What causes the plateau in action potential in the heart?
During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this maintains a prolonged period of depolarization, causing the plateau in the action potential.
What is the plateau phase of the cardiomyocyte action potential?
This variable conductance is partially responsible for the unique Plateau Phase of the Cardiomyocyte Action Potential. The initially low potassium conductance only allows for a partial repolarization of the membrane, making possible the Plateau Phase.
Is there a plateau phase in pacemaker action potentials?
There is no plateau phase present in pacemaker action potentials. During phase 3 (the "rapid repolarization" phase) of the action potential, the L-type Ca 2+ channels close, while the slow delayed rectifier (I Ks) K + channels remain open as more potassium leak channels open.
What is plateau phase and rapid repolarization?
Plateau Phase (PP): Characterized by a sustained membrane potential of roughly +10mV. Rapid Repolarization (RR): Characterized by a rapid shift in the membrane potential back to -90mV. Three basic ion channels are responsible for the cardiac action potential and explain its unique shape.
What is action potential with Plateau?
During the plateau phase of the action potential, calcium ions flow down this steep concentration gradient and enter the myocyte. Most of this calcium enters through the L-type channels, located primarily at sarcolemmal/sarcoplasmic reticulum junctions.
Why is there a plateau in cardiac action potential?
During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, and this activity maintains a prolonged period of depolarization,causing the plateau in the action potential.
What is depolarization and plateau?
Rapid depolarization phase – A rapid influx of Na+ into the cell causes the cell membrane potential to sharply increase from −90 to +30 mV. 2. Plateau phase – A sharp increase in the membrane potential ends with the closure of Na+ channels.
What happen during plateau phase?
The second phase of Masters and Johnson's four-stage model of physiological responses to sexual stimulation. Plateau follows the excitement or sexual arousal phase, and is characterised by increased heart and respiratory rates, increased sexual pleasure, increased muscle tension, and is followed by orgasm.
What event occurs during the plateau phase that initiates muscle contraction?
A plateau phase occurs during which Ca 2+ enters the cytosol of the muscle cell. Ca 2+ enters from the sarcoplasmic reticulum (endoplasmic reticulum) within the cell and also from outside the cell through slow‐opening Ca 2+channels in the sarcolemma.
Which segment of the ECG reflects the plateau phase?
The ST segment corresponds to the plateau phase of the ventricular transmembrane action potential. Normally, there are slow and relatively similar transmembrane voltage changes in the ventricular myocardial cells during this phase.
What is plateau current?
Plateau potentials, caused by persistent inward currents (PICs), are a type of electrical behavior seen in neurons.
Which phase of action potential is plateau phase?
Phase 2Phase 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.
What is the action potential of a heart cell?
The cardiac action potential is a brief change in voltage ( membrane potential) across the cell membrane of heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins called ion channels. The cardiac action potential differs from action potentials found in other types of electrically excitable cells, such as nerves. Action potentials also vary within the heart; this is due to the presence of different ion channels in different cells (see below).
What is rate dependence of action potential?
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. Action potential activity within the heart can be recorded to produce an electrocardiogram (ECG). This is a series of upward and downward spikes (labelled P, ...
Why does the membrane voltage increase in a pacemaker cell?
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.
What is phase 4 in a cell?
In these cells, phase 4 is also known as the pacemaker potential. During this phase, the membrane potential slowly becomes more positive, until it reaches a set value (around -40 mV; known as the threshold potential) or until it is depolarized by another action potential, coming from a neighboring cell.
How are cardiac muscle cells linked?
All cardiac muscle cells are electrically linked to one another, by structures known as gap junctions (see below) which allow the action potential to pass from one cell to the next. This means that all atrial cells can contract together, and then all ventricular cells.
Which type of calcium channel is activated more slowly than sodium channels?
The L-type calcium channels are activated more slowly than the sodium channels, in the ventricular cell, therefore, the depolarization slope in the pacemaker action potential waveform is less steep than that in the non-pacemaker action potential waveform.
How many action potentials does a San have?
They produce roughly 60-100 action potentials every minute. This action potential passes along the cell membrane causing the cell to contract, therefore the activity of the SAN results in a resting heart rate of roughly 60-100 beats per minute.
What is cardiac action potential?
