Where are the 3rd Cranial Nerves located?

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.

What is the function of the 3rd Cranial Nerve?

Each one of the two 3rd cranial nerves controls the parasympathetic response of the pupil on the same side (ipsilateral).  The parasympathetic response of the pupil (or "return to normal")  is constriction. 

The 3rd cranial nerve also controls eye muscle movement.  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.

The 3rd cranial nerve also controls the ability to open the eyelid.

What is the significance of monitoring for 3rd Cranial Nerve function?

Loss of 3rd cranial nerve function occurs with compression or injury to nerve.  In injuries or mass lesions located in the cerebral cortex (above the brainstem), a sudden loss of 3rd cranial nerve function can indicate extension of the swelling towards the brainstem.  Direct eye injuries can also produce 3rd cranial nerve symptoms.

What does loss of 3rd Cranial Nerve function cause?

ipsilateral - 

• inability to constrict the pupil in response to light 
• inability to rotate the eye and follow objects through the full visual field
• inability to open the eyelid

What are the findings associated with 3rd Cranial Nerve dysfunction?

 

• 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 the eyes fail to move in the same direction or are "discongegate") 
• 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).