Describe the way in which the normal heart beat is initiated and controlled

The heart muscle is myogenic, which means that it can contract without any input from the nervous system. The stimulus for the muscle to contract originates from a group of specialised cardiac cells, the sino-atrial node (SAN), in the wall of the right atrium. The electrical impulse from the SAN causes both the atria to contract. A ring of fibrous connective tissue prevents the impulse from passing directly from the SAN to the ventricles. This also prevents the atria and the ventricles from contracting at the same time. In order for the impulse to pass, it must pass through another group of specialised cardiac cells forming the atrio-ventricular node (AVN). The AVN passes the impulse along heart muscle fibres called the Bundle of His, which passes in between the two ventricles in the interventricular septum. The impulse passes along the Bundle of His to the apex of the ventricles and back upwards along the Purkinje fibres, causing a contraction of the ventricles from the apex upwards. While the heart does not need any nervous stimulation to contract, nerves and hormones can modulate the heart beat. For instance, intense exercise or hormones such as adrenaline can alter the heart rate.