Excitability, pacemaking and conduction

The initial event starting the normal heartbeat is the spontaneous activity in the SA node. Most cardiac cells do not possess normal automaticity, but must be excited by electric current flow. In order to generate an action potential, the cells must be depolarised to a critical potential (the threshold potential Etor Vt): sufficient channels carrying inward current (Na+or Ca2+current) must be opened to overcome the outward current. If this is the case, the cells generate a net inward current, which will cause further depolarisation and thereby start a regenerative action potential. Excitability of the cell depends on the balance of ionic currents flowing in the potential region between resting and threshold potential. Excitability is therefore influenced by many different factors affecting individual ionic currents.