How does sympathetic innervation affect the heart?

• A. It increases heart rate and force of contraction.
• B. It decreases heart rate and force of contraction.
• C. It stops heart contractions.
• D. It only affects the heart rate, not force.

Answer: (A)

Sympathetic innervation prepares the heart for action by increasing both how fast it beats and how strongly it contracts. Norepinephrine released from sympathetic nerves acts on beta-1 receptors in the SA node, AV node, and ventricular muscle. This raises cyclic AMP, which enhances calcium entry and calcium handling inside heart cells. The result is a faster heart rate (positive chronotropic effect), quicker conduction through the AV node (positive dromotropic effect), and stronger contractions (positive inotropic effect). This coordinated boost raises cardiac output to meet higher metabolic needs, such as during exercise or stress. It does not stop heart contractions; that would be contrary to its role. Parasympathetic input, in contrast, mainly slows rate and has a much smaller effect on ventricular contractility.