Electrical Conduction System
The heart is influenced by the autonomic nervous system which can increase or decrease the heart rate in line with the requirements of the body.
However, due to an intrinsic regulating system, called the conduction system it is possible for the heart to go on beating without any direct stimulus from the nervous system.
This system is composed of specialised muscle tissue that generates and distributes the conduction that causes contraction of the cardiac muscle. These tissues are found in the sinus (or sinoatrial) node, atrioventricular node, bundle of His, bundle branches, and conduction myofibres.
When stimulated by electrical activity, muscle fibres contract and produce motion. In the heart, this electrical activity is referred to as depolarisation. The contraction causes the blood to be pumped around the body. Contracted chambers within the heart are termed systolic. Relaxation of the heart muscle is caused by electrical repolarisation. Relaxed chambers within the heart are termed diastolic.
Heartbeat Origination in the Sinus Node
The normal resting rate of self-excitation of the sinus node is about 75 times per minute in adults. Since this rate is faster than that of other cardiac muscle fibres, the sinus node is called the pacemaker.
The conduction continues to travel in a wave both downwards and leftwards, through both atria, depolarising each cell in turn and causing the atria to contract. It is this depolarisation that can be seen as the P wave on the ECG.
Atrioventricular Nodal Depolarisation
Eventually this conduction of depolarisation meets the atrioventricular node near the centre of the heart. It is the atrioventricular node that is the main cause of delay in conducting the impulse from the atria to the ventricles. This delay allows the atria to fill the ventricles with blood before they contract.
The depolarisation travels down the septum and along the bundle of His, before splitting to follow the left and right bundle branches. It continues onward to the conduction myofibres which distribute the action potential and thus depolarise the muscle cells of the ventricles.
As the left bundle branch is activated first, the depolarisation proceeds from left to right and may give rise to a small negative deflection within the ECG, referred to as the Q wave. At the same time the atria and sinus node start to repolarise and relax.
Early Ventricular Depolarisation
The wave of depolarisation continues down the septum and into the ventricular wall. Since the mass of the left ventricular wall is significantly greater than the right, the mean vector of depolarisation of the ventricular wall is to the left.
Late Ventricular Depolarisation
The rim of the ventricular muscle is the last to contract. The direction of depolarisation leads to the S wave on the ECG.
Repolarisation is the return of the membrane potential to the baseline and relaxation of the muscle. This gives a deflection on the ECG in the same direction as depolarisation because both polarity and direction are negated — the T wave.
Atrial and Ventricular Relaxation
When the heart has completely repolarised, the chambers are relaxed and there is no electrical activity until the sinus nodetriggers the start of the next heartbeat.