Pacemaker

Pacemaker cells in the SA node receive direct input from the autonomic nervous system , the input can alter the frequency of action potentials which is basically the only thing that determines heart rate. Also the Sympathetic neurons project into the AV node. They influence the speed with which action potentials are conducted. This leads to a decrease in the duration of the systole which allows the diastole to be able to fill the heart with enough blood. Also the parasympathetic neurons can cause an increase in the length of the systole by decreasing the speed of conduction of the impulse.