The heart’s conduction system is regulated by the sinoatrial node (SAN), which sends impulses to other cardiac fibers. The SAN is the pacemaker of the heart and regulates heart rate. The conduction system uses bundled fibers to carry signals from the SAN throughout the heart, and the entire depolarization sequence for one heartbeat takes less than a third of a second. The electrical signals of the heart’s conduction system are measured by an electrocardiogram (ECG), and irregular wave patterns on the ECG can indicate heart disorders.
The conduction system of the heart propagates electrical activity through the heart muscles which pump blood. It is organized by the spontaneous pacemaker activity of the sinoatrial node (SAN), which sends impulses to other cardiac fibers that transmit them from cell to cell. All heart muscle, called cardiac myocyte, is electrically excitable and responds to impulses from the SAN. The relaxation and contraction phases of the heart all have specific electrical profiles that are graphically represented by the electrocardiogram (ECG).
The heart muscle is innervated by neurons of the autonomic nervous system, especially the vagus nerve. Cardiac myocytes, like neurons, communicate with each other through electrical signals called action potentials. A heartbeat is a cycle in the heart’s conduction system that causes the heart chambers to relax and contract. With each heartbeat, electrical stimulation opens ion channels in the cell membrane, allowing positively charged ions to depolarize the cell, setting off an action potential. Another set of ion channels depolarizes the cell with an influx of potassium, after a short period called the resting state.
In the right atrial wall of the heart, the sinus node regulates heart rate by sending electrical impulses to cardiac myocytes. The SAN is called a pacemaker because it regulates the conduction system of the heart through cells that are activated at regular intervals. Sometimes direct signals from the autonomic nervous system or hormonal swings change the rate of signaling during exercise and stress. While all cardiac myocytes transmit electrical activity, the pacemaker cells of the sinus node actively produce synchronized spontaneous impulses.
Impulses from the SAN travel to the AV node, the point at which further conduction through the heart begins. There is a short delay between contractions so that the blood in the atria flows into the ventricles. The conduction system of the heart uses bundled conduction fibers to carry signals from the SAN throughout the heart. The right and left atria contract first as the current spreads to the ventricles, which subsequently depolarize. The entire depolarization sequence for one heartbeat takes less than a third of a second.
The electrical signals of the heart’s conduction system are measured by an electrocardiogram which plots the activity of myocytes as a wave on a graph. Signals passing through the left and right atria appear on the ECG as a P wave, while sinus node activity is represented by the PR segment. The QRS wave of the ECG corresponds to the depolarization of the ventricles, while its T wave indicates their depolarization. Many disorders of the heart’s conduction system, called arrhythmias, produce irregular wave patterns that can be seen on the ECG. For this reason, some heart disorders are named based on their abnormal ECG readings.
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