The eye, what happens when iris contracts?

[ipodtouch]

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EK says this:

When it is dark, the sympathetic ns causes the iris to contract. This causes the pupil to dilate.

When it is bright, the parasympathetic ns causes the iris to contract. This causes the pupil to constrict.


Im fairly confused how this can happen. Are different parts of the iris contracting? Wondering if anyone could explain this to me...

 

[ridethecliche]

I think the iris relaxes in the first case.

Sent from my SGH-T959 using Tapatalk

 

[EnginrTheFuture]

EK says this:

When it is dark, the sympathetic ns causes the iris to contract. This causes the pupil to dilate.

When it is bright, the parasympathetic ns causes the iris to contract. This causes the pupil to constrict.


Im fairly confused how this can happen. Are different parts of the iris contracting? Wondering if anyone could explain this to me...

The iris consists of two main sphincter muscles, thus in this case "contraction" may not be referring to the iris itself (dilation and constriction) but to the muscles that make those action occur.

The Iris has a muscle that, upon contraction, dilates the iris during sympathetic NS response.

The iris also has a different muscle group that, ALSO upon contraction, constricts the iris during parasympathetic NS response.

These are antagonistic muscles so when one occurs the other relaxes.


I would just simply know that sympathetic = dilate, parasympathetic = constrict... both are stimulations of muscles that do opposite things upon contraction

 

[loveoforganic]

^👍, particularly in regard to the last sentence. All the other information was correct, but you shouldn't need to know more than that last bit

 

[MedPR]

Here you go.

 

[EnginrTheFuture]

Prettttyyy eyes