Ok, I had a lot of fun writing about the Trigeminal. Making it all about a Bollywood movie makes EVERYTHING better. Unfortunately, Bollywood movies do not apply to everything (but they DO apply to most things).
So now, we move on to Cranial Nerve VII, the Facial nerve. Why, you say, WHERE was Cranial Nerve VI? I covered it with cranial nerve IV, actually, the trochlear and abducens together, since both innervate very similar muscles in the eye.
And so now we move onward to cranial nerve VII, the facial nerve. This is another mixed nerve, one that does BOTH sensory and motor functions. And being the facial nerve, it’s ALL about the FACE.
The question becomes: can I illustrate this ENTIRE post using nothing but pictures of different facial expressions?
(This is an example of an internet meme known as rage comics. They are user made and thus only sometimes funny, but i discovered them via this, which is both funny and horribly, horribly true. Basically, you have a limited number of faces, male or female, and you make comics out of them, and they all look a little like this guy. And I hope whoever invented them (I think it’s Reddit) doesn’t hurt me, cause I love Reddit. If you think these are funny, you can build them yourself, using the memebuilder, which is what I used for all of these).
Most of the stuff that the facial nerve does is motor (you may recall that we get most of the sensory input from the face via the trigeminal). But it does carry some sensory information as well. It carries sensory information from the lobes of your ears. Just the lobes. So the pain you experienced when you got your ear lobes pierced:
Isn’t carried by the same nerves that hurt when you get the cartilage in your upper ear pierced (Sci admits these two felt different to her, but I imagine that was position and size of needle and numerous other things as well as which nerve was carrying the info).
Secondly, the facial nerve received information about TASTE from your TONGUE! This information in important, but actually a very large portion of what makes up our sense of taste is actually made up of our sense of smell. The information about taste runs up the facial nerve, and ends up in the solitary nucleus. So the next time you taste something COMPLETELY DISGUSTING:
Blame your facial nerve.
Finally, the facial nerve receives some sensory input from your ear, via a branch called the chorda tympani. What’s kind of cool is that this is the SAME nerve receiving the taste input from your tongue, it just runs by the ear on the way to the brain.
While the facial nerve does some service as a sensory nerve, it does a lot more as a motor one. Here, we start from the inside of the brain and move out.
(Source. This is by far the BEST image I’ve ever seen to illustrate any of the cranial nerves. Can we have more images like this, please?!?! There are so few of these. Look at it, facial innervation, all the way back to a section of the brain. So lovely.)
The main motor function here is to control the muscles for facial expression We start in the facial motor nucleus, which is a really rather large nucleus up there near the pons (prior to the faical motor nucleus, signals have come down from on high in the motor cortex, where the come down, partially cross at the level of the thalamus, and then synapse at the facial nucleus). In order for the nerves to exit out of the brain, they actually loop up and then flip around the abducens, which is just one of those hazards of evolution and developmental biology. It then exits the brain below and to either side of the pons.
And from there, the motor nerves head for the face. You can see there’s a big branch up there for your forehead.
That’s all your facial nerve moving stuff up there. Facial nerve inputs are ALSO what gets shut down in that area when people are given Botox, which targets the axon terminals of neurons (particularly SNAP and SNARE proteins, but we’re not going to get into that today), and thus prevents the facial nerve from firing, causing widespread forced muscle relaxation and smoothing out the wrinkles until the toxin wears off (a few months).
The facial nerve also does the rest of the face (cheeks, jaw, your ability to twitch your nose…). It ALSO actually controls the lacrimal glands, which aren’t so much of a muscle, but definitely have a strong and obvious point to make when stimulated.
Finally, one of the major things that doctors will look for when they are looking for possible damage to the facial nerve (say, from a stroke or something) is the corneal blink reflex. This is what happens when, as my friend was the other week, you are playing ultimate frisbee and a kamakazi gnat flies square into your eye. Right eye, or left eye, it doesn’t matter, BOTH eyes will blink. And then you’ll be on the sidelines with the stupid dead gnat somewhere neat your lacrimal gland, tearing furiously and finding it hard to open either of your eyes, while you beg the people nearby to GETITOUTGETITOUTGETITOUT.
The other things that can be done to test the facial nerve are things like asking you to wrinkle your forehead, frown, and purse your lips, but these are all things that might be affected by things like…too much Botox.
And thus, the facial nerve. With loads of faces.