14 Electrical activity of the heart

Question 1

State the electrical conduction pathway of the heart from the SA node.

Answer 1

SA node > both atria > AV node > bundle of His > Purkinje fibers

Question 2

How long is an action potential in SA node last?

Answer 2

150 ms

Question 3

In SA node, what is the sequence of phases during a pacemaker potential?

Answer 3

4 > 0 > 3

Question 4

What happens in phase 4 of SA node potential? (What type(s) of current)

Answer 4

• Inward Na+ current and T-type Ca2+ current

- Rate of phase 4 determines heart rate

Question 5

What ion is involved in phase 0 of SA node potential? What is the effect?

Answer 5

inward Ca2+ current (L-type)

Depolarization

Question 6

What ion is involved in phase 3 of SA node potential? What is the effect?

Answer 6

Outward K+ current

Repolarization

Question 7

Are there any phase 2 (plateau) in pacemaker potentials?

Answer 7

No

Question 8

What is the duration of action potential in atria and in ventricles respectively? Why is there a difference?

Answer 8

Atria: 150 ms
Ventricles: 250ms

long refractory period in ventricles due to Na+ channel inactivation that prevents summation of multiple cardiac contractions

Question 9

What are the phases involved in action potential in atria and the ventricle?

Answer 9

0,1,2,3,4

Question 10

What ion is involved in phase 0 of atrial/ventricular action potential? What is the effect?

Answer 10

upstroke

- inward Na+ current

Question 11

What ion is involved in phase 1 of atrial/ventricular action potential? What is the effect?

Answer 11

initial repolarization

- transient outward K+ current

Question 12

What ion is involved in phase 2 of atrial/ventricular action potential? What is the effect?

Answer 12

plateau - 0 net current

- inward Ca2+ current and outward K+ current

Question 13

What ion is involved in phase 3 of atrial/ventricular action potential? What is the effect?

Answer 13

Repolarization

- outward K+ current

Question 14

What ion is involved in phase 4 of atrial/ventricular action potential? What is the effect?

Answer 14

Resting membrane potential

Question 15

There is an AV delay of _______ ms to allow __________.

Answer 15

100;

sufficient ventricular filling time

Question 16

_______ form part of intercalated discs and join ends of cardiomyocytes. There is very low resistance among them to allow fast conduction of action potentials.

Answer 16

Gap junctions

Question 17

SA nodal action potentials normally occur between ___ to ____ beats per minute

Answer 17

60-100

Question 18

Which of the following are abnormal situations- ectopic pacemakers?

A. SA nodal firing is too slow

B. SA node is damaged or activity inhibited

C. latent pacemakers fire at a greater rate than SA node

D.conduction pathway is damaged, e.g. different types of heart blocks

Answer 18

all of the above

Question 19

In sympathetic systems, which 2 NT will activate the b1 receptors in the heart to increase contractility and heart rate?

Answer 19

Adrenaline and noradrenaline

Question 20

What is released in parasympathetic system? What receptor are activated?

Answer 20

Acetylcholine
M2 receptors in SA node, AV node and atria

(NOT in ventricles)

Question 21

How does activating M2 receptors in parasympathetic stimulation cause a negative chronotropic effect?

Answer 21

M2 receptors is coupled to Gi protein.
AC inhibition, thus less cAMP,
slower phase 4
increased threshold of action potential

Question 22

How does activating b1 receptors in sympathetic stimulation cause a positive chronotropic effect - increased conduction velocity?

Answer 22

b1 is coupled to Gs protein
increase AC increase cAMP > PKA
thus increase in Ca2+ for faster upstroke of action potential