(From Dr. Glasser’s Lecture, 12 July 2000, by Brian Buschman)
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The eye movements are controlled by LR6SO4R3 and the two muscles that have superior in their name are controlled contralaterally. All other eye muscles are controlled ipsilaterally. Eye movements must be coordinated and must function together. If not it’s called strabismus and we see double (diplopia).
We have four types of conjugate (together) eye movements and two types of vergence movements.
1) Saccadic eye movements involve a very fast scanning of both eyes together to acquire a target. It is controlled by area 8 (the frontal eye fields). You use the right area 8 to saccade to the left and so on. A legion to one of the area 8’s will cause a saccade to the injured side because of the unopposed action of the other side.
2) Smooth pursuit movements are controlled contralaterally by areas 17, 18 and 19 and involve a constant gaze at a specific target. If you are looking at a stationary target this will cause your eyes to have a constant drift and snap so that you do not continually use the same photoreceptors to look at the object. If a photoreceptor is constantly stimulated then it will accommodate out so it takes a small constant drift to be able to look directly at a stationary thing.
3) Vestibular oculomotor reflex is the drift and snap associated with the nystagmus of the vestibular reflex. It goes in on VIII and out on CN’s III, IV and VI.
4) Optokinetic reflex is a reflex drift and snap based on visual input such as when you lock on targets when looking out a train window. It goes in on II and out on III, IV and VI.
1) Convergence are when both eyes adduct to focus on an object moving closer to the eyes.
2) Divergence is dual abduction from an object moving away from the eyes.
We will look at the control associated with the drift of both eyes to a given side such as in any of the conjugate eye movements. It is complicated and you should look at a diagram drawn to explain it. The basic input to control such movements comes in on CN II or from areas 8, 17, 18 or 19 and goes into the superior colliculi. It is then carried in the MFL to the contralateral side down to the PPRF (paramedian pontine reticular formation) where it is joined by input from CN VIII. It sends fibers out to the abducent nucleus which gives CN VI and sends the internuclear neuron back up via the MFL to cross and take the signal to the oculomotor nucli to cause contralateral adduction. This is a complicated system that allows for both adduction of one eye and abduction of the other so that both eyes gaze in the same direction. If you stimulate area 8 on the right the resulting saccadic movement will involve abduction of the left eye and adduction of the right eye and hence looking to the left. Simple isn’t it. J
Note that the MFL carries fibers in both direction and crosses in both directions.
The internuclear neuron is one side that is likely to undergo demyelination. It cause a loss of adduction in horizontal gaze. It means that of you gaze to the left the right eye is unable to follow. When you test a patient you will see the right eye unable to make it past the midline and the left eye is able to abduct. There is a bunch of messed up input that makes the left eye want to both stay midline and abduct. What you will see if an eye that has a nystagmus between abduction and the midline. Know that when a patient has this problem they are still able to adduct both eyes with convergence.
Internuclear opthalmoplegia is the name for the above condition and it is usually an early warning sign of MS.
MS is a CNS demyelination disease that comes in attacks that go away and them come back. Each time it comes back there are a few more symptoms. The first symptom is usually blurry vision followed by internuclear opthalmoplegia. Later effects include loss of bladder control and autoimmune problems. The disease is progressive and the prognosis is poor.
This is another demyelination disease but this time of the PNS. It is acute and almost always has a full recovery if the patient is kept alive while recovering. The problems result from demyelination of the phrenic nerve. The patient must be put on a ventilator while the Schwann cells do their thing.
The upward gaze center is near the posterior commisure and works ipsilaterally.
The downward gaze center is controlled by the oculomotor nucli.
Oblique gaze is coordinated through the PPRF and the other gaze centers. Eye movements are purely in the basic directions. Oblique gaze is just a combination of other movements.
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