Question

Human Physiology question. Please answer this question in at least 2 paragraphs.

Describe in detail the action potential of a myocardiocyte (cardiac contractile cell). Be complete in your answer and explain all phases inclusive of all appropriate ions, their effect on the cell as well as the overall effect on the contractile cell as a whole. Lastly, what is the primary significant difference between the action potential of these cardiac cells as compared to a skeletal muscle cell?

Thank you for your help. :)

Answer 1

The action potential is the electrical stimulation which is created by a sequence of various ion flux via specialized channels in the membrane of cardiomyocytes. This in turn leads to cardiac contraction.

The action potential in cardiomyocytes consists of 5 phases (numbered 0-4), beginning and ending the phase 4.

Phase 4 – resting phase

In this phase, the resting potential is -90mV. This is due to constant outward leakage of potassium ions through the inward rectifier channels. The sodium and calcium channels are closed at the resting TMP.

Phase 0 – depolarization

In this phase, an action potential is triggered in neighbouring cardiomyocyte causing the TMP to increase beyond -90mV. The sodium channels begin to open up one after another and sodium ions leak into the cell, further increasing the TMP. So the TMP approaches -70 which is the threshold potential in cardiomyocytes. The large sodium ions rapidly depolarize the TMP to 0mV and slightly above 0mV for a transient period of time.

Phase 1 – early depolarization

In this phase, the TMP becomes slightly positive with some potassium ion channels opening briefly and an outward flow of potassium ions returning the TMP to nearly 0 mV.

Phase 2 – plateau phase

In this phase, the L-type calcium channels remain opened and there will be mild, constant inward current of calcium ions. This becomes significant in the excitation-contraction coupling process. The potassium ions leak out down the concentration gradient through delayed rectifier potassium channels. Hence, these two countercurrents become electrically balanced. The TMP will be maintained at a plateau slightly below 0mV throughout this phase.

Phase 3 – repolarization

During this phase, the calcium channels become increasingly activated. The constant flow of potassium ions, now exceeds the flow th calcium ions, bringing the TMP back to resting potential of -90 mV in order to prepare the cell for new cycle of depolarization. The normal transmembrane ionic concentration gradients are brought back by returning the sodium and calcium ions to the extracellular environment with the potassium ions to the interior of the cell. The pumps involved in the process include sarcolemmal Na+-Ca++ exchanger, Ca++-ATPase and Na-K-ATPase.

The difference between action potential in cardiac and muscle cells

1. Duration: it takes 2-5 ms in skeletal muscle cells and 200-400 ms in cardiac cells.
2. Role of calcium ions in depolarization: In skeletal muscles, the depolarization is caused by opening of sodium channels. In cardiac cells, the calcium ions are involved in the initial depolarization phase.