Action Potential TypesAction Potential Types

Fast- and Slow Response Action PotentialsFast- and Slow Response Action Potentials

Slow response: Ionic currentsSlow response: Ionic currents

Why slow response?

Fast response: Atria Slow response: Ventricles, Purkinje Fibers. SAN, AVN

• High dv/dt: >200 V/s• High conduction

velocity: 1-4 m/s• Specific drug

sensitivity: sodium channel blockers –

(lidocaine) and tetrodotoxin

• Low dv/dt: < 20 V/s• Low conduction

velocity: 0.02-0.05 m/s

• Specific drug sensitivity: calcium channel blockers –

nifedipine, verapamil• Spontaneous phase 4

depolarization

AutomaticityAutomaticity

Pacemakers hierarchy: Pacemakers hierarchy: SAN,AVN, Purkinje fibersSAN,AVN, Purkinje fibers

Why are fibers of the conducting system automatic?

Membrane potential of SA nodal cells

Pacemaker Frequency - Pacemaker Frequency - MechanismsMechanisms