Cardiac Contractility and the Cardiac Cycle

Question 1

What is the purpose of cardiac T-tubule mucopolysaccharides?

Answer 1

To enrich the environment in favour of calcium influx

Question 2

Explain contraction of cardiac muscle

Answer 2

• depolarisation travels across surface of cardiac muscle generated by SA node
• travels to T-tubules to open DHP, rush of influx of Ca, high plateau of stimulation causes a release of Ca from SR
• out floods Ca to interact with troponin which causes contraction
• amount of Ca released determines amount of force of contraction

Question 3

Explain relaxation of cardiac muscle

Answer 3

2 ways:

• Ca can go back into SR in energy dependent ATP process
• can also be moved out by transporter which brings sodium in - this creates sodium current so Na K ATP-ase restores membrane potential by bringing potassium in

Question 4

Explain how sympathetic innervation has a positive ionotropic effect on contraction

Answer 4

• enhances Ca influx
• promotes storage and release of Ca from SR
• increases contractility
• increases speed of relaxation

Question 5

Explain how parasympathetic innervation has an indirect negative ionotropic effect

Answer 5

doesn’t directly affect contractility just indirectly allows the heart to relax for longer
- innervates atria

Question 6

Explain why we cannot have summation or tetanaic contraction of cardiac activity

Answer 6

the plateau phase of contraction is driven by the calcium influx from DHP channels meaning that the AP extends almost as long as the contractile event its’s stimulating so by the time the cardiac AP is resolved, the muscle fibre is almost completely relaxed

Question 7

Relative refractory period

Answer 7

if conditions are strong enough, another AP can be generated but highly unlikely

Question 8

Supranormal excitability

Answer 8

lower than normal voltage change can open voltage gated Na channels

Question 9

Revise cardiac cycle graph

Answer 9

*

Question 10

What is stroke volume (SV)?

Answer 10

• EDV-ESV

- quantity of blood expelled per beat

Question 11

What is the cardiac output?

Answer 11

• SV x HR

- volume of blood pumped by the heart