CPR Cardiac Action Potentials & Conduction

Question 1

Describe how an action potential spreads in heart

Answer 1

SA node
AV node
Bundle of His
R/L bundle branches
Purkinje fibers

Question 2

What maintains the order of stimulus in the heart?

Answer 2

overdrive suppression of the SA node

Question 3

Why is the SA node in “control” of pacemaking properties?

Answer 3

the SA node generates action potentials most frequently (followed by AV node & then the Purkinje fibers)

Question 4

Where is the delay in the action potential?

Answer 4

@ the AV node

Question 5

Which cells have the highest velocity?

Answer 5

Purkinje cells because have the largest diameter

Question 6

Describe the order of contraction of the heart

Answer 6

RA contracts before LA

endocardium contracts before epicardium

RV epicardium contracts before LV epicardium

Question 7

What is unique about the SA node membrane potential?

Answer 7

resting Vm is more positive compared to resting Vm of ventricles & atria

Question 8

What is phase 4 assoc w/ in the SA node?

Answer 8

open of funny Na channels to cause rise in membrane potential in SA node

Question 9

What is responsible for phase 2?

Answer 9

combo of inward Ca2+ current, outward K+ current & inward rectifiers (K1)

inward Ca2+ current via L-type Ca2+ channels

Question 10

What is responsible for phase 1?

Answer 10

transient outward K+ current via K+ channels

Question 11

What is responsible for transition from phase 2 to phase 3?

Answer 11

voltage gated K+ channels (outward K+ current!)

Question 12

Does the AP or contraction occur first?

Answer 12

the AP occurs before the contraction

Question 13

What channels are always open?

Answer 13

K+ leak channels are open all the time

would have increased current outward during phase 2

Question 14

What provides the rhythmicity of contractions?

Answer 14

SA & AV nodes are primary source

Question 15

What occurs in phase 4 in SA & AV nodes?

Answer 15

resting Vm gradually depolarizes until reaches threshold & then fires (more slowly in AV node)

due to funny voltage-gated Na+ channels that open upon COMPLETE repolarization of membrane

Question 16

What occurs in phase 0 in SA node?

Answer 16

opening of voltage gated Ca+ channels (not Na+ channels) & closure of voltage gated K+ channels

Question 17

What occurs in phase 3 in SA node?

Answer 17

reversal of phase 0 w/ closure of voltage gated Ca+ channels & opening of voltage gated K+ channels

Question 18

How are the SA & AV nodes controllers of the contracting rhythm of heart?

Answer 18

will naturally depolarize to reach threshold w/o electrical stimulation

stimulation of AV node by SA node will trigger AV node to generate AP before it can make its own

Question 19

Why are refractory periods longer in cardiac cells?

Answer 19

help to prevent arrhythmias

Question 20

What is the supranormal period?

Answer 20

cell is more excitable than normal & easier to generate AP (may have abnormal conduction)

Question 21

How does an AP compare if stimulated in RRP or SNP?

Answer 21

conduction of AP will be weaker

Question 22

Describe parasympathetic innervation of heart

Answer 22

carried by vagus N to SA & AV nodes

ACh binds to muscarinic receptor (M2/M3)

Question 23

Parasympathetic effects on AP

Answer 23

negative chronotropic (slower opening of funny Na+ & hyperpolarization of SA node by increasing K+ outflow via special K+ channels)

negative dromotropic (reduced Ca+ inward current & hypopolarizes SA node b/c decreases outward + current)

Question 24

Describe sympathetic innervation of heart

Answer 24

to SA node, AV node & ventricular myocytes

Norepi binds to B1 adrenergic receptor

Question 25

Sympathetic effects on AP

Answer 25

positive chronotropic effects (more rapid opening of Na+ channels & hypopolarizes SA node)

positive dromotropic effects (increased inward Ca2+ current & hyperpolarization of SA node)

positive inotropic influence

Question 26

Sympathetic v Parasympathetic on HR

Answer 26

Sympathetic: hypopolarize & increase inward Na+

Parasympathetic: hyperpolarize & decrease inward Na+