Learning Objective
After completing this section, learners will be able to explain how parasympathetic stimulation modulates SA and AV node activity, identify the ionic mechanisms involved, and describe the functional effects on heart rate and AV conduction.
Activity
Parasympathetic Regulation of Nodal Cells
Neurotransmitter
- Acetylcholine (Ach) is released from postganglionic parasympathetic fibers
Receptors and Signaling
- M2 muscarinic receptors on SA and AV nodal cells
- Coupled to Gi/Go proteins
- Opens K⁺ channels → hyperpolarization
- Inhibits cAMP formation → reduces activity of HCN (funny) and L-type Ca²⁺ channels
Effect on Pacemaker Potential
- Hyperpolarizes nodal cells
- Reduces the slope of Phase 4 depolarization → reaches threshold more slowly
Functional Effects
- Negative chronotropy (SA node): decreased heart rate
- Negative dromotropy (AV node): decreased conduction velocity
