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Cardiac action potential
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As in other cells, the cardiac action potential is a short-lasting event in which the difference of potential between the interior and the exterior of each cardiac cell rises and falls following a consistent trajectory.
The cardiac action potential differs significantly in different portions of the heart. The heart is provided with a special excitatory system and a contractile system necessary to perform this function.
This differentiation of the action potentials allows the different electrical characteristics of the different portions of the heart. For instance, the specialized excitatory system of the heart has the special property of depolarizing without any external influence with a slow, positive increase in voltage across the cell's membrane (the membrane potential) that occurs between the end of one action potential and the beginning of the next action potential. This increase in membrane potential typically permits the membrane potential to reach the threshold potential at which it fires the next action potential (pacemaker potential). Thus, the pacemaker potential is what drives the self-generated rhythmic firing. This is known as cardiac muscle automaticity.
Pacemaker potentials are fired by sinoatrial node (SAN), but also by the other foci. However, the last ones have firing frequencies slower than the SAN's. When other foci attempt to fire at their intrinsic rate, they can't because they have been discharged by the previous electric impluse coming from the SAN before their pacemaker potential threshold is reached. This is called "overdrive suppression". Rate dependence of action potential is a fundamental property of cardiac cells. This is important for the QT interval, measured from the beginning of the QRS complex to the end of the T wave. This interval must be corrected for the cardiac rhythm QTc. A prolonged QTc, long QT syndrome, induced by drugs or disease congenital or acquired, increases the possibility of developing severe ventricular arrhythmias and sometimes sudden death.
The electrical activity of the specialized excitatory tissues is not apparent on the surface electrocardiogram (ECG). This is due to the relatively small time duration. It is not possible, for example, to see on the ECG the sinus node activity but the resulting atrial myocardium contraction is apparent as a wave: the P wave. The electrical activity of the conducting system can be seen on the ECG (for example the AV node delay and the so-called PR segment).
2 Phases of the cardiac action potential 2.1 Phase 4
2.2 Phase 0
2.3 Phase 1
2.4 Phase 2
2.5 Phase 3
3 Refractory period
4 Channels 4.1 Funny channels
4.2 The fast Na+ channel
4.3 Potassium channels
4.4 Calcium channels
5 Automaticity 5.1 Regulation by the autonomic nervous system
6 External links
7 See also
Intra- and extracellular ion concentrations (mmol/L)
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).
Action potentials are generated by the movement of ions through the transmembrane ion channels in the cardiac cells.
Cardiac muscle bears some similarities to skeletal muscle, as well as important differences. Like skeletal myocytes (and axons for that matter),...
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