The Heart's Electrical System

Your heart does not wait for the brain to tell it when to beat. Specialised heart cells generate electrical signals that organise each heartbeat, while nerves and hormones adjust the rate. Understanding that relationship explains what an ECG is actually recording.

Where each heartbeat starts Your heart can generate its own electrical signals without the brain sending a new nerve impulse for every beat. In a normal rhythm, a small group of specialised cells high in the upper right chamber ( right atrium ) starts the sequence. This group is the sinoatrial node (SA node) , often called the heart's natural pacemaker. Nerves and hormones can make the SA node generate signals faster or more slowly. They adjust the heart rate rather than starting each normal beat. How the signal reaches the lower chambers From the SA node, the electrical signal spreads through the two upper chambers ( atria ). It then reaches a small area between the upper and lower chambers called the atrioventricular node (AV node) . The signal slows briefly here. This gives the atria time to finish their contribution to filling the lower chambers before those chambers are activated. From the AV node, the signal travels rapidly through specialised electrical pathways in the lower chambers ( ventricles ). Together, these pathways form the ventricular conduction system . Detailed diagrams may label the bundle of His, bundle branches and Purkinje fibres within this system. Electrical activation comes before contraction When heart cells become electrically active ( depolarisation ), that electrical change helps trigger the muscle to contract. The electrical event and the contraction are connected, but they are not the same thing. Afterwards, the cells electrically recover ( repolarisation ), preparing them for another electrical cycle. What an ECG records An electrocardiogram (ECG) uses electrodes on the skin to record the voltage changes produced by the heart's electrical activity. In a typical beat during sinus rhythm, the P wave shows electrical activation of the upper chambers (atria). The QRS complex shows electrical activation of the lower chambers (ventricles). The T wave shows electrical recovery of the ventricles. The ECG therefore shows electrical events rather than the muscle squeezing itself. Feeling a pulse gives different information. It tells you the pulse rate and whether the beats feel regular, while the ECG shows the electrical pattern producing those beats. An irregular pulse may raise suspicion of an arrhythmia , meaning a problem with the heart's rhythm. The pulse alone usually cannot identify the exact rhythm. Some rhythm problems come and go. If the rhythm has returned to normal before a short ECG is recorded, the ECG may look normal. A clinician may then use longer monitoring when the timing and pattern of symptoms make an intermittent rhythm problem worth investigating.

The heart generates and conducts its own electrical signals. These signals help trigger contraction, but electrical activity and muscle movement are different events. An ECG records the electrical side of that sequence.