page_title: Cardiac physiology
section_title: Membrane potentials and ion movement in cardiac conductive cells
hierarchical_section_title: Cardiac physiology / Electrical conduction / Membrane potentials and ion movement in cardiac conductive cells
caption_attribution_description: Illustration from Anatomy &amp; Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013.
context_page_description: Cardiac physiology or heart function is the study of healthy, unimpaired function of the heart: involving blood flow; myocardium structure; the electrical conduction system of the heart; the cardiac cycle and cardiac output and how these interact and depend on one another.
context_section_description: Action potentials are considerably different between conductive and contractive cardiomyocytes. While sodium Na⁺ and potassium K⁺ ions play essential roles, calcium ions Ca²⁺ are also critical for both types of cell. Unlike skeletal muscles and neurons, cardiac conductive cells do not have a stable resting potential. Conductive cells contain a series of sodium ion channels that allow a normal and slow influx of sodium ions that causes the membrane potential to rise slowly from an initial value of −60 mV up to about –40 mV. The resulting movement of sodium ions creates spontaneous depolarization (or prepotential depolarization).
  At this point, calcium channels open and Ca²⁺ enters the cell, further depolarizing it at a more rapid rate until it reaches a value of approximately +5 mV. At this point, the calcium ion channels close and potassium channels open, allowing outflux of K⁺ and resulting in repolarization. When the membrane potential reaches approximately −60 mV, the K⁺ channels close and Na⁺ channels open, and the prepotential phase begins again. This process gives the autorhythmicity to cardiac muscle.
caption: The prepotential is due to a slow influx of sodium ions until the threshold is reached followed by a rapid depolarization and repolarization. The prepotential accounts for the membrane reaching threshold and initiates the spontaneous depolarization and contraction of the cell; there is no resting potential.[1]
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