During stress, heart rate increases primarily due to activation of the

• A. Parasympathetic
• B. Somatic
• C. Central nervous system
• D. Sympathetic

Answer: (D)

Stress triggers the sympathetic branch of the autonomic nervous system, which raises heart rate. Norepinephrine released by sympathetic nerves binds to beta-1 receptors in the sinoatrial node, increasing the rate of spontaneous firing and speeding up conduction through the heart. This leads to a faster heart rate and stronger contractions, preparing the body for quick action. The adrenal medulla also releases epinephrine into the bloodstream, which similarly stimulates beta-1 receptors on the heart to raise rate. In contrast, parasympathetic activity would slow the heart by releasing acetylcholine onto M2 receptors, not driving the stress response. The central nervous system coordinates this response, but the immediate driver of the increased heart rate is sympathetic activation.