Excitability is defined as the ability of a living tissue to give response to a stimulus. In all the tissues, initial response to a stimulus is electrical activity in the form of action potential. It is followed by mechanical activity in the form of contraction, secretion, etc.
Electrical Potentials In Cardiac Muscle:
Basics of electrical potentials in the muscle are:
- Resting Membrane Potential: Resting membrane potential in:
Single Cardiac Muscle Fiber : – 85 to – 95 mV
Sinoatrial (SA) Node : – 55 to – 60 mV
Purkinje Fibers : – 90 to – 100 mV
- Action Potential: Action potential in cardiac muscle is different from that of other tissues such as skeletal muscle, smooth muscle and nervous tissue. Duration of the action potential in cardiac muscle is 250 to 350 msec (0.25 to 0.35 sec).
- Phases Of Action Potential: Action potential in a single cardiac muscle fiber occurs in four phases:
- Initial Depolarization
- Initial Repolarization
- A plateau Or Final Depolarization
- Final Repolarization
- Initial Depolarization: Initial depolarization is very rapid and it lasts for about 2 msec (0.002. sec). Amplitude of depolarization is about + 20 mV.
- Initial Repolarization: Immediately after depolarization, there is an initial rapid repolarization for a short period of about 2 msec. The end of rapid repolarization is represented by a notch.
- Plateau Or Final Depolarization: Afterwards, the muscle fiber remains in depolarized state for sometime before further repolarization. It forms the plateau (stable period) in action potential curve. The plateau lasts for about 200 msec in atrial muscle fibers and for about 300 msec in ventricular muscle fibers. Due to long plateau in action potential, the contraction time is also longer in cardiac muscle by 5 to 15 times than in skeletal muscle.
- Final Repolarization: Final repolarization occurs after the plateau. It is a slow process and it lasts for about 50 to 80 msec before the re-establishment of resting membrane potential.
Ionic Basis Of Action Potential:
- Initial Depolarization: Initial depolarization (first phase) is because of rapid opening of fast sodium channels and the rapid influx of sodium ions, as in the case of skeletal muscle fiber.
- Initial Repolarization: Initial repolarization is due to the transient (short duration) opening of potassium channels and efflux of a small quantity of potassium ions from the muscle fiber Simultaneously, the fast sodium channels close suddenly and slow sodium channels open, resulting in slow influx of low quantity of sodium ions.
- Plateau or Final Depolarization: Plateau is due to the slow opening of calcium channels. These channels are kept open for a longer period and cause influx of large number of calcium ions. Already the slow sodium channels are opened through which slow influx of sodium ions continues. Because of the entry of calcium and sodium ions into the muscle fiber, Positivity is maintained inside the muscle fiber, producing prolonged depolarization i.e plateau. Calcium ions entering the muscle fiber play an important role in the contractile process.
- Final Repolarization: Final repolarization is due to efflux of potassium ions. Number of potassium ions moving out of the muscle fiber exceeds the number of calcium ions moving in. It makes negativity inside, resulting in final repolarization. Potassium efflux continues until the end of repolarization.
Restoration Of Resting Membrane Potential:
At the end of final repolarization, all sodium ions, which had entered the cell throughout the process of action potential move out of the cell and potassium ions move into the cell, by activation of sodium-potassium pump. Simultaneously, excess of calcium ions, which had entered the muscle fiber also move out through sodium-calcium pump. Thus, the resting membrane potential is restored.
Spread Of Action Potential Through Cardiac Muscle:
Action potential spreads through cardiac muscle very rapidly because of the presence of gap junctions between the cardiac muscle fibers. Gap junctions are permeable junctions and allow free movement of ions and so the action potential spreads rapidly from one muscle fiber to another fiber.
Action potential is transmitted from atria to ventricles through the fibers of specialized conductive system.