Final answer:
Norepinephrine acts on the heart by binding to beta-1 receptors, which increases the rate of depolarization and contraction, resulting in an increased heart rate.
Step-by-step explanation:
Norepinephrine acts on the heart by binding to the beta-1 receptors of the cardiac muscle cell membrane. This binding opens chemical- or ligand-gated ion channels, allowing an influx of positively charged ions. As a result, the rate of depolarization and contraction increases, leading to an increase in heart rate (HR).