Explain how a nerve impulse causes the chambers of the heart to contract (6).

The heart's excitation is myogenic and therefore can be started without any transmission from the brain. The sino-atrial node is found behind the posterior wall of the left atrium and is where the action potential (AP) is intiated.The AP travels away from the SA node in a wave of excitation across both atria causing them to contract first and push blood into the ventricles. THe electrical excitation wave reaches the atrio-ventricular node (found between the left atrium and ventricle in the walls) where it is delayed briefly to allow all the atrial blood to be pushed into the ventricles. The AV node then stimulates an AP to flow down through the septum wall in the Purkinje fibres where they reach the apex or the heart, flowing up the ventricular walls causing them to contract and force blood into the circulation through the semi-lunar valves.