Cardiac Ion Channels and Action Potentials

Question 1

Which cells have fast cardiac action potential

Answer 1

myocardial cells and cells of rapid conduction pathways

bundle of His, left and right bundle branches and Purkinje fibers

Question 2

Which cells have slow cardiac action potential

Answer 2

pacemaker cells of SA and AV nodes

Question 3

Fast Action Potential

Phase 0 = initial upstroke

Answer 3

Na+ in

INa = rapidly activated by depolarization

Question 4

Fast Action Potential

Phase 1 = partial repolarization

Answer 4

little K+ out

IKto = transient K+ current both activated and inactivated by depol at a slower rate than Na+ current

Question 5

Fast Action Potential

Phase 2 = prolonged plateau

Answer 5

Ca2+ in balanced by K+ out

ICa-L =activated by depol and inactivated by voltage and calcium dependent inactivation

IKr and IKs = rapid and slow delayed rectifiers
activated by depol

Question 6

Fast Action Potential

Phase 3 = rapid repol

Answer 6

K+ out

Inactivatate ICa-L and incr activation of IKr and IKs

Question 7

Fast Action Potential

Phase 4 = resting potential

Answer 7

Permeable to K+

IK1 = inward rectifier suited for holding cells at Ek (Nernst potential) by quickly correcting changes in potential through efflux or influx of K+

Question 8

Slow Action Potential

Phase 0 = upstroke phase

Answer 8

ICa-T and ICa-L = L-type and T-type calcium channels = slow action potential (NO INa)

L-type = High voltage Activated
T-type = low voltage activated

Question 9

Slow Action Potential

Phase 3 = repolarization

Answer 9

Balance between ICa, IKr/IKs

Question 10

Slow Action Potential

Phase 4 = pacemaker potential

Answer 10

If = funny current induced by hyperpolarization
permeable to Na+ and K+

deactivate IKr/IKs
activate ICa-T

Question 11

Do pacemaker cells require neuronal input?

Answer 11

No but they fire slow APs

Never reach stable resting potnetial–> continuosuly and repetitively fire slow APs

Question 12

Difference between Fast and Slow Action Potentials

CHANNELS

Answer 12

Fast = INa and Ik1
Slow = If (activ at hyperpolarization) and ICa-T (activ at low voltage)

Question 13

Function of IK1

Answer 13

No Gating

maintain cell at resting Ek by correcting changes in potential via influx/efflux of K+ (NO OUTWARD CURRENT ON DEPOLARIZATION)

Question 14

Function of If

Answer 14

Turned on ONLY AT HYPERPOLARIZED

impt for automaticity in nodal cells

Question 15

What is mechanism of overdrive suppression?

Answer 15

SA node controls electrical activity of other, less active and less frequent pacemakers in the heart (AV node…)

Question 16

What is absolute refractory period?

Answer 16

NO STIMULUS CAN EVOKE ACTION POTENTIAL after fast AP

because still inactivation of INa

Question 17

What is relative refractory period?

Answer 17

STRONGER THAN NORMAL STIMULUS REQUIRED

Question 18

When can you fire another action potential?

Answer 18

when you have inactivation of INa and deactivation of IKr and IKs before normal stimulus