21.7. Eye Movements Flashcards
What are the different muscles responsible for eye movement?
- Lateral rectus muscle -> Abduction
- Medial rectus muscle -> Adduction
- Superior rectus muscle -> Elevation + Adduction
- Inferior rectus muscle -> Depression + Adduction
- Superior oblique muscle -> Depression (counter-intuitive) + Abduction + Inward rotation
- Inferior oblique muscle -> Elevation (counter-intuitive) + Abduction + External rotation
What nerves innervate each of the extraocular muscles?
LR6SO4O3 -> This is a mock formula mnemonic for remembering:
- Lateral rectus -> 6th cranial nerve (abducens)
- Superior oblique -> 4th cranial nerve (trochlear nerve)
- Others -> 3rd cranial nerve (oculomotor nerve)
What are the main types of eye movement you need to know about?
To bring the image of a target onto the fovea:
- Saccades
- Vergence
To keep the image on the fovea:
- Smooth pursuit (during target movement)
- Vestibulo-ocular (during head movement)
What are saccadic eye movements (a.k.a. saccades)?
[IMPORTANT]
- They are rapid jerking movements of both eyes, used to change the area of a scene is focussed on the fovea
- Saccades are used when scanning an image for interesting details, to try and find something useful to focus on
Give some experimental evidence for saccadic eye movements.
[EXTRA]
(Yarbus, 1967):
- Presented participants with the painting “The Unexpected Visitor”
- The saccadic eye movements observed were different depending on whether a prompt was given and what this prompt was
- An example of a prompt was to assess the wealth of the figures in the drawing
Clinically, what is opsoclonus myoclonus?
[EXTRA]
A condition characterised by rapid, uncontrolled saccadic movements without intersaccadic pauses, which can lead to difficulty with vision and vertigo.
What is vergence?
[IMPORTANT]
Adducting (convergence) or abducting (divergence) both eyes to allow bilateral foveation of targets that are closer or further away respectively.
What is smooth pursuit?
[IMPORTANT]
When an object moves, the image can be kept still on the fovea by means of a smooth eye movement.
(i.e. moving your eyes to follow a moving object)
What is the vestibulo-ocular reflex?
[IMPORTANT]
It is the reflex that enables you to keep your gaze fixed on an object when you rotate your head.
Is the vestibulo-ocular reflex reliant on vision?
No, it works in the dark.
Which part of the brain is involved in the vestibulo-ocular reflex and how?
[IMPORTANT]
Vestibulocerebellum (flocculonodular lobe):
- Visual feedback is useful for calculating errors in the VOR.
- Although this is too slow to correct these errors in real time, there are connections to the cerbellum, which is responsible for adaptive changes to the VOR.
What is oscillopsia?
[EXTRA?]
- It is a visual disturbance in which objects in the visual field appear to oscillate
- It involved a change in the magnitude of the VOR due to vestibular disease
What is nystagmus? Give an example.
- This is a regular form of eye movement, comprising two alternating components – a slow and fast phase.
- Example : in a train where looking out of the window you look at a tree and follow it as it moves past (slow phase) then make a rapid movement back (fast phase) and fixate on the next object which again moves past you.
What are some different types of saccades?
[EXTRA?]
Describe the neuroanatomy of eye movements that you need to know.
[IMPORTANT]
- The oculomotor (III), trochlear (IV) and abducens (VI) nuclei control the extraocular muscles of eye movement
- The gaze centres are the final common pathway of control over these nuclei:
- Vertical gaze centre -> In the midbrain reticular formation. Controls oculomotor and trochlear nuclei (since VI has no role in vertical movement).
- Horizontal gaze centre -> In the paramedian pontine reticular formation (PPRF). Controls the abducens nuclei (which partly output to contralateral oculomotor nuclei, allowing both eyes to turn the same way at once).
- The gaze centres are influenced by various systems:
- Saccadic system (shown in diagram)
- Superior colliculus -> Is the final ‘gate’ to the gaze centres. Inhibits eye movement, so gaze centres must be disinhibited for saccades to happen.
- Frontal eye fields -> Compute and initiate the saccades by outputting to the superior colliculus. Also involved in smooth pursuit.
- Posterior parietal cortex -> Determines whether the visual target has been achieved and sends corrective signals to the frontal eye field and superior colliculus when the visual target has not come into view.
- Visual inputs
- Saccadic system (shown in diagram)
- The vestibular system (i.e. Vestibulo-ocular reflex) also influences eye movements since the vestibular nuclei (VIII) output to the abducens nuclei, etc.
What is unusual about the outputs from the abducens nucleus? Why is this important?
- It not only outputs to the muscle it innervates (lateral rectus), but also to the contralateral oculomotor nucleus.
- This means that both nuclei are activated at once, so that both eyes turn in the same direction (since the oculomotor nucleus innervates the medial rectus, among others).
Where are each of these nuclei:
- Oculomotor (III)
- Trochlear (IV)
- Abducens (VI)
- Oculomotor (III) and Trochlear (IV) nuclei are in the midbrain
- Abducens (VI) nuclei are in the pons
Draw a schematic of the main pathways involved in eye movement.
What are gaze control centres? Where is each?
- They are the centres that control the oculomotor, trochlear and abducens nuclei
- They receive input from the saccadic system and visual inputs
- There are two gaze control centres:
- Vertical gaze control centre -> In the midbrain
- Horizontal gaze control centre (a.k.a. paramedian pontine reticular formation (PPRF) -> In the pons
- You can remember the locations by remembering where the nuclei are. The oculomotor and trochlear are in the midbrain, so they are near the vertical gaze control centres. The abducens are in the pons, so they are near the horizontal gaze control centre (PPRF).
Describe how saccades are generated.
[IMPORTANT]
- Frontal eye fields compute and initiate the saccades by outputting to the superior colliculus.
- Superior colliculus usually inhibits the gaze control centres, keeping so their stimulation leads to disinhibition of the gaze control centres
- Posterior parietal cortex determines whether the visual target has been achieved and sends corrective signals to the frontal eye field and superior colliculus when the visual target has not come into view.
How does each abducens nucleus output to the contralateral oculomotor nucleus?
Via the medial longitudinal fasciculus.
