Week 4 - Vestibular Function

Question 1

what do vestibular labyrinths sense?

Answer 1

head motion, acceleration of head, and inertial forces

• each semicircular canal detects one of 3 axes of rotational movement
• an analagous arrangement in otoliths (utricle and saccule) detect 3 axes of translational movement and force of gravity

Question 2

how is the output of the vestibular system proportional to head velocity?

Answer 2

fluid dynamics of canals and neural computation in brainstem integrate acceleration
-so if head accelerates from 0-10 deg/sec, the VS will detect acceleration and integrate it over time to determine head velocity, then send signal to eye muscles to counter rotate eyes with velocity of 10 deg/sec

Question 3

what gives vestibular organs their directional sensitivity?

Answer 3

polarization of hair cells in ampulla (all one direction horizontal), sacculus (two sides face away from each other; oriented vertically), and utricle (face toward each other, oriented horizontaly)
-the striola separates hair cells with opposite polarity in sacculus and utricle

Question 4

how are vestibular hair cells polarized? what are kinocilia?

Answer 4

morphologically
-the kinocilia of a group of hair cells located on the same side are in the same orientation; they are considered the “leaders” that elicit depolarization or hyperpolarization

Question 5

what do cilia of hair cells project into?

Answer 5

gelatinous otolithic membrane

-membrane is covered otoconia (calcium carbonate crystals) to provide weight to membrane

Question 6

how is the displacement of the otolithic membrane and hair cells in relation to the inertial force?

Answer 6

displacement is in the opposite direction of force

Question 7

what are examples of depolarization of hair cells?

Answer 7

sustained head tilt backwards, or transient (no head tilt) forward acceleration

Question 8

what are examples of hyperpolarization of hair cells?

Answer 8

sustained head tilt forward, or transient (no head tilt) deceleration

Question 9

what happens to hair cells when one is upright?

Answer 9

no depolarization or hyperpolarization, but a resting discharge maintained by vestibular nerves

Question 10

what is the response of vestibular hair cells and vestibular nerves?

Answer 10

• if forces bend sterocilia of a hair toward the kinocilia (backward), yields depolarization and increases response of innervating CN8 fiber (excitation via increased impulse frequency)
• if forces bend sterocilia in the direction opposite the kinocilia (forward), yields hyperpolarization and decreases response of CN8 fiber

Question 11

what are the 3 degrees of rotation? what are they used to detect?

Answer 11

x axis (nose): roll
y axis (ear): pitch
z axis (head): yaw

used to detect angular acceleration or deceleration

Question 12

how is the orientation and integration of semicircular canals?

Answer 12

paired canals on either side of the head work together

• bilateral horizontal canals work together b/c lie in same plane (30 degrees)
• anterior canal on right side is on the same plane as posterior canal on left side, and vice versa
• -thus, anterior and posterior canals of opposite sides work as functional planes
• rotation in a particular direction will yield excitation of hair cells in one canal of a pair, and inhibition in the other

Question 13

what is the cupula?

Answer 13

a gelatinous membrane extending from the ampullary crest to the top of the ampulla

• displacced by inertia of fluid in canals due to angular acceleration or deceleration of head
• cilia of hair cells project into cupula, and each cilia is oriented in same direction towards kinocilium
• -binding of cupula puts shear stress on hair cells

Question 14

in what direction does fluid go if one turns left?

Answer 14

fluid (endolymph) moves to the right, and vice versa

Question 15

what happens to the horizontal canal if the head turns towards the right?

Answer 15

excites the right horizontal canal (depolarization of same side) and inhibits left (hyperpolarization)
-reflexes that result are driven by sum of left and right inputs

Question 16

what happens to the eye muscles when someone rotates the head to the left? (VOR)

Answer 16

excites left CN8 and vestibular nuclei, inhibiting right nuclei

• axons from left vestibular nuclei travel through medial longitudinal fasciculus to excite right abducens nucleus (to right lateral rectus) and left oculomotor nucleus (to left medial rectus)
• these paths in combination with simultaneous inhibition of antagonist eye muscles (left lateral rectus, right medial rectus) cause eye to rotate RIGHT (in an equal and opposite direction of rotation)

Question 17

how does spontaneous nystagmus occur?

Answer 17

pathologic condition where there is no head rotation, and no firing from one of the horizontal canals, while the other has regular baseline firing

• the comparison of activity makes it seem as if there is a rotation in the direction of baseline activity
• thus there would be slow eye movements to the left, and quick saccade resets to the right, as if rotating to the right

Question 18

how does information integrate if

• resting state
• rightward rotation
• resting with right canal pathology?

Answer 18

1. equal resting discharge activity, so no rotation or VOR
2. hair cells in right canal increase discharge, left decrease, so eyes rotate leftward and snap to right
3. unopposed resting activity in left is interpreted as leftward rotation, so eyes rotate rightward and snap left
   - happens if someone stands up from the chair and staggers to right (b/c left extensor muscles are excited)