Kenyon: Eye Movements Flashcards
The external muscles of the eye are controlled by lower motor neurons in what 3 brainstem nuclei?
3, 4, 6
What do eye movements accomplish?
they move the eye so that a point of interest is focused on the fovea
What’s intortion? What’s extorsion?
medial rotation (top towards nose) lateral rotation (top away from nose)
What actions does the superior oblique have?
depression; intorsion
What actions does the inferior oblique have?
elevation; extorsion
Where does the superior oblique muscle attach? What does contraction of the superior oblique do to the eye?
at the back of the eyeball;
depresses the eye and rotates it
The effect of the superior oblique muscle varies with the direction of the eye. Which ABDUCTION, contraction of the superior oblique does more (blank). With ADDUCTION, contraction of the superior oblique does more (blank)
intorsion; depression
The muscles of the eye are (blank) muscles. They have (blank) in brainstem nuclei and (blank) in brainstem and cortex.
lower motor neurons; upper motor neurons
1 motor neuron in the eye activates (blank) muscle fibers
3
What components does CN 3 have to the eye muscles?
somatic muscles of eyelid
autonomic to pupil
The nerve contains axons to contralateral superior oblique only
CN 4
Motor neurons to ipsilateral lateral rectus only
CN 6
What is unique about CN 4 and its exit from the brainstem?
exits the DORSAL surface of the brainstem, and then crosses over to the contralateral side
What is clinically significant about the CN 4 nuclei?
they are close together; a lesion does not have to be large to affect both of them!
What will paralysis of the superior oblique muscle result in? How will the patient compensate for the loss?
diplopia on looking downward; tilt head toward the weak muscle
Does CN 6 cross over?
no; remains on ipsilateral side
Most common palsy due to the long course of the nerve
loss of CN 6
What will damage to CN 6 result in? How do patients compensate?
loss of lateral rectus; so eye will fail to abduct on lateral gaze and can intort causing double vision; compensate by rotation of the head
Does CN 3 cross over?
no
What happens to the eye when there is damage to CN 3?
eye will go down and out
*due to unopposed action of SO and LR muscles
Neurons in the oculomotor nucleus also innervate the (blank) muscles of the eyelid.
levator
CN 3 also carries axons from (blank) that drive pupillary constriction
Edinger-Westphal nucleus
With damage to CN 3, ptosis occurs. Why? How can you compensate?
loss of levator palpebrae superioris; compensate by contracting frontalis muscle
What is the near reflex triad to test CN 3?
vergence adduction of eyeballs
accomodation of the lens
pupillary constriction
Rapid ballistic* movements that put an image on the fovea
saccades
Slow tracking movements to keep an image on the fovea
smooth pursuit
Align the fovea with targets at different distances (tracking something moving toward or away from you)
Move the eyes in opposite directions (conjugate movements) so that the image is positioned on both foveas.
vergence
Compensate for movements of the head - driven by the vestibular system
Hold images stable during brief head movements
vestibulo-ocular movements
When is the VOR reflex most important?
during acceleration or rapid rotation
Each semicircular canal transmits a tonic signal to the vestibular nerve even when the eyes are still.
A pathological imbalance in the signals from the vestibular system will result in (blank) when the head is still.
nystagmus
Rapid ballistic movements (figure) that fix on a target and continue afterwards - voluntary or reflexive
Shift the fovea rapidly to a target spotted at the periphery
The system figures out where the eye will point and snaps it to that position. This is not tracking.
saccades
Are saccades voluntary or reflexive?
can be either
- voluntary: reading
- involuntary: tomato
The (blank) controlling the eye muscles are located in CN 3, 4, and 6 nuclei. They receive input from (blank) in two gaze centers in the reticular formation that control the saccade.
lower motor neurons;
upper motor neurons
The (blank) of the reticular formation generates all kinds of horizontal movements including saccades
horizontal gaze center
The (blank) of the reticular formation generates all kinds of vertical movements including saccades
vertical gaze center
Why are there two kinds of neurons in the abducens nucleus? What are they?
abducens nucleus must send motor neurons to the lateral rectus on the ipsilateral side, and internuclear neurons across the MLF to the contralateral nucleus of 3 to activate the medial rectus
So, nucleus of 6 sends (blank) to lateral rectus, and (blank) to PPRF and ultimately the medial rectus
lower motor neurons; upper motor neurons
In addition to exciting the contralateral nucleus of 3, what do neurons in the PPRF do?
they drive inhibitory interneurons to the contralateral abducens nucleus to inhibit the opposing rectus muscles
Saccades are ballistic. Where are the targeting calculations done?
upper motor neurons in two higher structures
- superior colliculus
- frontal eye field
T/F: The saccade is to put a specific point in the visual field onto the fovea, not to move the eye a specific amount
True
*monkey experiment
Train a monkey to make a saccade, and then shift the eye to a further location, and the monkey will still look at the intended target
What do upper motor neruons in the frontal eye field and the superior colliculus do?
command saccades
What do upper motor neurons in the occipital and parietal lobes do?
command other movements
What do upper motor neurons in the cortex, frontal eye fields, and superior colliculus signal to?
upper motor neurons in the vertical and horizontal gaze centers
Where do upper motor neurons in the vertical and horizontal gaze centers signal to?
lower motor neurons *CN 3, 4, 6
Smooth pursuit movements are driven through the (blank)
horizontal gaze center
What areas provide sensory information that is essential for initiation and guidance of smooth pursuit movements?
visual areas in the parietal and occipital lobes
Our eyes are moving all around all the time. Therefore, the images sent to the visual centers are constantly changing.
Q: How can we make sense out of that information?
The visual centers also know about the commands moving the eye and use this information to construct a stable image
