Making muscles contract: molecular mechanisms

Question 1

What are the different cell types muscles are composed of?

Answer 1

• Muscle fibres (cells)
• Vascular cells
• Fibroblasts
• Satellite cells

Question 2

Where is calcium stored in the muscle?

Answer 2

Sarcoplasmic reticulum

Question 3

What is the relationship between muscles and motor units?

Answer 3

Muscles which need to produce a lot of force will have hundreds of motor units whereas muscles like eye muscles (less force produced) will only have 10s

Question 4

What are the 3 main parts of the neuromuscular junction?

Answer 4

• Axon terminal
• Motor end plate
• Synaptic cleft

Question 5

What is tropomyosin?

Answer 5

Chain surrounding actin filament, blocking binding sites for myosin to form a cross-bridge

Question 6

What is troponin?

Answer 6

Holds tropomyosin over binding site

Question 7

What happens when a muscle is activated, in relation to Ca2+ and troponin?

Answer 7

When muscle is activated, Ca2+ binds to Troponin which uncovers binding sites of actin filament and allows actin to bind to cross-bridge (allowing force to be generated and contraction to be produced)

Question 8

What happens when muscle is relaxed?

Answer 8

Ca2+ is transported back into sarcoplasmic reticulum

Ca2+ removal from troponin restores tropomyosin blocking action

Question 9

Does myosin move during the sliding filament mechanism?

Answer 9

No - Actin filament slides over myosin filament, myosin doesn’t move

Question 10

What are the 4 stages of the cross-bridge cycle?

Answer 10

1) Cross bridge formation
2) Power stroke
3) Cross bridge detachment
4) Reactivation of myosin head

Question 11

What happens during cross bridge formation?

Answer 11

Energized myosin head binds to actin to form a crossbridge, Pi is released and bond between myosin and actin becomes stronger

Question 12

What happens during power stroke?

Answer 12

ADP is released and activated myosin head pivots, sliding thin myofilament towards centre of sarcomere

Question 13

What happens during cross bridge detachment?

Answer 13

Another ATP binds to myosin head causing link between myosin head and actin to weaken and myosin head detaches

Question 14

What happens during reactivation of myosin head?

Answer 14

ATP bound to myosin is hydrolysed to ADP + Pi. Energy released during hydrolysis reactivates myosin head, returning it to cocked position

Question 15

What is ATPase?

Answer 15

An enzyme which determines the speed of ATP hydrolysis and resulting sarcomere shortening velocity