Cardiac Action Potential

Question 1

How does the AP spread in the heart?

Answer 1

SA node -> AV node -> bundle of HIS -> R/L bundle branches -> Purkinje fibers

Question 2

What are all the pacemaker ability structures from most frequent AP to least?

Answer 2

SA node > AV node > Purkinje fibers

Question 3

What is overdrive suppression?

Answer 3

-The SA node will fire before the AV or Purkinje fibers reach threshold = therefore controlling HR

Question 4

What increases the velocity of fibers?

Rank velocities fast to slower

Answer 4

larger diameter = faster

Purkinje > atrial and ventricular m. > AV node

Question 5

What contract first : the right or left atria?

Answer 5

right

Question 6

What contracts first endocardium or epicardium?

Answer 6

endocardium

Question 7

What contracts first: epicardium of the LV or epicardium of the RV?

Answer 7

epicardium of the RV

Question 8

Where do fast cardiac AP occur?

Answer 8

atria and ventricle myocytes

Question 9

What occurs during phase 0 of the fast AP?

Answer 9

• depolarization of myocytes via voltage gated Na channels
• L-type Ca2+ channels slow to open
• inward rectifier current (Ik) channels close

Question 10

What occurs during phase 1 of fast AP?

Answer 10

• rapid partial depolarization due to transient K+ channels

- inactivation gate of Na channel is closed by now

Question 11

What occurs in phase 2 of the fast AP?

Answer 11

• plateau in membrane potential as there is no net current flow
• plateau due to slow l-type Ca channel rain open

Question 12

What causes the no net current flow during the plateau phase?

Answer 12

The small influx of calcium is opposed by outward K+ (volt gated) flow of same magnitude

Question 13

What occurs in phase 3 of the fast AP?

Answer 13

l-type Ca channels close and inward rectifier current opens again to depolarize

Question 14

What occurs during phase 4 of the fast AP?

Answer 14

• leak channels remain open

- Na volt gate inactivation gate open and activation gate closed

Question 15

What contributes to the outward current during phase 2 of the fast AP?

Answer 15

K+ volt channels

Question 16

What processes are occurring that cause phase 2 in the fast AP to occur?

Answer 16

• inward Ca2+ current
• outward K+ current
• inward rectifiers (K+)

Question 17

What causes the atrial fast AP to be shorter than the ventricular?

Answer 17

• L-type Ca2+ close faster in the atria vs the ventricle

Question 18

Where does the slow cardiac AP occur?

Answer 18

SA & A V nodes

Purkinje fibers

Question 19

What occurs during phase 4 of the slow AP?

Answer 19

RMP is gradually reaching threshold due to “funny” Na channels

Question 20

What occurs during phase 0 of the slow AP?

Answer 20

-depolarization occurs vis Ca2+ voltage gated channels and closure of K+ rectifiers

(causes slow AP)

Question 21

What occurs during phase 3 of the slow AP?

Answer 21

depolarization via closure of the Ca2+ volt channels and opening of the K+ channels (rectifiers and traditional)

Question 22

What does overdrive suppression prevent?

Answer 22

-prevents bundle branches and Purkinje fibers from firing spontaneously

Question 23

Are refractory periods short or long in cardiac cells?

Why?

Answer 23

long, to prevent arrhythmia

Question 24

What is the absolute refractory period?

Answer 24

no AP can be generated b/c Na volt gate channels are inactivated and will not reopen till reach closer to RMP

Question 25

What is the relative refractory period?

Answer 25

AP can be generated but requires a greater stimulus or have abnormal conduction

Question 26

What is the supernormal period?

Answer 26

cell is more excitable that normal and easier to generate AP, may have abnormal conduction

(-70 to -85 mV)

Question 27

What does chronotropic mean?

Answer 27

• effect changes the HR

- slope in depolarization of slow phase 4 at SA node

Question 28

What is a positive chronotropic effect?

Answer 28

• HR = faster

- increase in slope of phase 4 (slow)

Question 29

What is a negative chronotropic effect?

Answer 29

• HR = decrease

- decrease in slope of phase 4 (slow)

Question 30

What does dromotropic mean?

Answer 30

effect speed of conduction (conduction velocity)

-effect slope of phase 0

Question 31

What does iontropic mean?

Answer 31

effect strength of muscular contraction

Question 32

What does lusitropic mean?

Answer 32

effect rate of muscular relaxation

Question 33

What are the parasympathetic and sympathetic n.s that innervate the heart?

Answer 33

para = vagus

symptoms = cardia splanchnic n.

Question 34

What structures does the vagus nerve effect on the heart?

What is its NT? receptor?

Answer 34

-SA & AV node

• Acetylcholine
• Muscarinic (M2/M3)

Question 35

What structures does the cardiac splanchnic n. affect?

What is its NT? receptor?

Answer 35

SA & AV node

• norepinephrine
• beta adrenergic (B1)

Question 36

What kind of effects does the parasympathetic system have on the heart?

Answer 36

hyperpolarize

negative chronotropic  (decrease HR)
negative dromotropic (decrease conduction velocity) 

-no changes on relaxation or force

Question 37

What kind of effects does the sympathetic system have on the heart?

Answer 37

“hypopolarize”

postive chronotropic (increase HR)

positive dromotropic (increase conduction velocity)

positive iontropic and lusitropic effects

Question 38

What channel is responsible to mitigating the effects of the parasympathetic and sympathetic nervous system in regards to K+ flow?

Answer 38

K+ - ACh channels