Cardiac Action Potential

Question 1

What all does the AP from the SA node go through?

Answer 1

spreads throughout atria causing depolarization (RA slightly before LA); spreads down to the AV node

Question 2

Describe the path of the AP from the AV node through the ventricles

Answer 2

AV node -> bundle of His -> right and left bundle branches in the interventricular septum -> down towards apex of heart and turns and moves back up along the lateral walls of the ventricles through Purkinje fibers

Question 3

Why are nerve fiber sizes in the heart important? Describe the order from large to small

Answer 3

larger fibers have faster AP transmission & greater velocity

Purkinje fibers/bundle branches/bundle of His > atria and ventricular myocytes > AV node

Question 4

What is the purpose of the conduction delay at the AV node?

Answer 4

to allow atria to completely empty into ventricles before ventricles contract

Question 5

What is resting membrane potential of the SA node?

Answer 5

-65 +/- (-5)

Question 6

What is resting membrane potential of the atria and ventricles?

Answer 6

-90 +/- (-5)

Question 7

Describe Na (m) channels

Answer 7

voltage gated Na channels that open to depolarize the cell during phase 0 (activated when membrane potential crosses threshold); contribute to phase 1 as well (inactivation gates close)

Question 8

Describe K (a) channels)

Answer 8

produce a transient outward current during phase 1; open briefly to cause partial repolarization (balancing act w/ Na (m) channels

Question 9

Describe L type Ca channels

Answer 9

voltage gated Ca channels that open after depolarization spike and bring Ca into the cell; establish plateau of phase 2 w/ K (b) channels; closing causes phase 3

Question 10

Describe K (b) channels

Answer 10

allow K out of cell except from phase 0 to phase 3; slow to close but prevent K from leaking out of cell too much as a result of Na influx

Question 11

Describe K (c) channels

Answer 11

K leak channels are always open and contribute to resting membrane potential

Question 12

Describe Na (f) channels

Answer 12

specific to the SA and AV nodes; cause slow depolarization during resting phase (allows nodes to have autonomous depolarization)

Question 13

Describe K (d) channels

Answer 13

voltage gated K channels; open and release K during phase 3 (repolarization)

Question 14

What causes phase 4 of the SA and AV nodes? What happens if the slope changes?

Answer 14

Phase 4 caused by Na (f) channels

If slope increases -> faster depolarization
If slope decreases -> slower depolarization

Question 15

What causes phase 0 of SA/AV node?

Answer 15

L type Ca channels open (Ca influx) and K (b) channels close

Question 16

What causes phase 3 of SA/AV node?

Answer 16

closing of L type Ca channels and opening of K (b) channels (reverse of phase 0)

Question 17

What do fast tissues involve for cardiac AP?

Answer 17

myocytes and Purkinje fibers

Question 18

What sustained resting membrane potential in phase 4 of fast tissues?

Answer 18

K (c) channels - leak channels continuously allow K out of cell

Question 19

What causes phase 0 of fast tissues?

Answer 19

depolarization caused by crossing threshold; voltage gated Na (m) channels open

Question 20

What causes phase 1 of fast tissues?

Answer 20

small repolarization caused by Na (m) channels closing and K (a) channels opening -> pulls potential down a little bit

Question 21

What causes phase 2 of fast tissues?

Answer 21

plateau phase -> sustained by slow opening of L type Ca channels which delays repolarization; balances w/ closure of K (b) channels to maintain plateau (sustain contraction)

Question 22

What causes phase 3 of fast tissues?

Answer 22

complete repolarization caused by closing of L type Ca channels and reopening of K (b) channels and opening of voltage gated K (d) channels

Question 23

What is absolute refractory period (ARP)?

Answer 23

no depolarization can occur

Question 24

What is relative refractory period (RRP)?

Answer 24

when an AP can be generated but the cell will have abnormal conduction (AP is weaker)

Question 25

What is supernormal period (SNP)?

Answer 25

when the cell is more excitable than normal but will have a lower phase 0 slope (slower velocity)

Question 26

Describe chronotropic effect

Answer 26

changes rate of depolarization of SA node (affects HR) ``` Positive = faster HR Negative = slower HR ```

Question 27

Describe dromotropic effect

Answer 27

changes speed of conduction ``` Positive = increased speed Negative = decreased speed ```

Question 28

Describe inotropic effect

Answer 28

changes strength of muscular contraction ``` Positive = heart contracts harder Negative = heart contracts softer ```

Question 29

Describe lusitropic effect

Answer 29

changes rate of muscular relaxation ``` Positive = increased relaxation Negative = decreased relaxation ```

Question 30

What carries parasympathetic fibers to the SA and AV nodes? Describe the receptors and NT involved in parasympathetics of the heart

Answer 30

Vagus N. carries parasympathetics to SA and AV node ``` NT = ACh Receptor = muscarinic (M2 or M3) ```

Question 31

Describe the receptors and NT involved in sympathetics of the heart

Answer 31

``` NT = NE Receptor = B1-andrenergic receptor ```