Sinoatrial node is the primary pacemaker of the heart Intrinsic pacemaking tissues Sa node av node and purkinje fibers Pacemaker activity: spontaneous time-dependent depolarization of the cell membrane that leads to an action potential in an otherwise quiescent cells Only composed of Phases 0, 3 and 4 The pacemaker with the highest frequency will trigger action potential A decreasing outward current of k and two increasing inward currents(of lca and ify control the phase 4 depolarization and intrinsic rhythmicity of the sa node The membrane currents are under the control of neural and circulating agents(acetylcholine, norepinephrine and epinephrine 100 100200300400500 Time(msec) 224 0 3 • Intrinsic pacemaking tissues: SA node, AV node and Purkinje fibers. • Pacemaker activity: spontaneous time-dependent depolarization of the cell membrane that leads to an action potential in an otherwise quiescent cells. • Only composed of Phases 0, 3 and 4. • The pacemaker with the highest frequency will trigger action potential. • A decreasing outward current of IK and two increasing inward currents (of Ica and If ) control the Phase 4 depolarization and intrinsic rhythmicity of the SA node. • The membrane currents are under the control of neural and circulating agents (acetylcholine, norepinephrine and epinephrine). Sinoatrial node is the primary pacemaker of the heart 22