Action potential of the myocardial cell

RESTING MEMBRANE POTENTIAL

Phase 4 ‘Diastole’

The resting membrane potential is about —90 mv. The onset of depolarisation (phase O) does not occur spontaneously but must be induced. This induced depolarisation (which gives rise to the inside of the cell becoming positive relative to the outside) is inevitably followed spontaneously by the slow & complex process of repolarisation (phases 2 & 3) before the resting potential (phase 4) is restored.

Triggering of cardiac cell

Contraction of cardiac muscle is triggered by action potential that sweep across the muscle cell membranes. Influx of Na+ from the extracellular fluid into the cardiac cell leads to a positive feedback cycle that initiates the rising phase of the AP (phase O) by opening voltage-regulated Na+ channels.

DEPOLARISATION

Phase 0 ‘Upstroke’

The induction of phase O of the action potential is produced by the sudden dramatic rise in the membrane permeability to Na⁺; resulting from the opening of the membrane Na⁺ channels. This period of increased Na+ is very brief because the Na+ gates are quickly inactivated & closed.

Note that it is the change in ion permeability rather than the ionic flux which induces the change in transmembrane voltage.

Note: an action potential will only ensue if the stimulus is sufficient to raise the membrane potential at least to a critical level (threshold potential: approx -60mv).

REPOLARISATION

Phase 1 ‘Early-fast repolarisation’

Phase 2 ‘Plateau’

Ca²⁺ Increased intracellular by 2 mechanisms; Both mechanisms serves (via troponin) as the signal for cross bridge activation and couples the depolarisation wave to myocardial cell contraction.

1) Transmission of the depolarisation wave down the T—tubules causes Ca²⁺ release into the sarcoplasm by the sarcoplasmic reticulum. Ca^2+.

2) The Na+ dependent membrane depolarisation also opens Ca²⁺ channels (slow channels) allowing Ca²⁺ to enter the cell from the extracellular space.

Although the Na+ permeability has plummeted to its resting level and repolarisation has already began at this point, the Ca²⁺ surge across the membrane prolongs the depolarisation potential producing a plateau in the AP. At the same time, K⁺ permeability decreases which also prolongs the plateau & prevents repolarisation.

Phase 3 ‘Repolarisation’

Eventually the slope of the AP falls rapidly. This results from closure of the Ca²⁺ channels and opening of the K⁺ channels, which allows a rapid efflux of K⁺ from the cell that restores the resting membrane potential.

During replolarisation, Ca²⁺ is pumped back into the SR and the extracellular space.

K. C. Potger
Copyright © 2001