Cardiac Action Potential & Conduction System

Question 1

What happens in phase 4 of cardiac AP for ventricles, atria, and the purkinje system?

Answer 1

Resting potential which is sustained by high K+ (c) conductance

Question 2

What happens in phase 0 of cardiac AP for ventricles, atria, and the purkinje system?

Answer 2

Rapid upstroke caused by crossing threshold and voltage-gated Na+ channels opening

Question 3

What happens in phase 1 of cardiac AP for ventricles, atria, and the purkinje system?

Answer 3

Small repolarization caused by start of Na+ (m) gates closing and some K+ (a) gates opening

Question 4

What happens during phase 2 of cardiac AP for ventricles, atria, and the purkinje system?

Answer 4

Plateau phase is sustained by slow Ca2+ channels opening and closing of special, voltage-gated K+ (b) channels

Question 5

What happens during phase 3 of cardiac AP for ventricles, atria, and the purkinje system?

Answer 5

Complete repolarization caused by slow Ca2+ channels closing and K+ channels opening

Question 6

What happens in phase 4 of cardiac AP for the SA node?

Answer 6

Resting membrane potential gradually depolarizes until it reaches threshold, then it “fires” albeit slower than other regions
This intrinsic, spontaneous depolarization makes the SA node the pacemaker as it has “automaticity”
Phase 4 is due to opening of funny voltage-gated Na+ (f) channels that open when membrane is repolarized

Question 7

How does phase 4 of cardiac AP differ in the AV node compared to in the SA node?

Answer 7

AV node is similar but phase 4’s slow depolarization is even slower
Thus, the AV node typically does not reach threshold until it receives a triggering signal from the SA node

Question 8

What happens during phase 0 of cardiac AP in the SA and AV nodes?

Answer 8

Due to opening of slow Ca2+ channels and closing of special K+ (b) channels

Question 9

What happens in phase 3 of cardiac AP for SA and AV nodes?

Answer 9

Due to closing of Ca2+ channels and closing of special K+ (b) gates

Question 10

What is the relationship between current and conduction velocity?

Answer 10

The higher the inward current (Na+ (m)) or slower the Ca2+ current, the faster the velocity
I.E. steeper the phase 0 slope, the faster the velocity

Question 11

Conduction Velocity

__________________>___________________>_________________

Answer 11

Purkinje (and atrial pathways)>atrial & ventricular muscle>AV node delay in AV allows atria to empty into ventricles before ventricles contract

Question 12

Refractory period is when the ________________ gates have not ‘reset’ sufficiently to allow a second AP to be generated

Answer 12

Electrolyte

Question 13

Refractory period is important to help prevent _________________, and are _______________ in cardiac cells than neurons for example

Answer 13

Arrhythmias

Longer

Question 14

Absolute refractory period

Answer 14

No depolarization

Question 15

Relative refractory period

Answer 15

AP can be generated but will have an abnormal conduction

Question 16

Supranormal period

Answer 16

Cell is more excitable than normal

Question 17

Conduction of action potential is ________________ if stimulated during RRP

Answer 17

Weaker

Question 18

Chronotropic

Answer 18

Effect changes rate of depolarization of SA node and therefore heart rate

• positive=faster
• negative=slower

Question 19

Dromotropic

Answer 19

Effect is speed of conduction

Question 20

Where is the cardiac PNS effect? What is the neurotransmitter? What is the receptor?

Answer 20

To AV and SA nodes and to insignificant extent to contractile myocytes
Neurotransmitter-acetylcholine (ACh)
Receptor-muscarinic

Question 21

How does the PNS effect gates?

Answer 21

Decreases opening of special Na+ (f) gates during phase 4 in SA and AV nodes

• decreases depolarization rate
• decreases HR-negative chronotropic effect

Question 22

PNS _______________ conduction velocity through AV node

Answer 22

Decreases

Question 23

Where is the cardiac effect of the SNS? What is the neurotransmitter? What is the receptor?

Answer 23

To all of heart
Neurotransmitter-norepinephrine
Receptor-beta-1

Question 24

How does SNS effect gates?

Answer 24

Increases opening of special Na+ (f) gates

• increases rate of depolarization
• increases HR-positive chronotropy

Question 25

SNS _____________________ conduction velocity through the AV node

Answer 25

Increases

Question 26

P wave on ECG represents ___________________________ of action potential

Answer 26

Depolarization of atria

Question 27

R wave (QRS complex) of ECG represents __________________ of action potential

Answer 27

Depolarization of ventricles

Question 28

T wave of ECG represents ______________________ of action potential

Answer 28

Repolarization of ventricles

Question 29

PVC (premature ventricular heartbeat)

Answer 29

Can be from development of new pacemaker foci in the ventricles

Question 30

PAV (premature atrial heartbeat)

Answer 30

Can be from development of new pacemaker foci in atrium away from SA node (e.g. left atrium)

Question 31

Ectopic pace makers

Answer 31

Can cause arrhythmias