Vestibular System

Question 1

what is the vestibular system?

Answer 1

• complex peripheral apparatus that signals position and movment
• detect acceleration and gravity
• series of membranous tubes in the temporal bone
• the precise physical arrangement of tubes and receptors allows for precise detection of any mvmt in any plane

Question 2

where are the targets of vestibular information?

Answer 2

• VPL of the thalamus –> cortex
• cerebellum
• spinal cord via LVST and MVST (vestibulospinal tracts) = postural changes
• CN’s: III, IV, VI = allows object to be maintained on retina
• reticular formation

Question 3

what is the transducer of the vestibular informaiton?

Answer 3

hair cell; mechanoreceptor

• hair cells synapse with primary affarents and release NT onto the primary affarents
• axons located in vestibulocochlear n.
• cell bodies located in vestibular nuclei

Question 4

what are hair cells bathed in?

Answer 4

apical portion: bathed in endolymph (high K+)

basilar portion: bathed in perilymph (low K+)

• sets up the driving force for potassium to move into the cell from the apical portion

Question 5

depolarization vs. hyperpolarization of hair cells

Answer 5

movement toward kinocilium = depolarized

movement away from kinocilium = hyperpolarized

• tip links connect the stereocillia and result in opening or closing the channel

Question 6

how does depolarization of hair cells occur?

Answer 6

• stereocilia move towards kinocilium and open the channel allowing K+ to influx from the apical portion.
• Results in increased depolarization and influx of Ca2+
• Ca2+ causes vesicles to be released and NT to be released, resulting in an AP in the affarent nerve to the brain

Question 7

Peripheral vestibular apparatus

Answer 7

• set of membranous tubes housed in bone that are composed of endolymph and are continuous with the cochlea
• composed of semicircular ducts (detect angular accel)
• otolith organs within utricle and saccule
• utricle (horizontal/linear accel)
• saccule (vertical accel.)

Question 8

how do semicircular ducts detect angular movment?

Answer 8

through the fluid movment within the canals

• fluid lags when head rotation occurs (and pushes in opposite direction)
• fluid mvmt is detected by hair cells in the ampulla via the movement of endolin
• tubes will rotate at the same time as the head, but the endolin will lag behind and results in deflection of stereocilia

Question 9

ampulla

Answer 9

the ampulla contain hair cells and are located at the base of the SCD’s

• fluid is blocked by a crista and cupula
• crista = where hair cell bodies are housed
• cupula = gel substance where stereocilia are embedded.

Question 10

example of what happens when head turns

Answer 10

when head turns to the right, the fluid within the utricle lags and deflect cupula to the left away from utricle. Causes cupula to be pushed left and pulling of kinocilia away from ventricle, thus decreasing activity and decreasing frequency of APs

Question 11

importance of functional pairs of the SCD’s

Answer 11

Ducts are located in right and left ear and respond oppositely to given stimulus

• together they each give a different signal
• CNS measures the difference between these to determine what extend you are rotating and at what speed.
• It increases the sensitivity of the system and allows for detection of minute changes in angular accelerations

Question 12

which SCD is depolarized?

Answer 12

the one in the direction that you are turning your head to.

if turn your head to the left, then fluid moves to the right in the left ear, pushing stereocilia toward kinocilia and thus depolarizing it. the right side will be hyperpolarized and the CNS will measure the difference between the two.

Question 13

Otolith Organs

Answer 13

• sense linear accel/gravity
• utricle = horizontal
• saccule = vertical
• Contain “macculae” (analagous to ampulla) that house hair cells
• covered by otolith organs, called “ear stones”: give the membrane greater mass, cause it to lag behind, allowing accel to be sensed.

Question 14

Otoliths

Answer 14

• “ear stones” gives membrane greater mass
• divided in halves by striola (changed direction of kinoclium)
• when head moves in a vertical or horizontal direction to the left the otoliths would pull on stereocilia embedded in gel to the right.
• macula on each side of the striola will give a different signal which allows for sensitivity of the system

Question 15

maculae

Answer 15

• contain hair cells that are oppositely polarized on each side of the striola
• allows for macula to give different signals

Question 16

Peripheral apparatus

Answer 16

composed of SCD and otolith organs

- functions in detection/transduction of accel and gravity into electrical signals

Question 17

primary affarents

Answer 17

• cell bodies in vestibular ganglion (in temporal bone)
• axons form the vestibular nucleus (CN VIII)
• most synapse in the vestibular nuclei
• some synapse in ipsilateral cerebellum via inferior cerebellar peduncle

Question 18

vestibular nuclei

Answer 18

• four on each side
• located ventrally and laterally to 4th ventricle (in caudal pons, dorsal medulla)
• distribute information and regulate reflexes to counteract accel. in any one direction
• all have baseline activity

Question 19

targets of vestibular nuclei?

Answer 19

1. cerebellum : allows for balance
2. reticular formation
3. VPL –> thalamus –> Cx (consciously aware of mvmt)
4. spinal cord (LVST, MVST) - regulate axial and proximal limb muscles in response to accel.
5. CN III, IV, VI : control extraocc. eye muscles, keep image focused on retina

Question 20

Vestibuloocular Reflex

Answer 20

= VOR

fn: to maintain gaze during head movements
- vestibular nuclei axons ascend in medial longitudinal fasciculus (MLF)
- pathway will act bilaterally
- influences cranial nuclei with motor input to extraoccular eye mm. via synapsing on nuclei of CN III, IV, VI

Question 21

what are three targets of vestibuloocular reflex?

Answer 21

abducent nuclei
trochlear nuclei
occulomotor nuclei

Question 22

nystagmus

Answer 22

= back-and-forth eye movments

• slow in one direction, fast in the other
• slow is controlled by VOR (VOR operates slowly in one direction to maintain gaze)
• fast is controlled by visual motor system
• named for the direction of the fast phase (i.e. eyes tracking slowly to the right = left-beating nystagmus)

Question 23

Vestibulocollic reflex

Answer 23

• mediated through medial vestibulospinal tract
• bilaterally projects into the MLF
• targets LMNs that innervate neck and thoracic limb muscles
• produces extension ipsilateral to depolarized hair cells (counteracts shift of body weight)

Question 24

Vestibulospinal reflex

Answer 24

• Mediated through Lateral Vestibulospinal Tract
• Arises from lateral vestibular nucleus
• Ipsilateral projection
• Targets LMNs supplying ipsilateral axial mm and antigravity mm of limbs
  (i. e. if there is angular accel to left then body weight will shift to that side. VS reflex will activate ipsilateral antigravity muslces without cortical input for postural adjustment)

Question 25

irritative lesion

Answer 25

• less common

- results in increased activity

Question 26

destructive lesion

Answer 26

• more typical
• the normal side APPEARS excessively active
• fast phase of the nystagmus is AWAY from the lesion
• head/body til, leaning and falling TOWARD the lesion.

with right vestibular lesion, results in no activity coming from right side, CNS thinks you are turning to the left side. VOR goes in the opposite direction. Head and body will lean to the right, thinking you are being turned to the left.