Chapter 10 Kinematics: Muscle Actions

Question 1

C10

Make a table of the six extra ocular muscles, listing each muscle’s primary and subsidiary actions from the primary position of gaze.

Answer 1

Muscle - Primary - Subsidiary

medial rectus - adduction -
lateral rectus - abduction -
superior rectus - supraduction- incycloduction,adduction
inferior rectus - infraduction - excycloduction, adduction
superior oblique - incycloduction - infraduction, abduction
inferior oblique - excycloduction - surproduction, abduction

Question 2

C10

List the cardinal direction of gaze for each extra ocular muscle.

Answer 2

medial rectus - nasal
lateral rectus - temporal
superior rectus - superior from temporal
inferior rectus - inferior from temporal
superior oblique - inferior from nasal
inferior oblique - superior from nasal

Question 3

C10

Using the H-pattern of the motility test as a basis, list the muscles from right to left eyes that are tested in each target position.

Answer 3

(looking right) (looking left)

RSR LIO RIO LSR

RLR LMR H RMR LLR

RIR LSO RSO LIR

Question 4

C10

When the left superior oblique acts in the cardinal direction of gaze, its yoke muscle is the _______.

Answer 4

right inferior rectus

*yoke muscles: two muscles, one in each eye, that cause the two eyes to more in the same direction

Question 5

C10

What does ‘cardinal direction of gaze’ mean?

Answer 5

The cardinal direction of gaze of a muscle is the answer to the question, “If Muscle X were not functioning correctly, in which attempted position of gaze would this be most obvious?”

Question 6

C10

Evaluate the following statement: The actions of the inferior oblique from the primary position are to excycloduct, elevate and abduct the eye. Based on its actions of election and abduction, this muscle has a supero-temporal cardinal direction of gaze.

Answer 6

This person is trying to predict cardinal direction of gaze from actions in the primary position of gaze… THIS IS BAD! They do not correlate with each other.

Question 7

C10

Justina has a right lateral rectus palsy. Describe the type of magnitude of strabismus you would observe (or just draw pictures) when she attempts to look monocularly to her right with (a) her right eye covered, and (b) with her loft eye covered. Explain briefly the basis for these observations.

Answer 7

right eye covered:

• the right lateral muscle will not work well
• by Hering’s Law, the yoke muscle receives equal signals so because the right later rectus does not respond well she will have an esotropia
• this is the primary deviation

left eye covered:

• the right eye will need to make an excessive effort to make her palsied right eye look right
• by Hering’s Law, this will make the good left eye turn in a lot under the cover
• she will have a large esotropia
• this is the secondary deviation

Question 8

C10

Draw diagrams to show how the paths of the extra ocular muscles are altered by the fibro-elastic pulleys, and how the fibro-elastic pulleys move with muscle contraction. Using drawings, compare the muscle paths to those predicted by ‘pre-pulley’ models.

Answer 8

-the muscle action plane differs from that predicted by the traditional model where the anatomical origin was the functional origin