1.5.2 The action potential
Phase 0 represents the depolarisation process, and begins once a cell has reached a certain potential (the threshold potential), spontaneously in the case of pacemaker cells, or as a result of normal depolarisation of adjacent cells in the case of conduction pathway and myocardial cells. In fast-conducting cells such as the Purkinje network and the myocardium, a rapid influx of sodium is responsible for this change in potential. In the slower conducting pacemaker tissues of the SA and AV nodes, the resting potential of the cell membrane is less negative than non-pacemaker cells (around -60 mV). This results in a decreased rate of phase 0 depolarisation and relatively slow conduction. In pacemaker cells, during phase 0 the contribution of the fast inward current carried by sodium is small, and the relatively slow depolarisation process is largely due to movement of calcium ions. Consequently, these tissues are relatively sensitive to changes in calcium concentration and drugs which affect calcium channels.
The plateau phase
The depolarisation phase is followed by a relatively small but sharp drop in potential (phase 1) and a plateau phase (phase 2), during which a complex interaction of ion movements involving sodium, potassium, calcium, magnesium and chloride, associated with different channels and pumps, results in some decrease of the intracellular potential towards zero.
After the plateau phase, repolarisation takes place. Sodium is pumped out of the cell in exchange for potassium, resulting in a return of the membrane potential to the resting level (phase 3). Repolarisation must occur before another impulse can be transmitted, or before the myocardial cells can contract again. This gives cardiac tissue the property of refractoriness. Abnormalities of the rate of repo(and therefore the degree of refractoriness) have a very important role in the generation of some arrhythmia’s (section 7.4).
An important property of certain specialised conducting cells and to a lesser extent all conducting tissues within the heart is that they are capable of spondepolarisation and repolarisation independent of external neural stiThis property is known as automaticity. The cells with the fastest rate of automaticity are known as pacemaker cells. There is a hierarchy of cells with different rates of automaticity. Under normal circumstances, the SA node has the highest rate and it therefore acts as the pacemaker.
Automaticity depends on phase 4 of the cardiac action potential. The rate of change in potential is dependent on a time-related change in membrane potassium permeability and is influenced by autonomic tone, electrolytes, drugs and disease. The steeper the slope, the faster the rate of automaticity. Phase 4 is steepest in SA nodal cells but, if these cells fail to depolarise, other tissues which have automaticity will spontaneously depolarise to initiate an impulse. The AV node and junctional tissue also have automaticity and will become the dominant pacemaker if the SA node fails to depolarise. In some disease states, Purkinje tissue may also act as a pacemaker and abnormal automaticity may also occur in other cells.