In the last few posts we've covered the first two cranial nerves, the Olfactory and the Optic nerves (remember our mnemonic: Oh, Oh, Oh, To Touch And Feel Virginia's Gucci Vest, Ah Heaven). Both of these cranial nerves carry sensory information IN to the brain for processing, and don't really control motion. For nerves like the olfactory, you wouldn't expect much motion (can't really move our nose around very well), but for the visual system, eye movements are extremely important, allowing you to focus the eye on the things that are most important in the visual field. Being able to do this accurately and at speed is extremely for complete processing of our visual field.
So today is for the third "Oh", the oculomotor nerve. Move those eyeballs!
When we talk about sensory nerves like the olfactory, we start on the outside and move inward, following the route of the sensory information. But for the oculomotor and other cranial nerves, we're not bringing information in. Instead we're sending information OUT from the brain to the periphery of the body to move various parts. So instead of starting on the outside, we're staring deep in the brain, at the two nulcei where the third cranial nerve is formed.
Side Note: In this case, and in all other cases where we will discuss the cranial nerves, their places of origin are referred to as their nuclei. In this case, nuclei refers to a group of specialized nerve cells which are all located in the same small place in the brain and similar functions related to a single specialized general function. So when I say the "Edinger-Westphal nucleus", I'm not talking about the middle of a cell, I'm talking about a group of cells.
So let's begin in the brainstem.
Zoooooooooming in now...
Here you can see the brainstem lying on its side. This is the base of the brain, and the front (toward what would be the eyes) is pointing right. You can just see the oculomotor nerve poking out underneath near the center.
If we go all the way in, we get something that looks like this:
This is a stained section (it's stained for myelin. This means that all of the axons that have myelin on them will be DARK, while the nuclei and other cell bodies will be light) of the brainstem (the midbrain, actually). The oculomotor nucleus, one of the two nuclei for the oculomotor nerve, is one of the paler patches just off the midline. The other nucleus of the oculomotor nerve, the Edinger-Westphal nucleus, is close by the oculomotor nucleus, but is further in toward the midline.
The two sets of axons join together and head out of the brain just behind the optic nerve, headed for the eye.
Along the way, the nerve splits into two branches, the superior and inferior branches, heading to the top and the bottom of the eye, respectively. In total, the oculomotor nerve controls 4 out of 6 of the main muscles of the eye. The oculomotor nucleus controls the rectus muscles (including the superior, inferior, and medial, but they could as "one"), the inferior oblique muscle, and the levator palpebrae superioris. The Edinger-Westphal nucleus controls the pupillary sphincter and the ciliary msucle (which are usually classed together).
The rectus muscles are muscles located in the orbit around the eye. When they are all at rest, the eye will point straight ahead (ok, now look straight ahead). When the superior rectus muscle contracts, it pulls the eye upward (look up! You are now using your superior rectus). Now look ahead again. To use your medial rectus muscle, try to cross your eyes. You are now using your medial rectus. Its only purpose is to move the eye for adduction (which is when you move something closer to the midline of the body. Moving your pupil closer to the midline of your body, and thus crossing your eyes, is adducting). Relax your eyes and stare ahead one more time. Then cross your eyes, and then focus toward your peripheral vision. Both of these movements will use your inferior rectus, which helps adduction and also controls lateral eye movement.
The next muscle the oculomotor nucleus controls in the inferior oblique muscle. The primary action here is rotating your eyes outward, away from the midline of the body, like looking out toward you peripheral vision. It also has secondary actions of controlling up and down and eye crossing movements.
Finally, the oculomotor nucleus controls the levator palpebrae superioris. Sounds big, don't it? That's your eyelid. So in a way, it is very big indeed. It may seem kind of odd that the muscles for the eyelid and those for moving the eye are from the same source, but in fact it makes a lot of sense. This is why, when you look UP, your eyelid will lift higher along with it, and when you look down, your eyelid will relax. It's pretty useful for getting your eyelid out of the way when you need to see something.
So that's all major eye movements from the oculomotor nucleus. What about the Edinger-Wesphal? This nucleus controls the ciliary ganglion, which is a bundle of neurons located just behind the eye. These neurons then head out to the eye to innervate two eye muscles:
1) The sphincter pupillae. This is a muscle that shrinks the pupil, a very important response to changes in light intensity.
2) The ciliary muscle. This muscle actually changes the shape of the eye lens itself, helping to control focus at various distances.
There's a cool way to test and play with your pupillary muscles. Get a friend and a small flashlight (not TOO bright now, you're about to put that thing near your eye). Keeping both eyes open, shine the flashlight in one. You will be able to watch as BOTH your pupils contract in response to the light. This is the your pupillary reflex, and if your Edinger-Westphal nucleus is damaged, one of the pupils will not respond.
People do get lesions and malfunctions of the oculomotor nerve. Usually this is due to a small stroke. The nerve nuclei are themselves very small, so often anything bigger will take out both nuclei. If it doesn't, however, one eye may have the oculomotor nerve function damage. Not only will the pupillary reflex not work (if the Edinger-Westphal is gone), the eye itself will tend to stare off into space, sort of looking away from the midline. This is because the recti muscles are paralyzed, and while the superior and medial muscles will keep it pointing straight ahead, the other muscles in the area will now reign unopposed and force the eye out toward the side.
And that is cranial nerve three!! Next time, we'll begin on a cranial nerve that actually DOESN"T start with an "O". Just for a change. 🙂