Muscle Contraction

Question 1

Describe the structure of the skeletal muscle

Answer 1

Bundles of fibres and fascicles enveloped in connective tissue and fascia.
Long, cylindrical multinucleate cells (fused myoblasts).
Each cell is composed of myofibrils.
Myofibrils consist of repeated sarcomeres (functional unit).

Question 2

What is a sarcomere?

Answer 2

Length between two Z-disks. Unit of muscle. Contains thick and thin filaments, and the overlapping 3D structure gives bands.

Question 3

What is titin?

Answer 3

Molecular ‘spring’ that holds myosin in optimal position. Attaches from myosin to Z-band to hold in place, and aids in elasticity.

Question 4

Describe the stages (6) of the sliding filament theory

Answer 4

1) Myosin heads bind to sites on actin filaments
2) Binding forms cross-bridges
3) Myosin neck bends and exerts force (power stroke)
4) Releases by ATP and repeat - head moves up length of actin
5) Sarcomere shortens due to increasing overlap of myofilaments

Question 5

Describe the first event during muscle contraction

Answer 5

Nerve impulse causes calcium release into cytosol.
Binds to troponin and changes troponin’s shape.
This moves tropomyosin and the binding site for myosin on actin is revealed.

Question 6

Describe the second event during muscle contraction

Answer 6

Myosin heads bind to actin via revealed sites.
Stored energy released and bends the neck.
This pulls the actin filament and shortens the sarcomere. When bound to actin, ADP+Pi is released from myosin quicker.
The ATP binding site is left empty.

Question 7

Describe the third event during muscle contraction

Answer 7

ATP binds to empty site on myosin head.
Causes myosin to detach from actin.
ATP -> ADP+Pi
Straightens the myosin neck and stores energy.

Question 8

What happens if Ca2+ levels remain high in cytosol after contraction?

Answer 8

Contraction restarts. If no Ca2+, muscle relaxes.

Question 9

What are the 3 uses of ATP in the excitation-contraction coupling cycle?

Answer 9

1) Binding of ATP to myosin required to break the cross bridge and release myosin heads from actin binding sites
2) Hydrolysis of ATP-> ADP + Pi energises the myosin head, allowing it to generate force for a power stroke.
3) In the sarcoplasmic reticulum, Ca-ATPase uses ATP to re-uptake Ca2+ back into SR to end a muscle contraction.

Question 10

What type of contraction is the following?
1) Muscle shortening
2) Muscle lengthening
3) Muscle stays the same length

Answer 10

1) Concentric contraction
2) Eccentric contraction
3) Isometric contraction

Question 10

What is the difference between the muscles of short and long distance runners? Why?

Answer 10

Long distance = more dark oxidative fibres. Red colour. Lots of myoglobin and a good capillary supply and many mitochondria, so darker appearence.
Shorter distance = more white muscle so fast to fatige and less ATP produced, but reacts faster.

Question 10

What are the 3 methods to regenerate ATP?

Answer 10

1) Oxidative phosphorylation in mitochondria. (aerobic respiration). Requires oxygen and blood. Is slow/ limited by blood supply to the muscles.
2) Phosphorylation catalysed by CK enzyme in cytosol. (reversible, used at the beginning of reaction. Rapid as already in cytosol)
3) Glycolysis in the cytosol (anaerobic resp) - No oxygen needed, lactic acid byproduct. Relied on in intense exercise.