule and utricle known as
maculae
how are macula orientated
utricle - horizontal plane
saccule - vertical plane
what is the structure of the maculae
set of cilia (single kinocilium, series of stereo cilium) protrude into otolith membrane
crystals called otoliths are embedded on other side of the membrane
what detects tilting of the head
macula in the utricle (horizontal plane)
otoliths have greater density than endolymph - more affected by gravity - otoliths moving move membrane - membrane moving moves cilia
how is a backwards tilt detected
moves otoliths TOWARDS kinocilium
depolarisation and increased APs in vestibular nerve
how is a forward tilt detected
moves otoliths AWAY from kinocilium
hyperpolarisation and decreased APs in vestibular nerve
what detects vertical forces (e.g. movement in a lift)
macula in saccule
also provide info on orientation of the head when lying down
where do projections from the vestibular nuclei project
ipsilaterally, bilaterally and contra-laterally to descending motor pathways
also extraocular nuclei (extraocular muscles)
where do vestibular nuclei receive input from
proprioceptors signalling limb and body position
also from neck and eye muscles
how do we get kinaesthesia
kinaesthesia = perception of movement and body position
from vestibular nuclei projecting via the thalamus to the cerebral cortex
what tracts do vestibular system reflexes involve
vestibulocortical and vestibulospinal tracts
what are the three vestibular system reflexes
tonic labyrinthine relfex
dynamic righting relfex
vestibulo-occular reflexs
what is the tonic labyrinthine reflex
keeps axis of head in constant relationship with the body - use info from maculae and neck proprioceptors
what is the dynamic righting reflex
rapid postural adjustments made to stop you falling when you trip - long reflexes involving extension of all limbs
how does info from the vestibular system affect eye movement
afferents from the semi circular canals connect in the vestibular nuclei to afferents travelling to the extraocular nuclei
= influence eye movement
why might you have increased difficulty balancing when closing your eyes
visual system sends descending projections which control posture
how is the balance of people with damaged vestibular apparatus affected
can still maintain balance when eyes open
immediately lost on closing eyes
what are the two vestibulo-ocular reflexes
static reflex
dynamic vestibular nystagmus
what is the static reflex
when you tilt your head - eyes intort/extort to compensate
what is dynamic vestibular nystagmus
series of saccadic eye movements - rotate eye against the direction of rotation of the head so that original direction of gaze is preserved
what is the conventional direction of nystagmus
right rotation = right nystagmus
vice versa
how can nystagmus be used to test vestibular function
post -rotatory nystagmus
caloric stimulation
what is seen during post-rotatory nystagmus
left rotation - during acceleration = left nystagmus
end of rotation - deceleration = right nystagmus
due to endolymph catching up and pushing chapel in opposite direction
what is caloric stimulation nystagmus
test for horizontal SSC
when outer ear washed with warm or cold water - temp gets through thin bone and sets up convection current to move endolymph
warm into R ear = R nystagmus
cold into R ear = L nystagmus
when can nystagmus be seen pathologically
lesions to peripheral or central vestibular pathways
what is kinetosis and what causes it
motion sickeness
caused by maintained stimulation of the vestibular system
if visual and vestibular inputs to cerebellum are in conflict e.g. vestibular indicates rotation but visual does not
what are the symptoms of kinetosis and how are they brought about
nausea, vomiting, deceased BP, dizziness, sweating, pallor
when conflict between visual/vestibular - cerebellum generates “sickness signal” to hypothalamus - brings about ANS changes
what does labyrinthitis cause
acute interference with normal vestibular function = all ANS symptoms and vertigo
what is vertigo
perception of movement in the absence of movement
what is involved in menieres disease
overproduction of endolymph causing increase pressure = vertigo, nausea, nystagmus, tinnitus
what is tinnitus
subjective noise
give an example of a drug that can attack the inner ear
streptomycin
how can chronic vestibular function be compensated for
by the visual system - learning circuits can be set up in the cerebellum
*problems if subjects cannot use visual info
what can lad to nystagmus at rest
lesions of the brainstem
