The sympathetic and parasympathetic nervous systems are opposing
forces that affect your heart rate. Both systems are made up of very tiny
nerves that travel from the brain or spinal cord to your heart. The sympathetic
nervous system is triggered during stress or a need for increased cardiac
output and sends signals to your heart to increase its rate. The
parasympathetic system is active during periods of rest and sends signals to
your heart to decrease its rate.
During stress or a need for increased cardiac output, the adrenal
glands release a hormone called norepinephrine into the bloodstream at the same
time that the sympathetic nervous system is also triggered to increase your
heart rate. This hormone causes the heart to beat faster, and unlike the
sympathetic nervous system that sends an instantaneous and short-lived signal,
norepinephrine released into the bloodstream increases the heart rate for
several minutes or more.