How does sympathetic stimulation affect heart rate and contractility?

• A. It decreases heart rate and contractility
• B. It increases heart rate and contractility
• C. It increases heart rate but decreases contractility
• D. It has no effect

Answer: (B)

Sympathetic stimulation increases both heart rate and contractility. When the sympathetic nerves release norepinephrine (and circulating epinephrine), it binds to beta-1 receptors in the heart. This receptor activation raises the intracellular cAMP level, which activates protein kinase A. PKA enhances calcium entry through L-type calcium channels and increases calcium release from the sarcoplasmic reticulum. The greater availability of intracellular calcium strengthens each heartbeat (positive inotropy), making contractions more forceful. At the same time, beta-1 stimulation speeds up the heart’s pacemaker activity by increasing the slope of the diastolic depolarization in the SA node, which raises the heart rate, and it also boosts conduction through the AV node. All of this prepares the heart to pump more blood during stress or exercise.