Eye and head movements

Question 1

What are the components that control gaze and stabilise an image on the retina?

Answer 1

Oculomotor system: moves eyes in orbit whilst head is still

Head-movement system: moves eye sockets as a whole whilst head moves

Question 2

Describe the different types of eye movements?

Answer 2

Saccadic: shift fovea rapidly to new visual target

Smooth pusruits: keep image of a moving target on fovea

Vergence: moves eyes in opposite directions

Vesticular ocular: holds image still on retina during brief head movements

Optokinetic: holds image stationery during sustained head rotation or translation

Question 3

Describe the extraocular muscles?

Answer 3

4 rectus muscles: superior, inferior, lateral, medial

2 obliques: inferior, superior

Question 4

Describe the movements that can be performed by the eye?

Answer 4

Elevation/depression

Abduction/adduction

Intorsion/extorsion

Question 5

What determines the actions of the rectus muscles?

Answer 5

Their insertion onto the eyeball

Question 6

Describe the primary actions of the rectus muscles?

Answer 6

SR: elevation

IR: depression

MR: adduction

LR: abduction

Question 7

Describe the insertions of the oblique muscles?

Answer 7

Insert a long way down the back of the eyeball

SO: well behind equator at an oblique angle close to LR (tendon runs through trochlea)

IO: behind equator close to LR

Question 8

Describe the primary actions of the oblique muscles?

Answer 8

SO: torsion (also involved in depression)

IO: torsion (also involved in elevation)

Question 9

How can each of the six eye muscles be tested in isolation?

Answer 9

Question 10

Which lower motor neurons are involved in the neural control of eye movements?

Answer 10

Oculomotor nerve (CN III): SR, IR, MR, IO

Abducens nerve (CN VI): LR

Trochlear nerve (CN IV): SO

Question 11

Describe the neural control of eye movements?

Answer 11

Hierarchical control of eye movement involving:

LMNs (CNs)

Brainstem centres (reticular formation)

Higher cortical areas

Question 12

How does the neural supply of CN IV to the SO muscle differ to the innervation of the other extraocular muscles?

Answer 12

CN IV innervates contralateral SO

Others all have ipsilateral innervation

Question 13

Which structures are responsible for coordinating the movements from one eye with the other?

Answer 13

Medial longitudinal fasiculus

Reticular formation: PPRF (pontine paramedian reticular formation) and MPRF (mesencephalic paramedian reticular formation)

Question 14

What is the MLF?

Answer 14

Medial longitudinal fasiculus

White matter tract that connects the various cranial nerve nuclei

Question 15

What is the function of the pontine paramedian reticular formation?

Answer 15

Horizontal gaze centre

Coordination of MR and LR of each eye (CN III and CN VI)

Question 16

What is the function of the mesencephalic paramedian reticular formation?

Answer 16

Vertical gaze centre

Coordination of SO and SR of each eye (CN IV and CN III)

Question 17

Where is a patient’s lesion if they cannot coordinate horizontal movement of the eyes?

Answer 17

Pons

Question 18

Where is a patient’s lesion if they cannot coordinate vertical movement of the eyes?

Answer 18

Midbrain

Question 19

Describe the neural control of horizontal saccades?

Answer 19

Requires simultaneous excitation of burst neurons and inhibition of omnipause neurons

Message from cortex > excites burst neurons > stimulates ipsilateral abducens > contraction of ipsilateral LR

At same time, activates neuron in MLF > stimulates part of contralateral oculomotor nucleus > contraction of contralateral MR

Communication from PPRF inhibits contralateral abducens nucleus > contralateral LR inhbited

At same time, via MLF, inhibitory signal from contralteral abducens nucleus to ipsilateral oculomotor nucleus > ipsilateral MR inhibited

Question 20

Describe the function of burst neurons in the PPRF?

Answer 20

Fire at high frequency just before movement

Two types: excitatory and inhibitory

Excitatory: provide excitatory connections with ipsilateral abducens

Inhibitory: suppress activity of contralateral abducens

Question 21

Describe the function of omnipause neurons in the MPRF?

Answer 21

Fire continuously during the saccade, excpet when the eyes move (tell eyeballs when to stop moving)

Project to contralteral abducens nucleus

Question 22

Which neurotransmitter do omnipause neurons in the MPRF use?

Answer 22

GABA

Question 23

What drives saccadic eye movements?

Answer 23

Higher cortical processing

Frontal eye fields, superior colliculus and basal ganglia play important roles

Question 24

How is the abducens nucleus connected to the contralteral oculomotor nucleus?

Answer 24

MLF

Question 25

How are we able to maintain focus on a stationary object while moving our heads, without loss of visual focus or dizziness?

Answer 25

Input from vestibular system

Question 26

How does the vestibular system help us to maintain focus on a stationary object when our head is moving?

Answer 26

Three semiciruclar canals provide information about the head in space (fluid movement across hair cells \> neural signal) In addition, otolith organs give information about linear acceleration

Question 27

How does neural signalling within semicicrcular canals alter eye movements?

Answer 27

When fluid goes in one direction through the semicircular canal \> increase in AP firing At the same time, fluid going through the oppsoite ear will cause a decrease in AP firing This signalling goes to the vestibular nucleus in the brainstem \> integrates with eye movement system (CN III, CN VI nuclei)

Question 28

Describe the vestibular-ocular reflex?

Answer 28