Submitted by Nirvighnam_noor t3_yrepih in askscience
Why did abducens(cn 6) nerve evolve to control just the lateral rectus and trochlear nerve(cn 4) controls the superior oblique when occulomotor nerve(cn 3) controls all other muscles of eye movement
Short answer: we don't know.
Longer answer: the layout of the cranial nerves only makes any kind of sense once you realize all vertebrates are just modified fish. It seems like the cranial nerves used to just be like the spinal nerves, with a dorsal and ventral branch, with the ventral branch mostly innervating the gill arches. This is still the case, with 5, 7, 9, and 10 innervating structures derived from the first, second, third, and posterior gill arches, and their dorsal partners spreading elsewhere.
The problem is that we know the locations of the nerves and their targets can and do shift over evolutionary time, and many of the species which could help us resolve which is connected to what and how and why they've shifted are either highly modified due to diverging from us 500+ million years ago (e.g. lampreys, hagfish) or just plain extinct and rarely fossilized with enough skull detail to help (e.g. ostracoderms, conodonts, acanthodians).
There's an exceptionally detailed look here: https://academic.oup.com/book/37442/chapter/331583157
https://acrobat.adobe.com/link/track?uri=urn:aaid:scds:US:b4e8a211-ea54-3cca-b7db-bea38df5c65b Read last paragraph on page 4 on left side you will see that we can deduce similarities based on the fact that abducens nerve controls the lateral rectus muscle and as usually in most organisms eye remains in the centre but to look for a predator lurking on the lateral extremities of the field(which I think they usually do to escape the prey's visual field) a quick and precise response from the muscle pulling the eye on the lateral side might help in spotting the predator quickly. So a separate nerve controlling the muscle might give the added control that one nerve controlling all the muscles might not be able to provide some similar explanation might be given for the superior oblique being controlled by the trochlear nerve.
The link is broken. But that rationale sounds weak - why would a separate nerve improve either response time or control? Is there actual evidence that the lateral rectus response faster or with greater precision than the rest of the oculomotor nerves, either in humans or other species?
I just want to mention that simple and rationally satisfying explanations for evolutionary modifications that we observe today may not always be possible. We aren't always able to infer all the evolutionary pressures that these modifications underwent over hundreds of millions of years. Also remember that evolution cannot go back to the drawing board. It can only work with incremental modifications of existing systems.
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[deleted] t1_ivtpxsw wrote
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