Describe the path of electrical activity through the heart, starting at the sino-atrial node.

The electrical activity is initiated by the pacemaker of the heart, the sino-atrial node at the top of the left atrium. This spreads across the atria before reaching the atrio-ventricular node. Here there is a slight pause before the electrical wave continues down through the septum through purkinje fibres in the bundle of HIS to the apex of the heart where it splits into 2 branches. One branch goes to the left ventricle, the other to the right. Here the electrical activity is released into the cardiac muscle causing it to contract from the bottom up, squeezing blood into the aorta and pulmonary artery.