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Flashcards in Cardio - Physio (Action Potentials) Deck (22)
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1

In what areas do the ventricular action potential occur?

Bundle of His and (other) Purkinje fibers

2

How many phases are there in the ventricular action potential? Name them.

5; Phase 0 - Rapid upstroke, Phase 1 - Initial Repolarization, Phase 2 - Plateau, Phase 3 - Rapid Repolarization, Phase 4 - Resting Potential

3

Again, what is the name of Phase 0? What occurs here?

Rapid upstroke; Voltage-gated Na+ channels open

4

Again, what is the name of Phase 1? What occurs here?

Initial repolarization; Inactivation of voltage-gated Na+ channels & Voltage-gated K+ channels begin to open

5

Again, what is the name of Phase 2? What occurs here?

Plateau; Ca2+ influx through voltage-gated Ca2+ channels: (1) balances K+ efflux (2) triggers Ca2+ release from sarcoplasmic reticulum & myocyte contraction

6

Again, what is the name of Phase 3? What occurs here?

Rapid repolarization; Massive K+ efflux due to opening of voltage-gated slow K+ channels and closure of voltage-gated Ca2+ channels

7

Again, what is the name of Phase 4? What occurs here?

Resting potential; High K+ permeability through K+ channels

8

What are 3 points to contrast cardiac muscle action potential against that of skeletal muscle?

(1) Cardiac muscle AP has a plateau, which is due to Ca2+ influx and K+ efflux; Myocyte contraction occurs due to Ca2+-induced Ca2+ release from sarcoplasmic reticulum (2) Cardiac nodal cells spontaneously depolarize during diastole, resulting in automaticity due to If channels ("funny current" channels responsible for a slow, mixed Na+/K+ inward current) (3) Cardiac myocytes are electrically coupled to each other by gap junctions

9

What are the funny current channels associated with the heart? What role do they play?

If channels allow funny current = slow, mixed Na+/K+ inward current; Cardiac nodal cells spontaneously depolarize during diastole, resulting in automaticity due to If channels (Note: If = I with subscript f = funny current)

10

Where do pacemaker action potentials occur?

SA and AV nodes

11

What is/are the key difference(s) that set pacemaker action potential apart from the ventricular action potential in Phase 0?

Upstroke (not Rapid)- opening of voltage-gated Ca2+ channel. Fast voltage-gated Na+ channels are permanently inactivated because of the less negative resting voltage of these cells --> Results in a slow conduction velocity that is used by the AV node to prolong transmission from the atria to the ventricles. (Recall: In ventricular action potential, Phase 0 - Rapid Upstroke involved Voltage-gated Na+ channels open)

12

What is/are the key difference(s) that set pacemaker action potential apart from the ventricular action potential in Phase 2?

Plateau is absent (in addition to Phase 1 being absent); (Recall: In ventricular action potential, Phase 1 - Initial repolarization involved Inactivation of voltage-gated Na+ channels & Voltage-gated K+ channels begin to open... Phase 2 - Plateau involved Ca2+ influx through voltage-gated Ca2+ channels: (1) balances K+ efflux (2) triggers Ca2+ release from sarcoplasmic reticulum & myocyte contraction)

13

What is/are the key difference(s) that set pacemaker action potential apart from the ventricular action potential in Phase 3?

Inactivation of Ca2+ channels and Increased activation of K+ channels --> K+ efflux; (Recall: In ventricular action potential, Phase 3 - Rapid repolarization involved Massive K+ efflux due to opening of voltage-gated slow K+ channels and closure of voltage-gated Ca2+ channels)

14

What is/are the key difference(s) that set pacemaker action potential apart from the ventricular action potential in Phase 4?

Slow diastolic depolarization - membrane potential spontaneously depolarizes as Na+ conductance increases (If different from INa in phase 0 of ventricular action potential) = Accounts for automaticity of SA and AV nodes (Recall: In ventricular action potential, Phase 4 involved Resting potential - High K+ permeability through K+ channels)

15

What accounts for the automaticity of SA and AV nodes?

Phase 4 of Pacemaker action potential = Slow diastolic depolarization - membrane potential spontaneously depolarizes as Na+ conductance increase due to funny current (different from Na+ current in phase 0 of ventricular action potential)

16

What determines heart rate?

Slope of Phase 4 (i.e., slow diastolic depolarization) in SA node

17

What decreases rate of diastolic depolarization and consequently also decreases heart rate?

ACh/adenosine

18

What increases rate of diastolic depolarization and consequently also increases heart rate?

Catecholamines

19

What effect does sympathetic stimulation have on heart rate, and why?

Sympathetic stimulation increases the chance that If channels are open and thus increase heart rate

20

Draw out the ventricular action potential, labeling its key phases and the effective refractory period (ERP). Be sure to include voltage references on the y axis.

See p. 260 top of the page

21

Draw a schematic representing the movement of currents across myocyte membranes during the ventricular action potential.

See p. 260 top of the page

22

Draw out the pacemaker action potential, labeling its key phases and threshold. Be sure to include voltage references on the y axis.

See p. 260 bottom of the page