Cardiovascular systems 5 - Cardiovascular control 1

Question 1

What is the purpose of the long refractory period in cardiac muscle?

Answer 1

• This allows the process of contraction to be well underway until a new action potential can be generated
• This ensures cardiac muscle cannot be tetanised

Question 2

List the phases of the action potential in cardiac muscle.

Answer 2

• Phase 0 - upstroke
• Phase 1 - early repolarisation
• Phase 2 - plateau
• Phase 3 - repolarisation
• Phase 4 - resting membrane potential

Question 3

Describe the ion movement in the upstroke of cardiac muscle action potentials

Answer 3

• Large increase in permeability to sodium ions
• Calcium influx triggers calcium release from intracellular stores
• Upstroke is more dependent on sodium

Question 4

Describe the ion movement in repolarisation of cardiac muscle.

Answer 4

• Initial repolarisation caused by transient outward potassium channels.
• Gradual activation of potassium currents (outward movement), which eventually balances flow of calcium, then overcomes it
• Following partial repolarisation, there is a large potassium current (inactive in the plateau). This is due to K1 channels, which fully repolarises the cell

Question 5

What causes the plateau in the cardiac muscle action potential?

Answer 5

Balance between the movement of potassium out and calcium in

Question 6

Why do different parts of the heart have different action potentials?

Answer 6

As there are different ion channels present

Question 7

What channels are present in the SA node?

Answer 7

• No Ik1 channels
• Little sodium influx, the upstroke is produced by calcium influx
• T type calcium channels are present, which activate at more negative potentials than L type
• Pacemaker current is present (If)

Question 8

How does sympathetic stimulation affect heart rate?

Answer 8

Heart rate increases, as depolarisation occurs faster

Question 9

How does parasympathetic stimulation affect heart rate?

Answer 9

Heart rate slows, as depolarisation occurs slowly

Question 10

What is the resting potential in the sinoatrial node?

Answer 10

No resting potential - constantly oscillating

Question 11

What is another name for the parasympathetic nerve?

Answer 11

Vagus

Question 12

Where is the cardioregulatory centre and vasomotor centre?

Answer 12

Medulla

Question 13

What is chonotropy?

Answer 13

Heart rate

Question 14

What is inotropy?

Answer 14

Contractility

Question 15

Describe the location of the sinoatrial node.

Answer 15

• Just below the epicardial surface, at the boundary between the right atrium and superior vena cava

Question 16

Describe the pathway of the cardiac conduction system.

Answer 16

• Starts at the sinoatrial node
• Moves to the internodal fibres
• The AV node slows down the action potential, allowing ventricular filling to complete
• Rapid conduction to the apex of the heart via the bundle of his, and then up the heart via then ventricular fibres

Question 17

How does the cardiac action potential propagate?

Answer 17

• Passive spread of current and the existence of a threshold which, once reached, allows the cell to generate its own action potential.
• Gap junctions reduce membrane resistance, so the current can easily leak from one cell to the neighbouring cell.

Question 18

How do gap junctions form?

Answer 18

At intercalated discs- two connexons, one from each cell, join together to form a tube