Cardio: Ventricular and Pacemaker APs

Question 1

Ventricular: Phase 0

Answer 1

Rapid upsroke and depolarization:
Voltage gated Na⁺ channels open (I_NA)

I_Na

Question 2

Ventricular: Phase 1

Answer 2

Initial Repolarization:
Inactivation of voltage-gated Na⁺ channels (I_NA)
Voltage-gated K⁺ channels begin to open (I_K)

I_Na & I_K

Question 3

Ventricular: Phase 2

Answer 3

Plateau:

Ca²⁺ influx through voltage-gated Ca²⁺ channels
balances K⁺ efflux

Ca²⁺ influx triggers Ca²⁺ release from Sarcoplasmic Reticulum (SR) and myocyte contraction

I_Ca & I_K

Question 4

Ventricular: Phase 3

Answer 4

Rapid Repolarization

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

I_K

Question 5

Ventricular: Phase 4

Answer 5

Resting Potential:

High K⁺ permeability through K⁺ channels

I_K

Question 6

Pacemaker: Phase 0

Answer 6

Upstroke:

Opening of voltage-gated Ca²⁺ channels

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 ventricles

I_Ca

Question 7

Pacemaker: Phase 1

Answer 7

Absent

Question 8

Pacemaker: Phase 2

Answer 8

Absent

Question 9

Pacemaker: Phase 3

Answer 9

Innactivation of Ca²⁺ channels and
Increased activation of K⁺ channels –> K⁺ efflux

I_K

Question 10

Pacemaker: Phase 4

Answer 10

Slow Diastolic Depolarization: I_Na

Membrane potential spontaneously depolarizes as Na⁺ conductance increases (If different from I_Na in phase 0 of ventricular action potential)

Accounts for automaticity of SA and AV nodes

The slope of phase 4 in the SA node determines the HR

Beta1: Hypokalemia and Mechanical stretch *increases *HR

ACh / Adenoisine / Digoxin decrease the rate of
disastolic depolarization and decreased HR
Catecholamines increase depolarization and increase HR

Sympathetic stimulation* increases* the chance that I_f channels are open and thus increases HR