[15.8] contraction of skeletal muscle

Question 1

what type of force does muscle contraction generate?

Answer 1

• pulling force via tendons (muscle to bone)
• pushing motion is only generated by a pulling force contraction
• relaxation is the absensce of contraction and returning to its original state

Question 2

what does antagonistic action mean?

Answer 2

• muscles work in pairs
• when one contracts, the other relaxes

Question 3

what evidence is there for the sliding filament mechanism?

Answer 3

when a muscle contracts:

• M and Z lines get closer together (sarcomere shortens)
• I band narrows
• H zone narrows
• A band stays same so myosin fibres have not become shorter, only relative positions have changed
• inside A band, proportion of overlap areas increases

Question 4

what is tropomyosin?

Answer 4

long, thin strands that are wound around actin filaments

Question 5

how is a muscle stimulated?

Answer 5

1. AP reaches many neuromuscular junctions simultaneously
2. calcium IPC open
3. calcium ions diffuse into synaptic knob
4. calcium ions cause synaptic vesicles to fuse with the pre SM and release their acetylcholine into the synaptic celft
5. acetylcholine diffuses across synaptic cleft
6. acetylcholine binds with receptors on muscle cell membrane, causing it to depolarise

Question 6

describe the sliding filament mechanism

Answer 6

1. tropomyosin prevents myosin head from attaching to binding site on actin molecule
2. calcium ions released from ER cause tropomyosin to change shape and so pull away from the bindings sites on actin
3. myosin head binds to binding site on actin filament
4. binding causes ADP to be released
5. myosin head changes angle, swinging the actin filament along as it does so
6. ATP molecule binds to myosin head, causing it to detach from the actin filament
7. calcium ions activate ATPhydrolase, which are in myosin head, so ATP is broken into ADP + Pi
8. this provides the energy for the myosin head to return to its original position
9. head of myosin reattaches to a binding site further along the actin filament and the cycle is repeated

Question 7

how do you relax a muscle?

Answer 7

1. nervous stimulation is stopped
2. calcium ions are actively transported back into ER using energy from ATP hydrolysis
3. reabsorption of calcium ions allows tropomyosin to block actin filament again
4. no actin-myosin crossbridges can form
5. actin and myosin are detached so no further contraction is possible

Question 8

what do muscles require energy for?

Answer 8

• movement of myosin heads
• reabsorption of calcium ions into ER via active transport

Question 9

what is the role of phosphocreatine in muscle contraction?

Answer 9

• stored in muscle as a reserve supply of phosphate
• can combine with ADP to re-form ATP
• store is replenished using phosphate from ATP when muscle is relaxed