The pararecurrent nerve in dogs, and sometimes people too

Altounian et al. (2015: fig 6).

As has been discussed here before, the recurrent laryngeal nerve (RLN) does not only innervate the larynx, but also parts of the esophagus and trachea (see this post, and in particular this comment). You can see that in this cadaver photo, in which the RLN is sending nice big visible branches into both the esophagus and trachea on its way to the larynx. Why is it doing this? Because the embryonic gut tube, which gives rise to both the digestive and respiratory systems, is serially innervated by the nerves of the pharyngeal arches that the gut tube passes through. Parts of the esophagus and trachea pass through the 4th to 6th pharyngeal arches, so they are innervated by the nerve that serves those arches, which is the recurrent laryngeal nerve. As discussed in the post and comment linked above, the recurrent course of the RLN to the esophagus and trachea is just as dumb as its recurrent course to the larynx, and equally strong evidence of a developmental constraint.

Although all tetrapods have an RLN that innervates the larynx, the axons to the esophagus and trachea aren’t always bound up with it. In dogs and many other mammals, those nerve fibers to the esophagus and trachea form a second recurrent nerve, the pararecurrent nerve or recurrent pharyngeal nerve. In this wonderful, complicated figure by Lemere (1932), the recurrent laryngeal nerve is labeled ‘r’, and the pararecurrent nerve is labeled ‘pa’.

Here’s Lemere’s figure with the RLN and pararecurrent nerve highlighted for easier comparison. The pattern of axonal wiring here is the same as in humans–all the axons have the same connections at the brainstem end on one hand, and at the pharynx and larynx end on the other hand–but the bundling of axons into what we recognize as peripheral nerves is different.

Interestingly, Lemere (1932) mentioned that having the recurrent pathways split into two nerves was the most common pattern in dogs, but occasionally he saw a case in which all of the axons had been bundled into a single RLN that served both the larynx and the esophagus and trachea, as in humans.

Modified from Altounian et al. (2015: fig. 4).

That door of variation swings both ways: a few years ago in our lab, we had a cadaver in which the left RLN only went to the larynx, and the vagus fibers to the esophagus and trachea were carried in a second, variant nerve. I didn’t know what that nerve was for a long time, until I stumbled onto the work of Lemere. So it seems that two nerves is the usual pattern for dogs, with one nerve as a rare variant, and the opposite is true in humans. 

Incidentally, I didn’t find the variant nerve in our lab, my students did. We got as far as putting together a manuscript, which we posted as a preprint (here), but we haven’t gotten it formally published yet. One of my goals for this year is to get some of these old, stalled projects dusted off and properly published. Watch this space.

I also discussed the pararecurrent nerve in my “How to make new discoveries in (human) anatomy” talk from SVPCA 2019, which is also a PeerJ preprint (here).

Other posts on the recurrent laryngeal nerve, and on the peripheral nervous system in general:

• The world’s longest cells? Speculations on the nervous systems of sauropods
• Oblivious sauropods being eaten
• Oblivious sauropods being eaten, part 2: Bakker’s snoozing brontosaur
• The longest cell in Andy Farke
• Path of the recurrent laryngeal nerve in frogs, giraffes, and elasmosaurs
• When sensory nerves from the thigh end up in the feet (
• Off topic: your gonads are innervated by your cranial nerves

References

• Altounian, D., Tran, C.M., Tran, C., Spencer, A., Shendrik, A., Kraatz, B.P., and Wedel, M.J. 2015. A variant nerve that mimics the left recurrent laryngeal nerve: a case study in human anatomy. PeerJ PrePrints 3:e781v1. DOI: 10.7287/peerj.preprints.781v1
• Lemere, F., 1932. Innervation of the larynx. I. Innervation of laryngeal muscles. American Journal of Anatomy 51(2): 417-437.