Action Potentials Many cells in the body have the ability to undergo a transient depolarization and repolarization that is either triggered by external mechanisms e. There are two general types of cardiac action potentials. Non-pacemaker action potentialsalso called "fast response" action potentials because of their rapid depolarization, are found throughout the heart except for the pacemaker cells. The pacemaker cells generate spontaneous action potentials that are also termed "slow response" action potentials because of their slower rate of depolarization.
Two main forces drive ions across cell membranes: The transmembrane potential TMP is the electrical potential difference voltage between the inside and the outside of a cell.
Ion channels help maintain ionic concentration gradients and charge differentials between the inside and outside of the cardiomyocytes. Properties of cardiac ion channels Selectivity: Therefore, specific channels open and close Cardiac action potential the TMP changes during cell depolarization and repolarization, allowing the passage of different ions at different times.
The different types of cardiac ion channels are discussed below, throughout the description of the phases of action potentials in different cardiac cells. Action potentials and impulse conduction Physiol Rev.
Action potential in cardiomyocytes The action potential in typical cardiomyocytes is composed of 5 phasesbeginning and ending with phase 4. Early repolarization TMP is now slightly positive. This becomes significant in the excitation-contraction coupling process described below.
These two countercurrents are electrically balanced, and the TMP is maintained at a plateau just below 0 mV throughout phase 2. Action potential in cardiac pacemaker cells Pharmacol Ther.
The funny current Automaticity: This property is known as automaticity, whereby the cells undergo spontaneous depolarization and an action potential is triggered when threshold voltage is reached.
Pacemaker cells have an unstable membrane potential and their action potential is not usually divided into defined phases. No rapid depolarization phase: Cardiac cell types displaying pacemaker behavior. Implications of pacemaker activity on global cardiac depolarization Synchronous contraction: An action potential in one cell will cause all neighbouring cells to depolarize, allowing the heart chambers to act as a unit.
Refractory period Defined as the time from phase 0 until the next possible depolarization of a myocyte, i. This is a physiological mechanism allowing sufficient time for the ventricles to empty and refill prior to the next contraction.
Absolute refractory period ARP: Effective refractory period ERP: Relative refractory period RRP: Cells in this phase are particularly susceptible to arrhythmias when exposed to an inappropriately timed stimulus, which is why one must synchronize the electrical stimulus during cardioversion to prevent inducing ventricular fibrillation.
Sequence of depolarization The SA node normally initiates electrical activation. The impulse propagates through atrial tissue to the AV node.
There is no direct electrical connection between the atrial and ventricular chambers other than through the AV node, as fibrous tissue surrounds the tricuspid and mitral valves. AV node allows a very short delay in conduction approximately 0. This pause has two important purposes: Allows the atria time to contract and fully empty prior to ventricular stimulation.
Allows the AV node to act as a gatekeeper, limiting the transmission of ventricular stimulation during abnormally rapid atrial rhythms.
Non-nodal action potentials, sometimes referred to as "fast response" action potentials, are characteristic of atrial and ventricular myocytes, and the fast-conducting Purkinje system in the ventricles. These action potentials have a true resting potential, a fast depolarization phase, and a prolonged plateau phase as shown below to the right. Action potential: electrical stimulation created by a sequence of ion fluxes through specialized channels in the membrane (sarcolemma) of cardiomyocytes that leads to cardiac contraction. Action potential in cardiomyocytes. Cardiac action potential consists of four distinct phases (Figure 2a). In phase 0, upstroke occurs due to rapid transient influx of Na +. Later, Na + channels are inactivated, combined with .
After crossing the AV node, the impulse spreads into the rapidly conducting bundle of His and through the bundle branches to the Purkinje fibers. The electrical impulse is distributed throughout the bulk of the ventricular myocyte for precisely timed stimulation and contraction of the ventricles.
Excitation-contraction coupling represents the process by which an electrical action potential leads to contraction of cardiac muscle cells. This is achieved by converting a chemical signal into mechanical energy via the action of contractile proteins.
Contractile proteins Main contractile elements: It prevents contraction in the resting state by inhibiting the interaction between myosin heads and actin. Troponin T TnT — ties troponin complex to actin and tropomyosin molecules.
Troponin C TnC — binds calcium ions that regulate contractile process. The strength of cardiac contraction is proportional to the number of crossbridges formed. Myocyte relaxation As with myocyte contraction, this process is synchronized with the electrical activity of the cell.
Neural modulation of contractility Adv Physiol Educ. Heart is innervated by both parasympathetic and sympathetic afferent and efferent neurons. Both sympathetic and parasympathetic tone is exerted on the heart at rest, but parasympathetic tone predominates.Non-nodal action potentials, sometimes referred to as "fast response" action potentials, are characteristic of atrial and ventricular myocytes, and the fast-conducting Purkinje system in the ventricles.
These action potentials have a true resting potential, a fast depolarization phase, and a prolonged plateau phase as shown below to the right. In the cardiac myocyte, the membrane potential is dominated by the K+ equilibrium potential.
An action potential is initiated when this resting potential becomes shifted towards a more positive value of approximately ~60 to ~70 mV (Figure 3). • The cardiac contractile cell relies on the autorhythmic cell to generate an action potential and pass the impulse down the line before the cell can contract.
• Like the autorhythmic cell, it has protein transport channels, but they are slightly different. The cardiac action potential is a transient reversible electromagnetochemical wavefront responsible for the generation of the cardiac impulse. It is based on a complex series of transmembrane ion fluxes that result in a net flow of electric current across the .
The trajectory of the cardiac action potential is divided into five distinct phases, which reflect changes in the predominant ionic current flowing during the cardiac cycle. Perioperative cardiac arrhythmias: an approach. The cardiac action potential is a short-lasting event in which the membrane potential (the difference of potential between the interior and the exterior) of a cardiac cell rises and falls following a consistent trajectory, similar to the action potential in other types of cells. In the cardiac myocyte, the membrane potential is dominated by the K+ equilibrium potential. An action potential is initiated when this resting potential becomes shifted towards a more positive value of approximately ~60 to ~70 mV (Figure 3).
The cardiac action potential is a short-lasting event in which the membrane potential (the difference of potential between the interior and the exterior) of a cardiac cell rises and falls following a consistent trajectory, similar to the action potential in other types of cells.
Another difference between cardiac and nerve and muscle action potentials is the role of calcium ions in depolarization.
In nerve and muscle cells, the depolarization phase of the action potential is caused by an opening of sodium channels.