The vagus(parasympathetic)nerves release乙酰胆碱(ACh)作为他们的主要neurotransmitter。
ACh binds to muscarinic receptors (M2) that are found principally on cells comprising the sinoatrial (SA) and atrioventricular (AV) nodes.
The interaction of ACh in SA and AV nodes decrease the pacemaker rate by increasing potassium and decreasing calcium and sodium movement. Or in the other word, Ach slow the rate of spontaneous depolarization, which extends repolarization and increases the time before the next spontaneous depolarization occurs.
As the pacemaker slow, so does the heart rate.
Figure 1.Signaling pathway of parasympathetic stimulation. The acetylcholine binds to muscarinic (M2) receptors, activating Inhibitory G protein (Gi) by dissociating the α subunit of the Gi。Giinhibits adenylyl cyclase, thus reduce the cAMP formation. The protein kinase A is inhibited. Thus the contractile machinery and ion channels are also inhibited.