Depolarization- Repolarization

Depolarization- Repolarization

Resting membrane Potential (RMP)
Electrical potential of cell at rest.  Above example is cardiac muscle tissue.

Action Potential
The five – phase process characterized by the movement of K+, Na, and Ca++( primarily) across cell membrane.

Threshold potential (TP)
The point at which a stimulus will initiate a response helps determine Automaticity of pacemaker cells.

Phase 0
Depolarization–fast NA+ channels open–Na+ rushes in to replace “lost” positive charges.

Phase 1
Overshoot–rush of positive Na+ ions causes polarity to be positive for a short time.

Phase 2
Plateau Phase–Na channels close, K= channels decrease permeability, slow Ca++ channels open–Ca++ goes into cell, Ca++ maintains cell in depolarized state for a prolonged period of time.
(Verapamil works here)

Phase 3
Repolarization — rapid loss of K+ ions restores and
electrogenic pump RMP to 90 mV.

Phase 4
Polarized (resting) Cell–negative polarity inside slowly K+ leaves IC space (concentration gradient).  Decreasing RMP to TP (Note; in SA node, CA++ influx responsible for decay of RMP to TP) K+—Electrolyte most likely to affect electrophysiology.

Resting Membrane Potential (RMP)

SA Node        -50 to -60 mV
AV Node        -65 to -70 mV
Muscle            – 90 to – 100 mV

Threshold Potential (TP)

SA Node        – 40 mV
AV Node        -55 to – 60 mV
Muscle            – 60 mV