The heart as an electrical pump

Question 1

Describe the pathway of the heart’s conduction system.

Answer 1

SA node -> AV node -> Bundle of His -> Right/left bundle branch -> Purkinje fibres

Question 2

Which conduction if slower?

1. SA to AV or
2. AV to Bundle of His

Answer 2

AV to bundle of his - allows time for ventricular filling to ensure adequate SV
Contraction of ventricles from base to apex

Question 3

Is there a higher concentration if K+ inside or outside of cell?

Answer 3

Inside

Na+ conc is higher outside

Question 4

Describe the action potentials in pacemaker cells.

Answer 4

Phase 4 - slow influx of Na+ and Ca2+
Phase 0 - L-type Ca2+
Channels open and increased influx of Ca2+
Phase 3 - voltage-gated K+ channels open and efflux of K+

Question 5

Describe the action potentials in fast depolarising cells (non-pacemaker cells).

Answer 5

1. Influx of Na+ from adjacent AP
2. Phase 0 - voltage-gated Na+ channels open
3. Phase 1 – Na+ channels inactivated, voltage-gated K+ channels open
4. Phase 2 - L-type Ca2+ channels open (Ca shoulder)
5. Phase 3 - Ca2+ channels inactivated, additional K+ channels open
6. Voltage-gated K+ channels inactivated
7. Phase 4 – resting membrane potential restored

Question 6

What is the difference between an absolute and relative refractory period?

Answer 6

Absolute - no new AP can be generated (voltage gated Na+ channels closed)
Relative - larger stimulus needed for AP (some voltage gated Na+ channels closed)

Question 7

Is the refractory period longer in cardiac muscle or skeletal muscle?

Answer 7

Cardiac muscle

Question 8

What is it called when cardiomyocytes contract as a unit?

Answer 8

Syncytium

Question 9

How do cardiomyocytes branch and connect with other cells?

Answer 9

Gap junctions and desmosomes

Question 10

How do action potentials propagate?

Answer 10

When a cell depolarises, ions leak into adjacent cells causing their membrane to depolarise to threshold, initiating an AP for that cell

Question 11

Explain the excitation - contraction coupling in myocytes.

Answer 11

Mechanism that translates a cardiac AP into muscle contraction.
Excitation - AP originates in pacemaker cells, passes along membranes of myocyte syncytium
Coupling - depolarisation - release of Ca2+ onto cytoplasm
Contraction - Ca2+ causes contraction