Cardiac Action Potentials and Conduction

Question 1

what is the order an action potential spreads in the heart

Answer 1

1) SA node
2) AV node
3) Bundle of His
4) Right/left bundle branch
5) Purkinje fibers

Question 2

what is the overdrive suppression

Answer 2

it helps maintain order in the contraction of a action potential

this is because SA node, AV node, and Purkinje fibers all can generate their own action potentials

however since SA node is the most frequent it holds the true pacemaker rthym

Question 3

what fibers/cells have the highest velocity

Answer 3

Velocity is the cells/fibers with the largest diameter conduct the fastest

Purkinje > atrial and ventricular muscle > AV node

Question 4

what is the order of chambers of contraction as well as epicardium vs endocardium

Answer 4

Right atrium prior to left atrium

Endocardium prior to epicardium

Right ventricle epicardium before left ventricle epicardium

Question 5

out of the Ventricle, atrium and Sinoatrial node which one has the longest action potential

Answer 5

Ventricle over the atrium

the sinoatrial node has the shortest but its phase 4 will slowly increase to threshold due to its pacemaker capability

Question 6

Phase, Action and Activated when: INA

Answer 6

Phase: 0,1

Action: open gate

when: THreshold is reached

voltage gated sodium channel
-sodium into cell

Question 7

Phase, Action and Activated when: Ito

Answer 7

Phase: 1

Action: open

when: cell depolarizes

Very rapid partial repolarization
K+ channel
-K+ out of cell

Question 8

Phase, Action and Activated when: ICa

Answer 8

Phase: 2 and 3

Action: opens

when: cell depolarizes

Slow to open and remains open for a determined amount of time then closes causing phase 3
L-type Ca channel
-Ca+ into cell
-causes the plateau on AP

Question 9

Phase, Action and Activated when: IK1

Answer 9

Phase: 2 and 3

Action closes

when: cell depolarizes

inward rectifies, slow to close and remains closed for a determined amount of time then opens helping the phase
Voltage sensing K+ channel
-closed during phase 2 and 3 but opens during phase 0
-prevents too much K+ from leaving
-when open K+ flows out

Question 10

Phase, Action and Activated when: IKr, IKs

Answer 10

Phase: 3

Action: opens

when: Peak

Contributes to phase 3 repolarization

• Voltage gated K+ channels
• Helps transition 2 to 3
• Allows K+ out of the cell

Question 11

Phase, Action and Activated when: IK

Answer 11

phase: 4
action: open
when: remain open

K+ leak channels
Open all the time but during phase 2 they have an increased current outward

Question 12

Phase, Action and Activated when: INaf

Answer 12

phase: 4
action: close
activated: threshold

these contribute to the rise in membrane potential in SA and AV node
Close at threshold but open during repolarization to being the slow depolarization from Resting membrane potential
-creates pacemaker affect

Question 13

what contributes to the plateauing phase 2 in the ventricle and atrium

Answer 13

Combination of Ca+ current, outward K+ , and inward rectifiers holding back K+

Question 14

is there a slight delay between action potential and contraction

Answer 14

Yes!!

Question 15

what is the difference between Phase 0 for SA and AV nodes vs atrium and ventricles

Answer 15

rather than using Na+ voltage gated channels and closure of voltage gated K+ channels to have the quick depolarization

the SA and AV nodes have voltage gated Ca+ channels

Question 16

what is the difference between Phase 3 for SA and AV nodes vs atrium and ventricles

Answer 16

rather than the opening of voltage gated K+ channels (both rectifiers and traditional)

there is just closure of the voltage gated Ca+ channels

Question 17

what are the two types of refractory periods

Answer 17

Absolute refractory period (ARP)
-No AP can be generated

Relative Refractory Period (RRP)
-AP can be generated but requires greater stimulus and/or have abnormal conduction

Question 18

what is the difference in refractory periods for Cardiac Cells

Answer 18

they are much longer, this way it helps prevent arrhythmias

Question 19

what is the supranormal period

Answer 19

supranormal period (SNP)
-is a period where the cell is more excitable than normal and easier to generate AP, may have abnormal conduction

Question 20

what happens if a cardiac cell is stimulated during RRP or SNP

Answer 20

conduction of that AP will be weaker

Question 21

Chronotropic

Answer 21

an effect changes in the heart rate, slope of depolarization in phase 4 at SA node

Positive chronotropic effect= faster
Negative chronotropic effect= slower

Question 22

Dromotropic

Answer 22

effect speed of conduction (conduction velocity) slope of phase 0

usually paired with chronotropic

Question 23

Inotropic

Answer 23

effect strength of musculature contraction

Question 24

Lusitropic

Answer 24

effect rate of muscular relaxation

usually paired with Lusitropic

Question 25

Parasympathetic stimulus on the heart
what are the two effects
receptor
neurotransmitter

Answer 25

to SA and AV node via acetylcholine on a muscarinic receptor (M2/M3)

Negative chronotropic effect:

• slower opening of sodium channels during phase 4
• hyperpolarization of SA node by increasing K+ current via special K+ channels (K+ ACh channel)

Negative dromotropic effects:

• reduce inward Ca+ current
• increase outward K+ churrent via special K+ channels (K+ ACh channel)

Question 26

Sympathetic stimulus
what are 4 effects
receptor
neurotransmitter

Answer 26

to SA node, AV node and ventricular myocytes via norepinephrine and receptor B1-adrenergic

Positive Chronotropic effects:
-more rapid opening of funny Na+ channels during phase 4

Positive dromotropic effect:
-increased inward Ca+ current

Positive inotropic or lusitropic influence