Eye Movements
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Introduction
Types of eye movements
Congugate eye movements are those when the eyes are aligned.
fast eye movements have a velocity of 300-700 deg/sec and include saccades and the quick phase of nystagmus.
Saccades are generated in the retiucluar formation.
slow eye movements have a velocity of 20-50 deg/sec and include:
- smooth pursuit
- vestibular response
- vergence
- optokinetic
Control of Eye Movements
There are nine cardinal eye movements, controlled by 6 muscles.
CN III (oculomotor) controls the superior, inferior, and medial rectus and inferior oblique. CN IV (trochlear) controls the superior oblique, while CN VI (abducens) controls the lateral rectus.
Gaze Centres
- lateral gaze centre is in midbrain
- eye fields are in frontal cortex
a right frontal lobe lesion causes deviation to the right
a brain stem stroke causes contralateral paralysis and eye deviation
Images are maintained on the fovea through two mechanisms - the oculomotor system, which controls our eyes, and the head movement system.
The vestibuloocular reflex is controlled by the vestibular system, which sends signals from the vestibular nucleibilaterally along the medial longitudinal fasciculus, through the PPRF (paramedian pontine reticular formation), to the oculomotor, trochlear, and abducens nuclei.
Lateral gaze is controlled by the pontine gaze centre, which sends signals from the PPRF to CN VI and CN III nuclei via the medial longitudinal fasciculus. Its disruption leads to horizontal disorder.
There are also internuclear connections among the PPRF and nuclei of CN III and VI which also involve the MLF. During lateral gaze, signals are sent from CN VI to CN III to control the medial rectus muscle. An MLF lesion will cause internuclear opthalmoplegia, leaving CN III free to work its magic during convergence but not on lateral gaze.
Resources and References
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