Normal Cardiac Electrical Activity

Question 1

What is the Nernst Equation?

Answer 1

E = (61/z)(log([C]_o/[C]_i))

Question 2

Intracellular/Extracellular concentrations of sodium and membrane potential

Answer 2

IC = 10 mM

EC = 145 mM

Eq potential = 71 mV

Question 3

Intracellular/Extracellular concentrations of potassium and membrane potential

Answer 3

IC = 120 mM

EC = 4.5 mM

Eq potential = -88 mV

Question 4

Intracellular/Extracellular concentrations of calcium and membrane potential

Answer 4

IC = 10^-4 mM

EC = 2 mM

Eq potential = 132 mV

Question 5

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

Answer 5

NA+/K+ ATPase

Question 6

What are some membrane channel control mechanisms?

Answer 6

Gating

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

Modulation

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

Question 7

What is the equation for conductance?

Answer 7

g = 1/R or g = I/E

Question 8

Phase 4 Depolarization

Answer 8

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

Question 9

Phase 0 Depolarization

Answer 9

upstroke of the action potential

I_Na and I_Ca(L-type)

Question 10

m gate

Answer 10

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

Question 11

h gate

Answer 11

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

prevents excess influx of sodium

Question 12

L-type calcium channel

Answer 12

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

the f gate closes much more slowly than the h gate

Question 13

Phase 1 Depolarization

Answer 13

inactivation of I_Na and more slowly I_Ca

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

Question 14

Phase 2 Depolarization

Answer 14

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 I_Kr and I_Ks

ultrarapid component I_Kur in atria - rapid activation, slow inactivation

slow activation gives rise to the plateau

Question 15

inward rectification

Answer 15

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 V_m as a result, and only an inward curent can occur

Question 16

Phase 3 Repolarization

Answer 16

inactivation gates (f gates) for calcium close

delayed rectifier, I_K, gradually activates

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