Muscular Contraction

Question 1

At Rest

Answer 1

High concentration of Mg2+ and ATP within myofibril but low concentration of Ca2+. Tropomyosin and troponin block the actin-myosin binding sites. Myosin heads are rotated away from actin filaments.

Question 2

1.

Answer 2

Depolarisation spreads along the sarcolemma and along tubules of sarcoplasmic reticulum.

Question 3

2.

Answer 3

This stimulates the release of Ca2+ from the sarcoplasmic reticulum into the sarcoplasm.

Question 4

3.

Answer 4

Ca2+ ions bind to troponin causing tropomyosin to move away from actin-myosin binding sites.

Question 5

4.

Answer 5

Myosin heads rotate towards actin filaments and form cross linkages with them.

Question 6

5.

Answer 6

Heads bend pulling actin filaments inwards so they overlap more with myosin. This is known as the powerstroke.

Question 7

6.

Answer 7

ATP is hydrolysed to ADP + Pi and energy released is used to move myosin heads away from actin so they form new cross linkages further along actin filament.

Question 8

7.

Answer 8

This process is repeated with myosin heads binding to actin, bending, pulling actin, releasing and binding again.

Question 9

When stimulation stops

Answer 9

Ca2+ is actively transported back into the sarcoplasmic reticulum using ATP. The actin and myosin cross linkages are broken. Troponin moves back, causing tropomyosin to move and block binding sites. The muscles doesn’t change in length until an antagonistic muscle pulls it back to its relaxed length.