Cardiac Electrophysiology

Question 1

Three States of Ion Channels

Answer 1

• open
• closed
• inactivated

Question 2

Transition from Open to Inactivated

Answer 2

-function of time

Question 3

Transition from Inactivated to Closed

Answer 3

-function of membrane potential

Question 4

Resting Membrane Potential of Cardiomyocyte

Answer 4

• close to -90 mV
• higher resting permeability to potassium ions than in skeletal muscle cells
• more negative resting potential important because it facilitates the switch from inactivated to closed to switch quickly

Question 5

Can cardiomyocytes spontaneously depolarize?

Answer 5

No, if they’re healthy, they cannot generate spontaneous depolarization

Question 6

Phase 0

Answer 6

• voltage gated sodium channels open, greatly increasing I(Na), allowing sodium to rush into the cell and producing depolarization
• membrane potential moves toward E(Na)

Question 7

2 Reasons why E(Na) is not reached during Phase 0

Answer 7

• VG sodium channels inactivate

- potassium channels carrying transient outward current, I(to), open and partially repolarize membrane

Question 8

Phase 1

Answer 8

• VG sodium channels quickly inactivate
• transient outward current potassium channels open and slightly repolarize the cell…I(to)
• get slight repolarization of ~10mV

Question 9

Phase 2

Answer 9

• VG calcium channels open, causing influx of calcium into cell, attempting to depolarize the cell
• influx of calcium known as slow inward current (I(si))
• early in phase, depolarization due to calcium is balanced by repolarization due to I(to)
• late in phase, depolarization due to calcium influx is balanced by rectifier currents (rapid and slow potassium currents) (I(kr) and I(ks))

Question 10

Phase 3

Answer 10

• VG calcium channels inactivate, no longer balancing depol-repol forces
• delayed rectifiers are still fully open and cause strong repolarization
• strong repolarization results in inward rectifier potassium channels to open (Ikir)
• all of the inactivated channels begin to close at end of phase (fast sodium, calcium channels, transient outward potassium channels)

Question 11

3 Reasons why calcium membrane potential never reached

Answer 11

• calcium channels inactivate
• early in phase 3, delayed rectifiers oppose calcium influx with potassium efflux and repol.
• late in phase 3, inward rectifiers oppose calcium influx with potassium efflux and repol.

Question 12

Phase 4

Answer 12

• inward rectifier is fully open, maintains membrane potential close to E(k)
• all inactivated channels close