The heart is composed of pacemaker, conducting and contractile tissue. Each has a different action potential morphology allowing the heart to function as a coordinated unit.

The sino-atrial (SA) node is in the right atrium, and of all cardiac tissue it has the fastest rate of spontaneous depolarisation so that it sets the heart rate. The slow spontaneous depolarisation (pre-potential or pacemaker potential) of the membrane potential is due to increased Ca2+ conductance (directed inward). At −40 mV, slow voltage-gated Ca2+ channels (L channels) open, resulting in membrane depolarisation. Na+ conductance changes very little. Repolarisation is due to increased K+ conductance while Ca2+ channels close (see Figure 15.1a).