Cardiac action potential
Unlike the
Rate dependence of the action potential is a fundamental property of cardiac cells and alterations can lead to severe cardiac diseases including cardiac arrhythmia and sometimes sudden death.[3] Action potential activity within the heart can be recorded to produce an
Overview
Element | Ion | Extracellular | Intracellular | Ratio |
---|---|---|---|---|
Sodium | Na+ | 135 - 145 | 10 | 14:1 |
Potassium | K+ | 3.5 - 5.0 | 155 | 1:30 |
Chloride | Cl− | 95 - 110 | 10 - 20 | 4:1 |
Calcium | Ca2+ | 2 | 10−4 | 2 x 104:1 |
Although intracellular Ca2+ content is about 2 mM, most of this is bound or sequestered in intracellular organelles (mitochondria and sarcoplasmic reticulum).[5] |
Similar to skeletal muscle, the
The action potential begins with the voltage becoming more positive; this is known as
There are important physiological differences between the
Cardiac automaticity
Cardiac automaticity also known as autorhythmicity, is the property of the specialized
Phases
The standard model used to understand the cardiac action potential is that of the ventricular myocyte. Outlined below are the five phases of the ventricular myocyte action potential, with reference also to the SAN action potential.
Phase 4
In the ventricular myocyte, phase 4 occurs when the cell is at rest, in a period known as
The activity of these pumps serve two purposes. The first is to maintain the existence of the resting membrane potential by countering the depolarisation due to the leakage of ions not at the electrochemical equilibrium (e.g. sodium and calcium). These ions not being at the equilibrium is the reason for the existence of an electrical gradient, for they represent a net displacement of charges across the membrane, which are unable to immediately re-enter the cell to restore the electrical equilibrium. Therefore, their slow re-entrance in the cell needs to be counterbalanced or the cell would slowly lose its membrane potential.
The second purpose, intricately linked to the first, is to keep the intracellular concentration more or less constant, and in this case to re-establish the original chemical gradients, that is to force the sodium and calcium which previously flowed into the cell out of it, and the potassium which previously flowed out of the cell back into it (though as the potassium is mostly at the electrochemical equilibrium, its chemical gradient will naturally reequilibrate itself opposite to the electrical gradient, without the need for an active transport mechanism).
For example, the
During this phase the membrane is most permeable to K+, which can travel into or out of cell through leak channels, including the inwardly rectifying potassium channel.[13] Therefore, the resting membrane potential is mostly equal to K+ equilibrium potential and can be calculated using the Goldman-Hodgkin-Katz voltage equation.
However,
The pacemaker potential is thought to be due to a group of channels, referred to as
Another hypothesis regarding the pacemaker potential is the 'calcium clock'. Calcium is released from the sarcoplasmic reticulum within the cell. This calcium then increases activation of the sodium-calcium exchanger resulting in the increase in membrane potential (as a +3 charge is being brought into the cell (by the 3Na+) but only a +2 charge is leaving the cell (by the Ca2+) therefore there is a net charge of +1 entering the cell). This calcium is then pumped back into the cell and back into the SR via calcium pumps (including the SERCA).[15]
Phase 0
This phase consists of a rapid, positive change in voltage across the cell membrane (depolarization) lasting less than 2 ms in ventricular cells and 10–20 ms in SAN cells.[16] This occurs due to a net flow of positive charge into the cell.
In non-pacemaker cells (i.e. ventricular cells), this is produced predominantly by the activation of
In pacemaker cells (e.g. sinoatrial node cells), however, the increase in membrane voltage is mainly due to activation of L-type calcium channels. These channels are also activated by an increase in voltage, however this time it is either due to the pacemaker potential (phase 4) or an oncoming action potential. The L-type calcium channels are activated more slowly than the sodium channels, therefore, the depolarization slope in the pacemaker action potential waveform is less steep than that in the non-pacemaker action potential waveform.[11][20]
Phase 1
This phase begins with the rapid inactivation of the Na+ channels by the inner gate (inactivation gate), reducing the movement of sodium into the cell. At the same time potassium channels (called Ito1) open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative. This is referred to as a 'notch' on the action potential waveform.[11]
There is no obvious phase 1 present in pacemaker cells.
