The cardiac action potential is a specialized action potential in the heart, with unique properties necessary for function of the electrical conduction system of the heart. The mole (symbol: mol) is the SI base unit that measures an amount of substance. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... The heart and lungs, from an older edition of Grays Anatomy. ... The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to (and stimulate) the myocardium (Cardiac muscle). ...

The cardiac action potential differs significantly in different portions of the heart. This differentiation of the action potentials allows the different electrical characteristics of the different portions of the heart. For instance, the specialized conduction tissue of the heart has the special property of depolarizing without any external influence. This is known as automaticity or myogenic rythmicity. In biology, depolarization is a decrease in the absolute value of a cells membrane potential. ... Automaticity is the the ability to do things without occupying the mind with the low level details required. ...

The electrical activity of the specialized conduction tissues are not apparent on the surface electrocardiogram (ECG). This is due to the relatively small mass of these tissues compared to the myocardium (muscle of the heart). “QRS” redirects here. ... Myocardium is the muscular tissue of the heart. ...

Overview

Cardiac muscle has some similarities to neurons and skeletal muscle, as well as important unique properties. Like a neuron, a given myocardial cell has a negative membrane potential when at rest. Stimulation above a threshold value induces the opening of voltage-gated ion channels and a flood of cations into the cell. When the threshold is met, an action potential initiates. This causes the positively charged ions to enter the cell [depolarization]. Like skeletal muscle, depolarization causes the opening of voltage-gated calcium channels and entry of Ca²⁺ from the t-tubules. This influx of calcium causes calcium-induced calcium release from the sarcoplasmic reticulum, and the increase in myoplasmic free Ca²⁺ concentration causes muscle contraction. After a delay (the absolute refractory period), Potassium channels reopen and the resulting flow of K⁺ out of the cell causes repolarization to the resting state. This article or section is in need of attention from an expert on the subject. ... Look up Threshold in Wiktionary, the free dictionary. ... Voltage-gated ion channel is a ion channel that is specifically activated, or gated, by the surrounding potential difference near the channel (or near the cell, neuron or synapse). ... An ion is an atom or group of atoms with a net electric charge. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Also called transverse tubules, t tubules are deep invaginations of the plasma membrane found in skeletal and cardiac muscle cells. ... Ryanodine receptors (RyRs) form a class of calcium channels in various forms of muscle and other excitable animal tissue. ... ... A top-down view of skeletal muscle A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) lengthens or shortens. ... It has been suggested that this article or section be merged into Refractory period. ... In cell biology, potassium channels are the most common type of ion channel. ... In neuroscience, repolarization refers to the change in membrane potential that returns the membrane potential to a negative value after the depolarization phase of an action potential has just previously changed the membrane potential to a positive value. ...

Note that there are important physiological differences between nodal cells and ventricular cells; the specific differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most importantly the spontaneous depolarizations (automaticity) necessary for the SA node's pacemaker activity.

Major currents

HERG (Human Ether-a-go-go Related Gene). ... Inwardly rectifing potassium channels (Kir or IRK) are potassium selective ion channels. ... The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger or exchange protein) is an antiporter ion pump membrane protein which removes calcium from cells. ... Simplified Diagram of the sodium pump Na+/K+-ATPase (also known as the Na+/K+ pump or Na+/K+ exchanger) is an enzyme (EC 3. ... Calcium ATPase is a form of ATPase which transfers calcium after a muscle has contracted. ...

Calcium channels

Two voltage-dependent calcium channels play critical roles in the physiology of cardiac muscle: L-type calcium channel ('L' for Long-lasting) and T-type calcium channels ('T' for Transient) voltage-gated calcium channels. This article needs to be cleaned up to conform to a higher standard of quality. ... Cardiac muscle is a type of involuntary mononucleated, or uninucleated, striated muscle found exclusively within the heart. ... The L-type calcium channel is a type of voltage-dependent calcium channel. ... The T-type calcium channel is a type of voltage-dependent calcium channel. ...

