The cardiac cell is a polarised membrane . It has a resting membrane potential of –80 to –90mV, and a high Na+ ion concentration outside the membrane and K +ion concentration inside the membrane. Upon excitation, depolarisation occurs as the cell membrane permeability to Na + ions increases, the negativity of resting
potential is lost, and a positive current is gen erated inside the cell. The characteristics of action potential rely on the type of the cell -myocardial contractile cell, or pacemaker, or potential pacemaker cell. There are five phases of the action potential of cardiac cells (figure 1.1):
1) Phase 0: In t his phase, rapid depolarisation of the cell membrane is observed as the sodium ions rapidly enter the cell through sodium channels. This phase is followed by re-polarisation.
2) Phase 1: In this short and initial phase, rapid re-polarisation is observed as the potassium ions move out of the cell.
3) Phase 2: This prolonged plateau phase is observed as the calcium ions enter the cell slowly through the calcium channels . This phase of action potential is present only in the cardiac cells.
4) Phase 3: This phase is th e second period of rapid re -polarisation as the potassium ions move out of the cell.
5) Phase 4: This is the resting phase as the resting membrane potential is re – established when the potassium ions return into the cell and sodium and calcium ions move out of it. The cells do not depolarise in response to another impulse during the phases 1 and 2; and this is called the absolute refractory period. The cells depolarise in response to a powerful impulse d uring the phases 3 and 4; and this is called the relative refractory period.
The heart rate and stroke volume are used for determining the cardiac output. The stroke volume depends on the preload (which itself depends on venous return) , afterload, and contractility. The load on the heart due to the blood volume reaching the left ventricle is called preload; while the resistance to the left ventricular ejection, i.e., the total peripheral resistance is called afterload.