Pupil Constriction and PNS association

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RSK25

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Why is pupil constriction considered a part of parasympathetic nervous system?

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It's more important to memorize this fact - the short, unscientific answer is: because it is.

But the short scientific answer is: because there exist neuromuscular junctions that use and respond to acetylcholine (PNS neurotransmitter).

A good way to remember that pupil constriction is part of the PNS is to actually remember the opposite - that pupil dilation is part of the SNS shown during our "fight or flight" response and that...
"The diameter of pupil of the eye is regulated by two opposing sets of muscles. Activation of the sphincter muscle of the iris by ACh (muscarinic) constricts the pupil while stimulation of the radial muscle by NE (alpha 1) opens it."
Source: http://www.life.umd.edu/classroom/bsci440/higgins/PNSPharmacology.html

The aforementioned quote has too much detail (knowing the muscle that each NS operates on is unnecessary for the MCAT).


Cheers!


http://forums.studentdoctor.net/forums/next-step-mcat-tutor-office-hours.970/
 
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Ok, and for clarification, the neurotransmitter for PNS is Ach and norepinephrine or epinephrine for SNS. Is PNS always excitatory?
 
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Ok, and for clarification, the neurotransmitter for PNS is Ach and norepinephrine or epinephrine for SNS. Is PNS always excitatory?
Neurotransmitters have no inherent meaning. It is how the cell interprets it that matters, which means the receptors for the neurotransmitters. That's how you can have reduced blood flow to some parts of the body while increasing other areas' blood flow with a single messenger/neurotransmitter/hormone.

And, for the MCAT, acetylcholine is the neurotransmitter for the somatic, preganglionic sympathetic, pre- and postganglionic parasympathic nervous system. The only place where epinephrine/norepinephrine is used is the postganglionic sympathetic nervous system.

@NextStepTutor_1 's explanation is extremely solid, and, like he said, beyond the expectations of the MCAT, so it may be beneficial to just memorize rather than understand.
 
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PNS = "rest and digest" is what my Neuro professors says. No need for a mnemonic to understand why pupils constrict, they do to restrict light from entering your eyes making you see much worse at night where predators around vs dilated pupils ("fight or flight") where you can find predators stalking you down in the nighttime.
 
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