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Overview of Cardiac Pacemaker Action Potential

This action potential (AP) graph shows the fluctuations in the membrane potential of the nodal cells of the cardiac pacemaker. These fluctuations occur because of the activities of voltage-gated channels and ion pumps in the cell membrane (called the 'membrane clock') and activities of channels and pumps in the sarcoplasmic reticulum membrane (called the 'calcium clock'). This tutorial describes these activities.

The graph at the right shows how the charge at the inner surface of the cell membrane relative to the charge at the outer surface (membrane potential) changes over time. This pattern is repeated over and over. This cyclical pattern consists of:

• Depolarization (red) initiated by the opening of calcium channels leading to a rapid increase positive charges to the inner surface. It begins at a membrane potential (vertical axis in millivolts, mV) of ~-40mV (threshold) as shown on the graph.
• Repolarization (blue) is due to the loss of potassium ions that removes positive charges from the inner-membrane surface.
• Diastolic depolarization (green) (a.k.a.; pacemaker potential) begins at ~-60mV (mean diastolic potential, MDP) as shown on the graph. It initially involves both a gain of sodium ions through membrane channels leading to an interaction between calcium ions from the sarcoplasmic reticulum and external environment with sodium calcium exchangers in the cell membrane.

Depolarization

At 'threshold' calcium channels in the cell membrane open rapidly allowing a large influx of calcium ions. These ions strongly activate sodium calcium exchangers in the cell membrane that rapidly return one calcium ion to the outside in 'exchange' for three sodium ions. This occurs very rapidly causing the sharp upswing in the graph.

In addition to activating the exchangers, the 'external' calcium ions also trigger calcium channels of the sarcoplasmic reticulum to open. These add to the activation of the exchangers. The exchangers cease to function as the membrane potential enters the positive range and calcium ions remaining in the cytoplasm are returned to the sarcoplasmic reticulum.

Repolarization

Membrane calcium channels close very soon after opening while potassium channels are simultaneously opening. As the loss of positive becomes greater than the gain, the curve changes direction and heads downward. As the membrane potential becomes more negative the potassium channels to begin closing. Simultaneously, sodium (pacemaker) channels begin opening causing the steepness of this downward portion of the curve to begin to level out.

Diastolic Depolarization

• Early diastolic depolarization begins when the gain of positive charge (sodium ions) through pacemaker channels exceeds the loss of positive charge (potassium ions) causing the curve to change from a downward to an upward direction. This usually occurs at ~-60mV (MDP) as indicated in the graph.

  As the membrane potential becomes more positive calcium channels open allowing a small influx of ions. These few ions mildly activate sodium calcium exchangers in the cell membrane to remove one calcium ion (Ca²⁺) from the cell in exchange for three sodium ions (Na⁺).

• Late diastolic depolarization is characterized by the added activity of the sarcoplasmic reticulum. This organelle cyclically releases small 'spurts' of calcium ions, the size of which dependents on how many ions are stored within the organelle. These calcium ions increase the activation of the sodium calcium exchangers thus enhancing the rate of sodium entering the cell. This is reflected in the change in shape of the curve from linear to exponential.

Last updated: 2/13/2016

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