electrocardiography Electrocardiography is the process of producing an electrocardiogram (ECG or EKG), a recording of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of t ...

, the ventricular

cardiomyocyte Cardiac muscle (also called heart muscle or myocardium) is one of three types of vertebrate muscle tissues, the others being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that constitutes the main tissue of the wall of ...

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. It equals the interior potential minus the exterior potential. This is th ...

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

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

reversal potential. When an

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

is generated, the membrane potential rises above this level in five distinct phases. # Phase 4: Resting membrane potential remains stable at ≈−90 mV. # Phase 0: Rapid depolarisation, shifting the voltage to positive. Specialised membrane proteins ( voltage-gated sodium channels) in the

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

selectively allow

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

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 convent ...

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. # Phase 1: Rapid repolarisation. # Phase 2: Plateau, the longest phase, approximately 100ms. # Phase 3: Rapid repolarisation, which returns the membrane potential to resting potential. 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; it has 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 to it ...

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 (biology), cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolar ...

. 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.

References

{{DEFAULTSORT:Ventricular Action Potential Cardiac electrophysiology Action potentials

See also

References