Cardiac Action Potentials

Question 1

What is the route of the cardiac conduction system?

Answer 1

SA Node –> Atria (which contracts/depolarizes) –> AV node –> Bundle of His –> RBB and LBB –> Purkinje Fibers on each side –> lateral walls of the ventricles

Question 2

Which layers receives AP first?

Answer 2

Endocardium of ventricles receive AP prior to epicardium. Right epicardium before left.

Question 3

Describe importance of fiber size

Answer 3

Large fibers have a faster AP transmission and greater velocity

Question 4

Conduction velocity from greatest to least? Relevance?

Answer 4

purkinje fibers > atrial and ventricular muscle > AV node

Delay in AV allows atria to empty into ventricles before ventricles contract

Question 5

Difference between ventricle and atrium action potentia?

Answer 5

Phase 2 of ventricle has a longer length of time in it’s plateau form and stays more positive than atrium AP.

Question 6

Why does the Ventricle and Atrium AP have a longer refractory period?

Answer 6

To make sure that the cardiac muscle relaxes before responding to a new stimulus

Question 7

Na+ (m)

• activates when?
• action, phase
• features

Answer 7

• activate when membrane potential reaches threshold
• Na+(m) channels open
• phase 0 and 1
• rapid depolarization then automatically closes to allow partial repolarization

Question 8

K+ (a)

• activation
• action, phase
• features

Answer 8

• activated when cell depolarizes
• K+(a) channels open
• phase 1
• rapid partial repolarization

Question 9

Ca2+

• activation
• action, phase
• features

Answer 9

• activated when cell depolarizes
• Ca2+ channels open
• phase 2&3
• opens slowly and remains open for a determined amount if time, when it closes it leads to phase 3

Question 10

K+ (b)

• activation
• action, phase
• features

Answer 10

• activated when cell depolarizes
• phase 2/3
• closes slowly and remains closed for a determined amount of time, then it opens (start of phase 3)

Question 11

K+ (c)

• activation
• action, phase
• features

Answer 11

• remains open (always active)
• phase 4
• leak channel maintains RMP

Question 12

Na+ (f)

• activation
• action, phase
• features

Answer 12

• activation via SA or AP node repolarization
• Funny Na+ channels opens
• phase 4
• specific to SA and AV node only, causes slow depolarization during resting phase (this is basically how it depolarizes during resting when it reaches threshold and how it can keep up maintaining its own AP)

Question 13

Describe phase 4 in the SA node

Answer 13

In the SA node RMP gradually depolarizes until it reaches threshold, then it “fires” (although slower than other regions. Intrinsic spontaneous depolarization makes the SA node the pacemaker because of its automaticity)
Phase 4 happens due to the opening of voltage-gated Na funny channels

Question 14

Phase 0 of SA node

Answer 14

In SA node - phase 0 occurs due to slow opening of Ca2+ channels and closing of K+(b) channels. Balance between Ca2+ coming in and K+ going out

Question 15

Phase 3 of SA node

Answer 15

due to the closing of Ca2+ channels and opening of K+(b) channels (basically a reversal of phase 0)

Question 16

Difference between SA node vs. AV node AP potential

Answer 16

AV node has a slower depolarization during phase 4

If the AV node takes over, the rate is much slower

Question 17

What is the resting potential in fast tissues and what is it sustained by?

Answer 17

Phase 4 is the resting potential and is sustained by high K+ (c) conductance

Question 18

What happens in phase 0? (AV node)

Answer 18

rapid upstroke by crossing threshold

VG Na+ (m) channels open

Question 19

What happens in phase 1? (mostly AV node)

Answer 19

small repolzarization caused by closing of VG Na+ (m) channels and opening of K+ (a) channels

Question 20

What happens in phase 2 of AV node AP?

Answer 20

plateau phase is sustained by slow opening of VG Ca2+ channels and closing of special VG K+(b) channels

this event prolongs contraction

Question 21

What happens in phase 3 of AV node AP?

Answer 21

complete repolarization (steep curve down) caused by slow VG Ca2+ channels closing and K+ (b) channels opening

Question 22

AV node damage effect

Answer 22

further delays conduction - AV Block

Question 23

Purkinje fiber damage effect

Answer 23

disrupts natural conduction and contraction of ventricles - Arrthymias)

Question 24

Refractory period importance

Answer 24

Refractory periods is when gates have not fully reset so it prevents a second AP to be generated.
RP is important to help prevent arrhythmias and are longer in cardiac cells than neurons

Question 25

ARP

Answer 25

no depolarization

Question 26

RRP

Answer 26

AP can be generated but will have an abnormal conduction. Conduction of AP is weaker if stimulated during RRP

Question 27

SNP

Answer 27

cells is more excitable than normal (phase 4)

Question 28

Chronotropic effect: positive and negative

Answer 28

-changes rate of depolarization of SA node and heart rate
(+) –> faster HR
(-) –> slower HR

Question 29

Dromotropic effect

Answer 29

speed of conduction changes

Question 30

Inotropic effect

Answer 30

changes strength of muscle contraction

Question 31

Lusitropic effect

Answer 31

changes rate of muscle relaxation

Question 32

Parasympathetic stimulus

Answer 32

NT: Ach
Receptor: muscarinic (M2 or M3)
To SA node and AV node

Question 33

Negative chronotropic effects

Answer 33

• Parasympathetic autonomic effect on AP
• slow opening of funny Na channels during phase 4
• hyperpolarization bu increasing outward K+ current

Question 34

Negative dromotropic effects

Answer 34

• parasympathetic
• reduced Ca2+ inward current
• increased outward K+ current

Question 35

Sympathetic stimulus on AP

Answer 35

to SA node and AV node and to myocytes
NT: NE
receptor: beta-1 adrenergic receptor

Question 36

positive chronotropic effects

Answer 36

• sympathetic NS effect on AP
• increase opening of funny Na channels during phase 4
• increase inward current of Ca

Question 37

positive dromotropic effects

Answer 37

• increased Ca2+ inward current during phase 2

- faster AP seen on graph