SCIENCE 101: Cranial Nerves IV and VI, the Trochlear Nerve and Abducens Nerve

Today we continue ONWARD with the series on the Cranial Nerves. But we're doing something a little different. Because last time we covered the Oculomotor Nerve, which innervates FOUR of the SIX muscles which control the eye, today we're going to round out the eye (heh, round, eye, heh) and do the other TWO. But we're going to have to go out of order. The order of the Cranial Nerves (with convenient mnemonic!) goes like this:

Oh: Olfactory
Oh: Optic
Oh: Oculomotor
To: Trochlear
Touch: Trigeminal
And: Abducens
Feel: Facial
Virginia's: Vestibulocochlear
Gucci: Glossopharyngeal
Vest: Vagus
Ah:Accessory
Heaven: Hypoglossal

We've been through the Oh, Oh, Oh (cranial nerves I, II, and III), but the two cranial nerves that control motor movements of the rest of the eye are NOT IV and V. Instead, they are IV and VI, the Trochlear Nerve and the Abducens. And we're going to cover them together, because each one is a motor nerve that moves exactly one muscle, and each one sends signals out from exactly one nucleus. Nice and relatively simple.

So here we go. And we'll start out seeing where they peek out from the brain:

You can see the trochlear nerve poking out, all thin, just to either side of the top of that rounded trapezoid that is the basal pons, while the abducens comes out of the bottom of the basal pons.

We'll start with the trochlear nerve. Moving from the outside in (this nerve is a motor nerve and will send signals to make a muscle move, so it only makes sense to go from the inside of the brain outward), the trochlear nerve begins at the aptly named trochlear nucleus.

Here we're looking at the back end of the midbrain.


(Source)

Inside the back of the midbrain, you're heading toward the pons, but not quite there yet. Instead at the bottom of the slide is an area called the reticular formation, and the open area on the top, which is the cerebral aqueduct (through which flows cerebrospinal fluid). And it's just below this aqueduct, at the rear of the midbrain, that we will find the trochlear nucleus.


(This is a myelin stain. A myelin stain stains WHITE MATTER, like myelin. So cell bodies and nuclei, like the cranial nerve nuclei, will be lighter because they don't have myelin. You can see the light chunk there that's the trochlear nerve)

Right there. You have one on each side. The fibers from this nucleus will first go backward to get around the periacqueductal gray there, and then arch over and above it, crossing on the way, and head out, above the pons, toward the eyes. Keep in mind that the fibers CROSSED. This means your right eye will get your left trochlear nucleus signals and vice versa.

When it reaches the eye, the trochlear nerve innervates the superior oblique muscle (the muscle, by the way, ends in a little pulley system behind the eye on the inside of the orbit, and pulley in Latin is Trochlea, so that's where the name comes from). The superior oblique muscle pulls the eye inward medially (toward the midline), downward, and can also move it outward (when it is stretched rather than contracted). Cross your eyes, and look DOWN your nose. That's your superior oblique and thus your trochlear nerve.

On the other side, the abducens nerve comes from the word "to abduct". While the superior oblique muscle and the trochlear nerve draw the eye IN toward the median (in other words, toward your nose), the abducens nerve moves the lateral rectus muscle, which abducts. Abduct means to move AWAY from the median. So whenever you side-eye someone, or shift your eyes to see out of your peripheral vision, you are using your abducens on that side to pull the eye outward toward your ear.

The abducens nucleus begins to the tail of where the trochlear nucleus ENDS! While the trochlear nucleus is just ahead of the pons, the abducens nucleus is further back and right above the pons.


(From the outside, the pons is the big stripey lump just behind center on the bottom of the brain)

From here the abducens nerve will head down and out the front at the bottom of the pons. Unlike the trochlear nucleus, the abducens fibers do not cross on the way to the eye, so your right abducens moves your right lateral rectus, and the left your left lacteral rectus.

BUT, though the abducens primarily controls the lateral rectus, it ALSO sends fibers to the oculomotor nucleus (which controls the other rectus muscles and needs the extra info), and THOSE fibers cross on the way. Dang abducens can't make up its mind.

So the abducens controls what happens when you orient your eye toward your ear, right? Well, the abducens, because of where the fibers run in the brain, is actually pretty susceptible to injury. It can occur due to stroke anywhere along the nerve, and is one of the most common problems seen in people with tuberculosis, of all things. You can also getting a cutting off of this nerve in thaimine deficiency, which is a problem associated most often with alcoholism. Finally, because this nerve comes out near the bottom of the brain, it's one of the first to suffer from an increase in intracranial pressure. So a lot of stuff can happen to your abducens, but thankfully none of it is life-threatening.

When the abducens nerve is screwed up, what you get is something called lateral gaze paralysis. For example, if your LEFT abducens was screwy. When you shifted your gaze to the right, you'd be fine. Looking straight ahead, fine. But you wouldn't be able to move your gaze to the LEFT. You'd have to turn your head. Another problem associated with abducens nerve damage is something called lateral gaze palsy, where the eye that is affected is basically stuck in the middle, moving only in a limited way toward the median, and not moving at all toward the ear.

Next time, we shall move to the Trigeminal nucleus, a nerve that has actually been giving Sci some trouble IRL recently (for the record, yup, still hurts). We'll talk about what it does, and possibly, why it's hurting. :) Until next time!

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