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What are the parts of equilibrium and balance?
- vestibular system
- equilibrium triad
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What is the function of the vestibular system?
detects changes in motion and position of the head (respect to gravity and horizon)
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Where does information come into the vestibular system from?
equilibrium triad
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What happens when the equilibrium triad is not in sync?
problems with equilibrium
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What are the parts of the equilibrium triad?
- vision
- proprioception
- vestibular system
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Which cranial nerve is associated with vision?
CN II: Optic Nerve
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What is the dominant sensation in humans?
vision
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What characteristics of vision must be able to be maintained in order to have normal equilibrium?
constant, coordinated eye gaze
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What determines head position?
plane of vision; adjust head to coordinate gaze
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VOR:
vestibulo-ocular reflex
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What is the VOR?
keeps your eyes pointed in a particular direction despite body movement
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Which column deals with proprioception?
dorsal column
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What is proprioception?
body position sense in context to gravity
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What does proprioception influence?
muscle tone of neck to maintain plane of vision
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Which cranial nerve is associated with vestibular system?
CN VIII: Vestibulochochlear nerve
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Where is the vestibular system found?
inner ear
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What does the inner ear detect?
motion and position changes of the head
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What anatomy makes up the vestibular system?
- bony labyrinth
- membranous labyrinth
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Where is the bony labyrinth located?
lateral portion of petrous portion of temporal bone
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How many parts make up the bony labyrinth?
3
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What are the 3 parts of the bony labyrinth?
- cochlea
- vestibule
- semicircular canals/ducts
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What does the cochlea do?
hears
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What is the vestibule?
small oval chamber composed of utricle and saccule
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What do the utricle and saccule detect?
linear acceleration
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How many semicircular canals are there?
3
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What are the 3 semicircular canals?
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What do the semicircular canals detect?
angular acceleration
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How are the semicircular canals arranged?
set at right angles to occupy 3 planes of space
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What shape do the semicircular canals form?
2/3 of a circle
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Where do the semicircular canals lie?
posterior to vestibule
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What is the diameter of the semicircular canals?
about 1.5mm, except at ends where they are dilated
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What are the dilated ends of the semicircular canals referred to as?
ampulla
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Ampulla:
bulge along semicircular canals that contain hair cells of semicircular canals
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Which way do the ampulla face?
towards the vestibule
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Perilymph:
fluid w/in the bony labyrinth
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Where is the membranous labyrinth?
inside bony labyrinth
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What is the membranous labyrinth?
system of sacs and ducts that communicate w/ and are suspended w/in the bony labyrinth
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What type of fluid in in the membranous labyrinth?
endolymph--characteristics are identical to perilymph
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What are the 3 parts of the membranous labyrinth?
- utricle and saccule
- 3 semicircular ducts
- cochlear duct
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Where are the utricle and saccule found?
vestibule of bony labyrinth
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What system are the utricle and saccule a part of?
vestibular system
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Do the utricle and saccule communicate with each other?
yes
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What system are the semicircular ducts a part of?
vestibular system
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Where are the semicircular ducts found?
in semicircular canals
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What are the enlarged ends of the semicircular ducts referred to as?
ampulla
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What is the sensory area of the ampulla?
crista ampullaris
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Crista ampullaris:
sheet of hair cells of the semicircular canals
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Where is the cochlear duct?
sits in cochlea (part of bony labyrinth)
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What is the cochlear duct responsible for?
hearing sound
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What is the functional area of the semicircular canals?
crista ampullaris
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Cupula:
gel that spans the lumen of the canal w/in the ampulla; cilia project into cupula
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What do the cilia of the cupula make up?
sensitized hair cells (75-100 stereocilia/microvilli)
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Kinocilium:
one tall (large) cilium per each hair cell that detects the direction in which the hair cells bend as a result of head movement
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What is another name for Kinocilium?
true cilium
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What does movement of the head cause?
endolymph to move and bend the hair cells
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What happens when the hair cells bend toward the kinocilium?
- cell membrane stretches, opening K channel (K influx)
- depolarization that results in opening of voltage-gated Ca channels (Ca influx)
- Ca influences exocytosis process
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What happens if the hair cells bend away from kinocilium?
hyperpolarizaiton (inhibits cell)
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What happens if the hair cells bend toward kinocilium?
depolarization (excitatory receptor potential)
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What does depolarization cause?
membrane to release NT
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What does the NT influence?
1st order sensory neurons in equilibrium pathway (CN VIII)
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What type of potential occurs at the 1st order sensory neurons in the equilibrium pathway?
- generated potential (type of graded potential)
- AP occurs w/ enough depol
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What does the crista ampullaris respond to?
angular acceleration and deceleration, but not constant movement
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While rotation excites the hair cells of one canal, it:
inhibits the hair cells of contralateral partner
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All bending of hair cells occurs in:
same direction
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What do the utricle and saccule contain?
hair cells similar to the semicircular canals
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otoliths:
tiny crystals of calcium carbonate that move in the endolymph function to add energy dissipation and amplify the message for movement
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What do the utricle and sccule together detect?
linear acceleration (moving forward/backward)
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How is the utricle positioned?
horizontally
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What does the utircle detect?
- horizontal movement
- lateral bending of head/neck
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How is the saccule positioned?
vertically
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What does the saccule detect?
- vertical movement
- flexion/extension of head/neck
- up and down changes
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There is some overlap b/w the crista amullaris/utricle/saccule--but for the most part:
each is responsible for the functions listed
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What are hair cells?
modified neurons
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Hair cells depolarize to dump:
NT into synaptic cleft, which influences receptor organ at distal end of 1st order neuron (CN VIII)
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Where are the cell bodies of hair cells located?
vestibular ganaglia
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Where does CN VIII go to synapse w/ 2nd order?
pons
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Where is nystagmus seen?
eyes
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Nystagmus:
movement of eyes beginning w/ slow movement to one side then rapid movement in opposite direction
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What are the stages of nystagmus?
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What is the rapid phase called?
saccades
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What is the slow phase called?
slow component
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How are nystagmus named?
according to the rapid phase direction (right or left)
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Nystagmus is malfunctioning vestibular system including problems with:
- inner ear
- CN VIII
- vestibular ganglia
- vestibular nuclei in pons
- cerebellum
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What is caloric nystagmus based on?
temperature in inner ear
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How do you test for caloric nystagmus?
use cold or warm water in syringe
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What does the water cause the endolymph to do?
move w/in semicircular canal more than saccule and more than utricle
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As the endolymph moves in response to water temperature, what do the hair cells do?
move and result in nystagmus
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What does cold water in the R ear do?
- nystagmus of L
- contralateral
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What does warm water in R ear do?
- nystagmus of R
- ipsilateral
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If there is no nystagmus with this test, there is a possible:
neurological issue going on
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Rotational nystagmus:
- as head and body pivot/circle, the eyes attempt to fix on an object in space (slow component)
- as head and body continue to circle, the eyes snap quickly in the direction that the head is circling (saccade)
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The action from rotational nystagmus is similar to what happens when you watch:
telephone poles from a moving train: saccade occurs in direction train is moving
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What happens to eye movements during rotational nystagmus?
repeat throughout duration of circling
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Which way does nystagmus occur in rotational nystagmus?
direction you spin
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