Which hormone stimulates the sympathetic nervous system and speeds up heart rate?

Which hormone stimulates the sympathetic nervous system and speeds up heart rate?

• A. Norepinephrine
• B. Acetylcholine
• C. Serotonin
• D. Dopamine

Answer: (A)

The heart speeds up during sympathetic activation because adrenaline-like signaling increases the rate at which pacemaker cells in the SA node fire. This effect is mediated mainly by norepinephrine binding to beta-1 adrenergic receptors on the heart. When these receptors are stimulated, the cells raise their cyclic AMP levels, which makes the pacemaker potential rise more quickly. That shortens the time between beats, speeds conduction through the AV node, and boosts the force of contraction, all contributing to a faster heart rate. Acetylcholine, in contrast, comes from the parasympathetic system and slows the heart by opening potassium channels in pacemaker cells, reducing the rate of spontaneous depolarization. Serotonin and dopamine have important roles in other systems, but they do not directly drive the sympathetic increase in heart rate through the heart’s adrenergic receptors the way norepinephrine does.

The heart speeds up during sympathetic activation because adrenaline-like signaling increases the rate at which pacemaker cells in the SA node fire. This effect is mediated mainly by norepinephrine binding to beta-1 adrenergic receptors on the heart. When these receptors are stimulated, the cells raise their cyclic AMP levels, which makes the pacemaker potential rise more quickly. That shortens the time between beats, speeds conduction through the AV node, and boosts the force of contraction, all contributing to a faster heart rate.

Acetylcholine, in contrast, comes from the parasympathetic system and slows the heart by opening potassium channels in pacemaker cells, reducing the rate of spontaneous depolarization. Serotonin and dopamine have important roles in other systems, but they do not directly drive the sympathetic increase in heart rate through the heart’s adrenergic receptors the way norepinephrine does.