SA nodal firing rate is primarily controlled by which autonomic inputs?

• A. Sympathetic and parasympathetic nerves
• B. Only sympathetic nerves
• C. Only parasympathetic nerves
• D. Intrinsic rate only

Answer: (A)

Firing rate of the SA node is determined by how fast the pacemaker cells depolarize during phase 4, and this pace is set by autonomic tone. Sympathetic input speeds up the rate: norepinephrine acting on beta-1 receptors raises cAMP, enhances the funny current and calcium currents, and quickens diastolic depolarization, so the SA node fires more rapidly. Parasympathetic input slows the rate: acetylcholine on muscarinic receptors lowers cAMP, increases potassium efflux, and reduces the depolarization slope, so the SA node fires more slowly. In normal life, both arms of the autonomic nervous system shape the heart rate; the intrinsic rate of SA nodal cells provides a baseline, but it’s the balance of sympathetic and parasympathetic influence that primarily sets the actual firing rate.