Electrical cardiac cycle

Answer 1

The heart is myogenic - it is able to beat on its own even after the organism that posessed the heart has died, if placed in well oxygenated Ringer's solution at 37 degrees Celsius. The heart controls the heartbeat electrically, as follows:

* The heartbeat originates at the sinoatrial node (SAN) which is made up of diffusely orientated cardiac fibres and nerve endings from the Autonomic Nervous system (ANS). The SAN initiates the heartbeat, but the rate at which the heart beats can be altered by stimulation from the ANS (the vagus nerve slows heart rate whilst the sympathetic nerve increases heart rate). * The cells of the SAN maintain a differential ionic gradient of -90mV across their membranes. When Sodium ions diffuse into the cells, it leads to depolarisation, and a wave of excitation is spread across the cells of the SAN and is transmitted across the atria at a rate of 1m/s. The atria contract simulataneously. * There is a septum of non-conduction tissue between the atria and the ventricles. This prevents the wave of excitation from spreading directly from the atria to the ventricles. * Instead, the wave of excitation has to travel through a small area of conductive tissue between the atria and ventricles known as the Atrio-ventricular node (AVN).

* This takes longer - around 0.1-0.15 seconds for the wave of excitation to be spread from atria and ventricles, and it means that there is enough time for atrial systole to be completed before ventricular systole occurs. (N.B systole means contraction). * The wave of excitation then travels along the Purkynje fibres which transmit the wave of excitation down the septum of the heart, to the apex, and then spreads upwards. As a result the ventricles contrct simulataneously. * Once the heart has started to contract it is unable to respond to any other stimulus until it has started to relax - it is said to be in the refractory period. * Where there is lack of coordination in the contraction of heart tissue, fibrillation is said to occur - the heart does not contract and relax as a whole - instead it simply 'flutters' and is unable to pump oxygenated blood around the body effectively. Fibrillation can be reversed by using defibrillators to give an electric shock to the heart which makes the heart contract as a whole again.