Ventricular action potential
Encyclopedia
In electrocardiography, the ventricular myocyte membrane potential is about -90 mV at rest, which is close to the potassium
reversal potential
. When an action potential
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
) in the cell membrane
selectively allow sodium
ion
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 repolarisation.
The depolarization of the membrane allows calcium
channels to open as well. As sodium channels close calcium provides current to maintain the potential around 10 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
. This current is observed in human and dog action potentials, but not in guinea pig action potentials.
Repolarization
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.
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
reversal potential
Reversal potential
In a biological membrane, the reversal potential of an ion is the membrane potential at which there is no net flow of that particular ion from one side of the membrane to the other...
. When an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
is generated, the membrane potential rises above this level in four distinct phases.
The beginning of the action potential, phase 0, specialized membrane proteins
Integral membrane protein
An integral membrane protein is a protein molecule that is permanently attached to the biological membrane. Proteins that cross the membrane are surrounded by "annular" lipids, which are defined as lipids that are in direct contact with a membrane protein...
(voltage-gated sodium channels
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...
) in the cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
selectively allow sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
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 repolarisation.
The depolarization of the membrane allows calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
channels to open as well. As sodium channels close calcium provides current to maintain the potential around 10 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
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
. This current is observed in human and dog action potentials, but not in guinea pig action potentials.
Repolarization
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
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
- Cardiac action potentialCardiac action potentialIn electrocardiography, the cardiac action potential is a specialized action potential in the heart, necessary for the electrical conduction system of the heart....