Lecture 13: Cardiac Action Potentials and Conduction

Question 1

What order does electrical impulse travel through the heart?

Answer 1

SA node > AV node > bundle of his > r/l bundle branches > purkinje fibers

Question 2

How frequently do these generate action potentials?
SA node
AV node
Purkinje fibers

Answer 2

SA - most frequent, spontaneous
AV - slower, non spontaneous
Purkinje - slowest

Question 3

How fast does AP travel in these parts of the heart?
Purkinje
Atrial and Ventricle myocytes
AV node

Answer 3

Fastest
Middle
slowest

Question 4

What affects conduction velocity?

Answer 4

Larger cell diameter = larger conduction velocity

Question 5

What is synchronicity ?

Answer 5

All cardiac myocytes need to contract in sync or you’ll have an arrhythmia.

Question 6

What contracts first, endocardium or epicardium?

Answer 6

Endocardium

Question 7

What channels close when activated?

Answer 7

I K1

I Naf

Question 8

What happens on the phase 4 of SA Node depolarization?

Answer 8

“funny” Na channels (I Na) open slowly letting Na+ in gradually and reaching threshold

Question 9

What happens at phase 0 of SA Node depolarization?

Answer 9

Voltage gated Ca2+ channels open causing large influx of Ca2+ in cell

Question 10

What happens at phase 3 of SA Node depolarization?

Answer 10

Ca2+ voltage channels close,
K+ voltage channels open
Overall positive charge decreases (repolarization)

Question 11

How does the AV node normally reach threshold? What is this concept called?

Answer 11

Overdrive suppression: It can spontaneously reach threshold on its own, but it is usually brought to threshold by SA Node potential before it can do so

Question 12

How are refractory periods different in cardiac cells? What does this accomplish?

Answer 12

Longer. Helps prevent arrhythmias

Question 13

What is the supranormal period (SRP)? What happens to action potentials generated during this period?

Answer 13

• Type of refractory period which is a reverse. Cardiac cell is more excitable to AP, so next AP happens before repolarization is over
• have weaker conduction

Question 14

What does chronotropic mean?
What does dromotropic mean?
What does inotropic mean?
What does lusitropic mean?

Answer 14

Affects heart rate
Affects speed of conduction (slope of phase 0)
Affects contraction strength
Affects relaxation strength

Question 15

*How is the SA and AV node parasympathetically innervated and how does this affect circulation?

Answer 15

Vagus > Ach > M2/M3 muscarinic receptors

Negative chronotropic and dromotropic effects (decreases intracellular Na+)

Question 16

How does parasympathetic stimulation achieve negative effects?

Answer 16

Chronotropic: Ach decreases rate of funny Na+ channel opening and increase K+ efflux through K+ Ach channel = hyperpolarization
Dromotropic: decrease Ca2+ influx, increase K+ efflux

Question 17

How are the SA, AV nodes and cardiac myocytes sympathetically innervated and how does this affect circulation?

Answer 17

Sympathetic fibers > NE > B1 receptors

Positive chronotropic and dromotropic effects (increases intracellular Na+)

Question 18

How does sympathetic innervation achieve positive effects?

Answer 18

Chronotropic: NE increases rate of funny Na+ channel opening and reduce K+ efflux = harder to hyperpolarize
Dromotropic: increase Ca2+ influx and decrease K+ efflux = hypopolarization