Phase 2
This phase is also known as the "plateau" phase due to the membrane potential remaining almost constant, as the membrane slowly begins to repolarize. This is due to the near balance of charge moving into and out of the cell. During this phase delayed rectifier potassium channels (Iks) allow potassium to leave the cell while L-type calcium channels (activated by the influx of sodium during phase 0) allow the movement of calcium ions into the cell. These calcium ions bind to and open more calcium channels (called ryanodine receptors) located on the sarcoplasmic reticulum within the cell, allowing the flow of calcium out of the SR. These calcium ions are responsible for the contraction of the heart.
Calcium also activates chloride channels called Ito2, which allow Cl− to enter the cell. Increased calcium concentration in the cell also increases activity of the sodium-calcium exchangers, while increased sodium concentration (from the depolarisation of phase 0) increases activity of the sodium-potassium pumps. The movement of all these ions results in the membrane potential remaining relatively constant, with K+ outflux, Cl− influx as well as Na+/K+ pumps contributing to repolarisation and Ca2+ influx as well as Na+/Ca2+ exchangers contributing to depolarisation.[21][11] This phase is responsible for the large duration of the action potential and is important in preventing irregular heartbeat (cardiac arrhythmia).
There is no plateau phase present in pacemaker action potentials.
Phase 3
During phase 3 (the "rapid repolarization" phase) of the action potential, the L-type
Ionic pumps as discussed above, like the
In the sinoatrial node, this phase is also due to the closure of the L-type calcium channels, preventing inward flux of Ca2+ and the opening of the rapid delayed rectifier potassium channels (IKr).[23]
Refractory period
Cardiac cells have two refractory periods, the first from the beginning of phase 0 until part way through phase 3; this is known as the absolute refractory period during which it is impossible for the cell to produce another action potential. This is immediately followed, until the end of phase 3, by a relative refractory period, during which a stronger-than-usual stimulus is required to produce another action potential.[24][25]
These two refractory periods are caused by changes in the states of
Gap junctions
Channels
Current (I) | α subunit protein | α subunit gene | Phase / role | |
---|---|---|---|---|
Na+ | INa | NaV1.5 | SCN5A[31] | 0 |
Ca2+ | ICa(L) | CaV1.2 | CACNA1C[32] |
0-2 |
K+ | Ito1 | KV4.2/4.3 | KCND2/KCND3 | 1, notch |
K+ | IKs | KV7.1 | KCNQ1 |
2,3 |
K+ | IKr | KV11.1 (hERG) | KCNH2 |
3 |
K+ | IK1 | Kir2.1/2.2/2.3 |
3,4 | |
Na+, Ca2+ | INaCa | 3Na+-1Ca2+-exchanger | NCX1 ( SLC8A1 ) |
ion homeostasis |
Na+, K+ | INaK | 3Na+-2K+-ATPase |
ATP1A | ion homeostasis |
Ca2+ | IpCa | Ca2+-transporting ATPase | ATP1B | ion homeostasis |
Ion channels are proteins that change shape in response to different stimuli to either allow or prevent the movement of specific ions across a membrane. They are said to be selectively permeable. Stimuli, which can either come from outside the cell or from within the cell, can include the binding of a specific
Each channel is coded by a set of DNA instructions that tell the cell how to make it. These instructions are known as a gene. Figure 3 shows the important ion channels involved in the cardiac action potential, the current (ions) that flows through the channels, their main protein subunits (building blocks of the channel), some of their controlling genes that code for their structure, and the phases that are active during the cardiac action potential. Some of the most important ion channels involved in the cardiac action potential are described briefly below.
HCN channels
Hyperpolarization-activated cyclic nucleotide-gated channels (HCN channels) are located mainly in pacemaker cells, these channels become active at very negative membrane potentials and allow for the passage of both Na+ and K+ into the cell (which is a movement known as a funny current, If). These poorly selective, cation (positively charged ions) channels conduct more current as the membrane potential becomes more negative (hyperpolarised). The activity of these channels in the SAN cells causes the membrane potential to depolarise slowly and so they are thought to be responsible for the pacemaker potential. Sympathetic nerves directly affect these channels, resulting in an increased heart rate (see below).[35][14]
The fast sodium channel
These sodium channels are voltage-dependent, opening rapidly due to depolarization of the membrane, which usually occurs from neighboring cells, through gap junctions. They allow for a rapid flow of sodium into the cell, depolarizing the membrane completely and initiating an action potential. As the membrane potential increases, these channels then close and lock (become inactive). Due to the rapid influx sodium ions (steep phase 0 in action potential waveform) activation and inactivation of these channels happens almost at exactly the same time. During the inactivation state, Na+ cannot pass through (absolute refractory period). However they begin to recover from inactivation as the membrane potential becomes more negative (relative refractory period).
