QS Study

Pacemaker Potential

Rhythmically discharging cells have a membrane potential that after each impulse, declines to the firing level and triggers the next impulse. This potential is called pacemaker potential or prepotential. Pacemaker cells are resolute at two ‘nodes’: the sinoatrial node (SA node), placed in the right atrium near the higher vena cava; and the atrioventricular node (AV node), placed at the base of the right atrium near the junction of the four heart chambers. They are exceptional because they can impulsively generate action potentials in rhythm to direct the heartbeat. They do this because they exhibit specialized membrane potential- the pacemaker potential.

Fig: Pacemaker Potential Mechanism

(i) At the peak of each impulse, K+ channels begins and brings about repolarization.

(ii) Ik then declines and as K+ efflux decreases the membrane begins to depolarize forming the first part of pacemaker potential.

(iii) Ca++ channel open. There are 2 types of Ca2+ channel in the heart, T and L channel. The calcium current (I ca) due to an opening of T channel completes the pacemaker potential and Ica due to an opening of L channels (Ica) produces the impulse.