initiation and regulation of heartbeat

• the cardiac muscle in the heart is myogenic. It contracts and relaxed automatically and does not depend on stimulation by nerves
• the initial stimulus originates form the SAN (pacemaker). it consist of group of specialized autorhythmic cells located in the right atrial wall
• the membrane of the autorhythmic cells of SA node are permeable to sodium ions. Sodium ions enter into the cells and the cell membrane are depolarised. Excitatory electrical impulses are generated and spread rapidly throughout the two atria, causing the atria to contract simultaneously. A slowing down occurs as depolarisation of the atrioventricular node is delayed for about 0.1 second. This allows the atria to complete their contraction and empty the blood into the ventricles.
• After the brief delay, electrical impulses are conducted through specialised branched internodal junctional fibres to the AV node. Electrical impulses from AV node are conducted rapidly by bundle of His in the interventricular septum to the right and left bundle branches. The bundle of His fibres then divides into Purkinje fibres.
• Electrical impulses are conducted rapidly by Purkinje fibres toward the heart apex and then spread throughout the ventricular walls. The contraction of both ventricles begins at the heart apex and then upwards to other parts of the ventricles. This is to force the blood into the aorta and pulmonary artery.