Which influence decreases heart rate?

• A. Parasympathetic
• B. Sympathetic
• C. Somatic
• D. Endocrine

Answer: (A)

Parasympathetic control is the main influence that lowers heart rate. When the vagus nerve releases acetylcholine onto the sinoatrial and atrioventricular nodes, it binds to muscarinic receptors (M2). This opens potassium channels and makes the pacemaker cells more negative (hyperpolarized), which slows the slope of the pacemaker potential (the rate at which it reaches threshold). As a result, fewer action potentials fire per minute, and heart rate decreases.

In contrast, sympathetic input releases norepinephrine (or epinephrine) that binds to beta-1 receptors, increasing cAMP, boosting funny current and calcium currents, and speeding up depolarization—this raises heart rate. The somatic nervous system doesn’t regulate heart rate, as it controls voluntary skeletal muscles. Endocrine signals like circulating adrenaline tend to increase heart rate rather than decrease it. So the direct, rhythm-lowering influence comes from parasympathetic activity on the heart’s pacemaker.