Topic 11: Extra Ocular Muscles

Question 1

describe extra ocular muscles

Answer 1

originate from annulus of zinn

the four recti muscles are seen anteriorly - arrangement: spiral of tillaux

the two oblique muscles are seen posteriorly

Question 2

state the 9 directions of gaze

Answer 2

1 primary position: looking straight ahead focused at infinity

4 secondary positions: rotations around either vertical or horizontal axis (left, right, up, down)

4 tertiary positions: around both vertical and horizontal axes (up-left, up-right, down-left, down-right)

NOTE: up=supraversion, down=infraversion, right=dextro, left=levo

Question 3

explain monocular eye movements

Answer 3

aka DUCTIONS: movement of one eye

NOTE: move.. up=supraduction/elevation, down=infraduction/depression, in/nasally=adduction, out/temporally=abduction

rotations of upward cornea NOTE: rotate.. in/nasally=InCycloduction, out/temporally=ExCycloduction

^ intorsion / extorsion

Question 4

state & describe the two types of binocular eye movements

Answer 4

versions: both eyes move together towards same direction. AKA conjugate eye/tracking eye movements
vergence: both eyes move in opposite directions. AKA dis-conjugate eye movements

Question 5

explain versions

Answer 5

4 secondary gaze positions + 2 rotation versions (InCycloversion & Excycloversion)

Question 6

explain vergence

Answer 6

horizontal plane rotations: convergence (towards midline) & divergence (laterally)

anterior-posterior axes: incyclovergence & excyclovergence

Question 7

explain herring’s law

Answer 7

herrings law of equal innervation to yoke muscles
-YM: one muscle from each eye that work together in the same direction (eg, RLR & LMR)

eg. dextroversion: for both eyes to move to the right, RLR & LMR have to contract equally

Question 8

explain sherrington’s law

Answer 8

sherrington’s law of reciprocal innervation to antagonistic muscles
-AM: 2 muscles of the same eye that work in opposite directions. when one muscle is stimulated, its antagonist is inhibited. (eg. RLR & RMR)

eg. dextroversion: for RE to move right, RLR contracts & RMR relaxes

Question 9

summarise the monocular actions of EOM (primary & secondary)

Answer 9

OLAB2nd SIE RADEL

OLAB2nd - ‘all obliques are abductors secondary’

SIE - ‘superiors are intorsioners’ & ‘inferiors are extorsioners’

RADEL - ‘recti are adductors except lateral (abuctor)’

Question 10

state nerve supply

Answer 10

(LR6 SO4)3

CN3=oculomotor, CN4=trochlear, CN6=abducens

Question 11

state blood supply

Answer 11

muscular branches of ophthalmic artery

NOTE: inferior EOM (IO, IR) also supplied by infraorbital artery (ECA). superior EOM (SO, SR) also partly supplied by supraorbital artery (OA < ICA)