Norepinephrine's main functions are controlling rest cycles, alertness and memory in the central nervous center. It also functions as a hormone and neurotransmitter according to the Macalester College website. It is often used medically to treat people who suffer from severe hypotension.
Norepinephrine can also affect the heart by increasing the contractions of the muscle. This happens when it is released as a neurotransmitter from the sympathetic neurons inside the sympathetic nervous system. It is also a part of the fight or flight response along with epinephrine. They release glucose, increase the heart rate and increasing the blood flow to the skeletal muscles. Norepinephrine is synthesized by the body from dopamine in the medullary chromaffin cells.