Electrical Activity in the Heart
The heart is a pretty clever organ - it can contract on its own without being stimulated by nerve impulses. It beats in a regular pattern almost continuously but sometimes things do go wrong, such as an abnormally fast or slow rhythm. Irregularities in our heart beat are detected by medics using an electrocardiograph.
Myogenic contraction
Cardiac muscle is myogenic, which means that it can contract on its own accord without being stimulated by nerve impulses. It is coordinated by a nodule in the wall of the right atrium called the sino-atrial node (SAN). The SAN is referred to as the ‘pacemaker’ of the heart because it sets the rhythm of heart contractions by creating regular waves of electrical activity which spread out to the atrial walls. This wave of electrical activity causes the right and left atria to contract at the same time. There is a band of non-conducting tissue between the atria and the ventricles which prevents the wave of electricity from reaching the ventricles. The electrical wave is instead directed towards the atrioventricular node (AVN) which is located between the atria and the ventricles. The AVN passes the wave of electrical activity onto a group of muscle fibres located in the centre of the heart, called the bundle of His. There is a delay before the wave of excitation passes onto the bundle of His to ensure that the ventricles only contract after the atria have finished emptying with blood. The bundle of His then passes on the wave of electrical activity to the Purkyne fibres, found along the bottom of the heart around the ventricular walls. This causes the left and right ventricles to contract simultaneously, from the bottom up.
Electrocardiographs
An electrocardiograph is a machine which can record the electrical activity of the heart. Whenever a part of the heart contracts, the membrane of muscle cells becomes depolarised (it loses charge) and repolarises (gains charge) when the muscle relaxes. This change in charge in the heart muscle can be detected by an electrocardiograph, which uses electrodes placed against the patient’s chest. The change in electrical activity is displayed as an electrocardiogram (ECG) which can be used by doctors to diagnose heart problems.
The first, small bump on an ECG is called the P wave which is caused by the atria contracting. The large, sharp spike is called the QRS complex and is caused by the ventricular contraction. The final bump is referred to as the T wave and results from repolarisation of the ventricles as they relax. You can pick out any of these points on the ECG to measure the time taken for one heart beat to occur - e.g. the time taken from one P wave to the next P wave. The height of the wave is proportional to the strength of contraction - the higher the wave, the greater the depolarisation of the muscle cell and the stronger the contraction.
ECG can be used to diagnose heart problems such as tachycardia (abnormally fast heart rate), bradycardia (abnormally slow heart rate), ectopic heartbeats (‘extra’ heartbeats) and fibrillation (complete loss of rhythm). The image below shows how the ECG from patients with some of these heart problems would look. Abnormal heart rates or fibrillation may be a consequence of cardiovascular disease or other heart conditions, such as damage to the heart muscle.
Did you know…
The heart pumps a hell of a lot of blood - over five litres every minute. Over a lifetime that’s enough blood to fill a whole football stadium. Once you have reached middle age, the heart shrinks slightly and has a reduced capacity - every year after middle age the volume of blood pumped by the heart reduces by about 5 per cent.
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