the cell (which is a movement is known as a funny current, If). These poorly selective, cation (positively charged ions) channels conduct more current as the membrane potential becomes more negative (hyperpolarised). The activity of these channels in the SAN cells causes the membrane potential to depolarise slowly and so they are thought to be responsible for the pacemaker potential. Sympathetic nerves directly affect these channels, resulting in an increased heart rate (see below). The fast sodium channel These sodium channels are voltage-dependent, opening rapidly due to depolarization of the membrane, which usually occurs from neighboring cells, through gap junctions. They allow for a rapid flow of sodium into the cell, depolarizing the membrane completely and initiating an action potential. As the membrane potential increases, these channels then close and lock (become inactive). Due to the rapid influx sodium ions (steep phase 0 in action potential waveform) activation and inactivation of these channels happens almost at exactly the same time. During the inactivation state, Na+ cannot pass through (absolute refractory period). However they begin to recover from inactivation as the membrane potential becomes more negative (relative refractory period). Potassium channels The two main types of potassium channels in cardiac cells are inward rectifiers and voltage-gated potassium channels. Inwardly rectifying potassium channels (Kir) favour the flow of K+ into the cell. This influx of potassium, however, is larger when the membrane potential is more negative than the equilibrium potential for K+ (~-90 mV). As the membrane potential becomes more positive (i.e. during cell stimulation from a neighbouring cell), the flow of potassium into the cell via the Kir decreases. Therefore, Kir is responsible for maintaining the resting membrane potential and initiating the depolarization phase. However, as the membrane potential continues to become more positive, the channel begins to allow the passage of K+ out of the cell. This outward flow of potassium ions at the more positive membrane potentials means that the Kir can also aid the final stages of repolarisation. The voltage-gated potassium channels (Kv) are activated by depolarization. The currents produced by these channels include the transient out potassium current Ito1. This current has two components. Both components activate rapidly, but Ito,fast inactivates more rapidly than Ito, slow. These currents contribute to the early repolarization phase (phase 1) of the action potential. Another form of voltage-gated potassium channels are the delayed rectifier potassium channels. These channels carry potassium currents