Natural pacemaker

The natural pacemaker is the heart’s special rhythm maker. It uses special cells called pacemaker cells that send out tiny electrical signals, known as cardiac action potentials. These signals tell the cardiac muscle — the muscle of the heart — when to squeeze, which controls the heart rate.

In most people, these cells are found in a part of the heart called the sinoatrial (SA) node. This is the main pacemaker and keeps the heart beating in a steady pattern called sinus rhythm.

Sometimes, if the SA node gets damaged or if the heart’s electrical system has trouble, another part of the heart may take over as the pacemaker. If the heart’s rhythm gets too irregular, called cardiac arrhythmias, a doctor might use a special machine called an artificial pacemaker to help the heart keep a steady beat.

Control

Primary pacemaker

The sinoatrial node, also called the SA node, is the main pacemaker of the heart. It is found in the upper part of the right atrium, close to where a large blood vessel enters. Special cells in the SA node, known as pacemaker cells, create tiny electrical signals on their own. These signals travel through the heart’s system, telling the heart muscle when to contract.

Because the SA node’s cells create signals the fastest, they control the heart’s rhythm. The signals move through the heart, making sure all parts contract together. This keeps the heart beating smoothly. Normally, the heart beats about 70 times each minute thanks to these signals.

Secondary pacemaker

If the main pacemaker isn’t working, another part of the heart, the atrioventricular node, can take over. It usually creates signals a bit slower than the main pacemaker.

Atrioventricular junction

Below the atrioventricular node is the Bundle of His, which sends signals to other parts of the heart. If both the main and secondary pacemakers stop working, these areas can also create signals, but they will be slower.

Generation of action potentials

Pacemaker cells in the heart create electrical signals that control the heartbeat. These signals, called action potentials, have three main stages.

In phase 4, pacemaker cells slowly become more electrically charged on their own, without needing help from the nervous system. This happens because potassium flows out of the cell less, and sodium slowly flows in. When the cell reaches a certain level of charge, it is ready to create an action potential.

In phase 0, the cell quickly becomes more charged as calcium flows in, causing the action potential to rise. Finally, in phase 3, the cell returns to its resting state as potassium flows out and pumps move ions back to their normal positions, readying the cell to start the process again.

Clinical significance

Damage to the SA node

If the main part of the heart that sets the rhythm, called the SA node, stops working or its signal gets blocked, another part of the heart can take over. This backup is usually found in the AV node, a spot between the upper and lower chambers of the heart. If the AV node also stops working, special cells called Purkinje fibers can sometimes act as a last backup to keep the heart beating.

Ectopic pacemaker

Sometimes, cells outside the main rhythm setter can cause the heart to beat too early. This is called an ectopic pacemaker. If this happens often, it can lead to irregular heart rhythms. In such cases, doctors might use a special machine called an artificial pacemaker to help the heart keep a steady beat.

Artificial pacemakers

Main article: Pacemaker

An artificial pacemaker is a small device placed inside the body. It sends tiny signals to the heart to make sure it beats at the right pace. This helps when the heart’s natural rhythm setter isn’t working properly.