Describe the electrical activity that occurs during the cardiac cycle?

The cardiac cycle, by convention, is considered to begin with depolarisation of the sino-atrial node (SAN). This depolarisation spreads across the two atria, causing the right and left atria to contract. The depolarisation then reaches the atrio-ventricular node (AVN), where there is a pause to allow time for the ventricles to fill with blood. After this short gap, the atrio-ventricular node depolarises. This depolarisation then travels through special conductive tissue called the bundle of His, which carries the charge through the non-conductive atrioventricular septum.This conductive tissue then splits into two branches - called the left and right bundle branches - which travel down the interventricular septum to the bottom of the ventricles. From here, branches of conductive tissue called purkinje fibres travel up and around the ventricles. These branches release this depolarisation to cause contraction of the ventricles. As a result of the structure of the conductive tissue, the ventricles contract from the bottom upwards.