Cardiac APs

Question 1

Order the conducting tissues in order of conduction velocity from fastest to slowest

Answer 1

His/Purkinje > Atria + Ventricles > AV node

Question 2

Why is important for the AV node to have the slowest conduction?

Answer 2

Slow AV node conduction provides a delay that allows sufficient time for the ventricles to fill with blood before a contraction can occur.

Question 3

Which ventricular epicardium receives the AP first from the purkinje system?

Answer 3

The right ventricular epicardium receives the AP before the left ventricular epicardium.

Question 4

Which tissues have pacemaker potential, what does this mean exactly, and which ones are considered latent pacemakers?

Answer 4

SA node, AV node, bundle of His, and Purkinje fibers have pacemaker potential.

Having pacemaker potential means these tissues all have the capacity to produce a spontaneous phase 4 depolarization.

The latent pacemakers are the AV node, bundle of his, and purkinje fibers.

Question 5

Why don’t we normally see the latent pacemakers spontaneously have a phase 4 depolarization?

Answer 5

Because the SA node does so before any of the latent pacemakers can, which is why the SA node controls the heart rate

Question 6

Order the tissues with pacemaker potential in order of fastest rate of phase 4 depolarization from fastest to slowest

Answer 6

SA node > AV node > bundle of His + Purkinje fibers

Question 7

What happens in Phase 4 of the SA node action potential?

Answer 7

Slow depolarization due to opening of “funny” voltage- gated Na+ (f) channels

Special K+ (b) channels are also open at this time

Question 8

What happens during phase 0 of the SA node action potential?

Answer 8

Rapid depolarization due to OPENING of slow Ca2+ channels and CLOSING of special K+ (b) channels

Question 9

What happens during phase 3 of the SA node AP?

Answer 9

Repolarization occurs due to CLOSING of Ca2+ channels and OPENING of special K+ (b) channels

Question 10

What happens during phase 4 of the cardiac cell AP?

Answer 10

Resting potential is sustained by high K+ (c) leak channel conductance at around -80 mV

K+ (b) channels are also open at this time

Question 11

What happens during phase 0 of the cardiac cell AP?

Answer 11

Rapid depolarization due to crossing threshold and OPENING of voltage-gated Na+ (m) channels

At this point K+ (c) leak channels and K+ (b) channels are also open

Question 12

What happens during phase 1 of the cardiac cell AP?

Answer 12

Small repolarization occurs due to CLOSING of Na+ (m) channels and OPENING of K+ (a) channels

K+ (c) leak channels and K+ (b) channels remain open at this time

Question 13

What happens during phase 2 of the cardiac cell AP?

Answer 13

Plateau phase occurs due to closing of K+ (a) channels, OPENING of slow opening voltage-gated Ca2+ channels, OPENING of K+ (d) channels, and closing of K+ (b) channels

The influx of Ca2+ and outflow of K+ balance out to form the plateau

At this time, K+ (c) leak channels are still open

Question 14

What happens during phase 3 of the cardiac cell AP?

Answer 14

Complete repolarization occurs due to CLOSING of voltage-gated Ca2+ channels, CLOSING of K+ (d) channels, and OPENING of K+ (b) channels.

K+ (c) leak channels are also open at this time

Question 15

What is a refractory period?

Answer 15

A period of time after Athens firing of an AP where the electrolyte gates have not “reset” sufficiently to allow a second AP to be generated

Helps prevent arrhythmias

Question 16

Absolute Refractory Period? (ARP)

Answer 16

Not depolarization can occur

Begins at the upstroke (Phase 0)

Ends after the plateau (Phase 3)

Question 17

Effective Refractory Period (ERP)

Answer 17

Conducted AP cannot be elicited

Slightly longer than the ARP

Begins at Phase 0

Ends at Phase 3

Question 18

Relative Refractory Period (RRP)

Answer 18

AP can be generated, but will have weaker conduction

AP requires greater than normal inward Na+ current to occur

Begins right after ARP ends (Phase 3)

Ends when repolarization is nearly complete

Question 19

Supranormal Period (SNP)

Answer 19

Cell is more excitable than normal

Begins right after RRP ends (when repolarization is nearly complete)

Question 20

What do Chronotropic effects do?

Answer 20

Change the rate of Depolarization of SA node, effectively increasing or decreasing the heart rate

Question 21

What do Dromotropic Effects do?

Answer 21

Changes the conduction velocity (primarily in the AV node), effectively increasing or decreasing the delay in activation of the ventricles

Question 22

Parasympathetics: What nerve carries this signal? What tissues does it innervate? What neurotransmitter does it use? What receptors does it activate?

Answer 22

Vagus N.

Innervates the SA node, AV node, and the atria (DOES NOT innervate the ventricles)

Neurotransmitter: Acetylcholine

Receptor: Muscarinic (M2 or M3)

Question 23

Sympathetics: what tissues are innervated? What neurotransmitter is used? What receptors are activated?

Answer 23

SA node, AV node, and myocytes

Neurotransmitter: Norepinephrine

Receptor: Muscarinic (Beta-1 adrenergic receptors)

Question 24

How do negative Chromotropic effects work?

Answer 24

Initiated by parasympathetic signaling

Slowed opening of the Na+ (f) channels during phase 4 of the SA node AP

Hyperpolarization by increasing K+ outward current via K+-Ach channel

Heart rate decreases

Question 25

How do negative Dromotropic Effects work?

Answer 25

Initiated by parasympathetic signaling

Reduced inward Ca2+ current during Phase 0 of AV node AP

Increased outward K+ current via K+-Ach channels

Occurs at AV node (slowing the AP transmission from atria to ventricle)

Question 26

How do positive Chronotropic Effects work?

Answer 26

Initiated by sympathetic signaling

Increased opening of Na+ (f) channels during Phase 4 of SA node AP

Increased inward Ca2+ current during Phase 0

Cause

Question 27

How do Positive Dromotropic Effects work?

Answer 27

Initiated by sympathetic signaling

Increased inward Ca2+ current during Phase 0 of AV node AP