The heart’s electrical control system

The constant pumping action of the heart is maintained and co-ordinated by electrical messages relayed through the heart wall along specialized conducting pathways. These pathways are not true nerves, but modified muscle tissue. The messages, in the form of tiny electrical impulses, initiate muscular contractions in the various chambers of the heart in the correct sequence.

At the beginning of each heartbeat, impulses start at a small collection of tissue known as the sinoatrial node, high in the right atrium. This is the cardiac ‘pacemaker’ – so-called because it initiates each contraction and so sets the pace of the heartbeats. This ‘pacemaker’ rhythmically discharges small electrical impulses which spread through the walls of the two atria, causing them to contract.

High in the interventricular septum, at the junction between atria and ventricles, is a similarly specialized area called the atrioventricular node. This acts as a relay station, picking up the signal and passing it to electrically conducting tracts in the ventricles, which stimulate the muscles, causing them to contract strongly and squeeze blood into the circulation. This phase of high pressure is known as systole. Once this impulse has passed, the heart muscle relaxes, allowing passive refilling of the chambers with blood. This phase of low pressure is called diastole. Once the ventricles have been stimulated, the conductive system is for some time no longer active. However, it restores itself quickly, after which the next stimulus is generated and conducted. In this way, the rhythm of the heart is regulated by the sinus. That is why it is sometimes refered to as the pacemaker. The sinus is again influenced by the autonomic or involuntary nervous system. This system can stimulate the sinus and make it act faster or slower, depending on the needs of the body. If the stimulation conductive system is blocked in one way or another, the heart can still contract. But the flow of blood is reduced and irregular, sometimes resulting in unconsciousness because of an insufficient supply of oxygen to the brain. Repeated bouts of unconsciousness caused by heart block is the condition known medically as Stokes-Adams syndrome. Even with a heart block, stimuli occur in the tissue of the heart muscle itself, but the muscle does not have normal co-ordinated contractions.

An artificial pace-maker, such as the type implanted through surgery, can be used to take over the function of the sinus. At a certain rhythm it sends stimuli to the ventricles, so that these contract at regular intervals. The fine adjustment of the rhythm, which happens under normal circumstances, cannot be induced with an artificial pacemaker. However, nowadays there are pacemakers that can be adjusted to a number of speeds, so that most of the various needs of the human body can be met.