Which factors regulate SAN pacemaker activity and conduction velocity?

• A. Autonomic nerves, hormones, electrolytes
• B. Autonomic nerves only
• C. Hormones only
• D. Temperature and pH

Answer: (A)

The factors regulating SAN pacemaker activity and conduction velocity come from three broad influences: autonomic nerves, hormones, and electrolytes. Autonomic input rapidly tunes heart rate and nodal conduction: sympathetic stimulation raises cAMP, increases the funny current (If) and L-type Ca2+ currents, and speeds diastolic depolarization, increasing heart rate; parasympathetic signaling lowers cAMP, enhances certain potassium currents, and slows diastolic depolarization, decreasing heart rate and slowing conduction through the nodal tissue. Hormones in the bloodstream produce similar effects in a sustained way, with catecholamines acting on beta-adrenergic receptors to raise rate and excitability, and thyroid hormone increasing overall adrenergic responsiveness over time. The ionic environment—electrolytes like calcium, potassium, and magnesium—directly shapes the pacemaker currents and the action potential upstroke, influencing both pacing rate and the speed of conduction. Temperature and pH can modulate ion-channel behavior and excitability, but the principal regulators in normal physiology are autonomic nerves, hormones, and electrolytes.