These channels respond differently to voltage changes across the membrane: L-type channels respond to higher membrane potentials, open more slowly, and remain open longer than T-type channels.

Because of these properties, L-type channels are important in sustaining an action potential, while T-type channels are important in initiating them.

Because of their rapid kinetics, T-type channels respond better to rhythmic stimulation and are also found in some neuron cell bodies, where they play an important role in rhythmic processes such as heartbeat, breathing, and spinal cord pattern generators used in walking.

L-type channels are selectively blocked by dihydropyridines. Pyridinium cation of pyridine. ...

Resting membrane potential

The resting membrane potential is caused by the difference in ionic concentrations and conductances across the membrane of the cell during phase 4 of the action potential. The normal resting membrane potential in the ventricular myocardium is about -85 to -95 mV. This potential is determined by the selective permeability of the cell membrane to various ions. The membrane is most permeable to K⁺ and relatively impermeable to other ions. The resting membrane potential is therefore dominated by the K⁺ equilibrium potential according to the K⁺ gradient across the cell membrane. The membrane potential can be calculated using the Goldman-Hodgkin-Katz voltage equation. The maintenance of this electrical gradient is due to various ion pumps and exchange mechanisms, including the Na⁺-K⁺ ion exchange pump, the Na⁺-Ca²⁺ exchanger current and the I_K1 inwardly rectifying K⁺ current. In biological cells that are electrically at rest, the cytosol possesses a uniform electric potential or voltage compared to the extracellular solution. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... In a biological membrane, the reversal potential of a particular ion is the membrane voltage at which there is no net flow of ions from one side of the membrane to the other. ... The Goldman-Hodgkin-Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the potential across a cells membrane taking into account all of the ions that are permeant through that membrane. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... Na+/K+-ATPase (also known as the Na+/K+ pump or Na+/K+ exchanger) is an enzyme (EC 3. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Atomic mass 40. ... Inwardly rectifing potassium channels (Kir or IRK) are potassium selective ion channels. ...

Intracellularly (within the cell), K⁺ is the principal cation, and phosphate and the conjugate bases of organic acids are the dominant anions. Extracellularly (outside the cell), Na⁺ and Cl^- predominate. A cation is an ion with positive charge. ... Above is a ball-and-stick model of the inorganic hydrogenphosphate anion (HPO42−). Colour coding: P (orange); O (red); H (white). ... Within the Brønsted-Lowry (protonic) theory of acids and bases, a conjugate acid is the acid member, HX, of a pair of two compounds that transform into each other by gain or loss of a proton. ... An organic acid is an organic compound that is an acid. ... An anion is an ion with negative charge. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... The chloride ion is formed when the element chlorine picks up one electron to form an anion (negatively-charged ion) Cl−. The salts of hydrochloric acid HCl contain chloride ions and are also called chlorides. ...

Phases of the cardiac action potential

The cardiac action potential has five phases.

The standard model used to understand the cardiac action potential is the action potential of the ventricular myocyte. The action potential has 5 phases (numbered 0-4). Phase 4 is the resting membrane potential, and describes the membrane potential when the cell is not being stimulated. Schematic of an action potential, drawn by User:Diberri in Adobe Illustrator. ... Schematic of an action potential, drawn by User:Diberri in Adobe Illustrator. ... Myocyte is the technical term for a muscle cell. ...

Once the cell is electrically stimulated (typically by an electric current from an adjacent cell), it begins a sequence of actions involving the influx and efflux of multiple cations and anions that together produce the action potential of the cell, propagating the electrical stimulation to the cells that lie adjacent to it. In this fashion, an electrical stimulation is conducted from one cell to all the cells that are adjacent to it, to all the cells of the heart. A cation is an ion with positive charge. ... An anion is an ion with negative charge. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ...

Phase 4

Phase 4 is the resting membrane potential. This is the period that the cell remains in until it is stimulated by an external electrical stimulus (typically an adjacent cell). This phase of the action potential is associated with diastole of the chamber of the heart. In biological cells that are electrically at rest, the cytosol possesses a uniform electric potential or voltage compared to the extracellular solution. ... Diastole is the period of time when the heart relaxes after contraction. ...