Cardiac action potential. Typically described cardiac action potential is that of the myocardial cell. Action potential of tissues like sinus node will be different and characterized by diastolic depolarization which contributes to the automaticity.
What is the action potential of myocardial cells?
It may be noted that the cardiac action potential is different from the surface electrocardiogram which represent the sum total of all electrical activity of the heart as recorded from the body surface. Myocardial action potential is recorded with intracellular electrode under experimental conditions.
What is cardiac action potential?
The cardiac action potential describes the molecular basis of electrical activity within the heart's cardiomyocytes. The action potential is a sudden positive shift in the cardiomyocyte's cellular membrane potential, termed depolarization. Depolarization not only initiates contraction within the affected cardiomyocyte ...
Which channel is responsible for the plateau phase?
The Slow Calcium Channels are most responsible for the Plateau Phase, because they allow a long time-scale influx of positive ions which exactly balances the initially low efflux of potassium. The balance between calcium influx and low potassium efflux is ultimately the basis of the sustained positive membrane potential observed in ...
What is the role of potassium channels in cardiomyocytes?
Cardiomyocyte potassium channels are induced to open following rapid depolarization and allow egress of positively-charged K + from the cell which is responsible for the cell's eventual repolarization . However, these potassium channels initially display low potassium conductance which increases gradually but slowly.
Which type of calcium channel is slow to open?
Cardiomyocytes uniquely possess a type of Slow Calcium Channel known as the Long, L-type calcium channel . These calcium channels are slow to open following the rapid depolarization phase but remain open for a long time afterwards (i.e. several tenths of a second).
Does depolarization spread in a wave-like fashion?
Consequently, depolarization tends to spread in a wave-like fashion throughout the heart as depolarized cardiomyocytes initiate depolarization of adjacent cells. Shape. Overview. The resting membrane potential of cardiomyocytes is roughly -90mV and during full depolarization the membrane potential reaches +20mV.
What happens at the end of the plateau of the cardiac action potential?
At the end of the plateau of the cardiac action potential, the influx of calcium ions to the interior of the muscle fiber is suddenly cut off, and the calcium ions in the sarcoplasm are rapidly pumped back out of the muscle fibers into both the sarcoplasmic reticulum and the T tubule-extracellular fluid space.
Where does the action potential go when it passes over the cardiac muscle membrane?
As is true for skeletal muscle, when an action potential passes over the cardiac muscle membrane, the action potential spreads to the interior of the cardiac muscle fiber along the membranes of the transverse (T) tubules. The T tubule action potentials in turn act on the membranes of the longitudinal sarcoplasmic tubules to cause release ...
What is the action potential of the T tubule?
The T tubule action potentials in turn act on the membranes of the longitudinal sarcoplasmic tubules to cause release of calcium ions into the muscle sarcoplasm from the sarcoplasmic reticulum.
How long does cardiac muscle contraction last?
As a result, the contraction ceases until a new action potential comes along. Duration of Contraction. Cardiac muscle begins to contract a few milliseconds after the action potential begins and continues to contract until a few milliseconds after the action potential ends. Therefore, the duration of contraction of cardiac muscle is mainly ...
What are the differences between cardiac and skeletal muscle?
At least two major differences between the membrane properties of cardiac and skeletal muscle account for the prolonged action potential and the plateau in cardiac muscle. First, the action potential of skeletal muscle is caused almost entirely by sudden opening of large numbers of so-called fast sodium channels that allow tremendous numbers ...
How does the second population of channels differ from the fast sodium channels?
This second population of channels differs from the fast sodium channels in that they are slower to open and , even more important , remain open for several tenths of a second. During this time, a large quantity of both calcium and sodium ions flows through these channels to the interior of the cardiac muscle fiber, ...
What is the refractory period of the heart?
The normal refractory period of the ventricle is 0.25 to 0.30 second, which is about the duration of the prolonged plateau action potential.
How long is cardiac action potential?
Typical neural AP duration is around 1ms and those of skeletal muscle are roughly 2-5ms, whereas cardiac action potentials range from 200-400ms.
Which cell uses Ca ions in depolarization?
