Vestibular System Flashcards
Vestibular system functions
. Detect the position and movement of our head in space
. Coordinate eye movements
. Control our equilibrium
Vestibule function
. Detects linear acceleration
. Saccule and utricle components
Vestibule saccule
. Dilated portion of vestibule
. Macula: in floor, sensory organ made of hair cells and supporting cells
. Detects up and down movements of the head and forward and backward (linear acceleration in AP axis and occipitocaudal/front back axis)
Vestibule utricle
. Dilated portion of vestibule
. Macula in its lateral wall
. Detects side to side movement
How do saccule and utricle work?
Hair cells are embedded w/in gelatinous matrix (otolithic membrane)
. Ca carbonate crystals embedded on matrix
. If you move your head the rocks move causing the otolithic membrane to move
. This causes hair cells to deflect and the nerve to fire
Semicircular duct function
. Detects angular acceleration/head rotation
. Ducts located in bony semicircular canals
Ample of semicircular ducts
. Dilated end of the duct
. Crista ampullaris: sensory organ w/ hair cells
. Kinocilia of hair cells in any given crista are oriented in the same direction
. Kinocilia of hair cells in the horizontal ducts are oriented toward the midline
Semicircular ducts position
. Ant.: vertical plane
. Post.: vertical plane
. Horizontal: tilted 30 degrees post.
. Each duct is positioned at right angles to each other
. Each duct is in same directional plane as one of the ducts on opposite side of the head (they work in pairs)
. Able to detect all head movements and head positions
How do the semicircular ducts work?
. At rest neurons have discharge rate of 90 spikes per second
. When head is turned to left, the endolymph in the L horizontal canal moves and deflects kinocilium towards midline (stereocilia move towards kinocilium) and firing rate inc.
. Endolymph in R horizontal canal deflects the kinocilium away from the midline and dec. the firing rate
. Signals are sent to the cortex and give the cortex an idea of where your head is in space
Vestibular ganglion
. Scarpa’s ganglion
. Contains sensory neurons assoc. w/ vestibular system
. Located near the internal acoustic meatus
. Ganglion cells are bipolar
. Peripheral process synapse on receptor hair cells
. Central axonal processes form the vestibular portion of the vesitbulocochlear n. (CN VIII)
. CN VIII attaches to brainstem at pontomedullary junction in the pontocerebellar angle
Central axonal processes will project ipsilaterally to ____
. Vestibulocerebellum
. Vestibular nuclei in the rostral medulla and caudal pons
Vestibulocerebellum
. Maintenance of equilibrium and the coordination of eye movements w/ head, neck, and body position
. Consists of flocculonodular lobe and fastigial nucleus (archicerebellum)
. Receive direct inputs from ipsilat. Vestibular ganglion
. Some ganglion cells send their central processes into brainstem through CN VIII and enter the cerebellum via the juxtarestiform body located on the med. side of inc. cerebellar peduncle
. Receives inputs from ipsilat. Vestibular nuclei
Vestibular nuclei
. Most primary vestibular afferents terminate in vestibular nuclei in the rostral medulla and caudal pons
. 4 different nuclei
. Located in lat. aspect of floor of 4th ventricle
Lateral vestibular (deiter’s) nucleus
. Contains large multipolar neurons
. Has salt and pepper appearance due to presence of numerous myelinated axons intermingled w/ neuronal cell bodies of nucleus
. Gives rise to lateral vestibulospinal tract
Lateral vestibulospinal tract (LVST)
. Courses ipsilaterally in ventral part of ventral funiculus of spinal cord
. Projects to all levels of spinal cord
. Tract will terminate in ventral horn of spinal cord on neurons that innervate extensor musculature
Functions of LVST
. Exerts powerful excitation on LMNs that innervate axial muscles
. Important tract in maintenance of one’ balance
. Allows for postural changes necessary to compensate for tilts and movements of the body which alter one’s balance
Medial vestibulospinal tract
. Courses bilaterally in descending portion of med. longitudinal fasciculus (MLF) in ant. Funiculus of spinal cord
. Projects to cervical spine only
