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06.06.06 (2 years ago)
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The optic nerve is responsible for the afferent limb of the pupillary reflex, or in other words, senses the incoming light. The oculomotor nerve is responsible for the efferent limb of the pupillary reflex; in other words, it drives the muscles that constrict the pupil.
The oculomotor nerve is the third of twelve paired cranial nerves. It controls most of the eye movements (cranial nerves IV and VI also do some), constriction of the pupil, and holding the eyelid open.
The oculomotor nerve arises from the anterior aspect of mesencephalon (midbrain). The muscles it controls are the ciliary muscle (affecting accommodation), and all extraocular muscles except for the superior oblique muscle and the lateral rectus muscle. In addition, it supplies parasympathetic fibres - which originate in the Edinger-Westphal nucleus - to the eye via the ciliary ganglion, and thus controls pupil constriction.
Anatomy
of the oculomotor nerve
On emerging from the brain, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater in front of and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli. It runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal. It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.
The superior ramus, the smaller, passes medialward over the optic nerve, and supplies the Rectus superior and Levator palpebrę superioris. The inferior ramus, the larger, divides into three branches. One passes beneath the optic nerve to the Rectus medialis; another, to the Rectus inferior; the third and longest runs forward between the Recti inferior and lateralis to the Obliquus inferior. From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the Obliquus inferior, which enters the muscle at its posterior border.
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Testing the oculomotor nerve
Cranial nerves III, IV and VI are usually tested together. The examiner typically instructs the patient to hold his head still and follow only with the eyes a finger or penlight that circumscribes a large "H" in front of the patient. By observing the eye movements and eyelids, the examiner is able to obtain more information about the extraocular muscles, the levator palpebrae superioris muscle, and cranial nerves III, IV, and VI.
Since the oculomotor nerve controls most of the eye muscles, it may be easier to detect damage to it. Damage to this nerve, termed oculomotor nerve palsy is known by the down n' out symptoms. The affected eye will be looking downward, because the superior oblique (innervated by CN IV), is unantagonized by the paralyzed superior rectus and inferior oblique and looking outwards, because the lateral rectus (innervated by CN VI) is unantagonized by the paralyzed medial rectus. There will also be strabismus.
They may show a degree of ptosis, or drooping of the eyelid, because the levator palpebrae superioris muscle (eyelid lifting muscle) is also innervated by the oculomotor nerve.
The oculomotor nerve also controls the constriction of the pupils. This can be tested in two main ways. By moving a finger towards a person's face to induce accommodation, as well as them going cross-eyed, their pupils should constrict.
Shining a light into their eyes should also make their pupils constrict. Both pupils should constrict at the same time, independent of what eye the light is actually shone on.
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