Depolarization and repolarization

Virtually every cell in our body is electrically charged. Also there is a difference in charge between the inside of the cell (the intracellular space) and the outside of the cell (the extracellular space). This difference in electric charge between the intracellular space and extracellular space is a result of the difference in the concentration of positive and negatively charged substances between the intracellular space and extracellular space.

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The intracellular space is said to be at rest when no specific activation processes take place. At rest the intracellular space is negatively charged relative to the extracellular space. This is because of 3 main reasons:

  • The higher concentration of potassium in the intracellular space causes potassium diffusion to the extracellular space which creates a negative charge intracellularly
  • The Sodium-potassium pump in the cell membranes of cells continuously pumps 3 sodium ions to the extracellular space for every 2 potassium ions pumped into the intracellular space resulting in a net deficit of positive ions intracellularly
  • Proteins and phosphate ions, which are negatively charged substances, are more concentrated in the intracellular fluid

In the resting state the cell is “polarized” with two poles, a negative pole (the intracellular space) and a positive pole (the extracellular space). The difference in charge between the intracellular space and the extracellular space during the resting phase generates an electrical potential difference across the cell membrane; and this is known as the resting potential. In cardiac cells the resting potential is approximately -0.90 milliVolts (mV).

When a cell is electrically activated, various biochemical processes create a condition in which the inside of the cell becomes positively charged in relation to the outside, thus completely changing the initial difference in charge.

In this state the polarity of the cell is inverted and the cell is said to be “depolarized”. In the depolarized state, the inside of the cell becomes the positive pole and the outside of the cell becomes the negative pole.

The increase in membrane potential created between the inside and the outside of the cell during depolarization is called the action potential.

Once a cell is excited (depolarized), it can excite adjacent cells and "pass on" its action potential, spreading depolarization to more and more cells.

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Cell in rest

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Depolarized cell

This is a simple way of explaining the conduction or transmission of cardiac impulse from one cell to another. This mechanism governs multiple processes in the body, including the conduction of impulses in nerve cells and the controlled contraction of the cardiac muscle.

In contrast to depolarization, the process of repolarization does not actively trigger any mechanical activity, such as muscular contraction or relaxation. Repolarization is strictly an electrical process in which the resting potential is restored.