Certain cells of the heart have the ability to undergo spontaneous depolarization, in which an action potential is generated without any influence from nearby cells. This is also known as automaticity. The cells that can undergo spontaneous depolarization the fastest are the primary pacemaker cells of the heart, and set the heart rate. Usually, these are cells in the SA node of the heart. Electrical activity that originates from the SA node is propagated to the rest of the heart. The fastest conduction of the electrical activity is via the electrical conduction system of the heart. The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... The normal electrical conduction in the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to (and stimulate) the myocardium (Cardiac muscle). ...

In cases of heart block, in which the activity of the primary pacemaker does not propagate to the rest of the heart, a latent pacemaker (also known as an escape pacemaker) will undergo spontaneous depolarization and create an action potential. A heart block is a disease in the electrical system of the heart. ...

The mechanism of automaticity involves the so-called pacemaker channels of the HCN family, Hyperpolarization-gated, Cyclic Nucleotide-gated channels. These poorly selective cation channels conduct more current as the membrane potential becomes more negative, or hyperpolarized. They conduct both potassium and sodium. The activity of these channels in the SA node cells causes the membrane potential to slowly become more positive (depolarized) until, eventually, calcium channels are activated and an action potential is initiated. The term pacemaker has multiple meanings: In sports, a pacemaker or pacer is a competitor who enters an athletics race with little or no intention of winning, but purely to set a fast pace for other competitors to follow. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ... Hyperpolarization has several meanings: In biology, hyperpolarization occurs when a cells membrane potential dips below its resting level. ... A cyclic nucleotide is any nucleotide in which the phosphate group is bonded to two of the sugars hydroxyl groups, forming a cyclical or ring structure. ...

Phase 0

Phase 0 is the rapid depolarization phase. The slope of phase 0 represents the maximum rate of depolarization of the cell and is known as V_max. This phase is due to the opening of the fast Na⁺ channels causing a rapid increase in the membrane conductance to Na⁺ (G_Na) and thus a rapid influx of Na⁺ ions (I_Na) into the cell; a Na⁺ current. Look up Slope in Wiktionary, the free dictionary. ... Ion channels are pore-forming proteins that help to establish and control the small voltage gradient that exists across the plasma membrane of all living cells (see cell potential) by allowing the flow of ions down their electrochemical gradient. ...

The ability of the cell to open the fast Na⁺ channels during phase 0 is related to the membrane potential at the moment of excitation. If the membrane potential is at its baseline (about -85 mV), all the fast Na⁺ channels are closed, and excitation will open them all, causing a large influx of Na⁺ ions. If, however, the membrane potential is less negative, some of the fast Na⁺ channels will be in an inactivated state insensitive to opening, thus causing a lesser response to excitation of the cell membrane and a lower V_max. For this reason, if the resting membrane potential becomes too positive, the cell may not be excitable, and conduction through the heart may be delayed, increasing the risk for arrhythmias. Cardiac arrhythmia is a group of conditions in which the muscle contraction of the heart is irregular or is faster or slower than normal. ...

The fast Na⁺ channel

The fast sodium channel can be modeled as being controlled by a number of gates. Each gate (or gating variable) can attain a value between 1 (fully open) and 0 (fully closed). The product of all the gates denotes the percentage of channels available to conduct Na⁺. Following the model of Hodgkin and Huxley, the sodium channel contains three gates: m, h, and j. In the resting state, the m gate is closed (zero) and the h and j gates are open (one). Hence, the product denoting the percentage of conducting channels is also zero. Upon electrical stimulation of the cell, the m gate opens quickly while simultaneously the h and j gates close more slowly. For a brief period of time, all gates are open (i.e. non-zero) and Na⁺ can enter the cell following its electrochemical gradient. If, as above, the resting membrane potential is too positive, the h or j gates may be considerably less than one, such that the product of m, h and j becomes too small upon depolarization. Sodium channels are integral membrane proteins that exist in a cells plasma membrane and regulate the flow of sodium (Na+) ions into it. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... The Hodgkin-Huxley Model is a set of non-linear ordinary differential equations, named after Alan Lloyd Hodgkin and Andrew Huxley, that approximates the electrical characteristics of excitable cells such as neurons and cardiac myocytes. ... In cellular biology, an electrochemical gradient refers to the electrical and chemical properties across a membrane. ...

