Cardio - Physio (Action Potentials)

Question 1

In what areas do the ventricular action potential occur?

Answer 1

Bundle of His and (other) Purkinje fibers

Question 2

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

Answer 2

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

Question 3

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

Answer 3

Rapid upstroke; Voltage-gated Na+ channels open

Question 4

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

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

Resting potential; High K+ permeability through K+ channels

Question 8

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

Answer 8

(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

Question 9

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

Answer 9

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)

Question 10

Where do pacemaker action potentials occur?

Answer 10

SA and AV nodes

Question 11

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

Answer 11

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)

Question 12

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

Answer 12

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)

Question 13

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

Answer 13

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)

Question 14

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

Answer 14

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)

Question 15

What accounts for the automaticity of SA and AV nodes?

Answer 15

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)

Question 16

What determines heart rate?

Answer 16

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

Question 17

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

Answer 17

ACh/adenosine

Question 18

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

Answer 18

Catecholamines

Question 19

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

Answer 19

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

Question 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.

Answer 20

See p. 260 top of the page

Question 21

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

Answer 21

See p. 260 top of the page

Question 22

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

Answer 22

See p. 260 bottom of the page