L9 - Muscle Contraction

Question 1

What is a muscle fibre?

Answer 1

Cells that convert ATP into mechanical energy, specialised in force and movement

Question 2

What are myofibrils?

Answer 2

fibres made up of contractile proteins actin and myosin, and some control proteins

Question 3

What is the basic functional and structural unit of a muscle fibre called?

Answer 3

Sarcomere

Question 4

Where does ATP bind to in order to provide energy for muscle contraction?

Answer 4

Myosin head

Question 5

Why would you ‘skin’ cold glycerol or Triton-X?

Answer 5

This removes cells and other lipid membranes, allowing an experimenter to bath actin and myosin filaments in other fluids.

Concentrated salt solution (with no Ca2+) causes actin and myosin to separate from each other

Question 6

What do myosin molecules do when bathed in dilute salt?

Answer 6

They assemble into bi-polar filaments (1.6 um long)

Question 7

How many subunits does Troponin have, and what does each bind?

Answer 7

3

1 binds actin, 1 binds tropomyosin, and 1 binds Ca2+

Question 8

Does the thickness of filaments change when the muscle contracts?

Answer 8

No - but the extent at which they overlap each other does

Question 9

What was a critical test of the sliding filament theory?

Answer 9

To demonstrate that the force produced by a muscle fibre when it is activated is proportional to the overlap between thick & thin filaments, therefore to the number of cross bridges that could form between the actin and myosin heads.

Question 10

Who proposed the ‘sliding filament’ theory?

Answer 10

Huxley and Huxley independently proposed a sliding filament theory of muscle contraction

Question 11

What methods did Huxley and Huxley individually use to find out about muscle contraction?

Answer 11

Electron and Interference microscopy

Question 12

What is the cross-bridge cycle?

Answer 12

1) Myosin head binds to actin forming a crossbridge & its orientation changes, exerting a force that causes actin to slide along myosin.
2) Myosin head then binds to ATP & releases actin. It then returns to its original formation ready for next cycle.
3) ATP is needed to break the actin–myosin bonds, which accounts for rigor mortis. .

Question 13

What protein hold myosin in the sarcomere?

Answer 13

Titin

Question 14

What roles does Calcium play in muscle contraction?

Answer 14

Ca2+ binds to troponin, this causes the troponin molecule to move, pulling tropomyosin out of the way, allowing the myosin heads to bind to actin.

Question 15

What makes myosin heads release from actin after contraction

Answer 15

the binding again to ATP

Question 16

What controls tension, and therefore the amount of force of a muscle fibre?

Answer 16

The muscle fibre membrane potential

Question 17

How does electrical excitation cause Ca2+ release?

Answer 17

1) An AP arrives at the axon terminal causing vesicles of ACh to be released.
2) The postsynaptic membrane generates an AP that spreads down the T-tubule
3) Depolarisation of T-tubules causes the release of Ca2+ from the SER into the cytoplasm.
4) Released Ca2+ diffuses in the sarcoplasm, stimulating muscle contraction

Question 18

Where are T-tubules associated with sarcoplasmic reticulum’s?

Answer 18

At triads

Question 19

How long does the cross bridge cycle continue and how does it stop?

Answer 19

The cycle repeats as long as Ca2+ is high.

When electrical excitation stops, the SER mops up Ca2+

Question 20

What is the evidence that electrical excitation of a muscle cell outer membrane controls development of force?

Answer 20

p

Question 21

What is the evidence that electrical excitation of a muscle cell outer membrane controls development of force?

Answer 21

p

Question 22

Does the length of a muscle fibre affect its strength?

Answer 22

No. It is the degree of overlap between actin and myosin filaments in individual sarcomeres that matters. Force in one sarcomere does not add to force within the next in a muscle fibre – they are independent functional units. Because sarcomere length does not vary much between different muscles in vertebrates, their individual muscle fibres also do not vary much in strength. The only way to make a muscle stronger is to make it more wide, increasing the number of filaments pulling in parallel by either adding more muscle fibres or making individual fibres wider. A long muscle fibre will be able to contract further than a short muscle fibre because the distance of shortening by individual sarcomeres does add up.

Question 23

What would be the difference in speed of contraction between a fibre with short compared with long sarcomeres?

Answer 23

the fibre with short sarcomeres will contract the fastest. When the muscle is activated to shorten, the individual sarcomeres will each shorten by (roughly) the same amount: more sarcomeres means a greater length of shortening in a given time. (The muscle with longer sarcomeres will be able to generate the greater force – a long sarcomere has more cross-bridges)