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Acetylcholine

If acetylcholine is removed from the circulation faster than norepinephrine is, which of the following autonomic processes would be most rapidly inactivated?
A. Dilation of the pupils
B. Dilation of blood vessels in the skeletal muscles
C. Rise in blood pressure
D. Stimulation of digestive secretions


Answer explanation says “Choices A, B, and C all describe sympathetic effects”. How is dilation of blood vessels in the skeletal muscles a sympathetic effect?
Original post by freestyler01
If acetylcholine is removed from the circulation faster than norepinephrine is, which of the following autonomic processes would be most rapidly inactivated?
A. Dilation of the pupils
B. Dilation of blood vessels in the skeletal muscles
C. Rise in blood pressure
D. Stimulation of digestive secretions


Answer explanation says “Choices A, B, and C all describe sympathetic effects”. How is dilation of blood vessels in the skeletal muscles a sympathetic effect?




Acetylcholine is the chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.


@Emily_B
(edited 2 years ago)
Original post by Tracey_W
Acetylcholine is the chief neurotransmitter of the parasympathetic nervous system, the part of the autonomic nervous system (a branch of the peripheral nervous system) that contracts smooth muscles, dilates blood vessels, increases bodily secretions, and slows heart rate.


@Emily_B

Yes, so if acetylcholine is responsible for dilating blood vessels, then B would be correct, but apparently it's not.
Reply 3
Original post by freestyler01
Yes, so if acetylcholine is responsible for dilating blood vessels, then B would be correct, but apparently it's not.

Blood vessels do not contain receptors for acetylcholine! The level of sympathetic stimulation determines blood vessels diameter i.e. more adrenaline/noradrenaline = vasoconstriction, less adrenaline/noradrenaline = vasodilation.

However, that’s not the main point of the question. While adrenaline/noradrenaline cause vasoconstriction in most vessels throughout the body, they actually cause vasodilation in skeletal muscles (they have different receptors) to increase blood flow in muscles. If you think about the fight or flight response, this makes sense, it increases blood flow to muscles to allow us to run or fight.
Original post by Jpw1097
Blood vessels do not contain receptors for acetylcholine! The level of sympathetic stimulation determines blood vessels diameter i.e. more adrenaline/noradrenaline = vasoconstriction, less adrenaline/noradrenaline = vasodilation.

However, that’s not the main point of the question. While adrenaline/noradrenaline cause vasoconstriction in most vessels throughout the body, they actually cause vasodilation in skeletal muscles (they have different receptors) to increase blood flow in muscles. If you think about the fight or flight response, this makes sense, it increases blood flow to muscles to allow us to run or fight.

Don't blood vessels have smooth muscle, which is controlled by acetylcholine? As this source (https://pubmed.ncbi.nlm.nih.gov/11475798/) mentions: "Acetylcholine (ACh) may induce the relaxation and the contraction of human blood vessels. These effects involve the activation of muscarinic receptors located on endothelial or smooth muscle cells." But maybe as you said, most blood vessels don't have acetylcholine receptors.

As an aside, does sympathetic innervation cause mainly pressure increase or vascular resistance (i.e vessel diameter reduction)? Which is it more likely to affect?
(edited 2 years ago)
Reply 5
Original post by freestyler01
Don't blood vessels have smooth muscle, which is controlled by acetylcholine? As this source (https://pubmed.ncbi.nlm.nih.gov/11475798/) mentions: "Acetylcholine (ACh) may induce the relaxation and the contraction of human blood vessels. These effects involve the activation of muscarinic receptors located on endothelial or smooth muscle cells." But maybe as you said, most blood vessels don't have acetylcholine receptors.

As an aside, does sympathetic innervation cause mainly pressure increase or vascular resistance (i.e vessel diameter reduction)? Which is it more likely to affect?

Perhaps I should have said that most blood vessels in the body have sympathetic innervation only. Some vessels will also be innervated by parasympathetic cholinergic fibres such as those in the genitals (e.g. causing erection). However, the acetylcholine causes vasodilation indirectly by inducing the formation of nitric oxide.

An increase in sympathetic drive will cause most blood vessels in the body to constrict, increasing systemic vascular resistance/total peripheral resistance. This is because these blood vessels contain mainly alpha-1 receptors. However, the blood vessels within skeletal muscle contain beta-2 receptors, which causes vasodilation. This has the effect of diverting blood away from places such as the skin and GI tract to the skeletal muscles instead.
I see, thank you.

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