Describe the pathway of electrical activity in the heart during contraction.

The sino-atrial node (SAN) is located within the wall of the right atrium, and acts like a pacemaker (it instigates the rhythms of the heart). The SAN sends out regular waves of electrical activity along the atrial walls, this causes them to contract simultaneously. Between the atrial and ventricular tissue is a band of non-conductive collagen tissue, therefore the stimulation doesn't simply spread across the entire heart, instead it is relayed to the atrioventricular node (AVN). The AVN passes the electrical activity to the 'bundle of His' after a short delay (to allow the blood to have emptied from the atria). The bundle of His then conducts the electrical energy to fine muscle fibres called 'Purkinje fibres' which are within the walls of the left and right ventricles. They then cause muscular contraction from the bottom of the ventricles to the top, hence allowing the blood to be expelled from the heart via either the pulmonary artery or aorta.