Cardiac Ion Channels & AP

Question 1

Electrical signal originates in ____ and propagates through ___ and ___

Answer 1

originates in myocardium propagates through atria and specialized conduction pathways

Question 2

HR is controlled by which cells located where?

Answer 2

pacemaker cells in SA node

Question 3

Intrinsic firing rate of SA nodal cells? Modulated by what?

Answer 3

100/min; modulated by ANS

Question 4

What effect does the parasympathetic tone have on firing rate?

Answer 4

Depresses it to 60-80/min

Question 5

What is overdrive suppression

Answer 5

SA nodal cell pacemaking is faster than other regions. As a result, it drives and dominates over the other regions (overdriven by SA node)

Question 6

Ectopic pacemakers

Answer 6

Cells outside of the SA node that can take on initiation of heartbeat under abnormal circumstances (ie damage, hypoxia)

Question 7

Where does cell-to-cell propagation occur?

Answer 7

Via gap junctions - direction of propagation is controlled by junction position and by the CT that “insulates”

Question 8

Where can APs propagate from SA node to the ventricles?

Answer 8

at the AV node

Question 9

Slow AP occurs where?

Answer 9

SA node and AV node

Question 10

Fast AP occurs where?

Answer 10

Atrial muscle
Ventricular muscle
Purkinje fiber
Bundle of His
L & R bundle branches

Question 11

Vm < Eion current flows _____ cell; causes _____

Vm > Eion current flows _____ cell; causes _____

Answer 11

Vm < Eion flows into cell causing depolarization

Vm > Eion flows out of cell causing hyperpolarization

Question 12

What is unique about If (or Ih) channels?

Answer 12

Opposite from the other channels in that it is activated by hyperpolarization. Allows cation fluxes and can lead to reversal potential. Can help generate rhythm.

Question 13

Fast cardiac AP include what channels?

- list the current that they represent

Answer 13

INa
ICa-L

IKto (transient outward current)
IKr + IKs
IK1

Question 14

Slow cardiac APs include what channels?

Answer 14

ICa-T + ICa-L
If
IKr + IKs

Question 15

What is the delayed rectifier current?

What is the inward rectifier channels?

Answer 15

Delayed recifier: IKr + IKs

Inward rectifier: IK1

Question 16

Funny current

Answer 16

funny channel, or If, or IKf, or pacemaker current) refers to a mixed sodium-potassium current in the heart that activates upon hyperpolarization

Question 17

Fast AP:

- Phase 0

Answer 17

○ Initial upstroke, rapid depolarization

- caused by entry of INa through voltage activated sodium channels.

Question 18

Fast AP:

- Phase 1

Answer 18

○ Small, partial repolarization

- because of inactivation of INa and activation of transient IKto

Question 19

Fast AP

- Phase 3

Answer 19

Termination of plateau by rapid repolarization back to ~-90mV
○ Continued outward Potassium current (IKr+Iks)
○ Low membrane permeability for other cations
○ Inactivation of Na and Ca are removed

Question 20

Fast AP

- Phase 4

Answer 20

○ Cell is held near EK by the inward rectifier channel (IK1)

○ Negative voltage/resting potential

Question 21

Slow AP

- Phase 0

Answer 21

○ Upstroke from activation(opening) of ICa-T and ICa-L

○ Slow because no INa

Question 22

Slow AP

- Phase 1

Answer 22

Does not exist in slow APs

Question 23

Slow AP

- Phase 2

Answer 23

Does not exist in slow APs

Question 24

Slow AP

- Phase 3

Answer 24

Rapid repolarization
○ Occurs shortly after peak of AP because of ICa-T and ICa-L channels close and K channels open (effluxes)

-Membrane potential goes towards K( ~-60)

(Na is still entering the cell at very low amts)

Question 25

Slow AP

- Phase 4

Answer 25

“the peacemaker potential”
initiated by Funny current
- (or funny channel, or If, or IKf, or pacemaker current)- mixed sodium-potassium current that activates upon hyperpolarization and supplies inward current

Na+ slowly leaks into the cell
(peak slightly rises until phase 0 initiated)
○ Brings cell back to threshold for generation of another AP