Are cardiac action potentials initiated by nerves and neurotransmitters?

• A. Yes
• B. No
• C. Only in atrial tissue
• D. Sometimes influenced by nerves

Answer: (B)

Cardiac action potentials originate from the heart’s intrinsic automaticity, not from nerves firing to start each beat. Pacemaker cells, especially in the sinoatrial node, spontaneously depolarize due to their ion-channel properties (like the funny current, If, and T-type calcium channels) and reach threshold to generate an action potential. Nerves don’t initiate these impulses; they modulate how fast the heart beats and how quickly impulses propagate by releasing neurotransmitters that change excitability and conduction. Sympathetic input (norepinephrine) speeds up the rate and increases excitability, while parasympathetic input (acetylcholine) slows the rate. So the action potentials are set by the heart’s own pacemaker cells, with nerves tuning the timing and rate rather than starting each impulse.