which ions (including Na+, Ca2+ and K+) can pass. As potassium is highest within the cell, it is mainly potassium that passes through. This increased potassium in the neighbour cell causes the membrane potential to increase slightly, activating the sodium channels and initiating an action potential in this cell. (A brief chemical gradient driven efflux of Na+ through the connexon at peak depolarization causes the conduction of cell to cell depolarization, not potassium.) These connections allow for the rapid conduction of the action potential throughout the heart and are responsible for allowing all of the cells in the atria to contract together as well as all of the cells in the ventricles. Uncoordinated contraction of heart muscles is the basis for arrhythmia and heart failure. Channels Ion channels are proteins that change shape in response to different stimuli to either allow or prevent the movement of specific ions across a membrane. They are said to be selectively permeable. Stimuli, which can either come from outside the cell or from within the cell, can include the binding of a specific molecule to a receptor on the channel (also known as ligand-gated ion channels) or a change in membrane potential around the channel, detected by a sensor (also known as voltage-gated ion channels) and can act to open or close the channel. The pore formed by an ion channel is aqueous (water-filled) and allows the ion to rapidly travel across the membrane. Ion channels can be selective for specific ions, so there are Na+, K+, Ca2+, and Cl− specific channels. They can also be specific for a certain charge of ions (i.e. positive or negative). Each channel is coded by a set of DNA instructions that tell the cell how to make it. These instructions are known as a gene. Figure 3 shows the important ion channels involved in the cardiac action potential, the current (ions) that flows through the channels, their main protein subunits (building blocks of the channel), some of their controlling genes that code for their structure, and the phases that are active during the cardiac action potential. Some of the most important ion channels involved in the cardiac action potential are described briefly below. HCN channels Hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels) are located mainly in pacemaker cells, these channels become active at very negative membrane potentials and allow for the passage of both Na+ and K+ into