Cardiac Electrophysiology

Question 1

What is Phase 0 characteristic of in the ventricular myocyte AP?

Answer 1

Rapid Depolarization and Influx of Na+ ions

Question 2

What is Phase 1 characteristic of in the ventricular myocyte AP?

Answer 2

Early Repolarization through Cl- ion influx and Na+ efflux.

Question 3

What is Phase 2 Characteristic of in the ventricular myocyte AP?

Answer 3

AKA Plateau phase, and effective refractory period (ERP). K+ channels start repolarizing and SLOW opening Ca2+ channels open and potential plateaus.

Question 4

What is Phase 3 Characteristic of in the ventricular myoctye AP?

Answer 4

Repolarization: by closing Ca2+ channels and opening up K+ channels.

Question 5

Which potassium channel is responsible for Phase 1 and remains active until after phase 2?

Answer 5

ito potassium channels.

Question 6

Which potassium channel is slow activating during phase 2 and begins to dominate once Ca2+ channels close?

Answer 6

ikr potassium channels.

also somewhat responsible for phase 3 hyperpolarization

Question 7

Which potassium channel is responsible for flux in phase 3 & 4?

Answer 7

ik1 potassium channels.

Question 8

Which potassium channel is relevant with respect to the regulation of cardiac AP via the parasympathetic nervous system.

Answer 8

ikAch potassium channels. (activated by acethylcholine and is Gi-coupled)

Question 9

What are the characteristics of the L-Type calcium channels?

Answer 9

They are LONG lasting and SLOW inactivation.

Question 10

What are the characteristics of the T-Type calcium channels?

Answer 10

They have FAST inactivation and are transient. They are also less abundant.

Question 11

Which calcium channels are the target for Calcium Channel Blocker drugs?

Answer 11

L-Type Ca2+ channels.

Question 12

What happens to force of contraction after the Plateau phase contraction is accomplished? (AKA Refractory Period)

Answer 12

The Ventricular myocyte begins to relax and during that period it CANNOT depolarize more and CANNOT bring up another contractile response.

Question 13

What can happen during the relative refractory period?

Answer 13

The myocyte is relaxing and is partially repolarized and a premature depolarization can happen and cause a weaker contraction. (Premature contraction)

Question 14

Why is a Premature Contraction possible during the relative refractory period?

Answer 14

1. Ca2+ hasnt moved out
2. # of Ca2+ channels that can be depolarized is lower
3. Less Ca2+ in the cell itself.

Question 15

True or False. A premature contraction causes a faster heart rate.

Answer 15

True

Question 16

True or False. If you are experiencing a faster heart rate than normal while exercise it is due to a premature contraction.

Answer 16

False ( After the relative refractory period has passed you can a have a full contraction that is earlier than normal through induced mechanisms (during exercise)

Question 17

When depolarization occurs where does Ca2+ flow?

Answer 17

It flows into the cytosol of the ventricular myocyte.

Question 18

What is Calcium Induced Calcium Release?

Answer 18

It is when the Ca2+ flowing into the cytosol attaches to voltage gated Ca2+ channels on the SR and induce the release of stored Ca2+.

Question 19

Once stored Ca2+ from the SR is released what happens?

Answer 19

Since the Ca2+ level is elevated in the cytosol it interacts with the sarcomere and causes contractions.

Question 20

What are the 2 ways that cardiac myocytes sequester Ca2+ after contraction?

Answer 20

1. Ca2+/ATPase on the SR

2. Ca2+/Na+ exchanger and Na+/K+ ATPase (secondary active transport)

Question 21

Why is Extracellular Ca2+ important for cardiac myoctyes?

Answer 21

1. The SR in cardiac muscle is less developed and does not store enough Ca2+
2. T tubules are larger and allow Ca2+ to enter easily

Question 22

Why is skeletal muscle not affected by extracellular Ca2+ but cardiac muscle is not?

Answer 22

Because skeletal muscle stores enough Ca2+ in the SR and the action potential itself causes the release of Ca2+

Question 23

Which pacemaker of the heart is located at the top of the right atrium?

Answer 23

The SA node

Question 24

How is the SA node and AV node connected?

Answer 24

Through Internodal Pathways

Question 25

Once at the bottom of the ventricles, the fibers extending from the AV node further divide into what?

Answer 25

Purkinje Fibers.

Question 26

Why is there a AV nodal delay?

Answer 26

Because the fibers narrow while crossing the atrial/ventricle wall and contains REDUCED gap junctions and a more NEGATIVE resting membrane potential. This allows fro ventricles to “top off” before contraction.

Question 27

How can the PNS control the electrophysiology of the heart?

Answer 27

Vagus nerve stimulation on SA & AV nodes LOWERS HR, ACh opens K+ channels and causes HYPERPOLARIZATION which causes a greater time to threshold.

Question 28

How can the Sympathetic nervous system control the electrophysiology of the heart?

Answer 28

It innervates the SA node and Ventricles which INCREASES HR. NE opens Ca2+ channels causes DEPOLARIZATION and a shorter time to threshold.