Special cells often referred to as "pacemaker" cells produce electricity in the body by rapidly changing their electrical charge from positive to negative and back again. When the heart muscle is relaxed the cells are electrically polarized, meaning the inside of each cell has a negative electrical charge. The environment outside the cells is positive. Cells depolarize as some of their negative atoms are allowed through the cell membrane, and it's this depolarization that causes electricity in the heart. Once one cell depolarizes it sparks a chain reaction and electricity flows from cell to cell. When cells return to normal it's called repolarization, and the process is repeated with every heartbeat.
The SA node is regulated by the autonomic nervous system, which controls all of the automatic functions of the body including heartbeat, breathing and digestion. The sympathetic nervous system and the parasympathetic nervous system are part of the autonomic nervous system and work together to control how fast the pacemaker cells spontaneously depolarize and increase and decrease the rate the SA node sends out electrical signals.
"The sympathetic nervous system is responsible for increasing the heart beat during exercise, while the parasympathetic nervous system lowers the pulse during periods of rest," explains Marshal Fox, M.D., electrophysiology cardiologist at Baystate Health in Massachusetts.
When the SA node fires off an electrical impulse the pulse of electricity first travels through the top chambers of the heart and continues through the AV node where it's slowed down. By slowing down the electrical signal the AV node allows time for the upper chambers of the heart to contract first, before the ventricles. Once through the AV node's gate the pulse continues to move through the bundle branches and Purkinje fibers and then finally ends in the ventricles, which contract and pump blood through the body.
"People may have different intrinsic resting pulse rates and the reason for this is due to the balance between the sympathetic and parasympathetic nervous systems," explains Fox. "Athletes, for instance, develop higher parasympathetic tone with continued training and therefore while at rest will have a lower pulse than their counterpart couch potatoes."