Abstract:
The resting membrane potential is expressed in mill volts (mV) and intracellular potential is expressed relative to extracellular potential. The resting membrane potential of cardiac cells is determined primarily by potassium ions. The conductance (permeability) to K ion at rest is high; the conductance to sodium at rest is low. Myocardial fibers have a resting membrane potential of approximately – 90 mV.
Action potential of single cardiac muscle cell in these tissues is characterized by rapid depolarization, a plateau, and a slow depolarization process. The action potential duration is long, varies from 150 ms in the atria to 250 ms in the ventricle to 300 ms in purkinje system and consequently the long refractory period.
The benefits of plateau to cardiac cell are:
A- It will prolong the duration of excitation (absolute refractory period).
B- Prolong the duration of high cytoplasm Ca concentration, so it increase the force of cardiac muscle contraction.
C- It gives more time that all cardiac cells can be activated together and contract as one unit in the atria and ventricles.
Refractory period:
1- Absolute refractory period (ARP): It is the period during which the membrane can not be re-excited by an outside stimulation whatever the level of external voltage applied. It is prolong in cardiac cell due to presence of plateau. It occurs in phase 0, 1, 2 and half phase 3 until membrane reaches approximately – 50 mV during repolarization.
2- Relative refractory period (RRP): It is the time during which action potential can be generated with stimulus larger than normal.
The filling of the ventricle which is about (120-130 ml) is called (end diastolic volume). The ventricle empties about 70 ml only (stroke volume). The remaining volume (50-60 ml) is called (end systolic volume). Stroke volume differs depending on age and size of body.
Ejection fraction = Stroke volume/ end diastolic volume.
Normal ejection fraction of left ventricle (55-80%), right ventricle is (45%).