6.3 SKELETAL MUSCLES

Question 1

What is the structure of the skeletal muscle?

Answer 1

• Made up of microfibrils.
• Many small microfibril bundles exert the most force.
• Cells in the microfibrils fuse together to share nuclei and cytoplasm (called sarcoplasm)
• Lots of mitochondria & RER

Question 2

What are the myofibril fibres made up of?

Answer 2

Thick and thin filaments of actin & myosin.

Question 3

What is the structure of actin?

Answer 3

• Thin filament of muscle

- Consists of two filaments wound around each other/

Question 4

What is the structure of myosin?

Answer 4

• Thicker filament of muscle

- Longtails with bulbous heads that can form cross-bridges with actin.

Question 5

How are actin and myosin arranged together? (naming)

Answer 5

The entire length of ‘one’ structure is called the sarcomere.

Question 6

What is the actual arrangement of actin and myosin together?

Answer 6

(From LHS to RHS)

Z line, I band, A band, H band, M line, H band, A band, I band, Z line.

Question 7

What is the Z line of the sarcomere?

Answer 7

The end on ‘one length’ of actin & myosin/

Question 8

What is the I band of the sarcomere?

Answer 8

The area where there are only actin filaments. LIGHT BAND (isotropic)

Question 9

What is the A band of the sarcomere?

Answer 9

The area where there are both actin & myosin filaments. DARK BAND (anisotropic)

Question 10

What is the H band of the sarcomere?

Answer 10

The area where there are only myosin filaments.

Question 11

What is the gross image of what the sarcomere looks like?

Answer 11

|——— | ————|
| ======|====== |
|——— | ————|
| ======|====== |

• is the actin filament
  = is the myosin filament

Question 12

What happens when the muscles contract?

Answer 12

The sarcomere shortens and filaments slide past each other and bringing them closer together.

Question 13

Explain the changes in the sarcomere as the muscles contract?

Answer 13

• I band gets shorter
• H band disappears
• A band overlaps more
• Z lines get closer together

Question 14

How do muscles contract and filaments slide over each other? (cross-bridge cycle)

Answer 14

1. ATP is hydrolysed and the energy causes myosin heads to cock and change angles.
2. Mysoin heads then attach to actin to form cross-bridges.
3. When ADP + Pi come back together to form ATP, myosin heads detach from the actin.
4. The process of attachment and detachment repeat very fast, and causes actin & myosin to slide over each other.

Question 15

What controls and initiates the muscle contraction and cross-bridge cycle?

(opposite for muscle relaxation)

Answer 15

1. Action potential arrives at the neuromuscular junction.
2. The junction releases acetylcholine which stimulates an AP in the muscle cell membrane.
3. AP travels down T tubules in the muscle’s sarcoplasm.
4. Tubules are connected to the sarcoplasmic reticulum, which has calcium ion channels.
5. Action potential opens the calcium ion channels and forces calcium ions to move out into the sarcoplasm.
6. Here the calcium ions will bind to tropomyosin. (which covers myosin-binding sites on actin)
7. Once it is bound, the tropomyosin will move and expose the myosin-binding sites.
8. Now myosin cross-bridges can form and the cross-bridge cycle occurs. (see flashcard 14)
9. Now the muscles contract.

Question 16

What is the role of ATP in muscle contraction?

Answer 16

• Moves myosin heads to form cross-bridges.

- Actively transports calcium ions back to sarcoplasmic reticulum during relaxation of muscles.

Question 17

What is the role of phosphocreatine in muscle contraction?

Answer 17

• Regenerates ATP.

- Stored in muscles for quick access to phosphates for ADP - ATP.

Question 18

What are the slow-twitch muscles?

Answer 18

• Muscles contract slowly with less power over time.
• Have lots of mitochondria and blood supply.
• Respire aerobically.

Question 19

What are the fast-twitch muscles?

Answer 19

• Muscles contract quickly with more power in a short time.
• Have lots of phosphocreatine to regenerate ATP.
• Respire anaerobically.