Which influence increases heart rate and the force of contraction?

• A. Sympathetic
• B. Parasympathetic
• C. Somatic
• D. Enteric

Answer: (A)

Activation of the sympathetic nervous system drives both a faster heart rate and stronger contractions. It releases norepinephrine that binds to beta-1 adrenergic receptors on heart cells, raising cAMP levels. This speeds up depolarization of the pacemaker cells (increasing heart rate) and enhances calcium availability in the heart muscle (increasing the force of contraction). This is the fight-or-flight adjustment that prepares the heart to pump more blood.

In contrast, parasympathetic input slows the heart by acetylcholine acting on muscarinic receptors, mainly reducing heart rate and having little effect on contraction strength. The somatic and enteric systems control skeletal muscle and the GI tract, respectively, and do not regulate heart rate or contractility.