An action potential (AP) is the transient depolarization of a cell as a result of activity of ion channels. The cardiac AP is considerably longer than those of nerve or skeletal muscle (∼300 vs ∼2 ms). This is due to a plateau phase in cardiac muscle, lasting for 200–300 ms.

Ventricular Muscle Action Potential (Figure 11a)

Initiation of the Action Potential

At rest, the ventricular cell membrane is most permeable to K⁺ and the resting membrane potential (RMP) is therefore close to the K⁺ equilibrium potential (E_K), ∼–90 mV (see Chapter 10). An AP is initiated when the membrane is depolarized to the threshold potential (∼−65 mV). This occurs due to transmission of a depolarizing current from an adjacent activated cell through gap junctions (see Chapter 2). At threshold, sufficient voltage-gated Na⁺ channels are activated to initiate a self-regenerating process – the inward current caused by entry of Na⁺ (I_Na) through these channels causes further depolarization, which activates more Na⁺ channels, and so on. The outcome is a very large and fast I_Na, and therefore a very rapid AP upstroke (phase 0; ∼500 V/s).

Activation of Na⁺ channels during phase 0 means that the Na⁺ permeability is now much greater than that for K⁺, and so the membrane potential moves towards the Na⁺ equilibrium potential (E_Na, ∼+65 mV) (see Chapter 10). It does not reach E_Na because the Na⁺ channels rapidly inactivate as the potential nears +40 mV (see Chapter 10); this, and activation of a transient outward K⁺ current, can lead to a rapid decline in potential, leaving a spike (phase 1), best seen in Purkinje fibres (Figure 11d). The inactivated Na⁺ channels cannot be reactivated until the potential returns to less than −60 mV, so another AP cannot be initiated until the cell repolarizes (refractory period). The refractory period therefore lasts as long as the plateau and contraction (Figure 11a), so unlike skeletal muscle, cardiac muscle cannot be tetanized.

The Plateau (Phase 2)

By the end of the upstroke all Na⁺ channels are inactivated, and in skeletal muscle the cell would now repolarize. In cardiac muscle, however, the potential remains close to 0 mV for ∼250 ms. This plateau phase is due to opening of voltage-gated (L-type) Ca²⁺ channels

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