Contraction

Question 1

How does Ca2+ get into the cardiac muscle cell?

Answer 1

There is an action potential and the cell membrane depolarises. The membrane potential rises. The threshold of voltage gated L-type Ca2+ channels is reached and these open. Ca2+ enters. This binds to the ryanodine receptor on the sarcoplasmic reticulum. This promotes further release of Ca2+ from the SR stores. The [Ca2+] in the cell increases hugely.

Question 2

What is it called what Ca2+ comes into the cardiac muscle cell via the L-type Ca2+ channels, bind to the ryanodine receptor and cause the release of Ca2+ from the SR?

Answer 2

calcium induced calcium release

Question 3

How does Ca2+ drive contraction?

Answer 3

Ca2+ binds to troponin C in the troponin complex. Tropomyosin moves to allow an actin/myosin interaction and there is muscle contraction

Question 4

What are the three troponin compounds that make up a troponin complex?

Answer 4

Troponin C, I, T

Question 5

Briefly describe sarcomere shortening

Answer 5

Initially, the actin binding sites on myosin are blocked.
Ca2+ binds to the Troponin C. There is movement of the troponin/tropomyosin complex exposing the myosin binding site on actin. There is interaction between actin and myosin (cross-bridge).
The myosin head flips. The actin moves towards the centre of the sarcomere. The sarcomere shortens

Question 6

How can we produce a more forceful contraction?

Answer 6

You can increase the number of cross-bridges (not increasing the number of cardiomyocytes because all of them are contracting at once)

Question 7

What are the three different mechanisms to reduce intracellular Ca2+ which relaxes the cardiac muscle cell?

Answer 7

1. SR Ca2+ pump (SERCA)
2. Na+/Ca2+ exchanger (NCX)
3. Ca2+ ATPase

Question 8

Explain how intracellular Ca2+ reduced by the SR CA2+ pump (SERCA)?

Answer 8

This pump uses ATP to pump cystoplasmic Ca2+ back into the SR

Question 9

How is SERCA activity regulated? Explain this

Answer 9

by Phospholamban.
When PLB is bound to SERCA, SERCA is partially inhibited so it takes up Ca2+ more slowly. When PLB is phosphorylated, it can dissociate from SERCA and Ca2+ uptake happens quicker

Question 10

Explain how the Na+/Ca2+ exchanger (NCX) reduces the intracellular Ca2+ concentration

Answer 10

This is a secondary active transporter which wants to move Ca2+ against its gradient (to the outside of the cell). It uses the Na+ gradient to provide the energy to do this

Question 11

Explain how Ca2+ ATPase reduces the intracellular Ca2+ concentration

Answer 11

This uses ATP to pump Ca2+ out of the cell

Question 12

What is the refractory phase?

Answer 12

The cell during the plateau phase is still depolarised so the cell is in-excitable. The cell is already depolarised so if another action potential comes along, it can’t be triggered to depolarise again. Cardiomyocytes have to wait a long time before they can activate again

Question 13

Why is summation of contractions not possible in the heart?

Answer 13

because each contraction has to partially relax before we move out of the refractory period

Question 14

What prevents re-excitation during most contraction periods and prevents circuitous recycling of AP?

Answer 14

There is a stable Ca2+ (about 20mV) makes cardiomyocytes inexcitable. There is a long absolute (250ms) and relative refractory periods (300ms)

Question 15

What parts need the ATP?

Answer 15

• SERCA
• deattachement of the actin-myosin cross-bridge
• Ca2+ extrusion by Ca2+ ATPase
• Na+/K+ ATPase
• primary active transport