Phase 1

Phase 1 of the action potential occurs with the inactivation of the fast Na⁺ channels. The transient net outward current causing the small downward deflection of the action potential is due to the movement of K⁺ and Cl^- ions, carried by the I_to1 and I_to2 currents, respectively. Particularly the I_to1 contributes to the "notch" of some ventricular cardiomyocyte action potentials. Sodium channels are integral membrane proteins that exist in a cells plasma membrane and regulate the flow of sodium (Na+) ions into it. ...

It has been suggested that Cl^- ions movement across the cell membrane during Phase I is as a result of the change in membrane potential, from K⁺ efflux, and is not a contributory factor to the initial repolarisation ("notch").

Phase 0 and 1 together correspond to the QRS complex of the ECG. However, note that the normal EKG's QRS deflection corresponds to the surface reading of the wave of electrical activity as myocytes are activated by the His-Purkinje system, while the tracings illustrated on this page have been recorded intracellularly from single myocytes. “QRS” redirects here. ...

Phase 2

Phase 2 of the action potential corresponds to the ST segment of the ECG.

This "plateau" phase of the cardiac action potential is sustained by a balance between inward movement of Ca²⁺ (I_Ca) through L-type calcium channels and outward movement of K⁺ through the slow delayed rectifier potassium channels, I_Ks. The sodium-calcium exchanger current, I_Na,Ca and the sodium/potassium pump current, I_Na,K also play minor roles during phase 2. This article needs to be cleaned up to conform to a higher standard of quality. ... In cell biology, potassium channels are the most common type of ion channel. ... KvLQT1 is a potassium channel protein coded for by the gene KCNQ1. ... The sodium-calcium exchanger (often denoted Na+/Ca2+ exchanger or exchange protein) is an antiporter ion pump membrane protein which removes calcium from cells. ... The title given to this article is incorrect due to technical limitations. ...

Phase 3

During phase 3 of the action potential, the L-type Ca²⁺ channels close, while the slow delayed rectifier (I_Ks) K⁺ channels are still open. This ensures a net outward current, corresponding to negative change in membrane potential, thus allowing more types of K⁺ channels to open. These are primarily the rapid delayed rectifier K⁺ channels (I_Kr) and the inwardly rectifiyng K⁺ current, I_K1. This net outward, positive current (equal to loss of positive charge from the cell) causes the cell to repolarize. The delayed rectifier K⁺ channels close when the membrane potential is restored to about -80 to -85 mV, while I_K1 remains conducting throughout phase 4, contributing to set the resting membrane potential. To meet Wikipedias quality standards, this article or section may require cleanup. ... KvLQT1 is a potassium channel protein coded for by the gene KCNQ1. ... In cell biology, potassium channels are the most common type of ion channel. ... This article or section is in need of attention from an expert on the subject. ... HERG (Human Ether-a-go-go Related Gene). ... Inwardly rectifing potassium channels (Kir or IRK) are potassium selective ion channels. ...

Phase 3 of the action potential in ventricles corresponds to the T wave on the ECG. “QRS” redirects here. ...

Image File history File links Pacemaker_potential. ...

Abnormal automaticity

The normal activity of the pacemaker cells of the heart is to spontaneously depolarize at a regular rhythm, generating the normal heart rate. Abnormal automaticity involves the abnormal spontaneous depolarization of cells of the heart. This typically causes arrhythmias (irregular rhythms) in the heart. Cardiac arrhythmia is a group of conditions in which the muscle contraction of the heart is irregular or is faster or slower than normal. ...

See also

• Electrical conduction system of the heart
• Action potential
• Antiarrhythmic agents
• Cardiac arrhythmia
• Cardiac pacemaker
• Resting membrane potential
• Ventricular action potential