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Author Topic:   Recurrent Laryngeal Nerve: Part II
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Message 75 of 75 (835369)
06-22-2018 9:04 AM
Reply to: Message 1 by slevesque
08-06-2010 3:41 PM

Hello slevesque and everyone else.
I've read through this thread and the previous one on the RLN. It seems like the answer to the original question in part I was not found, and I've been debating this subject on another forum so have thought a bit about what the reason for the circuitous route might be. Apologies in advance if I waffle a bit.
As you say, the branch from the vagus that we call the recurrent laryngeal nerve supplies the trachea, upper 1/3 of the esophagus, and the lower part of the larynx. (It also sends a few filaments to the cardiac plexus).
There is already a nerve to the upper part of the larynx, also a branch of the vagus. This supplies cricothyroid muscle with efferent fibres and the mucosa of the larynx above the glottis (the gap between the vocal folds) with afferent fibres.
It makes sense for the inferior laryngeal nerve to approach the larynx from below, so that it can slip underneath muscles such as cricopharyngeus, on its way to the rest of the laryngeal muscles and the mucosa below the glottis. That's fine; however, why does it need to come up all the way from the bottom of the trachea, rather than just from say the first tracheal cartilage?
We need to bear in mind that the trachea starts off as a tiny outpouching of the foregut called the respiratory diverticulum. Suppose that the nerve supply to the portion of the larynx formed from the sixth branchial arch, becomes attached before the trachea and esophagus start to elongate. In order for it to maintain its orientation from below, it could be made secure by an attachment to the foregut.
As the trachea and esophagus lengthen, the nerve will also. By looping under the subclavian artery or aorta, a tethering effect is created that maintains the caudal-cranial direction of the nerve so that it enters the larynx from below.
Also bear in mind that the cell bodies for the nerve fibres contained in the RLN are found in one region of the medulla (the nucleus ambiguus). This suggests that they are a functional unit, which explains why they travel in the same nerve.
With most vertebrates, the neck is not so long that this arrangement would seem inappropriate. Only in some animals does it seem 'too long'.
The arrangement is not a complete waste of 'nerve length', as some would describe it: If there were lots of separate branches coming off the vagus for the lower larynx, trachea, esophagus and cardiac plexus, these branches would be longer than the branches that come from the RLN as they now do. They would have to travel further to get to the structure they supply. For an animal with a relatively short neck, the recurrent route may not add a great deal of nerve length overall, when alternative branching arrangements are considered.
So a common design would lead to a few animals having a very long nerve length, but that would not negate the overall logic of using such an arrangement.
That the heart may be part of this functional unit is suggested by the increase in heart rate we get when we breathe in. Vocalizing causes a 'change in the change' in heart rate, if you get my meaning.
I think that is all I had to say. I have made the assumption that an attachment from the vagus to the lower larynx is established early on.
Hope that is of some help.
Edited by Adminnemooseus, : Added blank lines between paragraphs.

This message is a reply to:
 Message 1 by slevesque, posted 08-06-2010 3:41 PM slevesque has not replied

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