Cardiac Conduction and Action Potentials

Question 1

The action potential spreads throughout the myocardium in the following sequence:

Answer 1

SA node AV node, bundle of his, purkinje system

Question 2

slow conduction through the AV node ensures what?

Answer 2

that the ventricles have sufficient time to fill with blood before they are activated and contract

Question 3

how is the velocity of the cardiac conduction system determined?

Answer 3

cells/fiber with larger diameter are faster (purkinje> atrial and ventricular muscle> AV node

Question 4

What is phase 0 in the ventricles, atria, and purkinje system known as?

Answer 4

upstroke/ depolarization

Question 5

what is phase 0 caused by in the ventricles, atria, and purkinje system?

Answer 5

a transient increase in Na + conductance ( I na)

Question 6

what happens when the Na+ conductance increases?

Answer 6

there is an inward Na+ current, which drives the membrane potential towards +65 mV

Question 7

what is the majority of phase 0 being driven by?

Answer 7

the opening of voltage gated “fast” Na+ channels activation gate

Question 8

what happens to the Na+ channels as the cell starts to depolarize?

Answer 8

the inactivation gates on the Na+ channels start to close in response to depolarization

Question 9

for repolarization to occur, what must happen?

Answer 9

there must be a net outward current

Question 10

what is phase 1 in the ventricles, atria, and purkinje system known as?

Answer 10

initial repolarization

Question 11

what is phase 2 in the ventricles, atria, and purkinje system known as?

Answer 11

plateau

Question 12

how is there a balance of inward and outward currents achieved during the plateau of phase 2 (aka what occurs in phase 2 of the cardiac fast action potential)?

Answer 12

there is opening of the voltage-gated L-type Ca2+ channels; closure of inward rectifying K+ channels; there is outward K+ current

Question 13

what is phase 3 in the ventricles, atria, and purkinje system?

Answer 13

repolarization

Question 14

what occurs during phase 3?

Answer 14

closure of the voltage-gated L-type Ca2+ channels; opening of the inward rectifying K+ channels; opening of the voltage-gated K+ channels

Question 15

what is the big driver of phase 3- repolarization?

Answer 15

the opening of the voltage-gated K+ channels (I Kr (rapid) and I Ks (slow))

Question 16

what causes Long QT syndrome?

Answer 16

transition from phase 2 to phase 3 is lengthened; it could be acquired or it can be congenital causes influencing I Kr or I Ks

Question 17

Outward K current makes the membrane potential what?

Answer 17

more negative

Question 18

in the cardiac fast action potential, what does the resting membrane approach?

Answer 18

the K+ equilibrium potential

Question 19

what occurs during phase 1 of the cardiac fast action potential?

Answer 19

there is closure of the inactivation gates on the voltage-gated Na+ channels AND opening of the transient outward K+ current

Question 20

what is phase 4 of the cardiac slow action potential?

Answer 20

resting membrane potential gradually depolarizes until reaches threshold

Question 21

what acts as the pacemaker of phase 4 of the cardiac slow action potential?

Answer 21

spontaneous depolarization

Question 22

what causes the spontaneous depolarization of the phase 4 of the cardiac slow action potential?

Answer 22

a slow influx of Na+ through “funny” voltage-gated Na+ channels

Question 23

when do the funny Na+ channels open?

Answer 23

upon complete repolarization of membrane

Question 24

what generates/drives the phase 0 of the cardiac slow action potential?

Answer 24

voltage-gated Ca2+ channels

Question 25

what occurs during the phase 0 of the cardiac slow action potential?

Answer 25

opening of the voltage-gated Ca2+ channels and closure of the inward rectifying K+ channels

Question 26

what is/ what occurs during phase 3 of the cardiac slow action potential?

Answer 26

closure of the voltage-gated Ca2+ channels and opening of the delayed rectifying K+ channels and closure of inward rectifying K+ channels

Question 27

what are the parasympathetics to the heart carried by?

Answer 27

the vagus nerve

Question 28

what is the parasympathetic neurotransmitter that acts on the heart? and what receptors does it act on?

Answer 28

acetylcholine acts on M2/M3

Question 29

what is the specific effect of the parasympathetics on the heart?

Answer 29

there is slower entry of Na+ through funny Na channels during phase 4; also hyperpolarization of SA node by increasing outward K+ current via special K+-ACh channels

Question 30

what is the sympathetic neurotransmitter that acts on the heart? and what is its receptor?

Answer 30

norepinephrine - Beta-1 adrenergic receptor

Question 31

what is a positive chronotropic effect?

Answer 31

speeding up of heart rate

Question 32

what is a positive inotropic effect?

Answer 32

increased contractility

Question 33

how specifically does the sympathetics speed up heart rate?

Answer 33

there is a more rapid influx of Na+ through funny Na+ channels

Question 34

how do the sympathetics increase contractility?

Answer 34

there is increased Ca2+ into myocytes

Question 35

what happens during the absolute refractory period of the heart?

Answer 35

all voltage-gated Na+ channels are inactivated

Question 36

what is the supranormal period?

Answer 36

cell is actually more excitable than normal

Question 37

what effect does hyperkalemia have on the cardiac action potential?

Answer 37

it slows phase 0 and speeds phase 3 (you are more likely to reach threshold)

Question 38

what effect does hypokalemia have on the resting membrane potential?

Answer 38

it will be more negative- you will need a stronger stimulus