Coordination of Eye Movements and VOR (Week 3--Bisley)

Question 1

Cranial nerve innervation of extraocular muscles

Answer 1

CN III (oculomotor): superior rectus, medial rectus, inferior rectus, inferior oblique; exits brainstem ventrally; innervates extraocular muscles on ipsilateral side

CN IV (trochlear): superior oblique; exits brainstem dorsally; innervates contralateral side

CN VI (abducens): lateral rectus; exits brainstem ventrally; innervates ipsilateral side

Question 2

Complex but well understood circuit that provides 2 inputs to the relevant CN nuclei

Answer 2

Step: sustained activity (tonic discharge) that holds eye in position; from motor neurons in relevant CN nuclei

Pulse: burst that moves the eye; from motor neurons in relevant CN nuclei

Note: when eye held in place, have step firing in abducens; when eye being moved medial to lateral, have pulse firing to contract LR

Question 3

4 classes of eye movements

Answer 3

Saccadic eye movements

Smooth pursuit eye movements

Vergence eye movements

Vestibulo-ocular eye movements

Question 4

Saccadic eye movements

Answer 4

Rapid eye movements that bring objects of interest onto fovea

Occur 2-3 times per second

Can be voluntary, but often automatic

Ballistic (once it starts you can’t stop it)

Elicited by low current stimulation of a number of cortical or subcortical areas that drive eye movements

Question 5

Smooth pursuit eye movements

Answer 5

Movement in single direction to track visual stimulus

Keep moving objects on fovea

May be initiated voluntary but cannot be performed voluntarily without a visual stimulus to track

Need to figure out trajectory, plan saccade to bring stimulus on fovea but prior to saccade, may be initial follwing response where eyes move along correct trajectory but stimulus not on fovea

When large field stimulus begins to move, “ocular following response” initiated and eyes move along correct trajectory as quickly as 60 ms after movement (still takes 100-300 ms for catch-up saccade)

Question 6

Vergence eye movements

Answer 6

Two eyes rotate in opposite directions to keep stimulus on fovea when moving in depth

Can be controlled voluntarily (Magic Eye pictures!)

Go cross eyed when you look at something close to you

Question 7

Vestibulo-ocular eye movements/reflex (VOR)

Answer 7

Keeps stimulus on fovea during head rotations

When head rotates left, eyes rotate right to keep gaze in same direction

(Reminder: copula toward utricle causes increased response) –> rotation of head CCW/left –> cupula toward utricle –> kinocilia closest to utricle in horizontal canals so increased activity in CN VIII –> central process of nerve excites second order neurons in vestibular nuclei –> excitatory neuron to contralateral CN VI to contract lateral rectus on right and excitatory neuron to contract ipsilateral medial rectus on left –> eyes turn right keeping gaze static

Also an inhibitory output from vestibular nucleus to left abducens so reduced activity/relaxed muscles that usually turn eyes left

Also equal but opposite response from right horizontal canal so reduced inhibition on right side (more excitatory info on right side abducens) and reduced excitation on left side (more inhibition on left side abducens)

Question 8

Nystagmus

Answer 8

Repeated movement of eye springing back to center after rotating too far

Defined by direction of fast phase

Question 9

Optokinetic nystagmus (OKN)

Answer 9

When stimulus moving rapidly, eyes track a feature within stimulus and then reset to track new object

Initial eye movement is ocular following response and has short latency tracking movement

Slow phase is smooth pursuit of stimulus

Fast phase is saccade back toward center

Direction of OKN is direction of fast phase

When big stimulation field, OKN is hard to inhibit (ocular following response completely automatic)

Question 10

VOR nystagmus

Answer 10

Happens if head rotates far enough for eyes to reach untenable angle in orbit (when you spin around)

Gaze returns to center to keep image on fovea

Nystagmus in same direction as rotation since slow phase moves in opposite direction of rotation (so fast phase must move in same direction of rotation)–eyes trying to catch up to your head spinning

Question 11

Caloric nystagmus

Answer 11

Uses circuitry for VOR

Rotate head back 60 degrees so horizontal canals are vertical

Irrigate left ear with warm water –> warm water heats endolymph in region closest to outer ear –> density current where warmed endolymph rises –> pushes cupula toward utricle –> hair cells react by inducing activity in left CN VIII –> circuit for VOR is activated and eyes move to the right –> endolymph continues to push against cupula so nystagmus sets in and eyes have to reset to left (then slowly rotate right again), giving nystagmus to the left (same direction as water in ear)

Question 12

How do you remember which way nystagmus goes if using cold vs. warm water in caloric nystagmus?

Answer 12

COWS

Cold water induces nystagmus in opposite direction as irrigated ear

Warm water induces nystagmus in same direction as irrigated ear

Question 13

Pathological nystagmus

Answer 13

Damage anywhere within circuit can disrupt balance (at rest hair cells are active so damage on one side stops balance normally seen at rest) and cause nystagmus

Damage to vestibular end organs causes nystagmus toward intact side (eyes slowly move toward side of deficit)

Damage to brainstem or cerebellum can cause nystagmus

Question 14

Do healthy individuals have nystagmus?

Answer 14

Yes!

Optokinetic, VOR, and caloric nystagmus are all normal