In electrocardiography, the ventricular

cardiomyocyte Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...

membrane potential is about −90 mV at rest, which is close to the

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

reversal potential In a biological membrane, the reversal potential is the membrane potential at which the direction of ionic current reverses. At the reversal potential, there is no net flow of ions from one side of the membrane to the other. For channels that are pe ...

. When an

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells, ...

is generated, the membrane potential rises above this level in four distinct phases. The beginning of the action potential, phase 0, specialized membrane proteins (

voltage-gated sodium channels Sodium channels are integral membrane proteins that form ion channels, conducting sodium ions (Na+) through a cell's membrane. They belong to the superfamily of cation channels and can be classified according to the trigger that opens the chann ...

) in the

cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

selectively allow

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s to enter the cell. This causes the membrane potential to rise at a rate of about 300 V/s. As the membrane voltage rises (to about 40 mV) sodium channels close due to a process called inactivation. The Na⁺ channel opening is followed by inactivation. Na⁺ inactivation comes with slowly activating Ca²⁺ channels at the same time as a few fast K⁺ channels open. There is a balance between the outward flow of K⁺ and the inward flow of Ca²⁺ causing a plateau of length in variables. The delayed opening of more Ca²⁺-activated K⁺ channels, which are activated by build-up of Ca²⁺ in the sarcoplasm, while the Ca²⁺ channels close, ends the plateau. This leads to repolarization. The depolarization of the membrane allows

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

channels to open as well. As sodium channels close calcium provides current to maintain the potential around 20 mV. The plateau lasts on the order of 100 ms. At the time that calcium channels are getting activated, channels that mediate the transient outward potassium current open as well. This outward potassium current causes a small dip in membrane potential shortly after

depolarization In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is ess ...

. This current is observed in human and dog action potentials, but not in guinea pig action potentials.

Repolarization In neuroscience, repolarization refers to the change in membrane potential that returns it to a negative value just after the depolarization phase of an action potential which has changed the membrane potential to a positive value. The repolarizat ...

is accomplished by channels that open slowly and are mostly activated at the end of the action potential ( slow delayed-rectifier channels) and channels that open quickly but are inactivated until the end of the action potential ( rapid delayed rectifier channels). Fast delayed rectifier channels open quickly but are shut by inactivation at high membrane potentials. As the membrane voltage begins to drop the channels recover from inactivation and carry current.

See also