W2: Principles and actions of eye movements

5 main types of eye movement

• Saccades

• Smooth pursuit

• Vergence

• Optokinetic

• Vestibular

Identify and define the 3 main categories of ocular positions

• Primary position: eyes looking straight ahead with the head vertical

• Secondary positions: any vertical or horizontal movement from the primary position

• Tertiary positions: oblique movement of the eyes, a combination vertical and horizontal movement

Ocular movement occurs along the _________.

Equator

Describe Axes of Rotation (Fick’s axes)

Eye movements occur along 3 axes.

• X axis - horizontal

• Y axis - antero-posterior

• Z axis - vertical

The different directions of eye movements occuring within the axes are:

• X axis - vertical eye movement

• Y axis - torsion

• Z axis - horizontal eye movement

Describe Listing’s plane

An imaginary plane passing through the rotational centre of the eye, including the X and Z axes of Fick. Any movement into a tertiary position involves this plane.

Define ductions and identify the actions.

Monocular movements of the eye.

• Adduction

• Abduction

• Supraduction

• Infraduction

• Incycloduction

• Exocycloduction

Define adduction/adduction and identify the type of action.

Ductions

• Adduction - towards the nose

• Abduction - towards the temple

Define supraduction/infraduction and identify the type of action.

Duction

• Supraduction - upwards

• Infraduction - downwards

Define incycloduction/exocycloduction and identify the type of action.

Version

• Incycloduction - rotation inwards (torsion) around anteroposterior axis

• Exocycloduction - rotation outwards (torsion) around anteroposterior axis

Define versions and identify the actions.

Binocular movements where the eyes are moving in the same direction - CONJUGATE

• Dextroversion

• Laevoversion

• Supraversion

• Infraversion

• Dextrocycloversion

• Laevocycloversion

Define dextroversion/laevoversion and identify the type of action.

Version

• Dextroversion - both eyes to the right

• Laevoversion - both eyes to the left

Define supraversion/infraversion and identify the type of action.

Version

• Supraversion - both eyes upwards

• Infraversion - both eyes downwards

Define dextrocycloversion/laevocycloversion and identify the type of action.

Version

• Dextrocycloversion - torsional rotation to right

• Laevocycloversion - torsional rotation to left

Define vergences and identify the actions.

Binocular movements where the eyes are moving in the opposite direction - DISGUGATE

• Convergence

• Divergence

• Supravergence

• Incyclovergence

• Exocyclovergence

Identify the Cardinal Positions of Gaze

• Primary position

• Elevation

• Depression

• Laevoversion

• Dextroversion

• Dextroelevation

• Laevoelevation

• Dextrodepression

• Laevodepression

Key components to the mechanics of ocular movements include:

• Centre of rotation

• Tangential point

• Arc of contact

• Pulleys

• Muscle plane

Describe the centre of rotation. Where is it located?

The eye is a sphere in a socket that rotates freely around this centre of rotation.

• Located 13.5mm behind cornea

Describe the Tangential Point.

It is the first point of contact of the muscle onto the globe, described at the ‘effective‘ insertion of the muscle.

• The position of this point varies when the muscle contracts/relaxes and the globe rotates

Describe the Arc of Contact.

The area between the anatomical insertion and point of first contact (tangential point). The muscle lies in direct contact with the globe.

What happens to the Tangential Point when the muscle contracts?

As muscle contracts, the muscle unrolls from the globe. This causes the Tangential Point to move to Insertion point.

Describe extraocular muscle pulleys and their significance in eye movement.

• They are connective tissue sleeves enveloping the rectus muscles.

• Considered to function as the mechanical origin of the muscle

• Stabilises rotation of the globe during complex eye movements

• Prevents excessive retraction of the globe

Identify the mechanical origin of extraocular muscles.

Muscle pulleys

Muscle plane

Determine by a line drawn between the anatomical insertion and tangential point when the eye is in primary position.

Visual axis

An imaginary line extending from the viewed object through the centre of the pupil to the fovea

Relationship between muscle plane and visual axis

They are positioned 25º to each other

Primary action

Occurs when the muscle plane and visual axis coincide and muscle contracts

Primary action of medial rectus and position of eye

• MP coincides with VA when eye is in primary position

• Primary action is adduction

Primary action of lateral rectus and position of eye

• MP coincides with VA when eye is in primary position

• Primary action is abduction

Primary action of superior rectus and position of eye

• Primary action is elevation

• MP coincides with VA when eye is in 25º abduction

Primary action of inferior rectus and position of eye

• Primary action is depression

• MP coincides with VA when eye is in 25º abduction

Primary action of superior oblique and position of eye

• Primary action is depression

• MP coincides with VA when eye is in 55º adduction

Primary action of inferior oblique and position of eye

• Primary action is elevation

• MP coincides with VA when eye is in 50º abduction

Draw the diagram relating to the fields of action.

Secondary actions

Occur when the visual axes and muscle plane are at 90º to one other

Secondary action of medial/lateral rectus and position of eye

Neither have a secondary action because the muscle planes coincide with the visual axes in primary position.

Secondary action of superior rectus and position of eye

• Secondary action is intorsion and adduction

• Occurs when eye adducts 65º

Secondary action of inferior rectus and position of eye

• Secondary action is extortion and adduction

• Occurs when eye adducts 65º

Secondary action of superior oblique and position of eye

• Secondary action is intorsion and abduction

• Occurs when eye abducts 35º

Secondary action of inferior oblique and position of eye

• Secondary action is extorsion and abduction

• Occurs when eye abducts 49º

Synergists

Two muscles working together to produce the same movement

Antagonists

Two muscles working against one another

• When the agonist relaxes, then by reciprocal innervation, its antagonist relaxes

Yoke muscles

Contralateral synergists - the opposing muscles required to move both eyes into a particular position of gaze

Hering’s Law

Equal and simultaneous innervation flows to yoke muscles (contralateral synergists)

• Whenever the impulse for a particular movement is sent out, corresponding muscles in either eyes recieve equal innervation to contract or relax

Sherrington’s Law

Whenever a change occurs in the innervation to an EOM, a proportionate but opposite change occurs in the antagonist.

• Whenever an agonist receives the impulse to contract, the antagonist receives an equal impulse to relax and lengthen