The Myogenic Control of Heart Rate

• Control of the basic heartbeat is myogenic, which means it originates in muscle tissue rather than from nerve impulses (neurogenic). This means that the heart will beat without any external stimulus.
• This intrinsic rhythm means the heart beats at around 60 times per minute.
• The rhythm of the heartbeat is set by a group of cells in the wall of the right atrium called the sinoatrial node (SAN), or the pacemaker.
• The SAN initiates a wave of depolarisation that causes atrial systole by stimulating the atria to contract.
• The Annulus fibrosus is a strip of non-conducting collagen tissue which prevents the depolarisation spreading straight to the ventricles. This ensures that the atria and ventricles do not contract at the same time. Instead, the depolarisation travels from the SAN to the atrioventricular node (AVN), a region of conducting tissue between atria and ventricles.
• There is a slight delay (0.1-0.2 seconds) before the AVN reacts, to ensure that the atria are completely empty before the ventricles contract.
• The AVN transfers the waves of electrical activity to the bundle of His, a group of muscle fibres responsible for passing the electrical activity down between the ventricles to the bottom of the heart (the apex).
• At the apex, the bundle of His divides into two conducting fibres, called Purkyne tissue, finer tissue that runs up the sides of the ventricle walls.
• The Purkyne fibres spread around the ventricles and initiate the depolarisation of the ventricle tissues, causing them to contract simultaneously from the apex up.
• This means that, during ventricular systole, blood is forced upwards and outwards through the pulmonary artery and aorta.
• Electrocardiography can be used to monitor the electrical activity of the heart.
• Electrodes that are capable of detecting electric signals are placed on the skin. These electrodes produce an electrocardiogram (ECG).
• An ECG shows the distinctive electrical waves produced by the activity of the heart. A healthy heart produces a distinctive shape in an ECG.
• The P wave is caused by the depolarisation of the atria, which results in atrial contraction (systole).
• The QRS complex is caused by the depolarisation of the ventricles, which results in ventricular contraction (systole). This is the largest wave because the ventricles have the largest muscle mass.
• The T wave is caused by the repolarisation of the ventricles, which results in ventricular relaxation (diastole).
• Scientists are still uncertain of the cause of the U wave, some think it is caused by the repolarisation of the Purkyne fibres.