Of Noses and Nerves

So I have had nosebleeds since college. For no reason at all, I would just start gushing. It was like I was a character in an anime, confronted with a lust-inspiring image (nosebleeds are very weakly connected to high blood pressure (which I don’t have) and probably not at all to sexual arousal). This isn’t usually a problem as I can just pinch my nose closed and after a couple of minutes, the bleeding stops. The bleed occurs in the front of my nose, so it is an anterior nosebleed. If it happened deeper in the nose, it would be a posterior nosebleed, and pinching wouldn’t help. If either type of nosebleed won’t stop, go to see a doctor. In fact, posterior nosebleeds often require some kind of hospital visit, and likely will need admission. These can be very serious, especially in the elderly. A neat little micro tidbit for you, and one that med students need to be aware of is that the iron in blood, if it is freed from dying cells, is like bacterial fertilizer. Iron is one of those nutrients that is rate limiting for bacterial growth. Your body does a good job keeping iron sequestered, and it has been suggested that this was an early evolutionary step to keep iron away from potentially unfriendly bacteria.

I had put up with this situation for long enough and made an appointment to get something done about it. I was pretty sure about what I could expect, some form of cauterization of the suspect tissue.

The human nose is a mess of engineering with easily broken bones, tissues prone to infection, nose hairs and connections to sinuses (more opportunities for infection!) and my personal bane, the tendency to grow capillary beds next to the surface of the nasal epithelium. Dry air? Cracks and bleeds. Allergies? Cracks and bleeds. Blow nose? Bleeds. Scratch nose? Bleeds.

The doctor proceeded to open my nostrils with a reversed set of tweezers (a nasal speculum? Yes, they do exist!), and immediately saw not one, but two capillary beds that were problem areas. What fun! The idea behind putting a stop to chronic nosebleeds is simple. Cauterize those vessels and capillary beds. Do some tissue damage to induce the formation of scar tissue to protect the capillary bed. However, I had no idea what kind of cauterization to expect. Chemical, electrical, heat? I usually research these things to death, but I decided to leave it as a surprise. (Here is an outline of the procedure)

The doctor soaked a small cotton ball with a local anesthetic and left to take a phone call. Shortly thereafter, he came back with a cardboard tube filled with little swabs. Chemical it was. The swabs were tipped with a little blob of black goo which was soon revealed to be silver nitrate. Scott at ratherberunning describes them a looking like fireplace matchsticks with a black head. It’s pretty close to the truth. I didn’t know that silver nitrate could be used as a cauterization agent, so here are the details.

Silver nitrate is an oxidizer, meaning that it loves to tear electrons away from things, thereby reducing itself. This bit of chemistry often confuses people, so this is how I explain it. Oxidizing used to be associated with oxygen, hence the name. Turns out, it isn’t just oxygen that is involved, but the name stuck. Oxidizers want more electrons, and since oxygen is one of the greediest electron thieves out there, it should be no surprise that scientists noticed oxygen doing this first. When oxidizers get an electron, their charge becomes more negative, so think about becoming more negative as being reduced. On the opposite side, reducing agents want to give up electrons and are rather insistent about it. Seriously insistent. They become more positively charged, and are therefore, oxidized.

So oxidizers oxidize things, and by doing so, become reduced. Reducing agents reduce things and become oxidized. You can see how these things go together, and for every oxidation, there must be a reduction. We call these paired reactions “redox reactions.” Red for reduce, ox for oxidize.

“So what,” you ask. “You got your nose fixed.”

Well, I tell you that story to tell you this. As soon as the local anesthesia wore off, I was able to feel a dull ache in my nose, weakly throbbing with my heartbeat. But not just there, but in my left central maxillary incisor, too.

Now that was a little odd, but it reminded me of something I had studied so many years ago… the mighty cranial nerves.

Your head is innervated by a dozen nerves on each side. This includes your optic nerve, auditory nerve and the one of interest right now, cranial nerve V, the trigeminal nerve. Seriously, go check out that link, especially if you are taking an anatomy class or need a refresher.

Trigeminal nerve

Trigeminal Nerve. Image from Florida Center for Instructional Technology. http://etc.usf.edu/clipart/

There it is, in all of its gory (ha ha).

Here is another way of looking at it.

Trigeminal nerve

Trigeminal nerve. Image from Psychiatric Zone

It divides into three main branches, named Opthamolic or V1, Maxillary  (V2) and Mandibular (V3). The V2 branch is the one I’m interested in. I find it useful to visualize what parts of the face are innervated by which nerves, so here is a picture of me with a friend I met in the back yard of the family we stayed with when we were on a research trip to Nairobi, Kenya.

Facial innervation

Roughly where the branches of the trigeminal nerve lead to. Picture credit, Carla Bevins, PhD

Branch V2 supplies both my nerve and the upper half of my teeth with sensation and motor directions. That means that both the left side inside of my nose and my left central incisor both share the same branch. If you go up to the previous two images, you can see that the incisor is actually getting part of its innervation from a nerve that actually descends from the nose! How cool is that?! It isn’t uncommon for people this procedure performed to complain of pain in the incisor on the same side of the face as the cauterization.

Nerves are wonderfully sensitive and excellent at conveying information to the brain at amazing speeds, but sometimes, they accidentally stimulate a neighboring nerve. This is called cross-excitation (warning, serious research article, if you aren’t used to them, concentrate on the abstract, introduction and discussion sections) or cross-depolarization. Basically, even though the different nerves are somewhat isolated from each other, if a nerve fires over and over, it can cause the nerve next to it to fire, too. And I have exactly that going on. My nose was just chemically burned and I have a first degree burn inside my left nostril. It hurts. The neighboring nerve leading to my left central incisor is getting a bit of stimulation, too and it hurts. That is the risk of sharing a ganglia. You can confuse messages and give faulty sensory data.

A good way to think about this is a busy highway with an HOV lane. There is lots of traffic in the regular lanes, and sometimes, people cross into the HOV lane even though they aren’t supposed to. For nerves that run right past each other, the constant depolarization and repolarization of one nerve is probably causing tiny disturbances in ion concentration or neurotransmitter concentration. if these disturbances manage to spread far enough, they can cause a neighbor to fire by accident, and once recruited, it can be difficult to shut down until the nerve doing all the real signalling calms down. Three days later, that seems to be happening a bit.

Life is so much cooler when you can understand something like this rather than just be puzzled.

———

Use in the classroom: This makes a good example in an anatomy and physiology class of cranial innervation, and while students can’t actually dissect their heads to trace their nerves, taking a picture of yourself and mapping the trigeminal branches is a great way to visualize what is going on under your skin. Combining this with visual aids showing the paths of the different cranial nerves and models of the skull can make this a good hands on learning exercise.

In an advanced neuro class, the research article linked to above makes a good discussion piece. I like to have a student or group of students present papers like these to the class. In small classes, especially of graduate students, distributing a paper a couple days before class and having each student describe and explain a part of a figure is a great way to teach students how to read a research article.

After you do this a few times, mix it up. Give the students the paper when they come in the class, give them ten minutes to read it, and have them explain it to you. Make sure everybody is involved. This will be invaluable experience for them when they start to write a thesis or dissertation. Quickly examining an article and figuring out if it will be useful lets you go through the millions of papers out there and find the hundred that you should be aware of and can help you find the dozen you absolutely need to know.

ResearchBlogging.org

Amir R, & Devor M (1996). Chemically mediated cross-excitation in rat dorsal root ganglia. The Journal of neuroscience : the official journal of the Society for Neuroscience, 16 (15), 4733-41 PMID: 8764660

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