Pacemaker cells are specialized cells that cause involuntary muscles and tissues to contract or dilate. They are found in all involuntary muscle groups, including both striated and smooth tissues. Striated involuntary muscles are found in the heart, while smooth involuntary tissue can be found in several organs, including the brain, blood vessels, and the digestive system.
Pacemaker cells have a specialized cell membrane that allows sodium and potassium to cross and trigger their electrical impulses. Although many of these cells can exist in a single location of the body, only one cell can fire at a time. Once the first cell generates an electrical impulse, it triggers a chain reaction in the other pacemaker cells.
These cells are usually found in the right atrium in an area known as the sinoatrial node. The electrical impulses generated by these cells cause the cardiac muscles to contract. They also control the rate of each contraction. Pacemaker cells generate an electrical impulse approximately 70 times per minute when the heart is at rest and up to 160 times each minute during vigorous exercise.
All of the heart’s cells are capable of acting as a pacemaker cell. The cells located in the sinoatrial node, however, are considered primary because they are faster than the other cells. This makes them responsible for initiating the chain reaction most of the time. The other cells are then secondary and will fire in the event that those in the sinoatrial node fail to start the chain reaction. When both primary and secondary pacemaker cells fail to keep the heartbeat regular, an artificial pacemaker might be considered.
Pacemaker cells in the brain have an impact on more than regulating muscle contractions. These cells can affect behavior, sleeping cycles, and can impact they way the brain perceives information. Other automatic functions, such as breathing, are also controlled by the brain’s pacemakers; they play a part in the involuntary contraction and dilation of blood vessels and pupils as well as in the peristalsis of the digestive system as well. However, in these systems, the automatic nervous system plays a larger role in regulation than the pacemaker cells.