Final answer:
Methamphetamine (meth) causes a racing heart and increased activation of sympathetic nervous system synapses. This is due to the release of norepinephrine onto target organs by sympathetic fibers.
Step-by-step explanation:
Methamphetamine (meth) causes a racing heart and increased activation of sympathetic nervous system synapses. When the cardiovascular center in the medulla initiates a signal, sympathetic fibers release norepinephrine onto target organs, including the heart. The release of norepinephrine increases the heart rate and heart contractility, leading to a racing heart.