Muscles

Question 1

What is a hierarchical structure of the muscle?

Answer 1

1) Muscle. Surrounded by deep fascia (connective tissue rich in lymphatic vessels and hyaluronan) and epimysium (dense irregular collagen rich connective tissue)

2) Fascicle (muscle fiber bundle). Surrounded with perimysium.

3) Muscle fiber (once very specialised cell - myocyte). Surrounded with endomysium.

4) Myofibrils

Question 2

What are the 3 types of muscle cells?

Answer 2

1) Skeletal: the bunches of very long cylindrical multinucleated cells that appear striped under microscope.

2) Cardiac muscle: branched arrangement of smaller mononucleated striped cells. Connected with intercalated disks and gap junctions.

3) Smooth muscle: small unnucleated spindle shaped cells, no stripes, no sarcomeres.

Question 3

What is the structure of a skeletal muscle fibre?

Answer 3

• Sarcolemma (specialised plasma membrane)
• myofibril (bundles of myosin and actin and supporting proteins)
• sarcoplasmic reticulum around each myofibril (specialised endoplasmic reticulum; stores Ca2+; no ribosomes there)
• lots of mitochondria between each myofibril because sliding of filaments requires lots of energy
• T-tubules between each sarcomere (basic contractile unit of myofibril). Conduct action potential inside and trigger Ca2+ release from sarcoplasmic reticulum.
• multiple nuclei
• few cytoplasmic ribosomes

Question 4

What are the proteins in myofibril?

Answer 4

Contractile proteins:
- myosin: its bundles form thick filament, 15 nm in diameter. Has bare regions and region with myosin heads. One myosin molecule: 4 light chains, 2 heavy chains.

• actin: thin filament, 7 nm in diameter

Associated proteins:
- troponin (troponin T: binds tropomyosin; troponin I: binds actin; troponin C: binds Ca2+ to move tropomyosin to expose active sites on actin)
- tropomyosin (covers binding sites on actin there is no signal for contraction)
- titin (connects myosin to Z disk)
- nebulin (keeps actin straight)

Question 5

What is triad?

Answer 5

T tubule between 2 terminal cisterna (ends of sarcoplasmic reticulum).

Question 6

What is the structure of sarcomere?

Answer 6

• 2 Z disks (actins from neighbouring sarcomeres connecting to each other in zig-zags)
• I bands (thin filaments only)
• A band (all length of thick filaments)
• H zone (thick filaments only)
• M line (attachment site for myosin)

When sarcomere contracts, l band and H zone become shorter because actin is pulled towards M line (centre). A band remains constant.

Question 7

What are the steps of excitation-contraction coupling?

Answer 7

Resting state:
- cytoplasmic Ca2+ is only 0.1 milliM because most of it is pumped to sarcoplasmic reticulum with Ca2+ ATPase.

1) from the firing motor neuron, acetyl choline triggers Nicotinic acetylcholine receptors on sarcolemma

2) sarcolemma is depolarised, action potential travels down to T tubules

3) in T tubules, there are voltage-gated DHP (dihydropyridine) receptors which are connected to RyD (ryanodine) receptors/channels on sarcoplasmic reticulum. Activation of DHP receptors open RyDs channels.

4) Ca2+ which was stored in sarcoplasmic reticulum is released in cytoplasm, increasing concentration to 30 milliM.

5) Ca2+ binds to troponin C

6) tropomyosin shifts and exposes binding sites on actin

7) myosin head fully binds to actin binding site (crossbridging), releasing phosphate and performing power stroke, moving actin closer to M line.

Question 8

How do filaments slide exactly?

Answer 8

1) Myosin head is tightly bound to the “first” actin at 45 degrees (rigor state), no ATP present

2) ATP binds to myosin head, making it detach from the “first” actin

3) ATP hydrolysis to ADP and Pi (both are still bound to myosin head)

4) Myosin head weakly binds to the “second” actin at 90 degrees (relaxed state)

5) Pi is released from myosin head (ADP is still bound) - results in power stroke, moving actin closer to M line.

6) ADP is released from myosin head, return to rigor state