Part 5 - Vestibular Reflexes

Question 1

What 3 reflexes are used to test the vestibular system?

Answer 1

VSR - vestibulospinal reflex (“ears to muscles”)
VCR - vestibulocollic reflex
VOR - vestibulo-ocular reflex

Question 2

Which reflex are cats good at?

Answer 2

VSR

Question 3

What are the 3 tracts of the VSR?

Answer 3

Lateral vestibulospinal tract (otolith + cerebellum)
Medial vestibulospinal tract (SCCs + vestibular nuclei)
Reticulospinal tract (sensor + motor systems)

Question 4

Describe the 4 balance strategies of the VSR?

Answer 4

1. Ankle
   - mildest
   - muscles recruited distal to proximal; head in phase with hips
2. Hip
   - faster than ankle
   - muscles recruited proximal to distal; head out of phase with hips
3. Stepping
   - fall prevention when other strategies fail
   - BOS catches up with COG
4. Sustpensory
   - “skier” posture - forward bend with hip/knee flexion
   - lower COG

Question 5

The VCR acts on neck muscles in response to detection of what?

Answer 5

Otolith Velocity

Question 6

How do we evaluate the VCR?

Answer 6

Cervical Vestibular Evoked Myogenic Potentials (VEMPs)

Question 7

What is the function of the VOR?

Answer 7

“Ears to Eyes”

To stabilize images on the fovea

Question 8

Regarding the VOR, what is Ewald’s 1st Law?

Answer 8

SCC stimulation causes eye rotation with respect to head in the plane of the canal being stimulated

Question 9

The VOR is a 3 neutron reflex. What 3 things do we need for it to work?

Answer 9

1. Good SCCs and otoliths
2. Good vestibular nuclei and brainstem pathways
3. Good ocular motoneurons
   (and of course, the cerebellum)

Question 10

Regarding the VOR, which extra ocular muscles would be stimulated by which nerve if the head were to turn right?

Answer 10

Right SCC stimulated:
CN VI contracts L. lateral rectus
CN III contracts R. medial rectus

Left SCC inhibited:
Relax R. lateral and L. medial recti

Question 11

What are the 2 components (types) of VOR?

Answer 11

Angular:
SCCs; gaze stabilization
Horizontal, Anterior Vertical, Posterior Vertical

Linear:
Otoliths; nearby targets
Translational (binocular fixation) and Tilt

Question 12

For Vertical Angular VOR, which extra ocular eye muscles would be stimulated/inhibited with stimulation of the Anterior SCC?

Answer 12

Stimulated:
Ipsi superior rectus and Contra inferior oblique

Inhibited:
Ipsi inferior rectus and Contra superior oblique