The pacemaker is the location in the heart which is responsible for firing action potentials. The pacemaker is usually found in the sinoatrial node, a region of muscle cells located in the right atrium. There are many pacemaker cells in this region. However, only one pacemaker can be active at one time. Whichever pacemaker has the quickest action potential stimulates the other cells. This, in turn, sets off a chain reaction, stimulating every cell and allowing for the contraction of the cardiac muscle. The pacemaker is stimulated and controlled by the autonomic nervous system.
Many cells found in other regions of the heart also have the capabilities of becoming pacemakers. The difference between these cells and those in the sinoatrial node is that their action potentials are significantly slower. Pacemaker cells do not have typical action potentials because they lack a steady resting potential, thus making them much quicker then the others. A pacemaker cell fires 70 times per minute at a resting heart rate, and up to 160 times per minute during exercise.
If the resting rate is fewer than 60 beats per minute, then the person may be suffering from bradycardia. In a situation such as this, it may be deemed necessary to implement an artificial cardiac pacemaker. These devices deliver electrical stimulus to the heart, allowing for the heart rate to reach normality. Artificial pacemakers run off of lithium batteries, lasting 6 to 10 years. Surgery is required to replace the battery.