Action potential in cardiac muscle
Cardiac muscle has an inherent property of generating its own action potentials rhythmically, independent of nerve stimulation. This occurs in the peacemaker cells of the S.A. node, which depolarizes faster than any other parts of the heart.
The onset of action potential in cardiac muscle is slower but prolonged than skeletal muscle which lasts for 150 msec. in atria and 300 m.sec. in ventricle. In cardiac muscle, repolarisation does not occur immediately after depolarisation The positivity generated as the spike potential by the depolarisation remains as a plateau near the peak. This plateau lasts for a few-hundred m.sec prolongs the contraction of the cardiac muscle.
The reason for the prolonged action potential in cardiac muscle cells is due to the presence of two separate channel systems- Voltage activated Na channel (fast channel) and Voltage activated Na – Ca channel (slow channel).
Activation of the fast Na channels causes the spike potential of the action potential, whereas the slow channel prolongs the passage of Ca ++ and Na+ ions into the interior of the cell, thus establishes the plateau in the action potential.
The inflow of Ca++ ions in to the cardiac muscle cells decreases K+ permeability by about 5 fold. This delays the K+ ion permeability to outside which in turn delays the re-polarization process of the action potential in cardiac muscle. This prolonged action potential (250 – 300 m.sec) provides longer contraction period in the cardiac muscle cells than the skeletal muscles. Hence, cardiac muscles do not functionally tetanized.
Action potential in pacemaker cell
In the cells of SA node, the resting potential is -55 mv only due to naturally leaky nature of these cells to Na+ ions cause spontaneous depolarisation and the resting potential gradually rise up to – 40 mv. At this voltage, the fast Na+ and Na+– Ca++ channels are opened leading to development of self-excitation and heart beat. This is followed by opening of the K+ channels causes diffusion of greater number K+ ions to out side the cell causing the hyperpolarization (-55 to -60 mv).
After this event, the K+ channels close and Na+ ion leaks to the interior of the cells to repeat the cycle.
In cardiac muscles, the action potential is prolonged and it is referred as plateau of action potential.