Learning Objective
After completing this section, learners will be able to explain how sympathetic catecholamines regulate SA and AV node excitability, describe the ionic mechanisms involved, and understand the functional effects on heart rate and conduction.
Sympathetic Regulation of Nodal Cells
Catecholamines
- Norepinephrine (NE) from postganglionic sympathetic nerves
- Epinephrine (Epi) circulating in the blood
Receptors and Signaling
- β1-adrenergic receptors on SA and AV nodal cells
- Coupled to Gs protein → cAMP pathway
- Stimulates opening of HCN channels (funny current, If) and L-type Ca²⁺ channels
Effect on Pacemaker Potential
- Increased slope of Phase 4 depolarization → reaches threshold sooner
- Faster spontaneous depolarization of the SA node
Functional Effects
- Positive chronotropy (SA node): increased heart rate
- Positive dromotropy (AV node): increased conduction velocity through the AV node
