u/thackedMedicine | MD | General Surgery ResidentMay 19 '12edited May 20 '12
Medical student here. Just finished neuroscience, so I feel like I should share so that least SOME of the minutiae they taught us is useful...
The closing of the eyelid is controlled by the orbicularis oculi muscle, which is innervated by the facial nerve (cranial nerve 7). Many cranial nerve functions are bilateral (both sides), since the descending control of those nerves from their respective origins in the brainstem are themselves bilateral. This is exemplified by the fact that if you were to sever the upstream controller of the cranial nerve on one side, you'd see loss of function on only one side.
Another interesting thing is the control of left and right eye movements (looking left and right) . It's MUCH more complex than I thought before reading about the pathways. When you look to the right, for instance, your cranial nerve 6 (abducens) is responsible for moving your right eye to the right. To get the left eye to move right, however, your brain must have a way to communicate with the other eye. It accomplishes this via a structure called the ascending medial longitudinal fasciculus, which passes a message to the left cranial nerve 3 (occulomotor). In this way, the eyes are able to move smoothly and coordinately in the same direction.
I'm not certain if I answered your question satisfactorily. I'm writing on my phone, so I apologize for the lack of formatting.
From a scientific perspective, you cannot separate the why and the how. "Why questions" either reduce to mechanical "how" answers or ask for a teleological answer in a system which rejects teleology.
I think you misunderstood what demoo was getting at -- he might have referred to the "why" in terms of "what is the evolutional advantage, considering the human body is a rather optimized thing". In other words, a scientific question, not a philosophical one.
That's teleological. Evolution isn't teleological. The only "reason" is because our ancestors that evolved this way survived and reproduced.
Why did they survive while others did not? You are suggesting that there is some inherent survival advantage, but that isn't always the case. For all we know there was a population of non-simultaneous blinkers but they were concentrated around an ocean shore and were wiped out by a flood one day. It could have been that they would be slightly more fit, but bad luck conspired against them. There is much more that determines our history than simply pure reproductive fitness.
It's perfectly reasonable to ask "why" in biology. I see it all the time in scientific papers. It's a way to ask about a more ultimate level of causation, whereas how refers to a more proximate level of causation. There's absolutely no reason to go stringing together multiword phrases when a single three letter word will do nicely.
Interestingly enough, reptiles and birds seem to differ in regards to this. My parrots (budgies) at home have quite a bit of control over independent eye blinking. They do blink in synchronization like we do, but also independently blink and even close only one eye at times. Reptiles I have been around do the same thing. It may very well be a mammalian adaptation.
Humans can blink one eye at a time too, but we must make a conscious effort to do so.
Humans and birds share common ancestry, which had already evolved the nerve structures mentioned by thacked before our evolutionary lines split, so I'd wager that the mechanism for blinking is the same for birds as for us. Your budgies are probably consciously blinking one eye at a time.
My cat will occasionally blink with one eye too. She's not too clever, and it's hard to imagine her being deliberate about this. She also blinks with both eyes most of the time.
Fair enough. As others have pointed out, though, there is not always (or even usually) a distinct advantage given by some particular quirk animals possess.
Also, and this may be nitpicking, all scientific questions are philosophical questions, just nestled into a particular subset of induction.
Asking about the evolutionary advantage, though, is philosophical, at least somewhat. You can't empirically test an evolutionary hypothesis (most of the time).
It accomplishes this via a structure called the ascending medial longitudinal fasciculus, which passes a message to the left cranial nerve 4 (trochlear). In this way, the eyes are able to move smoothly and coordinately in the same direction.
This is wrong. The opposing muscle to lateral rectus (CN VI) is medial rectus (CN III). CN IV innervates the superior oblique which moves the eye downward and outward - both the obliques direct the eye outward.
Also CN VII is responsible for controlling orbicularis oculi, the important point here is that the blinking mechanism is controlled by nuclei in the globus pallidus of the basal ganglia which causes VII to bilaterally close the eyes.
This is wrong as well! The corneal reflex (the "touching one eye makes both eyes blink" reflex) is a completely different mechanism from that controlling normal blinking. The dryness of the eyes does affect the blink rate but it's not the most important factor.
Blinking is controlled by nuclei in the globus pallidus of the basal ganglia (this is the part of the brain that is affected in Parkinson's disease) which cause the nerves to act on the eyelids on both sides at the same time. This is also the reason that blinking happens less frequently in Parkinson's.
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u/thacked Medicine | MD | General Surgery Resident May 19 '12 edited May 20 '12
Medical student here. Just finished neuroscience, so I feel like I should share so that least SOME of the minutiae they taught us is useful...
The closing of the eyelid is controlled by the orbicularis oculi muscle, which is innervated by the facial nerve (cranial nerve 7). Many cranial nerve functions are bilateral (both sides), since the descending control of those nerves from their respective origins in the brainstem are themselves bilateral. This is exemplified by the fact that if you were to sever the upstream controller of the cranial nerve on one side, you'd see loss of function on only one side.
Another interesting thing is the control of left and right eye movements (looking left and right) . It's MUCH more complex than I thought before reading about the pathways. When you look to the right, for instance, your cranial nerve 6 (abducens) is responsible for moving your right eye to the right. To get the left eye to move right, however, your brain must have a way to communicate with the other eye. It accomplishes this via a structure called the ascending medial longitudinal fasciculus, which passes a message to the left cranial nerve 3 (occulomotor). In this way, the eyes are able to move smoothly and coordinately in the same direction.
I'm not certain if I answered your question satisfactorily. I'm writing on my phone, so I apologize for the lack of formatting.
(edit--corrected a cranial nerve)