. Terminates in ventral horn of spinal cord on neurons innervating neck musculature
. Controls head movement and position in relegation to movements of rest of body
. Stabilizes head as other body movements take place
. Gives rise to cerebellar afferents that enter the cerebellum via juxtarestiform body to terminate in vestibulocerebellum
Medial vestibular nucleus
. Gives rise to an important descending tract (med. vestibulospinal tract)
Inf. Vestibular nucleus
. Has salt and pepper appearance from myelinated axons intermingled w/ neuronal cell bodies of nucleus
. Gives rise to cerebellar afferents that enter the cerebellum via juxtarestiform body to terminate in vestibulocerebellum
Superior vestibular nucleus
. Gives rise to cerebellar afferents that enter the cerebellum via juxtarestiform body to terminate in vestibulocerebellum
T/F there are extensive commissural connections btw the vestibular nuclei on either side of the brainstem
T
. Allows for direct comparison of activity on one side of brain w/ that on other side
All vestibular nuclei project to the motor nuclei of the extraocular mm. Via ___
Ascending limb of the medial longitudinal fasciculus (MLF)
. Vestibular nuclei also project to the contralateral paramedian pontine reticular formation (PPRF)
Paramedian pontine reticular formation
. Group of neurons located in RF adjacent to each abducens nucleus in caudal pons
. Center for horizontal gaze
. Receives input from contralat. Vestibular nuclei and sup. Colliculus
. Neurons of this project to the ipsilat. Abducens nucleus and contralat. Oculomotor nucleus via ascending portion of med. longitudinal fasciculus (MLF)
.acts as comparator of levels of activity in vestibular system
. Stimulation will produce a lat. or horizontal gaze of both eyes to the side of the stimulated PPRF
Medial longitudinal fasciculus (MLF)
. Has ascending and descending portions
. Ascending portion: interconnects PPRF and vestibular nuclei w/ motor nuclei of extraocular mm. And pretectum
. Connections important for lat. and vertical gaze
. Descending portion: contains med. vestibulospinal tract among other connections
T/F Vestibular system has connections w/ visceral centers of the brainstem, thalamus, and limbic areas since overstimulation of this system
T
. May cause nausea, vomiting, headaches, dizziness, and vertigo
Saccades
. Rapid movements from 1 point to another
. Brings new objects to interest onto fovea
. Conjugate movements of the eyes
. During this, visual system is suppresses incoming visual inputs bc they are out of focus
. Can be reflex or voluntarily generated
Smooth pursuit
. Slower movements that allow eyes to follow moving targets
. Holds image of moving target on the fovea
. Conjugate movements of the eyes
Vengeance movements
. Movements that adjust for target distance by changing the angle btw the eyes
. Convergent movement for distant to near vision
. Divergent movements fo near to distant vision
. Coupled w/ changed in lens and the size of the pupil
. Disconjugate movements of the eyes
Brainstem gaze centers
. PPRF in pons is center for lat. gaze
. Area of midbrain RF near the rostral end of the MLF and oculomotor nuclear complex is considered to be center for vertical gaze
. Oblique conjugate movements of the eyes appear to be produced when the horizontal and vertical gaze centers work simultaneous
Vestibulo-ocular reflex (VOR)
. Keeps eyes fixed on an object while the head is moving
. Elicits compensatory eye movements in response to movements of head
. Eyes and head move in opposite directions
. Utilizes vestibular inputs to hold images stable on retina during brief or rapid head movements
. Eye movements are slow and gradual
. Can be consciously suppressed
Vestibulo-ocular reflex when head turns left
. Endolymph in horizontal semicircular ducts flows toward kinocilium of hair cells on L side and away from kinocilium on R side
. Hair cells in L horizontal semicircular duct depolarize while those in R hyperpolarize
. Vestibular ganglion cells in L ganglion stimulated and send APs to L vestibular nuclei
. L vestibular nuclei projects to R CN VI nucleus through the PPRF
. CN VI nucleus activates R CN VI and R LR m.
. CN VI nucleus also sends a projection through MLF to L oculomotor nucleus which activities L MR m.
