What causes the heart to contract?

This type of question requires careful explanation and therefore would need to be broken down into chunks. One of the reasons for this is that it may be completely new concepts for students. My answer to a student would follow a similar style to that below.The heart muscle is formed from cardiac muscle cells, which are a specific type of muscle cell that is different to smooth or skeletal muscle cells. The heart contracts in a sequence where the upper chambers (atria) contract first followed by a slight pause. Next the lower chambers (ventricles) contract. The mechanism that causes the chambers to contract at different times is via an electrical conduction system within the muscle of the heart. This begins in the sino atrial node which is a group of cells near the opening of the vena cava in the right atrium. This group of cells has an array of ion channels - Na+, Ca+ and K+. The movement of ions freely though these channels causes a 'funny current' by which these cells become depolarized. Once they reach a threshold potential, adjacent cardiac muscle cells in the atria are depolarized and this leads the propagation of a wave of depolarization that causes cardiac muscle cells to contract. This is what causes the atria to contract. Next the depolarization wave reached the atrio ventricular node and here the depolarization moves very slowly through the cells, this is what allows for the slight pause. The purpose of this pause is to allow blood to flow from the atria to the ventricles. Then, instead of spreading through all of the cardiac muscle cells, the wave of depolarization is constrained to the conduction system. Initially, it is the bundle of His, which splits into left and right branches and then into the Pyrkinje fibres that run along the ventricle walls. This allows the wave of depolarization and therefore the speed of contraction of the cardiac muscle cells to occur at a greater rate. (Here I would also like to explain, with the use of a whiteboard, how the conduction through the pirkinje system up and out forces blood into the aorta and pulmonary arteries at high velocity).