tas8831
Well-Known Member
Or at least a curious part of it.
The recurrent laryngeal nerves in humans are branches of of the Vagus nerve(s), which originate in the brain (cranial nerve X), The Vagus exists the skull, split into 2 branches, and provides innervation to almost all thoracic and abdominal organs, including the larynx. However, the branches that supply the larynx* go past it, loop around blood vessels, and head back up toward the head to reach the larynx. These are the right and left recurrent laryngeal nerves (recurrent as they 'go back'), RRLN and LRLN for short.
The RRLN loops around the right subclavian artery, and heads back to the larynx, not a terrible long journey.
The LRLN, however, loops around the aortic arch, down in the upper part of the thorax, about twice as far as the right:
When one understands what happens during embryonic development, this makes perfect sense (I could also mention the Phrenic nerves, which innervate the diaphragm, whose origins are in the upper neck region, but that is another story).
During development, the circulatory system's basic shape is laid down before much of the peripheral nervous system's is (whereas the central nervous system is actually a bit ahead of the circulatory system for a while).
At this general stage of development, the embryo is unidentifiable as human, but it does, despite what Jon Wells claims, look pretty much like any other vertebrate embryo during this stage (and a little later, like any other mammal, etc.). In what will later be our neck region, the primordia of what will later become the diaphragm, the early heart (which is little more that a pair of tubes at this point) and the pharynx (throat) are all in one tight little blob, just in front of what will become the lower part of the brain. As the primordia of the nerves that will ultimately innervate various structures in the neck, thorax and even the abdomen start to grow out from the brainstem, they take, in effect, the path of least résistance, which for the RRLN and LLRN is straight out from the brainstem, within cylinders of tissue known as pharyngeal arches which contain early blood vessels (as well as nerve primordia, and a rod of cartilage which in us will later become bony parts of the facial skeleton for the most part) surrounding the primitive throat (the aortic arches), to the clump of tissue that will later be the larynx.
As development continues, the embryo elongates, the pharynx becomes the throat, in what we now recognize as the neck, and the nerves get, in effect, dragged along as this is happening, In addition, those aortic arches that these nerves have passed between to get to the larynx are changing - some are merging together to form adult blood vessels, some hang around until birth and then seal up, some are just lost.
On the left side, the aortic arch that the LRLN had gone 'underneath' is coopted to form the aortic arch**. On the right, that aortic arch is coopted for something else - this allows the RRLN to 'slip' upwards as development proceeds, whereas the left gets 'stuck' under the aorta.
Now in us and most other mammals, this is a curiosity, but not a huge deal, as nerve impulses typically travel fast enough that it does not cause an issue.
But the giraffe?
Early Giraffe development is basically identical to what happens in us, except for that neck... And thing of the Apatosaurs!
The fact that Giraffes (and presumably Apatosaurs) have nerves that suffer the same fates that could potentially cause major issues should make one paus when it comes to ascribing it all to a Grand Designer. Why not just "program" the sprouting LRLN to go with the nerve of the 4rd aortic arch instead of the 6th? Should be easy peasy for the Creator of the universe! But nope - just use the same plan laid out for fish in everything else....
Imagine an intelligent city planner/engineer making such circuitous routes for important services like sewer or water services, or electric lines.
*There are also a pair of superior laryngeal nerves that go, in essence, 'straight' to the larynx. We - and all tetrapods - having to simply re-use the plans for development that we inherited, have to suffer the idiosyncrasies of these altered plans. Plans altered by evolution.
** Yes, the language seems redundant/confusing, and it is - hopefully this will clear it up a little?
Or maybe not - as you can see, the aortic arches are also referred to as branchial (or pharyngeal) arches...
The recurrent laryngeal nerves in humans are branches of of the Vagus nerve(s), which originate in the brain (cranial nerve X), The Vagus exists the skull, split into 2 branches, and provides innervation to almost all thoracic and abdominal organs, including the larynx. However, the branches that supply the larynx* go past it, loop around blood vessels, and head back up toward the head to reach the larynx. These are the right and left recurrent laryngeal nerves (recurrent as they 'go back'), RRLN and LRLN for short.
The RRLN loops around the right subclavian artery, and heads back to the larynx, not a terrible long journey.
The LRLN, however, loops around the aortic arch, down in the upper part of the thorax, about twice as far as the right:
When one understands what happens during embryonic development, this makes perfect sense (I could also mention the Phrenic nerves, which innervate the diaphragm, whose origins are in the upper neck region, but that is another story).
During development, the circulatory system's basic shape is laid down before much of the peripheral nervous system's is (whereas the central nervous system is actually a bit ahead of the circulatory system for a while).
At this general stage of development, the embryo is unidentifiable as human, but it does, despite what Jon Wells claims, look pretty much like any other vertebrate embryo during this stage (and a little later, like any other mammal, etc.). In what will later be our neck region, the primordia of what will later become the diaphragm, the early heart (which is little more that a pair of tubes at this point) and the pharynx (throat) are all in one tight little blob, just in front of what will become the lower part of the brain. As the primordia of the nerves that will ultimately innervate various structures in the neck, thorax and even the abdomen start to grow out from the brainstem, they take, in effect, the path of least résistance, which for the RRLN and LLRN is straight out from the brainstem, within cylinders of tissue known as pharyngeal arches which contain early blood vessels (as well as nerve primordia, and a rod of cartilage which in us will later become bony parts of the facial skeleton for the most part) surrounding the primitive throat (the aortic arches), to the clump of tissue that will later be the larynx.
As development continues, the embryo elongates, the pharynx becomes the throat, in what we now recognize as the neck, and the nerves get, in effect, dragged along as this is happening, In addition, those aortic arches that these nerves have passed between to get to the larynx are changing - some are merging together to form adult blood vessels, some hang around until birth and then seal up, some are just lost.
On the left side, the aortic arch that the LRLN had gone 'underneath' is coopted to form the aortic arch**. On the right, that aortic arch is coopted for something else - this allows the RRLN to 'slip' upwards as development proceeds, whereas the left gets 'stuck' under the aorta.
Now in us and most other mammals, this is a curiosity, but not a huge deal, as nerve impulses typically travel fast enough that it does not cause an issue.
But the giraffe?
Early Giraffe development is basically identical to what happens in us, except for that neck... And thing of the Apatosaurs!
The fact that Giraffes (and presumably Apatosaurs) have nerves that suffer the same fates that could potentially cause major issues should make one paus when it comes to ascribing it all to a Grand Designer. Why not just "program" the sprouting LRLN to go with the nerve of the 4rd aortic arch instead of the 6th? Should be easy peasy for the Creator of the universe! But nope - just use the same plan laid out for fish in everything else....
Imagine an intelligent city planner/engineer making such circuitous routes for important services like sewer or water services, or electric lines.
*There are also a pair of superior laryngeal nerves that go, in essence, 'straight' to the larynx. We - and all tetrapods - having to simply re-use the plans for development that we inherited, have to suffer the idiosyncrasies of these altered plans. Plans altered by evolution.
** Yes, the language seems redundant/confusing, and it is - hopefully this will clear it up a little?
Or maybe not - as you can see, the aortic arches are also referred to as branchial (or pharyngeal) arches...
Last edited:
