Human Errors #3: The Crazy Long Nerve

Here’s how Nathan Lent describes a nerve in Human Errors:

“Nerves are bundles of tiny individually wrapped cables called axons that convey impulses from the brain to the body (and vice versa).”

 

Take the nerve with the initials RLN (Recurrent Laryngeal Nerve), which connects the brain to the larynx (voice box). Given how close the brain is to the voice box in your neck, you’d think the RLN would be a few centimetres long. But no. Instead, it winds its way from the brain to the upper chest, loops under the aorta (heart area) and reaches the voice box!

“The RLN is more than three times longer than it has to be. It winds through muscles and tissue that it need not.”

 

There is no functional benefit, so why does the RLN “travel this long, lonely road”? For the answer, we need to start with the ancient fish, which is where this nerve originated. From where it continued in all vertebrates. In those ancient fish, the RLN connected the brain to the ancestor of the voice box – the gills. The route via the heart “makes sense in a fish anatomy”, with no lungs or neck.

 

When fish evolved into tetrapods (which would give rise to humans), the heart began to move back and the neck started to grow.

“But the gills did not (move in relative terms from the brain).”

Remember how it was the gills that evolved into the voice box? So while the RLN shouldn’t have been affected by the heart’s movement, because it had initially gone via the heart route, the RLN was too intertwined with the heart route. And so the RLN just kept stretching to continue via the same route!

“There was no easy way for evolution to reprogram the embryonic development of this nerve so as to untangle it from the aorta.”

 

And so it has to come to be that the RLN in the long necked ostrich is a meter long; and in a giraffe, it can be up to 5 meters long! If that seemed long, check out how long the RLN must have been in the apatosaurus, brachiosaurus, and other sauropods. 


 Which is why Lent says:

“Maybe we shouldn’t scorn our own relatively puny RLN after all.”

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