Potassium channels
The two main types of potassium channels in cardiac cells are inward rectifiers and voltage-gated potassium channels.
The voltage-gated potassium channels (Kv) are activated by depolarization. The currents produced by these channels include the transient out potassium current Ito1. This current has two components. Both components activate rapidly, but Ito,fast inactivates more rapidly than Ito, slow. These currents contribute to the early repolarization phase (phase 1) of the action potential.
Another form of voltage-gated potassium channels are the delayed rectifier potassium channels. These channels carry potassium currents which are responsible for the plateau phase of the action potential, and are named based on the speed at which they activate: slowly activating IKs, rapidly activating IKr and ultra-rapidly activating IKur.[38]
Calcium channels
There are two
These channels respond to voltage changes across the membrane differently: L-type channels are activated by more positive membrane potentials, take longer to open and remain open longer than T-type channels. This means that the T-type channels contribute more to depolarization (phase 0) whereas L-type channels contribute to the plateau (phase 2).[39]
Conduction system
In the
In addition to the SAN, the AVN and Purkinje fibres also have pacemaker activity and can therefore spontaneously generate an action potential. However, these cells usually do not depolarize spontaneously, simply because action potential production in the SAN is faster. This means that before the AVN or Purkinje fibres reach the threshold potential for an action potential, they are depolarized by the oncoming impulse from the SAN[40] This is called "overdrive suppression".[41] Pacemaker activity of these cells is vital, as it means that if the SAN were to fail, then the heart could continue to beat, albeit at a lower rate (AVN= 40-60 beats per minute, Purkinje fibres = 20-40 beats per minute). These pacemakers will keep a patient alive until the emergency team arrives.
An example of premature ventricular contraction is the classic athletic heart syndrome. Sustained training of athletes causes a cardiac adaptation where the resting SAN rate is lower (sometimes around 40 beats per minute). This can lead to atrioventricular block, where the signal from the SAN is impaired in its path to the ventricles. This leads to uncoordinated contractions between the atria and ventricles, without the correct delay in between and in severe cases can result in sudden death.[42]
Regulation by the autonomic nervous system
The speed of action potential production in pacemaker cells is affected, but not controlled by the autonomic nervous system.
The sympathetic nervous system (nerves dominant during the body's fight-or-flight response) increase heart rate (positive chronotropy), by decreasing the time to produce an action potential in the SAN. Nerves from the spinal cord release a molecule called noradrenaline, which binds to and activates receptors on the pacemaker cell membrane called β1 adrenoceptors. This activates a protein, called a Gs-protein (s for stimulatory). Activation of this G-protein leads to increased levels of cAMP in the cell (via the cAMP pathway). cAMP binds to the HCN channels (see above), increasing the funny current and therefore increasing the rate of depolarization, during the pacemaker potential. The increased cAMP also increases the opening time of L -type calcium channels, increasing the Ca2+ current through the channel, speeding up phase 0.[43]
The parasympathetic nervous system (nerves dominant while the body is resting and digesting) decreases heart rate (negative chronotropy), by increasing the time taken to produce an action potential in the SAN. A nerve called the vagus nerve, that begins in the brain and travels to the sinoatrial node, releases a molecule called acetylcholine (ACh) which binds to a receptor located on the outside of the pacemaker cell, called an M2 muscarinic receptor. This activates a Gi-protein (I for inhibitory), which is made up of 3 subunits (α, β and γ) which, when activated, separate from the receptor. The β and γ subunits activate a special set of potassium channels, increasing potassium flow out of the cell and decreasing membrane potential, meaning that the pacemaker cells take longer to reach their threshold value.[44] The Gi-protein also inhibits the cAMP pathway therefore reducing the sympathetic effects caused by the spinal nerves.[45]
Clinical significance
Antiarrhythmic drugs are used to regulate heart rhythms that are too fast. Other drugs used to influence the cardiac action potential include sodium channel blockers, beta blockers, potassium channel blockers, and calcium channel blockers.
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External links
- Interactive animation illustrating the generation of a cardiac action potential
- Interactive mathematical models of cardiac action potential and other generic action potentials