The pararecurrent nerve in dogs, and sometimes people too

October 22, 2020

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:

References

8 Responses to “The pararecurrent nerve in dogs, and sometimes people too”

  1. Mike Taylor Says:

    Dude, we have lost it. Between me and you, we’ve posted three consecutive articles on mammal heads. I think we’re due a period of profound repentance and pennance.

  2. Matt Wedel Says:

    So lemme get this straight: you do two back-to-back posts on the stinkin’ heads of stinkin’ mammals, I do one, suddenly “we” have done three and it’s a problem? I think I’m owed at least one more before you get to raise a stink.

    That minor, petty point-scoring aside, you are of course correct. :-)

  3. Mike Taylor Says:

    At least I had the basic decency to limit my posts to bones. I’m not the one who came along introducing gloop into the equation. Shame on you, sir! Shame!

  4. Matt Wedel Says:

    At least I had the basic decency to limit my post to necks. I’m not the one who came along introducing pre-vertebral ephemera into the equation. I say shame on you sir! Shame, shame, shame.

  5. Mike Taylor Says:

    “Limit the post to necks” … and what, pray, does the recurrent larygyeal nerve connect the larynx to? Shame on you, sir! Shame!

  6. Matt Wedel Says:

    “and what, pray, does the recurrent larygyeal nerve connect the larynx to?”

    To development.

    To evolution.

    To sauropods.

    What’s your dumb old badger skull connect to? Memories of a tub of putrid water in the woodshed? Eh? Eh?

    (Also: really? No love for me slagging off heads as “pre-vertebral ephemera”? You’ve changed, man.)

  7. Mike Taylor Says:

    Yeah, you’ve got me bang to rights, I should have acknowledged the beauty of “pre-vertebral ephemera”, or PVE as I will in future refer to them.

    However, none of that changes the fact that the RLN connects not just to the head, but to gloop in the head. It’s not worthy of you, man. You’re better than that.

  8. Matt Wedel Says:

    Yeah, I got nuthin’.

    Curses! You win this round, Taylor. But I’ll back. Oh yes….


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