Cardiac Action Potentials & Conduction

Question 1

Delineate the conduction system of the heart.

Answer 1

SA node
AV node
AV bundle
R and L bundle branches
subendocardial branches

Question 2

List the cardiac cell areas that have intrinsic pacemaker abilities in order from most frequent action potential to least frequent.

Answer 2

SA node
AV node
subendocardial branches

Question 3

What is a problem with the subendocardial fibers if only they were left to generate action potentials for the heart’s contraction?

Answer 3

• subendocardial fibers’ action potentials are not synchronized
• cardiac muscle ctx would be chaotic

Question 4

True or False: fibers with a larger diameter deliver signals more quickly

Answer 4

True

Question 5

What happens in Phase 0 of ventricular and atrial action potentials?

Answer 5

rapid depolarization

-voltage-gated Na+ channels open

Question 6

What happens in Phase 4 of SA node action potential?

Answer 6

gradual depolarization

Question 7

What ionic conditions are responsible for the long action potential (dragging out of Phase 2) in cardiac muscle?

Answer 7

+calcium coming into the cell
+potassium is prevented from leaving the cell (IK1)
– voltage-gated K+ channels let K+ out of cell
–K+ leak channels let K+ out of cell
— combination of all this interplay

Question 8

What happens during Phase 1 of ventricular action potentials?

Answer 8

-transient outward K+ current

–keeps the Na+ channels from overshooting

Question 9

What happens during Phase 3 of ventricular action potentials?

Answer 9

-repolarization of the cell

• -increased activity of voltage-gated K+ channels
• -inward rectifier channels open to let K+ out (IK1)

Question 10

Can an action potential fire during the Absolute Refractory Period?

Answer 10

no

Question 11

Can an action potential fire during the Relative Refractory Period?

Answer 11

yes, but it requires a greater stimulus and/or will have abnormal conduction

Question 12

What is the Supranormal Period?

Answer 12

-when the cell is more excitable than normal and it’s easier to generate an action potential, but may have abnormal conduction

Question 13

What are chronotropic changes in reference to?

Answer 13

heart rate

Question 14

What are dromotropic changes in reference to?

Answer 14

speed of conduction

Question 15

What are inotropic changes in reference to?

Answer 15

strength of ctx

Question 16

What are lusitropic changes in reference to?

Answer 16

rate of relaxation

Question 17

What supplies parasympathetic innervation to the heart?

Answer 17

cardiac branches of the vagus N.

Question 18

True or False: parasympathetic innervation to the heart goes only to the SA and AV nodes, not to the ventricles

Answer 18

True

Question 19

What is the neurotransmitter and receptor combination used in parasympathetic innervation to the heart?

Answer 19

Ach
muscarinic receptors (M2 and M3)

Question 20

What is the negative chronotropic effect caused by parasympathetic innervation of the heart?

Answer 20

• slower opening of funny Na+ channels during Phase 4

- increase in outward K+ current via K(ach) channels

Question 21

What is the negative dromotropic effect caused by parasympathetic innervation of the heart?

Answer 21

• reduced inward calcium current
• increased outward K+ current
• hyperpolarization

Question 22

What provides sympathetic innervation to the heart?

Answer 22

cardiac splanchnic N.

T1-T5

Question 23

True or False: sympathetic innervation goes to SA and AV nodes, as well as to the ventricles

Answer 23

True

Question 24

What is the neurotransmitter and receptor combination for sympathetic innervation to the heart?

Answer 24

norepinephrine

beta-1 adrenergic receptors

Question 25

What is the positive chronotropic effects caused by sympathetic innervation of the heart?

Answer 25

-more rapid opening of funny Na+ channels during phase 4

Question 26

What is the positive dromotropic effect caused by sympathetic innervations of the heart?

Answer 26

increased inward calcium current

Question 27

Does sympathetic innervation has a positive or negative effect on inotropy and lusitropy?

Answer 27

positive

• increased contractility
• increased rate of relaxation