. R LR and L MR mm. Contract, eyes slowly gaze to the right
Neural pathway from light reflex when bright light appears in person’s far right visual field
. Light hits L temporal and R nasal hemiretinae
. Retinal ganglion cells excited, APs sent down optic nn. And info enters L optic tract
. Some fibers exit tract to enter left sup. Colliculus
. L sup. Colliculus projects to the R PPRF
. R PPRF projects to R CN VI nucleus and L oculomotor nucleus
. R LR and L MR mm. Contract
. Eyes slowly gaze to right towards visual stimulus
optokinetic reflex
. Reflex system that allows you to follow or track an object w/ head stationary
. Utilizes visual inputs to hold images stable on the retina
. Useful when tracking objects that leave visual field
. Slow component: movement of eyes in direction of object
. Fast component: when eyes near edge of orbit a quick saccade brings them back to primary position
. Set of alternating slow and fast eye movements (optokinetic nystagmus)
Railroad nystagmus
. Optokinetic nystagmus where. An individual develops these eye movements when focusing on telephone or fence poss that go by the window of moving train
Frontal eye fields
. Generates volitional/voluntary saccade
. Corresponds to area 8 in middle frontal gyrus
. Stimulation initiates a saccade of the eyes
. L FEF projects to L sup. Colliculus
. L sup. Colliculus projects to R PPRF
. R PPRF projects to R CN VI nucleus and L oculomotor nucleus via MLF
. Eyes move to side opposite of FEF stimulation saccadically
Occipital eye field
. Generates volitional smooth pursuit
. Non-specific cortical area of occipital lobe
. Important area for ability to track or smoothly pursue a moving object
. L OEF projects to L pontine nuclei
. L pontine nuclei projects to R flocculus of cerebellum
. Flocculus projects to R vestibular nuclei
. Vestibular nucleus project to L CN VI and R oculomotor nuclei
. Eyes move to same side as stimulation smoothly
T/F you can’t voluntarily move your eyes smoothly from 1 side to another unless you are tracking an object
T
. You can only scan across the room making a series of small jerky saccadic eye movements
Physiologic nystagmus
. Seen after large rotation of the head
. Due to vestibular activation
. Slow component is opposite direction of head turning (VOR)
. Fast component in direction of head turning (cortical-driven saccades)
. If oscillating eye movements persist for some time it is referred to as beating nystagmus
. Also seen as part of optokinetic reflex
Caloric nystagmus
. Small volume of warm or cold water introduced into external auditory canal
. Not done on conscious patients due to potential of these patients to vomit during test
Warm caloric test
. Sets up convection current so that endolymph flows toward ampulla exciting vestibular afferents on that side
. Through VOR eyes slowly deviate away from irrigated ear and fast, saccadic movements snap eyes back
. Nystagmus on same side as irrigation
Cold caloric test
. Set up convection so tha endolymph flows away from ampulla inhibiting vestibular afferents on that side
. Through ipsilat. PPRF eyes slowly deviate toward irrigated ear and fast saccadic movements snap eyes back
. Nystagmus on side opposite irrigation
Vertigo
. Illusion of movement of the body or the environment although no real movement is taking place
. Rotational or unidirectional
. Assoc. w/ nystagmus, nausea, vomiting, sweating, and gait ataxia
. Maybe due to central or peripheral lesions(central=vestibular nuclei, peripheral=inner ear labyrinth or CN VIII)
Peripheral vertigo
. 75% of cases
. Intermittent
. Nystagmus always present and is unidirectional (never vertical)
. Can be seen with auditory dysfunction
Central vertigo
. 25% of cases
. Nystagmus may/may not be present, in any direction
. Due to central lesions
. Assoc. w/ neuro deficits
Dix-Hallipike positional testing
. Help distinguish btw peripheral and central vertigo
. Patient lies back w/ head rotated to one side and extending over edge of bed
. Should be done rapidly
. Causes max. Stimulation of post. Semicircular duct that is down and ant. Duct that is up
.patient reports any vertigo while physician looks for nystagmus
Lesion of ascending MLF
. Referred to as intranuclear opthalmoplegia (INO)
. Actions of abducens and oculomotor nn. During voluntary gaze are uncoupled
. Inability of MR ipsilateral to lesion to adduct on attempted lat. gaze to opposite side
. Nystagmus on contralat. LR on attempted lat. gaze
Lesion of abducens nucleus in pons
. Affects PPRF and MLF too
. Due to PPRF loss, gaze preference away from side of lesion
. Inability to gaze to side of lesion w/ either eye
. Eye ipsilat. To lesion is immobile in horizontal plane
. Movement of contralat. Eye limited to abduction and may be assoc. w/ nystagmus
