Naked Science Forum

Life Sciences => Physiology & Medicine => Topic started by: annie123 on 30/11/2015 01:42:48

Title: WHy do eyelids stay shut when we're asleep?
Post by: annie123 on 30/11/2015 01:42:48

Are eyelids naturally open or shut? When we're asleep they stay shut even though we may not consciously shut them, and dead people have to have them shut manually if death happens when they're open. What is the mechanism behind the working of eyelids?

Title: Re: WHy do eyelids stay shut when we're asleep?
Post by: evan_au on 30/11/2015 08:23:28

Many muscles are disabled when we are asleep, perhaps so we don't hurt ourselves or innocent bystanders while we are dreaming.

Some muscles must keep working, like heart and lungs, or we die. A lot of these are smooth muscles, that can operate without conscious control.

Other muscles like the ones that control our eyeballs are quite active during REM sleep - I guess we can't hurt anyone with those.

Perhaps having eyelids shut is a protective mechanism - if our eyes were open, the cornea could dry out, and we could get dust in our eyes. This could lead to irritated & scratched cornea. Better to force our eyelids closed while we are asleep!

Title: Re: WHy do eyelids stay shut when we're asleep?
Post by: annie123 on 05/12/2015 06:59:01

Thanks. Looks as if no one else has an answer.
Do we 'force' our eyelids to close n sleep? I think they close without our conscious direction. And why don't they close when we die?

Title: Re: WHy do eyelids stay shut when we're asleep?
Post by: evan_au on 05/12/2015 08:44:16

Quote from: annie123

why don't eyelids close when we die?

I assume it takes nerve impulses acting on muscles to close our eyelids.
Both of these stop when we die.

Title: Re: WHy do eyelids stay shut when we're asleep?
Post by: Don_1 on 05/12/2015 15:46:02

What fascinating question.

Blinking is an involuntary action, that is it needs no conscious input to carry on. When awake our eyelids need no conscious input to stay open. They do need input to be held closed. When asleep they need no conscious input to stay closed, which is just as well since we are not then conscious.

Other involuntary actions, such as the beating of the heart and contraction and relaxation of the diaphragm keep going regardless of whether we are awake or not, in just the same way, albeit at differing rates.

But the eyelids seem to have two opposing involuntary parking positions which do not appear to be connected to the sympathetic and parasympathetic nerve system. Or are they?

There you sit reading a book or watching an interesting programme on the TV. The next thing you know, the programme has finished. Despite conscious attempts to keep your eyes open, you finally succumbed to tiredness and fell asleep, with your eyes firmly closed.

So there appears to be a switchable parked position for the eyelids.
Sympathetic = Awake the switch is to open
Parasympathetic = Asleep the switch is to closed.

So is it that somehow the Sympathetic nervous system switches off for the eyes during sleep?