Cardiac Action Potentials

Question 1

RMP is set-up by:

Answer 1

NA/K pump (3 Na out, 2 K in) and K leak channels.

Question 2

When peak action potential is reached and begins to return to RMP, what is going on? (3)

Answer 2

Na channel inactivation gates are closed.
VG K channels are open.
Membrane permeability to Na decreased and permeability to K increases.

Question 3

Overall conduction pattern:

Answer 3

SA node to AV node to bundle of His (common bundle) to Perkinje fibers (branch to RV and LV).

Question 4

Timing of transmission of the signal in the ventricles

Answer 4

Endocardium of ventricles receives AP first, then epicardium.
RV epicardium gets AP before LV.

Question 5

Conduction velocity (slow to fast)

Answer 5

AV node < atrial and ventricular muscle < Purkinje fibers (and atrial pathways).

Question 6

What is the reasoning for a delay in AV?

Answer 6

It allows atria to empty into ventricles before they contract.

Question 7

Phase 4 in SA node

Answer 7

RMP gradually depolarizes until threshold, then it “fires”.
It’s opening is due to funny VG Na+ channel.
Has intrinsic, spontaneous depolarization which makes it a “pacemaker” as it has automaticity.

Question 8

What causes Phase 0 in SA node?

Answer 8

Due to opening of the slow Ca 2+ gates and closing of K+ (b) gates.

Question 9

Why is there no Phase 1 or 2 in SA APs?

Answer 9

There is too much scarcity of traditional VG Na (m) channels.

Question 10

What causes Phase 3 in SA APs?

Answer 10

Due to closing of Ca 2+ and opening of K+ (b) gates (reversal of phase 0).

Question 11

Cardiac AP pacemaker summary

Answer 11

SA node naturally depolarizes to reach threshold w/o electrical stimulation.
AV node is similar to SA node but is much slower.
Bundle of His and Purkinje fibers will remain polarized.

Question 12

Phase 0 in fast tissues/fibers is caused by:

Answer 12

Rapid upstroke caused by crossing threshold and VG Na channels opening.
Passing threshold and AP is propagated.

Question 13

Phase 1 in fast tissues/fibers is caused by:

Answer 13

Brief repolarization
Small repolarization via VG Na+ (m) channels closing and K+ (a) channels opening.
Inactivation gates of VG Na+ channels are closed.

Question 14

Phase 2 in fast tissues/fibers is caused by:

What does it do functionally?

Answer 14

Plateau
Sustained by opening of VG Ca 2+ channels and closing of VG K+ (b) channels.
Prolongs contraction and is fundamentally different than observed in skeletal muscle.

Question 15

Phase 3 in fast tissues/fibers is caused by:

Answer 15

Slow VG Ca 2+ channels closing and K+ (b) channels opening.

Question 16

Relationship between phase 0 and velocity

Answer 16

More rapid phase 0 occurs, the steeper phase 3 and faster conduction.
This is also related to differences in the speed of either VG Na or Ca channels.

Question 17

AV block

Answer 17

AV node damage. Further delays in conduction.

Question 18

Arrhythmias

Answer 18

Purkinje fiber damage - disrupts natural conduction and contraction of ventricles.

Question 19

Refractory periods and arrhythmias

Answer 19

RPs are important to prevent arrhythmias and tend to be longer in cardiac cells.

Question 20

Absolute refractory period

Answer 20

No depolarization

Question 21

Relative refractory period

Answer 21

AP can be generated but will have abnormal conduction.

Question 22

Supranormal period

Answer 22

Cell is more excitable than usual.

Conduction of AP is weaker is stimulated during relative refractory period.

Question 23

Chonotropic effect

Answer 23

Affects changes rate of depolarization of SA node and therefore HR.

Question 24

Dromotropic effect

Answer 24

Affects speed of conduction.

Question 25

Inotropic effect

Answer 25

Changes the strength of muscular contraction.

Question 26

Lusitropic effect

Answer 26

Changes rate of muscular relaxation.

Question 27

Parasympathetic stimulus in cardiac AP generation

Answer 27

Carried by vagus n.
To SA node and AV node.
NT is ACh and receptor is muscarinic (M2 and M3).

Question 28

Negative chonotropic effects

Answer 28

Slowed opening of Na+ (f) channels in phase 4.

Hyperpolarization by increasing outward K+ current via K+ ACh channel.

Question 29

Negative dromotropic effects

Answer 29

Reduced Ca 2+ inward current.

Increased outward K+ current via K+ ACh.

Question 30

Sympathetic stimulus of cardiac AP generation

Answer 30

To SA node and AV node and to myocytes.

NT is NE and receptor is muscarinic.

Question 31

Positive chronotropic effects

Answer 31

Increased opening of Na+ (f) channels during phase 4.

Increased Ca 2+ current.

Question 32

Positive dromatotropic effects

Answer 32

Increased Ca 2+ inward during phase 2.