Lecture 3 Flashcards
What happens when an image we are looking at moves off the horopter?
Diplopia occurs, and we’ve got to use our motor fusion ability to fixate the image
Sensory vs motor fusion
Sensory fusion allows 2 similar images to be seen as one ‘fused’ together
- Central fusion: at the fovea (on the horopter)
- Peripheral fusion: fusion of images in Panum’s fusional area
Motor fusion allows the eyes to move together to maintain fusion of the 2 similar images.
Motor fusion
If you focus on a pen a partner is holding in front of you and they bring it forward towards the nose, what will happen?
- As the images moves forward off the horopter it will have disparity
- The eyes will make an inward movement to place the image back on the fovea
- Disparity is the trigger and the eye movement occurs to maintain a single fused image
- Once the movement of the eyes is no longer sufficient to fuse the images, diplopia occur
Describe the movements that occur with motor fusion
Motor fusion occurs in response to objects of regard moving forward or backward from fixation and having disparity
The inward and outward movement of the eyes that occurs during motor fusion are called vergence movements.
Convergence = Inward
Divergence = Ouward
What are the purpose of an eye movement?
How may it be described according to?
The purpose of an eye movement is:
- To place an object of fixation on the fovea
- To stabilise the image/ maintain fixation
Eye movements can occur voluntarily, due to reflex, fast or slow
Eye movements may be described according to:
- Whether it involves movements of both eyes or one eye
- And a description of the direction of movement
Terminology of eye movements
One eye only = ductions
Both eyes moving together =
Disjugate: eyes moving in opposite direction
Conjugate movements: eyes moving in the same direction
Dextro (right), Laevo (left), elevation, depression
Adduction (in), abduction (out), supraduction, infraduction
Duction of one eye only
The eye may move along three primary axes.
Z = side to side eye movements
X = up and down movements
Y = rotation of the eye in and out
Intorsion: rotation of the eye along the y axis inwards (nasally)
Extorsion: rotation of the eye along the y axis outwards (temporally)
Ductions of the right eye
Describe the muscles responsible for the movements of the eye
Movement of the eyes is controlled by the surrounding extraocular muscles. The extraocular muscles are skeletal muscle.
There are six muscles around each eye that are responsible for its movement:
2 horizontal muscles and 4 vertical muscles
Describe the innervation of the EOMs
- The cranial nerves responsible for eye movement stem from the cranial nerve nuclei in the brainstem and travel to the EOM in the orbit
III CN- Oculomotor nerve -
- Supplies, SR, MR, IR, IO, ciliary muscles & constrictor pupillae
IV CN - Trochlear nerve
- Supplies SO
VI CN - Abducens nerve
- Supplies LR
What are the roles of EOM in terms of primary position and anatomical position of rest?
- The EOm always hold a certain tone which contributes to aligning the eyes to straight ahead (primary position)
- If the EOM are completely devoid of tone, the eyes will move into the anatomical position of rest. Usually slightly upwards and outwards
- This can be due to a pretective phenomenon (Balls Phenom)
- This also occurs when someone is deceased or asleep
Describe the measurements of the rectus muscles?
What is the annulus of zinn?
All of the rectus muscles have their origin at the annulus of zinn
Describe the actions of the horizontal recti muscles
The horizontal recti are positioned on either side of the globe. It’s main action is to move the eye side to side along the z-axis.
Lateral rectus - temporal
Medial rectus - nasal
Describe the action of the vertical recti muscles
The vertical recti are positioned on top and bottom of the globe. The main action is to move the eye up and down along the x-axis.
Describe the superior oblique muscles
- Has its origin in the posterior orbit on the sphenoid bone, medial and superior to the optic fissure
- The SO is then very unique in that it travels forward in the orbit before passing through a U-shaped piece of cartilage called the trochlea
- It then travels backward to isnert in the superior posterior globe
Describe the inferior oblique
- The IO is the only EOM to have its origin in the anterior orbit
- It arises from the maxillary bone in the floor of the orbit
- It then travels backward to insert in the inferior posterior globe
The actions of the oblique muscles
Because the oblique muscles are inserted posteriorly in the globe, when they contract they wil pull from the back of the eye (e.g. pull the back up or down)
This means that the movement produced will be in the direction opposite to the muscle position.
What are the torsional movement of the oblique muscles
Because of the angle of the oblique muscle postion on the globe, it will cause a torsional movement
SO - Intorts the eye
IO - extorts the eye
What ar ethe primary actions of the recti muscles?
The horizontal and vertical actions are considered the primary actions of each of the rectus muscles.
What are the primary actions of the oblique muscles?
Torsional movement is considered their primary actions,
Vertical actions are considered secondary
Do the rectus muscles have secondary actions?
The vertical muscles have secondary actions because their muscle plane (the angle at which the muscle is straight and able to pull best) does not equal the midline of the eye.
As the SR and IR pull to their origin, it also creates abduction and adduction with the assistance of SO and IO which also causes intorsion and extorsion.
What is the maximal action for EOMs?
The muscle has its maximal action for the primary/vertical action when the muscle plane and the midline of the eye are at the same angle.
The superior and inferior rectus are abducted at 23o.
The inferior oblique is adducted at 51o.
The superior oblique is adducted at 54o.
The maximal action for secondary/horizontal and torsional action
The muscle has its maximal action for the secondary/horizontal/torsional action when the midline of the eye is 90o from the muscle plane.
Superior and Inferior rectus - adducted at 90o
Inferior and superior oblique - abducted at 90o
Describe Sherrington’s Law
Sherrington’s law of reciprocal innervation states that when a muscle is stimulated its antagonist is inhibited
What is versions?
Versions are an example of a conjugate eye movement: where both eyes move together (binocular)
Dextro-version - both eyes are moving into right gaze
Laevo-version - both eyes moving into left gaze
Infra-version - both eyes are moving into depression
Supra-version - both eyes are moving into elevation
What are the different fixation movements?
Saccades:
- Fast fixation movements to attent to an object
- Can be voluntary or involuntary
Pursuits:
- Slow following movements to track a moving object/ maintain fixation
- Elicited voluntarily
- Speed of target movment is matched by eye speed
9 position of gaze
What are yoke muscles?
To move both eyes into each position of gaze, a pair of muscles needs to be activated (one for each eye). These pairs are called yoke muslces.
What is Hering’s Law?
States that when innervation is going to one muscle to allow it to contract, equal innervation will be transmitted to its yoke muscle (contralteral synergist) in order to keep the visual axes aligned.
So, when a muscle is underacting for some reason and additional innervation is sent to that muscle to get it in to position, the same amount of innervation will be sent to the yoke muscle causing it to overact.
