Which stimulus increases heart rate and contractile force through sympathetic activation?

• A. Increased levels of carbon dioxide in the blood
• B. Vagus nerve
• C. Decrease in blood pH
• D. Sympathetic center in the medulla oblongata

Answer: (D)

Autonomic control of heart rate and contractility is determined by the balance between sympathetic and parasympathetic input. The stimulus that directly increases both heart rate and contractile force through sympathetic activation comes from the cardioaccelerator center in the medulla oblongata. When this center is activated, it sends sympathetic signals to the heart, releasing norepinephrine that binds to beta-1 receptors on the SA node, AV node, and cardiac muscle. This speeds up pacemaker activity, shortens conduction through the AV node, and increases calcium availability in the muscle, raising both rate and force of contraction. The vagus nerve, by contrast, provides parasympathetic input that slows the heart via acetylcholine on muscarinic receptors. Factors like rising CO2 or acidosis can modulate autonomic tone, but the direct mechanism described for increasing heart rate and contractile force is activation of the medullary cardioaccelerator center driving sympathetic outflow.