Electrical Conduction

Question 1

Which structure interconnects myocardial cells and allows action potentials to spread from one cell to another?

Answer 1

Gap junctions at intercalated discs

Question 2

Where is the SAN located?

Answer 2

Right atrium

Question 3

Which node is known as the pacemaker of the heart?

Answer 3

SAN

Question 4

Describe the pacemaker potential.

Answer 4

1. K+ channels gradually close
2. Pacemaker cells have F-type channels which open when the membrane potential is negative. Causes inward Na+ current, causing depolarisation
3. There is an inward current of Ca2+ which acts as a final depolarising boost
4. Action potential occurs at threshold

Question 5

Describe the pathway of depolarisation from the SAN to the AVN.

Answer 5

SAN > muscle cells of atria > AVN

Question 6

Where is the AVN located?

Answer 6

Base of right atrium

Question 7

Why is there a delay in propagation of action potentials through the AV node?

Answer 7

To enable the atria to full empty into the ventricles. Enables atrial contraction to be completed before ventricular excitation occurs.

Question 8

Where does the action potential travel to after the AVN?

Answer 8

Down the bundle of His. Divides into right and left bundle branches that separate at the apex and enter the walls of the ventricles.

Question 9

Which fibres transmit the impulse through the ventricles?

Answer 9

Purkinje fibres. These then pass the impulse to ventricular myocardial cells.

Question 10

What does each wave of an ECG represent?

Answer 10

P wave: Depolarisation of atria
PR interval: Delay of AV node to allow filling of ventricles
QRS complex: Depolarisation of ventricles
ST segment: beginning of ventricular repolarization - normally flat
T wave: Ventricular repolarization

Question 11

When hearing a heart sound, what does the ‘lub’ and ‘dub’ refer to?

Answer 11

Lub: Closing of mitral and tricuspid valves (AV valves)
Dub: Closing of aortic and pulmonary valves

Also third heart sound of the blood rushing into the left ventricle.

Question 12

Describe the phases of a myocyte action potential.

Answer 12

Phase 0: Cardiac myocyte reaches threshold potential via intercalated disc. Inflow of Na+ via voltage gated channels, rapid DEPOLARISATION
Phase 1: Partial depolarisation, sodium channels close but outflow of K+ via voltage gated channels
Phase 2: Plateau - slow inward Ca2+ via voltage gated channels
Phase 3: K+ outflow, deactivated Ca2+ inflow, REPOLARISATION
Phase 4: Pacemaker potential - slow Na+ inflow, slowing of K+ outflow. REFRACTORY PERIOD

Question 13

Which term is given to increased heart rate?

Answer 13

Tachycardia

Question 14

Which term is given to decreased heart rate?

Answer 14

Bradycardia

Question 15

Which term is given to the heart being on the right side of the chest instead of the left?

Answer 15

Dextrocardia

Question 16

Name the standard limb leads

Answer 16

I, II, III

Question 17

How are the standard limb leads positioned?

Answer 17

They form a triangle between electrodes on the wrist and the LEFT leg.

Question 18

In standard limb leads, which poles are reference electrodes and which poles are recording electrodes?

Answer 18

Reference electrodes = negative poles

Recording electrodes = positive poles

Question 19

Name the augmented leads

Answer 19

aVR, aVL, aVF

Question 20

How are the augmented leads positioned?

Answer 20

They bisect the angles of the triangle by combining two electrodes.

Question 21

Name the precordial leads

Answer 21

V1-V6

Question 22

How are the precordial leads positioned?

Answer 22

Electrodes are placed on the chest

Question 23

In an ECG graph, what does each big square and small square represent?

Answer 23

Big square: 0.2seconds

Small square: 40ms

Question 24

What effect is on the impulse, the closer it gets to an electrode?

Answer 24

It will get bigger

Question 25

Why are impulses from the atria smaller?

Answer 25

Atria are smaller than the ventricles so have less myocytes.