Cardiac Electrophysiology

Question 1

Routes of an electrical impulse in the cardiac system

Answer 1

1) SA Node
2) Internodal path (branches trigger atrial contraction)
3) AV Node
4) Bundle of His–directly below AV node
5) Purkinje fibers

*The endocardium is depolarized first, then the epicardium

Question 2

How is the electrical potential of the cardiac cell maintained and what are the properties of the mechanism?

Answer 2

Na+/K+ pump

a) uses energy (ATP)
b) electrogenic (causes slight inside-negative Vm to ~ -30 mV)
c) inhibited by digitalis (digoxin)
d) Responsible for the relatively high intracellular concentration of K+, and high extracellular concentration of Na+

Question 3

Which intracellular ion has the most impact on the membrane potential of cardiac cells?

Answer 3

Potassium

Question 4

Steps of a cardiomyocyte depolarization:

Answer 4

1) Myocyte receives a depolarizing signal from another source
2) Voltage gated ion channels open and sodium floods the cell
3) A slow ‘h’ gate closes and stops the influx of more sodium
4) Cell initiates a repolarization process using the Na/K pump

Question 5

Which cardiomyocytes exhibit fast action potentials? Which are slow?

Answer 5

Slow= AV node and SA node
Fast = Atrium, ventricle, Purkinje fibers, Bundle of His

Question 6

What are the phases in a fast action potential?

Answer 6

phase 0 – upstroke
phase 1 - partial, rapid repolarization
phase 2 - plateau
phase 3 – final repolarization
phase 4 - resting membrane potential

Question 7

What happens in phase 0 in a fast action potential?

Answer 7

voltage-gated, fast Na+ and Ca2+ channels open simultaneously allowing for rapid influx of Na+ and Ca2+ into the cell; creates action potential with a steep slope and large amplitude.

Question 8

What happens in phase 1 in a fast action potential?

Answer 8

a) fast Na+ channels close

b) Ito (“transient outward” K+ current) open

Question 9

What happens in phase 2 in a fast action potential?

Answer 9

a) continued ICa (Ca2+ current)–fast acting calcium channels close, but slow acting ones close gradually

Question 10

What happens in phase 3 in a fast action potential?

Answer 10

a) increase in IK potassium current (increased outward current) – delayed rectifier K+ channels
b) closure of ICa channels (decreased inward current)

Question 11

What are the phases of a slow action potential?

Answer 11

phase 0--upstroke
phase 1--doesn't exist
phase 2--doesn't exist
phase 3--repolarization
phase 4--diastolic depolarization

Question 12

What happens in phase 0 in a slow action potential and why is it different than a fast one?

Answer 12

Ca++ channels open; there is no voltage gated sodium channels in slow action potential cells

Question 13

What happens in phase 1 in a slow action potential?

Answer 13

Nothing–slow action potentials don’t have one.

Question 14

What happens in phase 2 in a slow action potential?

Answer 14

Nothing–slow action potentials don’t have one.

Question 15

What happens in phase 3 in a slow action potential?

Answer 15

a) increase in IK potassium current (increased outward current) – delayed rectifier K+ channels
b) closure of ICa channels (decreased inward current)

Question 16

What happens in phase 4 in a slow action potential?

Answer 16

Consistent, slow depolarization of the membrane (aka diastolic depolarization) until the cell reaches its threshold for another action potential

Question 17

Factors that determine changes in the heart rate

Answer 17

A change in the rate of diastolic depolarization (a small slope –> a lower heart rate), a decrease in the maximum diastolic potential (i.e. how depolarized can those cells get), fluctuations in the calcium channel presence in the pacemaker cells

Question 18

What are typical pacemaker levels for the SA node, the AV node, and the ventricular pacemakers?

Answer 18

a) SA node – 60-100 beats/min
b) Junction (AV nodal or His) – 40-50 beats/min
c) Ventricular pacemakers – less than 40 beats/min

Question 19

How does the parasympathetic nervous activity decrease heart rate?

Answer 19

1) open K+ “leak” channels resulting in a more negative maximal diastolic potential;
2) decrease If to reduce the steepness of diastolic depolarization
and 3) decrease ICa, to reduce the steepness and increase the threshold

Question 20

What neurotransmitter does the parasympathetic nervous system rely on and what are its target receptors in cardiomyocytes?

Answer 20

Acetylcholine and Muscarinic receptors

Question 21

What neurotransmitter does the sympathetic nervous system rely on and what are its target receptors in cardiomyocytes?

Answer 21

Norepinephrine and beta-adronergic receptors

Question 22

How does the sympathetic nervous activity increase heart rate?

Answer 22

Increase If and ICa, which will combine to increase the steepness of diastolic depolarization and lower the
threshold for action potential generation.

Question 23

How is the conductance of an action potential from one cell to another represented?

Answer 23

Ohm’s Law:

Conductance of current = change in voltage divided by the resistance of the flow

Question 24

Factors that affect the difference in voltage

Answer 24

• The amplitude (total voltage change) and rate and rise (slope of phase zero) of the action potential
• A more negative resting membrane potential

Question 25

Factors that affect the resistance of flow

Answer 25

• The presence of gap junctions (the more junctions, the faster a signal will travel)
• The diameter of the cell (the bigger the cell, the faster the conductance)

Question 26

What is the difference between effective refractory period and a relative refractory period?

Answer 26

Effective refractory periods are absolute, and no matter how strong a signal, a cell cannot generate a new action potential.
Relative refractory periods allow another action potential to occur, but may have less force than one generated from the normal excitability of a resting potential.