Electrophysiology Lecture 1 -- Ion Channels and Action Potential

Question 1

Define current

Answer 1

Movement of positive ions per unit time

Question 2

Reason for slow channel AP in AV node

Answer 2

To ensure proper timing (allows good LV filling)

Question 3

Fast channel tissue in the heart

Answer 3

Working atrial muscle His-Purkinje system Working ventricular muscle

Question 4

Slow channel tissue in the heart

Answer 4

Sinoatrial node Atrioventricular node

Question 5

Dominant current in phase 4

Answer 5

I_K1

Question 6

Dominant current in phase 0

Answer 6

I_Na

Question 7

Dominant current in phase 2

Answer 7

Balance between I_Ca and I_K1

Question 8

Dominant current in phase 3

Answer 8

I_Kr

Question 9

Describe the events of phase 1

Answer 9

A brief increase in K+ current rapidly but only partially repolarizes the cell to lead in to the plateau phase

Question 10

Describe the events of phase 3

Answer 10

Final rapid repolarization caused by opening of a specific type of K+ channels

Question 11

Name of K+ channels involved in phase 3

Answer 11

Delayed-rectifier channels

Question 12

Time constants for opening of the relevant ionic channels

Answer 12

Na+ = 1 ms

Ca+ = a few ms

K+ = 100s of ms

Question 13

Reason for long time constant of K+ channel opeing

Question 14

Define APD

Answer 14

Action Potential Duration = the interval between depolarization and complete repolarization

Question 15

Define refractory period

Answer 15

The interval between depolarization and repolarization until -60 mV before a new action potential can be fired

Question 16

Equilibrium potentials of relevant ions

Answer 16

K+ = -90 mV

Na+ = +60 mV

Ca++ = +60 mV

Question 17

Where is I_f expressed?

Answer 17

His-Purkinje system and SAN/AVN

Question 18

Membrane potential from end of phase 3 until the next action potential in working atrial and ventricular muscle

Answer 18

Constant (-80 or -90 mV)

Question 19

Phase 4 potential in His-Purkinje tissue

Answer 19

Gradually depolarizes due to inward Na+ movement through a pacemaker channel carrying I_f

Question 20

Resting potential of slow-channel tissues

Answer 20

-60 mV

Question 21

Significance of slow channel tissue resting potential

Answer 21

Sodium channels are deactivated when membrane potential is -60 mV or more, so the membrane relies on Ca++ influx for depolarization (innately slower reaction)

Question 22

Compare fast channel vs. slow channel tissues

Answer 22

Fast = Na+, fast activation, large max current, small sensitivity to ANS

Slow = Ca++, slow activation, small max current, high sensitivity to ANS

Question 23

Define the relative refractory period/post-repolarization refractoriness

Answer 23

Period after complete repolarization where the membrane is hyperpolarized past its resting potential

Question 24

What does automaticity depend on?

Answer 24

Spontaneous phase 4 depolarization

Question 25

3 major determinants of spontaneous automatic rate

Answer 25

• Maximum diastolic potential (most negative potential from which phase 4 depolarization takes place)
• Slope of phase 4 (rate of spontaneous phase 4 depolarization)
• Threshold potential

Question 26

Purpose of I_f

Answer 26

Open when cell repolarizes to allow spontaneous firing