Sauropod Vertebra Picture of the Week

Amazing diagram of the path of the vagus nerve and its branches in the neck, thorax, and abdomen, from Wilson-Pauwels et al. (1988). The gonads aren’t drawn here, but they do receive vagal innervation.

This isn’t new to science, it’s just one of the cool little quirks of human and comparative anatomy that more people should be aware of.

Quick-quick background: autonomic (unconscious, involuntary) innervation of the body comes in two flavors, sympathetic and parasympathetic. Sympathetic nerves mostly handle the fight-or-flight response, parasympathetics are feed-breed-and-read. You could also think of them as the “oh crap emergency” and “hum-drum housekeeping” branches of the nervous system. Sympathetic nerves to the whole body are derived from the spinal cord between T1 and L2, and parasympathetics come from certain cranial nerves and from the sacral part of the spinal cord. If a refresher on all of this would be handy, please see this.

The most awesome nerve in the body is cranial nerve X, the vagus nerve. “Vagus” means “wandering” in Latin; it’s the same root from which we get ‘vagabond’ and ‘vagrant’. As the name implies, the nerve gets around–various branches innervate just about all your viscera from the soft palate at the back of your mouth to the first half of your large intestine. And, as stated in the title of the post, your gonads.

That’s actually pretty weird, even for a nerve from your skull that innervates most of your digestive tract. The reason why it’s weird is that your embryonic hindgut (descending colon, sigmoid colon, and rectum) and most of your pelvic viscera and reproductive system get their parasympathetic innervation from the pelvic splanchnic nerves. The gonads are buried in all of that, and they may get some pelvic splanchnic innervation as well, but they also get innervated by the vagus nerves. (Note for fellow arch-pedants: yes, I know there’s evidence that the pelvic splanchnics should technically be considered sympathetic, but I’m not going down that rabbit hole right now.)

Like most of the weird stuff that goes on in our bodies, the reason why the gonads get vagal innervation is developmental. The gonads actually start developing pretty high up in the abdomen, not far below the diaphragm, and their nerve and blood supply are established at that point. High in the abdomen is firmly in vagus nerve territory, so the gonads get vagal innervation. Then later the gonads descend, in both sexes, and they drag their nerves and blood vessels along with them, which is why the gonadal vessels in both sexes come off the aorta near or with what end up being the renal arteries. (Kidneys do the opposite thing, developing down low and then climbing the aorta, swapping arteries as they go, but that’s a story for another day.) In males the testes take the final leap through the abdominal wall to descend into the scrotum, but ovaries descend almost as far, from up by the diaphragm down to the bowl of the pelvis, or at least to its rim. (Most of the time: just as males can have undescended testes, females can have incompletely-descended ovaries; they turn up now and then in the anatomy lab.)

Incidentally, fellas, this is why you feel sick to your stomach when you get kicked in the groin–the nerves to your testicles come out of the same plexus that serves your stomach and most of your intestines, and the pain fibers go back the same way.

For some super-interesting work on determining gonadal innervation using viral tracing, see Gerendai et al. (2005, 2009). I may quote some choice passages down in the comments.

Another great figure from Wilson-Pauwels et al. (1988), this time showing the path of vagal sensory fibers from the periphery to the brain (and again omitting the reproductive tract). Note the nucleus solitarius in the medulla oblongata, where vagal sensory fibers travel first on their way to the brain.

As is often the case in biology, things get stranger still. In females it’s not just the ovaries that are innervated by the vagus nerve, but part of the cervix and vagina as well. This was hypothesized by Komisaruk et al. (1997), based on the fact that some women with complete spinal cord injuries (‘complete’ here meaning ‘spinal cord cut all the way through’) could still experience genital sensation. It was confirmed by Komisaruk et al. (2004), who found that women with complete transection of the spinal cord could achieve orgasm from vaginal stimulation. Their fMRI study showed that the posterior part of the nucleus solitarius in the brainstem–which receives sensory fibers from the vagus nerve–was active in the process. I assume that there is some esoteric bit of embryology that explains how vagus fibers end up in the vagina, probably something to do with the mesonephric ducts. But I don’t know what that is off the top of my head, so I’ll have to go hit the books (again).

If you dig the diagrams I’ve used here, definitely go track down a copy of Wilson-Pauwels et al. (1988). There are newer versions of the same book that have full-color illustrations, but I think the single-color-nerve-on-black-and-white figures from the 1988 version are cleaner and more readable. It’s not just a good book on the cranial nerves, it’s a master class on clear visual presentation of complicated material. Unfortunately it is not cheap; even used paperback copies start around $40 unless you get lucky. UPDATE: However, through the generosity of Dr. Wilson-Pauwels, the illustrations and captions from the 2013 3rd edition are freely available for teaching purposes at this link. Go avail yourself of this phenomenal resource!

Anyway, the moral of the story is that, male or female, you have nerve fibers that exit your skull through the jugular foramen, pass down your neck behind your carotid arteries, follow your esophagus and stomach to the networks of nerves that run your guts, and run as impossibly slender fibers on the surfaces of the blood vessels that go to your testes or ovaries and vagina. You run your ‘nads from your brain, not just by way of the spinal cord but also by nerves that come straight out of your friggin’ head. Have fun with that thought.

References

• Gerendai, I., Banczerowski, P. and Halász, B. 2005. Functional significance of the innervation of the gonads. Endocrine 28(3): 309-318.
• Gerendai, I., Tóth, I.E., Boldogkői, Z. and Halász, B. 2009. Recent findings on the organization of central nervous system structures involved in the innervation of endocrine glands and other organs; observations obtained by the transneuronal viral double-labeling technique. Endocrine 36(2): 179-188.
• Komisaruk, B.R., Gerdes, C.A. and Whipple, B., 1997. Complete’spinal cord injury does not block perceptual responses to genital self-stimulation in women. Archives of Neurology, 54(12): 1513-1520.
• Komisaruk, B.R., Whipple, B., Crawford, A., Grimes, S., Liu, W.C., Kalnin, A. and Mosier, K. 2004. Brain activation during vaginocervical self-stimulation and orgasm in women with complete spinal cord injury: fMRI evidence of mediation by the vagus nerves. Brain Research 1024(1-2): 77-88.
• Wilson-Pauwels, L., Akesson, E.J. and Stewart, P.A. 1988. Cranial Nerves: Anatomy and Clinical Comments. Toronto: BC Decker.