Nervous and muscle cells (as well as non-pacemaker cardiac cells) use the opening of Na channels to facilitate the depolarisation phase, whereas cardiac pacemaker cells use Ca ions in depolarisation. The transfer of ions from the intracellular environment to the extracellular environment, and vice versa, is what allows for ...
What is the transmembrane potential?
The Transmembrane Potential is the voltage difference between the intra and extracellular environments. A net movement of positive ions out of the cell, causes the TMP to become more negative, and vice versa. Cardiac ion channels have various properties that enable them to carry out their function: Selective.
What are the forces that facilitate the transfer of ions across the cell membrane?
The main forces responsible for facilitating the transfer of ions across the cell membrane are: Chemical potential. causes an ion to move down its concentration gradient. Electrical potential. causes an ion to move away from similarly-charged particles.
What happens to the K+ channels during the plateau phase?
These transient opening K + channels rapidly repolarise the cell before the plateau phase. Therefore they set the membrane potential of the plateau phase. Greater K + currents during this notch phase allow more repolarisation so that the plateau occurs at lower voltages. Fewer K + currents means that less repolarisation occurs and the plateau phase occurs at higher voltages.
How does each phase of action potential work?
Like any action potential, each phase is driven by the opening and closing of a variety of specific ion channels. This is because opening an ion channel will push the membrane potential closer to the equilibrium potential of the ions it conducts.
Why do voltage-gated Na channels open?
Voltage-gated Na + channels open in response to depolarisation that spreads into the cell through gap junctions. The influx of Na + ions depolarises the cell further causing the opening of more Na + channels. This continues in a positive feedback mechanism to cause fast and steep depolarisation.
What is the action potential of ventricular myocytes?
Action potentials in ventricular myocytes trigger the Ca 2+ entry that is necessary for their contraction. Their synchronicity, characteristic shape and length protect the heart against abnormal electrical activity. When these protective mechanisms go wrong, it can be potentially life threatening.
How do ions spread in a cell?
As cells share cytosol, ions will quickly spread by passive diffusion down a large electrochemical gradient. This almost instantaneously evokes an action potential in the next cell. Gap junctions ensure a unidirectional spread of the action potential.
What is the main determinant of the resting membrane potential?
K + currents are the main determinant of the resting membrane potential as the membrane is far more permeable to K+ than any other ion. At rest K + channels are open, therefore resting membrane potential tends towards the equilibrium potential for K + (E K ).
How many phases are there in ventricular action potential?
The ventricular action potential is generally split into 5 phases ( phases 0-4 ). Rather confusingly, phase 4 is the baseline that the membrane potential begins and ends at. Like any action potential, each phase is driven by the opening and closing of a variety of specific ion channels.
Summary
Overview
Similar to skeletal muscle, the resting membrane potential (voltage when the cell is not electrically excited) of ventricular cells is around -90 millivolts (mV; 1 mV = 0.001 V), i.e. the inside of the membrane is more negative than the outside. The main ions found outside the cell at rest are sodium (Na ), and chloride (Cl ), whereas inside the cell it is mainly potassium (K ).
The action potential begins with the voltage becoming more positive; this is known as depolariza…
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.
In the ventricular myocyte, phase 4 occurs when the cell is at rest, in a period known as diastole. In the standard non-pacemaker cell the voltage during this …
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.
Gap junctions
Gap junctions allow the action potential to be transferred from one cell to the next (they are said to electrically couple neighbouring cardiac cells). They are made from the connexin family of proteins, that form a pore through which ions (including Na , Ca and K ) can pass. As potassium is highest within the cell, it is mainly potassium that passes through. This increased potassium in the neighbour cell causes the membrane potential to increase slightly, activating the sodium cha…
Channels
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 molecule to a receptor on the channel (also known as ligand-gated ion channels) or a change in membrane potential around the channel, detected by a sensor (als…
Autorhythmicity
Electrical activity that originates from the sinoatrial node is propagated via the His-Purkinje network, the fastest conduction pathway within the heart. The electrical signal travels from the sinoatrial node (SAN), which stimulates the atria to contract, to the atrioventricular node (AVN) which slows down conduction of the action potential, from the atria to the ventricles. This delay allows the ventri…
See also
• Electrical conduction system of the heart
• Excitation–contraction coupling
• Cardiac excitation-contraction coupling
• Action potential