Sometimes one system or the other can influence the heart in a biased way, for instance, the parasympathetic system can slow your heart rate way down, and it does this by influencing how fast the
heart's natural pacemaker
SA Node, discharges. Much of
this is handled by the
Vagus Nerve, a nerve that starts in your brain, goes down your esophagus, and on to the digestive system.
Please be advised that this may not be your problem, but the following article may be of some help.