4. Vestibular System Flashcards
key information provided by the vestibular system?
- conscious awareness of our orientation relative to gravity and of our own movement
- automatically activates reflexes to maintain upright posture and keep our eyes focused on objects of interest as our head moves
how does the vestibular system answer questions about:
- orientation in space
- maintaining upright posture/keep eyes focused
rapid estimate of head movement
what types of movements do the vestibular end organs detect?
- linear and angular accelerations and
- gravity
list the 5 perceptive elements of the vestibular end organ:
- 3 semicircular canals - oriented orthogonally
- superior
- posterior
- horizontal
- utricle - linear motion
- saccule - linear motion
how do vestibular hair cells transduce stimulus of head motion –> neural activity
- using specialized structureS: hair cells extend into endolymph
- deflection of hair cells TOWARDS kinocilium –>
- depolarizes the cell –>
- increased impulse frequency in primary afferent (axon in CN8)
which movement of hair cell results in excitation/depolarization?
movement of hair cells TOWARDS kinocilium
is there activity while the hair cells are at rest?
yes! even when there is no deflection of the stereocilia, there is a relatively high level of baseline (resting) activity generated in the afferent neurons
steady state firing:
define
- meaning system is tonically active at rest
• you can make it faster or slower
• firing frequency of things that innervate them (makes things faster or slower)
where and how is linear acceleration and gravity detected?
- utricle and saccule
- tilting your head back –> hair cells move back as much as you tilt –> but otolithic membrane continues to pull due to gravity towards kinocilia (depolarizing membrane)
how to detect ACCELERATION
- *no head tilt; transient
- change in linear motion –> there is an otolithic lag/delay due to inertia, so there is a force deflecting the stereocilium towards the kinocilium
how to detect DECELERATION
*no head tilt; transient
there is an otolithic lag/delay due to inertia, so there is a force deflecting the stereocilium AWAY from the kinocilium
where are rotational movements detected?
by the semicircular canals;
each has an ampulla (expanded region, where you find the receptive elements); each are open into the utricle
• important in understanding how the system works
what occurs when the cupula is displaced?
the cupula is not attached, and therefore doesn’t go automatically w/ the head movement:
- inertial lag of endolymph flow in opposite direction of acceleration
- relative displacement opposite that of rotation
- causes stereocilia to deflect towards the kinocilium –> depolarizing response
where is the kinocilium in relation to ampulla?
kinocilium in the horizontal semicircualr canals are always on the UTRICULAR SIDE of the ampulla
the endolymphatic fluid flow will be in which relation to the movement?
endolymphatic fluid flow will be opposite the movement
when will there be a depolarizing response/excitation during head rotation?
there will be depolarizing on the same side as the direction the head is moving; and hyperpolarizing on the opposite side;
e.g. if moving head to the left, then there will be inc. excitation to the left (same side), and inhibition to the opposite side (opposite)
how does the vestibulo-ocular reflex work?
by enabling counter rotation of the eyes at the same rate as rotation, which starts miliseconds after head rotation starts (almost immediately)
which muscles are contracted during horizontal head rotation to the LEFT?
To rotate the eyes to the RIGHT, requires coordinate contraction of:
- RIGHT lateral rectus muscle (mediated by right CN 6)
- LEFT medial rectus muscle (mediated by CN 3)
which two cranial nerves are involved in the vestibulo-ocular response?
CN 3: oculomotor nerve
CN 6: abducens nerve
abducens nerve:
location, and exit
- is located in the PONS, on the floor of the fourth ventricle, at the level of the facial colliculus
- exits the leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve
medial longitudinal fasciculus:
function
• connects vestibular nuclei w/ all others controlling vision
Where is the vestibular nuclear complex located?
what is it’s function?
- located dorsolaterally in the upper part of the medulla, and lower (caudal) half of the pons
- fxn: organizes the vestibuloocular reflex in response to input from vestibular end organ
circuitry for vestibulo-ocular reflex (VOR):
for leftward turning of head/ rightward turning of eyes
- activity to Vestibular nucleus (medulla/pons)
- synapses across midline to Right Abducens nucleus (2 types of neurons)
- Motor neuron: axons form the abducens nerve; exit the brain, and go to lateral rectus on that side (Right - ipsilateral)
- Internuclear neuron: axon leaves nucleus, ascends in MLF, to innervate medial rectus on Left side, causing them to contract –> pulling it to the Right
describe the vestibulo-ocular reflex (during) sustained rotation:
e.g. if head to right
can cause nystagmus, which can be part of a normal response
- Initial response (VOR) - eyes will go slowly opposite direction of movement; so if head is going right, eyes go left
- But as you continue to rotate, there is a resetting motion that bring the eyes back to the RIGHT
- (RESETTING is in the direction of rotation, and VOR is direction OPPOSITE to movement)
• Then the eyes continue to move back to the LEFT
- (RESETTING is in the direction of rotation, and VOR is direction OPPOSITE to movement)
what is the net effect from the inputs to the vestibular nuclear complex?
(mediated via the lateral vestibulospinal tract)
to rapidly change muscle activity to maintain the upright posture against gravity
which region of the human brain is involved in processing vestibular information?
parieto-insular regions
cc: lesion in vestibular system;
function
creates the imbalance b/w R and L side, and brain perceives this as head rotation (even if there isn’t head rotation) –> so you have the nystagmus, postural instability, and vertigo
