Cardiac Cycle

Question 1

How does the AV node connect parts of the heart

Answer 1

Right atrium and right ventricle connected, delaying impulses so that atria have time to eject their blood into ventricles before ventricular contraction

Question 2

What is pacemaker potential?

Answer 2

Ability to generate own electrical activity which is then distributed around the heart.

Question 3

What is phase 4 in the generation of an action potential by a pacemaker

Answer 3

Resting membrane potential (atypical because it is unstable.) The SA node cell is hyperpolarised, and Na+ voltage gated channels open (funny current), and the cell depolarises. Ca2+ voltage gated channels open at around -40mV.

Question 4

What is phase 0 of SA pacemaker potential

Answer 4

Ca2+ moves into the cell, cell depolarisation until inside is +20mV.

Question 5

What is phase 3 of SA pacemaker potential

Answer 5

Calcium channels switched off, potassium channels open. Potassium moves out of the cell, causing repolarisation to phase 4, and then hyperpolarisation to funny current.

Question 6

What is phase 4 of atrial/ventricular muscle action potential

Answer 6

Resting membrane potential. Contractile cardiac muscle cells very stable.

Question 7

What is phase 0 of atrial/ventricular muscle action potential

Answer 7

Depolarisation from SA node via adjacent cells, stimulating opening of voltage gated Na+ channels.

Question 8

What is phase 1 of atrial/ventricular muscle action potential

Answer 8

Na+ channels close again. Na/K exchanger is still active, can cause slight repolarisation of cells. Ca2+ channels open, stopping repolarisation.

Question 9

What is phase 2 of atrial/ventricular muscle action potential

Answer 9

Ca2+ channels open, inside cell stays positive (plateau phase). Calcium released from cell sarcoplasmic reticulum, plateau prolonged. Delays repolarisation and further action potentials.

Question 10

What is phase 3 of atrial/ventricular muscle action potential

Answer 10

End of plateau phase, slow repolarisation. Ca2+ channels close, opening of K+ channels. K+ leaves cell, repolarisation to resting potential.

Question 11

Although the SA node is responsible for pacemaker activity in the heart, why does the AV node itself also have pacemaker potential without stimulation from the SA node

Answer 11

So the ventricles can still continue to pump blood even when there is damage or loss of the conduction system upstream of the AV node

Question 12

What are cardiomyocytes and why are they important

Answer 12

Heart muscle cells, connected through gap junction-rich intercalated discs. Action potential event in one cardiomyocyte induces action potential event in adjacent cells with a slight time delay. Coordination of depolarisation through cardiac muscle, contracting atria/ventricles.

Question 13

Describe electrical conduction through the heart.

Answer 13

1. Action potential in SA node, pacemaker potential generated by funny current. Gap junction connects the cytoplasm of two cells. Atria depolarised, polarisation passes from cell to cell.
2. Wave of depolarisation reaches the AV node, delays conduction.
3. Depolarisation signal conducted from AV node to the apex of the ventricles via the bundle of his.
4. Contraction of the ventricles begins at the apex, depolarisation moves up through the ventricular walls via Purkinje fibres.

Question 14

Describe the features of electrical activity of the heart in an ECG

Answer 14

P wave = atrial depolarisation
PR = AV nodal delay
QRS = ventricular deparisations
ST = ventricles contracting and emptying
T = ventricular repolarisation
TP = ventricles repolarising and filling.

Question 15

Describe cardiac diastole

Answer 15

Chambers of the heart are relaxed. Blood fills in the right heart via great veins, and to left side via pulmonary vein. Tricuspid and mitral valves open.