The contraction of cardiac muscle (heart muscle) in all animals is initiated by electrical impulses known as action potentials that in the heart are known as cardiac action potentials. The rate at which these impulses fire controls the rate of cardiac contraction, that is, the heart rate. The cells that create these rhythmic impulses, setting the pace for blood pumping, are called pacemaker cells, and they directly control the heart rate. They make up the cardiac pacemaker, that is, the natural pacemaker of the heart. In most humans, the highest concentration of pacemaker cells is in the sinoatrial (SA) node, the natural and primary pacemaker, and the resultant rhythm is a sinus rhythm.
Sometimes a secondary pacemaker sets the pace, if the SA node is damaged or if the electrical conduction system of the heart has problems. Cardiac arrhythmias can cause heart block, in which the contractions lose any useful rhythm. In humans, and sometimes in other animals, a mechanical device called an artificial pacemaker (or simply "pacemaker") may be used after damage to the body's intrinsic conduction system to produce these impulses synthetically.
Control
Primary pacemaker
The sinoatrial node (SA node) is the primary pacemaker of the heart. It is a region of cardiac muscle on the wall of the upper right atrium near to the superior vena cava entrance. The cells that make up the SA node are specialized cardiomyocytes known as pacemaker cells that can spontaneously generate cardiac action potentials. These signals are propagated through the heart's electrical conduction system. Only one percent of the heart muscle cells are conductive, the rest of the cardiomyocytes are contractile.
The pacemaker cells are connected to neighboring contractile cells via gap junctions, which enable them to locally depolarize adjacent cells. Gap junctions allow the passage of positive cations from the depolarization of the pacemaker cell to adjacent contractile cells. This starts the depolarization and eventual action potential in contractile cells. Having cardiomyocytes connected via gap junctions allow all contractile cells of the heart to act in a coordinated fashion and contract as a unit. All the while being in sync with the pacemaker cells; this is the property that allows the pacemaker cells to control contraction in all other cardiomyocytes.
Cells in the SA node spontaneously depolarize, ultimately resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers via the autonomic nervous system, so that the average resting heart rate in adult humans is about 70 beats per minute.
Secondary (AV junction and Bundle of His) Impulses from the sinus node reach the atrioventricular node which acts as the secondary pacemaker. The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.
Further down the electrical conducting system of the heart is the Bundle of His. The left and right bundle branches, and the Purkinje fibers, will also produce a spontaneous action potential at a rate of 30-40 beats per minute, so if the SA and AV node both fail to function, these cells can become pacemakers. It is important to realize that these cells will be initiating action potentials and contraction at a much lower rate than the primary or secondary pacemaker cells.
The SA node controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarization, thus they initiate action potentials the quickest. The action potential generated by the SA node passes down the electrical conduction system of the heart, and depolarizes the other potential pacemaker cells (AV node) to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SA node. This is the normal conduction of electrical activity in the heart.
Generation of action potentials
There are 3 main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase 1 or 2, just phases 0, 3, and 4.
Phase 4 - Pacemaker potential
The key to the rhythmic firing of pacemaker cells is that, unlike other neurons in the body, these cells will slowly depolarize by themselves and do not need any outside innervation from the autonomic nervous system to fire action potentials.