Describe the conduction system of the heart and explain how the atria and ventricles contract sequentially.

The heart consists of four chambers, two atria and two ventricle. The conduction system of the heart mediates synchronised contraction of the chambers to allow the heart to effectively pump blood. The conduction system contains two 'nodes' and various connecting fibers. The conduction system starts at the sino-atrial node (SA node) located in the right atrium, the node will spontaneously generate an action potential without external input (which is why a heart can beat outside of the body) however the action potential can be modified by external nervous input. The action potential generated in the SA node then spreads across the atria causing depolarisation and atrial contraction. Between the atria and the ventricles is a non-conducting layer formed by the valves of the heart. This stops the depolarisation from simply conducting straight down to the ventricles, instead it has to go through the second node, the atrio-ventricular node (AV node). Once this node is depolarised the action potential travels down the intraventricular septum in fibers called the bundle of his, then in the right and left bundle branches until the depolarisation reaches the apex of the heart. The action potential then travels up the sides of the ventricles in fibers known as purkinje fibers causing depolarisation of the ventricle walls. This allows for depolarisation to start at the bottom of the ventricles and travel up - creating a more effective pumping action.