Striated muscle structure & the contractile mechanism

Question 1

Describe the structure of skeletal muscle

Answer 1

Multiple long contractile cells called myofibres

Within each myofibril are highly organised bundles of thin (actin) and thick (myosin) filaments - give rise to striations

Connective tissue which holds the myofibres together and provides an atachment to skeletal structures:

Epimysium

Perimysium

Endomysium

Question 2

Describe the structure and fundction of the sarcomere and its components.

Answer 2

Question 3

What causes the striated appearance of skeletal muscle cells?

Answer 3

Question 4

What are the 4 different types of muscle contraction?

Answer 4

1. Muscle shortens actively, causing a movement
2. Isometric contraction - muscle produces force but does not shorten
3. Concentric contraction - muscle may produce force and also shorten
4. Eccentric contraction - muscle may produce active force while it is being stretched by other muscles contracting

Question 5

Describe the steps involved in the cross-bridge cycle of muscle contraction.

Answer 5

1. Myosin heads are primed (ATP has bound and has been hydrolysed)
2. Ca2+ binds to troponin, which causes a confirmation change in tropomyosin, revealing myosin-binding sites on the actin filament.
3. Myosin heads bind to actin and the release of Pi and ADP causes the heads to ‘pull’ on the actin molecule - ‘power stroke’
4. New ATP binds to the myosin allowing the heads to detach from the actin. (In the absence of ATP, myosin heads cannot detach so rigor mortis occurs)

Question 6

What are the changes to the sarcomeric lines/bands during muscle contraction?

Answer 6

• Sarcomere shortens
• I-bands shorten
• H-zone shortens
• Length of A-band does not change but the overlap of thick and thin filaments in the A-band increases

Question 7

What does the ‘length-tension’ curve show?

Answer 7

The length of a muscle (how stretched it is) affects the amount of force it can produce.