which are responsible for the plateau phase of the action potential, and are named based on the speed at which they activate: slowly activating IKs, rapidly activating IKr and ultra-rapidly activating IKur. Calcium channels There are two voltage-gated calcium channels within cardiac muscle: L-type calcium channels ('L' for Long-lasting) and T-type calcium channels ('T' for Transient, i.e. short). L-type channels are more common and are most densely populated within the t-tubule membrane of ventricular cells, whereas the T-type channels are found mainly within atrial and pacemaker cells, but still to a lesser degree than L-type channels. These channels respond to voltage changes across the membrane differently: L-type channels are activated by more positive membrane potentials, take longer to open and remain open longer than T-type channels. This means that the T-type channels contribute more to depolarization (phase 0) whereas L-type channels contribute to the plateau (phase 2). Conduction system In the heart's conduction system electrical activity that originates from the sinoatrial node is propagated via the His-Purkinje network, the fastest conduction pathway within the heart. The electrical signal travels from the sinoatrial node (SAN), which stimulates the atria to contract, to the atrioventricular node (AVN) which slows down conduction of the action potential, from the atria to the ventricles. This delay allows the ventricles to fully fill with blood before contraction. The signal then passes down through a bundle of fibres called the bundle of His, located between the ventricles, and then to the purkinje fibers at the bottom (apex) of the heart, causing ventricular contraction. Other than the SAN, the AVN and purkinje fibres also have pacemaker activity and can therefore spontaneously generate an action potential. However, these cells usually do not depolarize spontaneously, simply because, action potential production in the SAN is faster. This means that before the AVN or purkinje fibres reach the threshold potential for an action potential, they are depolarized by the oncoming impulse from the SAN This is called "overdrive suppression". Pacemaker activity of these cells is vital, as it means that if the SAN were to fail, then the heart could continue to beat, albeit at a lower rate (AVN= 40-60 beats per minute, purkinje fibres = 20-40 beats per minute). These pacemakers will keep a patient alive until the emergency team arrives. An example of premature ventricular contraction, is the classic athletic heart syndrome. Sustained training of athletes causes a cardiac adaptation