Kaplan Psysiology Electrical Activity Of The Heart

Front 1

Back 1

Phase 0
• Fast channels open, i gNa' Sodium influx then causes depolarization.
• The channels open and dose quickly, and they have dosed by the time the main part
of the plateau phase is entered.
Phase 1
This slight repolarization is due to a transient potassium current and the dosing of
the sodium channels. Originally it was thought that CI was involved, though we now
know that is not the case.
Subendocardial fibers lack a phase 1.
~MEDICAL 41
Section II: Excitable Tissue
42 ~MED1CAl
Phase 2
• L-type Ca2+ channels are open, gCa t permitting a calcium influx.
Voltage-gated potassium channels, the iKI, are closed; gK -1. compared with resting
membrane.
Potassium efflux continues through the ungated potassium channels and possibly
other channels.
• If voltage-gated potassium channels did not close during depolarization, early repolarization
would occur, preventing the full development of the plateau phase.
• The development of the plateau phase is dependent on the closing of voltage-gated
potassium channels.
• Calcium channel antagonists shorten the plateau.
• Potassium channel antagonists lengthen the plateau.
Phase 3
L-type Ca2+ channels close,gCa -1.; this eliminates any influx through these channels.
Voltage-gatedpotassium channels, the delayed rectifier iK, then the iK j are opening, gK T.
Becausewe are a long way from the potassium equilibrium potential and conductance to
potassium is increasing, a large potassium efflux begins, and the cell quickly repolarizes.
If the voltage-gated potassium channels did not open, the cell would still repolarize
but more slowly,because of closure of calcium channels and potassium efflux through
the ungated potassium channels.
Phase 4
gK high; voltage-gated and ungated potassium channels open. The delayed rectifiers,
iK, gradually close but are responsible for the relative refractory period during early
phase 4.

Front 2

Back 2

Na" Permeability
(fast channel)

Front 3

Back 3

Ca2+ Permeability
(slow channel

Front 4

Back 4

K+Permeability : /
(vottage-gated channel