Vestibular System

Question 1

Vestibular system fxn

Answer 1

• detects stimuli that reflect rotation of the head (dynamic processes) AND
• head’s orientation w/respect to gravity (dynamic and static processes
• w/visual, proprioceptive and tactile info ==> sense of balance

Question 2

Components of the vestibular system

Answer 2

• sensory end organs @ inner ear
  
  • utricle and saccule
  • semicircular canals
• sensor types:
  
  • two pairs of otolith organs = utricle and saccule
    
    • detect linear acceleration and direction of gravity vector
  • three pairs of semicircular canals
    
    • detect head rotation (angular acceleration)

Question 3

Longest stereocilia =

“Axis of polarity” =

Answer 3

• “kinocilium”
• axis of polarity = the direction of the arrow that bisects the bundle toward the kinocilium

Question 4

Physiology of hair cell fxn

Answer 4

• Bending of hair cell bundle in direction of axis of polarity ==> depolariztion ==> NT release ==> ANF firing increases
• depolarization = via mechanical spring (“tip link”) stretching from tips of stereocilia ==> stretching and opening of K⁺ channels

Question 5

Structure/fxn of the utricle

Answer 5

• horizontal orientation like seat of chair → linear acceleration
• does a side to side tilt or a front to back rocking
• Senses forward acceleration like in a car
• Hairs lined up towards striola
• **roles reversed when lying down

Question 6

Structure/fxn of the saccule

Answer 6

• vertical orientation like back of chair → gravitational sensations
• elevator sensations
• bouncing
• falling sensations
• Hairs lined up away from striola
• **role reversed when lying down

Question 7

Characteristics of the macula

Answer 7

• =sensory epithelium @ succule and utricle
• = hair cells + associated supporting cells
• overlaying hair cells/bundles ==> gelantinous layer
• ==> fibrous structure = otolithic membrane
  
  • embedded otoconia = crystals of calcium carbonate

Question 8

Characteristics/properties of otoconia

Answer 8

• ==> otolithic membrane heavier than structures/fluids surrounding it
• increased weight ==> increased relative displacement when head is tilted (or undergoes linear acceleration)
• ==> shearing motion between otolithic membrane and macula ==> displacement of hair bundles

Question 9

Characteristics of semicircular canals

Answer 9

• 3 canals = anterior, horizontal and posterior
• @ base of each canal, there is a bulbous expansion = ampulla
  
  • houses sensory epithelium (“crista”) that contains hair cells
  • hair bundles extend ==> cupula (gelatinous mass)
  • cupula = bridges width of ampulla & prevents endolymph from passing

Question 10

Mechanism of rotational acceleration sensation

Answer 10

• occurs @ semicircular canals
• cupula is distorted by movements of the endolymphatic fluid.
• head turns in the plane of one of the semicircular canals ==>
• inertia of the endolymph produces a force across the cupula ==> distending it away from the direction of head movement
• ==> displacement of the hair bundles

Question 11

Vestibular motor reflexes

Answer 11

• vestibulo-ocular reflexes (VOR)
• vestibulocollic reflexes (VCR)
• vestibulospinal reflexes (VSR)

Question 12

Characteristics of vestibulo-ocular reflexes (VOR)

Answer 12

• vestibulo-ocular reflexes (VOR) allow the eyes to maintain focus on a point of interest during head movement
• If you’re missing VOR ==> “video-camera vision”
  
  • i.e. like having a bad video in front of you every minute

Question 13

Characteristics of vestibulocollic reflexes

Answer 13

• vestibulocollic reflexes (VCR) help maintain head stability and upright stance of the head

Question 14

Characteristics of vestibulospinal reflexes (VSR)

Answer 14

• vestibulospinal reflexes (VSR) assist balance and provide a framework for coordinated motion of the limbs during ambulation
• No conscious though is used on placement of arms and legs to help you balance, and if you do fall, they move to break the fall

Question 15

What is the three neuron arc?

Answer 15

• VOR
• Shortest reflex in body
• Allows eyes to stay focused when turn head