acetylcholine - effects on heart rate

[dmission]

I'm having a tough time understanding exactly why acetylcholine decreases heart rate. I thought that because it binds to receptors on the neurons, it would immediately trigger/spread the AP down the next neuron. Instead it seems, it does the opposite.

Would appreciate any clarification, thanks!

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[Chowdder]

You're probably thinking of Acetylcholine in control of neuro musclar junctions, which is involving the activation of motor neurons. Heart rate is not controlled by these groups of neurons, rather they are controlled by autonomic neurons. Which is divided into sympathetic and parasympathetic nervous systems.

Acetylcholine is the major post-synaptic neurotransmitter used by the parasympathetic nervous system.
Norepinephrine is the major post-synaptic neurotransmitter for the sympathetic nervous system.

Addition of Acetylcholine will activate parasympathetic response, thereby slowing down heart rate.

 

[ih2183]

Acetylcholine binds two types of receptors: at the neuromuscular junction, it binds ionotropic nicotinic receptors, which when ACh binds, it stimulates increase in Na+ flux into cell, therefore depolarizing it. This is most responsible for a muscle twitch.

In the heart, there exists the muscarinic G-protein metabotropic receptor that when ACh binds, a cascade occurs through second messenger to eventually open K+ channels longer. By doing this, the membrane potential drops greatly, hyperpolarizing the cell. This is why it is harder to stimulate another action potential, and why heart rate decreases--> parasympathetic response as mentioned above.