Parasympathetic Nervous System

Upon parasympathetic stimulation, the nerve cells release neurotransmitter acetylcholine. When acetylcholine binds to its receptor, K+ ion channels open resulting more K+ leaks out from these pacemakers cells. This will lead to hyperpolarisation with lower resting membrane potential at -70 mV.

This means that more inflow of Ca2+ are needed to reach the threshold (-40 mV) for the generation of impulse. Consequently it takes longer for the nerve impulse to be generated and the heart rate slows down (E.g.: 40 bpm).