Pacemaker Potential

Autorhythmicity is the ability of the heart to beat in the absence of external stimuli.

Pacemaker cells and the Pacemaker potential

Pacemaker (or autorhythmic) cells account for a small minority of cells in the myocardium.  The rest (99%) are contractile cells that are responsible for pumping the blood.  Pacemaker cells are concentrated at two ‘nodes’: the sinoatrial node (SA node), located in the right atrium near the superior vena cava; and the atrioventricular node (AV node), located at the base of the right atrium near the junction of the four heart chambers.  They are unique because they can spontaneously produce action potentials in rhythm to control the heart beat.  They do this because they exhibit a specialised membrane potential- the pacemaker potential.

The Pacemaker Potential

(see also membrane potential– this has background information)

The membrane potential in cardiac cells is different from the rest of the body due to 3 reasons:

  1. An increased influx of sodium (Na+)
    1. This is the main factor driving the pacemaker potential towards threshold.  Cardiac cells possess unique Na+ ion channels (‘funny channels’) that open in response to hyperpolarisation.  Therefore after an action potential, these open and allow sodium to flow into the cell, pushing the membrane potential towards the resting potential for sodium (60mV)
  2. A decreased efflux of potassium (K+)
    1. On top of the effect of sodium, potassium channels in the heart begin to close during hyperpolarisation (of the action potential).  This limits the amount of potassium that can leave the cell, pushing the membrane potential up further.
  3. Calcium flow is much more important
    1. Unlike other excitable cells, calcium plays an important role in the action potential of cardiac cells.  Transient calcium channels open just before threshold is reached (funny channels close at this point), pushing the membrane potential to threshold, then close.
    2. Once threshold is reached, voltage gated long-lasting calcium channels open to produce the action potential.

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