Action potentials

Generation of action potentials

Na+ and K+ conductance at resting potentials

Refractory periods

Refractory periods and Na+ Channels

Refractory periods

Involvement of other Ions in Action potential

Cardiac Conduction Generation of Action potential every 0.8 seconds, or 75 action potentials per minute at the SA node (Pacemaker of the heart) 10

1 2 3 4

Cardiac Muscle Action Potential Depolarization Repolarization Refractory period Contraction Membrane potential (mV) Repolarization due to closure of Ca2+ channels and K+ outflow when additional voltage-gated K+ channels open Rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open Plateau (maintained depolarization) due to Ca2+ inflow when voltage-gated slow Ca2+ channels open and K+ outflow when some K+ channels open 0.3 sec + 20 –20 –40 – 60 – 80 –100 2 1 3 Depolarization Repolarization Refractory period Contraction Membrane potential (mV) Rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open Plateau (maintained depolarization) due to Ca2+ inflow when voltage-gated slow Ca2+ channels open and K+ outflow when some K+ channels open 0.3 sec + 20 –20 –40 – 60 – 80 –100 2 1 Depolarization Repolarization Refractory period Contraction Membrane potential (mV) Rapid depolarization due to Na+ inflow when voltage-gated fast Na+ channels open 0.3 sec + 20 –20 –40 – 60 – 80 –100 1

Continuous Conduction in Unmyelinated axons