Cardiac Physiology: Autonomic Control of the Heart

Introduction

The autonomic nervous system (ANS) plays a crucial role in regulating cardiac function. It comprises the sympathetic and parasympathetic divisions, which exert opposing effects on heart rate and contractility.

Sympathetic Control

Sympathetic stimulation increases heart rate (chronotropy) and myocardial contractility (inotropy) via the release of norepinephrine, acting on beta-adrenergic receptors.

• Activation of beta-1 receptors increases heart rate.
• Enhanced calcium influx improves contractility.
• Facilitates faster conduction through the AV node.

Parasympathetic Control

Parasympathetic stimulation, mediated by the vagus nerve, decreases heart rate through acetylcholine's effect on muscarinic receptors.

• Reduces SA node firing rate.
• Decreases myocardial contractility.
• Slows AV node conduction.

Clinical Significance

Dysfunction of autonomic control can lead to arrhythmias, syncope, and heart failure. Conditions such as autonomic neuropathy or beta-blocker therapy impact cardiac autonomic regulation.