After somewhat of a hiatus, the cranial nerves RETURN! After all, we can’t quit now! Only four more to go. And here’s where we’re at:
Oh: Optic and Optic 2
Virginia’s: Vestibulocochlear and Vestibulocochlear 2
So we’re up to cranial nerve IX, the Glossopharyngeal.
And for function:
Some Say Marry Money But My Brother Says Big Brains Matter Most.
Where the first letter of each word (they are all S, M, or B), corresponds with the functional role, where S is sensory, M is motor, and B is both. So you can see that the glossopharyndeal is going to have both sensory and motor fibers, which is going to make it that much more complicated.
But that’s ok. Repeat after me: Glossopharyngeal. Glossopharyngeal. Doesn’t it just SOUND nice? Like an onomatopoeia of what happens when a sip of delicious Coca-Cola Slurpee slips over the tongue and fizzes a little at the back.
And speaking of Slurpees…well what did you THINK the glossopharyngeal did?
So, luckily for us, the glossopharyngeal nerve is MOSTLY sensory, with only one motor component. When we talk about the glossopharyngeal, “glosso” meaning tongue, and “pharyngeal” meaing throat, well, that’s basically what we’re after, though there’s a bizarre little branch of the nerve which for some reason takes sensory information from the middle ear.
But for right now we’re going to start with sensory. First of all, the glosspharyngeal nerve gets sensory input from the tongue. Not the WHOLE tongue, just the last 1/3 at the back. It also gets sensory information from the tympani part of the inner ear, the middle of the OUTSIDE part of the ear, and the walls of the pharynx. This is information on pain, touch, temperature, your basic sensory stuff. These nerves head into the skull and find…the glossopharyngeal ganglion. Note that this is a GANGLION and not a NUCLEUS. It’s a ganglion because…we’re not in the brain yet. We’re still outside it, inside the skull, and these neurons just happen to have their first synapse before heading on in. From there the sensory information is processed, and actually is processed in the trigeminal nucleus we discussed before. The difference that means this is glossopharyngeal and not trigeminal is that extra synapse in the ganglion while the neurons are still outside of the brain.
The glossopharyngeal nerve takes care of another kind of sensory information as well. This is VISCERAL sensory information, information, in this case, on co2 levels and blood pressure. These fibers get their information from the carotid sinus, which is near the carotid vein running down both sides of your neck. The information here is on things like changes in pH and pressure. They ALSO head to the glossopharyngeal ganglion (which has superior and inferior sections, and which I REALLY wish I could find a picture of), and then those neurons synapse again in an area called the nucleus solitarius. They then connect with areas like the hypothalamus to change breathing rates or blood pressure to help control blood pH (a sign of oxygen concentration) and pressure.
And then there’s the FINAL kind of sensory information. We’d got pH, pressure, pain, touch, temperature…what ARE we missing…TASTE!! This is the taste sensation from the back 1/3 of your tongue (not that part on your ear, that’d be weird). These fibers, like the other glossopharyngeal fibers, hit the glossopharyngeal ganglion for their first synapse on the way in. They then head to the nucleus solitarius (the solitary nucleus, it’s called this because it’s got the solitary tract running through and into it, which gives it the appearance of a little white dot of white matter surrounded by grey matter, all alone and sad). They then go up through the thalamus to the primary sensory cortex, where you can find out that Bertie Bott’s Every Flavor Beans really do come in “booger”.
But remember, the glossopharyngeal tract does both sensory AND motor stuff! Where’s the motor at?
First there’s a visceral motor part, which isn’t under conscious control. This is the motor control of your parotid gland, which is the largest of your salivary glands. Yes, drool. Starting in the salivary nucleus in the brain (did you know there’s a whole part of your brain devoted to drool?), they head out of the skull and toward the glands. They do NOT synapse in the glossopharyngeal ganglion like the sensory components, no time, there’s drool to do.
The other motor part here is the part that you probably think you use most often (though the control of blood pressure and CO2 levels is really pretty constant). There’s a branch of the glossopharyngeal nerve which controls the stylopharyngeus muscle, controlling the elevation of the pharynx. While that sounds like med-speak (and it is), the pharynx is the part of the throat behind the mouth and nasal cavity, and you can’t make sound without it. Of course, this is under voluntary control, so this whole pathway is going to start in your motor cortex at the top of the brain (literally, the top of the brain), and the signals head down through the thalamus (as most signals do), to a nucleus that…is vague…well it’s ambiguus. The nucleus ambiguus. It’s called that because it’s not really easy to make out on a microscope slide, but it’s also just…well it’s not easy to define. I mean, it’s fuzzy and indistinct, kind of difficult to make out and talk about what it does…
Ok, I’ll stop. That was terrible. Nucleus ambiguus. From there the nerve travels out to innervate the pharynx and allow you to read this out loud (were you so inclined).
So, to sum up: Glossophayngeal nerve gets pain, temperature, and touch from the tongue, ear, insides of the throat, etc. It gets TASTE from the back 1/3 of the tongue where it has chemoreceptors. It can sense pressure and pH to control how fast you breathe and how high your blood pressure is. It controls your saliva, and finally it helps you TALK. And yet, this is one of the least remembered cranial nuclei. But that’s a lot for one nerve to cover!
Next time we take on the big one. The big cheese. The VAGUS. Stay prepared!