Cardiac Electrophysiology

Question 1

What are the elements of an action potential?

Answer 1

1. Resting cell
2. Depolarization
3. Repolarization

Question 2

What happens during resting cell phase?

Answer 2

Inside of cell is negative relative to the outside. K+ is inside the cell, Na+ is outside kept in balance by NaK pump

Question 3

What happens during depolarization?

Answer 3

Resting cell becomes positive / excited.

Stimulus from SA node makes membrane increase semi-permeability which cases Na to rush in to cell making it more ++

Question 4

What happens during repolarization?

Answer 4

Cell interior slowly becomes negative again.
Cl- passively follows Na+ into cell while NaCa channels open allowing Ca++ to enter. K starts leaving cell just as NaCa channels close and cell becomes more negative again. NaK pump restores cell to resting state.

Question 5

What is refractoriness?

Answer 5

Extent to which a cell is able to respond to a stimulus

Question 6

What is the relative refractory period?

Answer 6

Cardiac cells are repolarizing but strong enough stimulus will cause depolarization again. Cells are “relatively still excited”.

Comprises 2nd half of the T-wave, ectopic beats going through may cause R-on-T

Question 7

What are the electrophys properties of pacemaker cells?

Answer 7

Resting membrane potential is not fixed/stable (approx -60mv)

NaCa channels remain open all the time allowing Na to slowly drift into cell until it reaches threshold at -40mV. Depolarization occurs (automaticity)

Question 8

What are Class 1 anti-arrhythmic drugs?

Answer 8

Slows Na+ influx by blocking Na channels

Decrease HR by slowing depolarization

E.g. Lidocaine, procainamide

Question 9

What are Class 2 anti-arrhythmic drugs?

Answer 9

Blocks beta receptors, makes RMP more negative therefore harder to reach threshold potential

Decrease HR

E.g. Metoprolol

Question 10

What are Class 3 anti-arrhythmic drugs?

Answer 10

Blocks K+ channels, prolonging action potential

Decrease HR by increasing refractory time (**May cause arrythmia!)

E.g. Amiodarone, ibutilide

Question 11

What are Class 4 anti-arrhythmic drugs?

Answer 11

Blocks Ca++ channels, decreases APs in SA node and prolongs AP in the ventricles

Decrease HR, contractility, BP

E.g. Diltiazem, Verapamil

Question 12

What are Class 5 anti-arrhythmic drugs?

Answer 12

Works magically.

E.g. Adenosine, MgSO4

Question 13

What is the sarcoplasmic reticulum?

Answer 13

Storage space for Ca++

Question 14

What are transverse tubules?

Answer 14

Passageways for Ca++

Question 15

What is actin and myosin?

Answer 15

Filaments in sarcomere (muscle cell) that combine during muscle contraction with help from troponin and tropomyosin

Question 16

What happens during cardiac contraction?

Answer 16

1) Ca+ binds to troponin using ATP
2) Troponin tells tropomyosin to change binding sites allowing
3) Actin and myosin filaments slide together

Question 17

What happens during cardiac relaxation?

Answer 17

Actin and myosin disengage when Ca++ removed / unbinds from troponin

Question 18

What is the role of troponin and tropomyosin?

Answer 18

1) Attached to actin filaments, binds with Ca++ to trigger contraction
2) Prevents actin and myosin from creating cross bridges (inhibits constant contraction)