Answer:
Two
3rd cranial nerves (oculomotor nerves) are located at the top of the brainstem
- one to the right and one to the left. Motor messages from the 3rd
cranial nerve nucleus controls eye muscles on the same side, or, ipsilateral
(medial rectus, superior rectus, inferior rectus and inferior oblique).
See
Diagram.
Pupillary
Control
The
3rd cranial nerve is a parasympathetic response. The parasympathetic
(return to normal) is responsible for pupillary constriction.
Eye
Movement
The
3rd cranial nerve controls the muscle that makes the eye move. The
4th cranial nerve controls the muscle that allows the eye to look downward
and inward towards the nose, and the 6th cranial nerve controls the muscle
that makes the eye look towards the temple on the same side. The
ability to move the eye in all other directions is controlled by the 3rd
cranial nerve.
See
diagram of Cranial Nerves responsible for eye movement.
Eyelid
Opening
The
3rd cranial nerve also controls the ability to open the eyelid.
Compression
of the 3rd cranial nerve prevents the transmission of the impulse to the
pupil, eyelid and eye muscles, on the same side as the nerve compressed.
Compression
of the 3rd cranial nerve causes the following problems (on the same side
as the compression):
-
inability
to constrict the pupil in response to light
-
inability
to rotate the eye and follow objects moved through the full visual field
-
inability
to open the eyelid
Clinical
findings of 3rd cranial nerve compression include:
-
ipsilateral
dilation and loss of reactivity of the pupil (because the 2nd cranial nerve
carries the light to the oculomotor nucleus, pupillary constriction requires
both the 2nd and 3rd cranial nerve to be functioning)
-
ipsilateral
ptosis (drooping of the eyelid - mild ptosis may be evident when the affected
eye appears to have a "thick" lid with less visible "white" than the unaffected
eye)
-
inability
to move the affected eye throughout the full ocular range
-
diplopia
(blurred vision - which occurs when both eyes fail to deviate in the same
direction)
-
inability
to move the eyes upward - diplopia may worsen when attempts are made to
look upward, as only the unaffected eye moves in that direction
-
the affected
eye is "down and out" -
the eyelid is down, and the eye is rotated downward and outward (due to
loss of opposing upward movement).
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