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Flashcards in Normal Cardiac Electrical Activity Deck (16)
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What is the Nernst Equation?

E = (61/z)(log([C]o/[C]i))


Intracellular/Extracellular concentrations of sodium and membrane potential

IC = 10 mM

EC = 145 mM

Eq potential = 71 mV


Intracellular/Extracellular concentrations of potassium and membrane potential

IC = 120 mM

EC = 4.5 mM

Eq potential = -88 mV


Intracellular/Extracellular concentrations of calcium and membrane potential

IC = 10-4 mM

EC = 2 mM

Eq potential = 132 mV


What is the primary transporter that maintains the cell membrane ion gradient?

NA+/K+ ATPase


What are some membrane channel control mechanisms?


  • voltage
  • time
  • direct agonist
  • G protein
  • calcium


  • increases in phosphorylation
  • oxidation-reduction
  • cytoskeleton
  • calcium
  • ATP


What is the equation for conductance?

g = 1/R or g = I/E


Phase 4 Depolarization

resting membrane potential

determined mainly by gK which is 100x greater than gNa

the predominant current is the inward rectifier K current

membrane potential is -85 mV to -92 mV


Phase 0 Depolarization

upstroke of the action potential

INa and ICa (L-type)


m gate

the activation gate of sodium channels that open when the threshold potential is reached (about -70 mV)

opening and closing of the gates are both time and voltage dependent


h gate

the inactivation gate that closes after the m gate opens for a brief period of time

prevents excess influx of sodium


L-type calcium channel

controleld by d and f gates, which represent their activation and inactivation probabilities

the f gate closes much more slowly than the h gate


Phase 1 Depolarization

inactivation of INa and more slowly ICa

transient outward potassium current

possible contribution by other species such as Cl-

creates notch in action potential

more prominent in ventricular epicardial than endocardial cells


Phase 2 Depolarization

plateau, mainly due to Ca2+ current because channels remain open much longer than the Na+ channels

residual, very small component of Na+ current

at positive potentials, resistance to K+ is high, so ions can't contribute as much - called inward rectification

main current during this phase is the delayed rectifier K channel - broken down into IKr and IKs

ultrarapid component IKur in atria - rapid activation, slow inactivation

slow activation gives rise to the plateau


inward rectification

inward rectifier K+ channels cannot conduct ions outward at positive membrane potentials because of an cytoplasmic Mg2+ block (also spermine and spermidine)

K+ can't contribute to Vm as a result, and only an inward curent can occur


Phase 3 Repolarization

inactivation gates (f gates) for calcium close

delayed rectifier, IK, gradually activates

as the cell repolarizes, the contribution of the inward rectifier increases