Muscular contraction

Question 1

What are the three types of muscles?

Answer 1

Skeletal (voluntary, striated); cardiac (involuntary, striated); smooth (involuntary, smooth/non-striated)

Question 2

What is the hierarchy of the muscle?

Answer 2

Actin and myosin filaments; sarcomere; myofibril; muscle fibre; muscle

Question 3

How many actin strands does one myosin strand interact with?

Answer 3

6

Question 4

How many myosin strands does one actin strand interact with?

Answer 4

3

Question 5

How do the zones of the sarcomere change with contraction?

Answer 5

I band and H zone decreases, but A band remains constant

Question 6

What is the difference between the I band and H zone, and the A band?

Answer 6

I band and H zone are relative distances between opposing actin and myosin filaments, whilst the A band is the length of the myosin filament

Question 7

What is the structure of the myosin filament?

Answer 7

Heavy chain as main part of the filament, with light chains holding the cross-bridge where the ATP and actin binding sites are situated

Question 8

Outline the sliding filament theory.

Answer 8

In relaxed muscle, the cytosolic Ca2+ present is low, and the cross-bridges are energised but cannot bind to actin due to tropomyosin covering the binding site; in activated muscle, cytosolic Ca2+ is high, and attaches to troponin; tropomyosin is removed from binding sites; myosin heads bind to actin by cross-bridges, and force can now be generated; myosin head moves, releasing ADP and Pi as it does so; actin filament is pulled past the myosin; ATP attaches to myosin head and hydrolyses the cross-bridges, reforming ADP and Pi; cross-bridges returned to energised state and myosin head returns to original positions

Question 9

How long do the muscle fibre action potential and muscle contraction last?

Answer 9

A.p. lasts 1-2ms, whilst the contraction lasts more than 100ms

Question 10

Is the response of a contraction immediate?

Answer 10

No, there is a latent period before the muscular contraction starts

Question 11

What is the mechanical activity of the muscle dependent on?

Answer 11

Depends on availability of intracellular Ca2+, instead of being related to electrical activity; removal of Ca2+ from cytosol requires energy from ATP, and the contraction lasts for a long time after the action potential has finished

Question 12

What are the proteins which enable to electric impulse to release Ca2+ from the SR?

Answer 12

Dihydropyradine and ryanodine

Question 13

Where is the dihydropyridine receptor located?

Answer 13

On the sarcolemma of the t-tubule

Question 14

Where is ryanodine receptor located?

Answer 14

On the membrane of the SR

Question 15

What is a motor unit?

Answer 15

A group of muscle fibres that is innervated by a single motor neuron

Question 16

What is the significance of the size of a motor unit?

Answer 16

Size relates to function; if the motor unit is smaller, the action of the muscle is likely much more precise

Question 17

Outline the process which occurs at the neuromuscular junction.

Answer 17

Action potential reaches axon terminal; depolarisation of the membrane causes v-gated Ca2+ channels to open; Ca2+ enters axon terminal; neurosynaptic vesicles of ACh fuse with membrane; ACh diffuses across cleft; ACh binds to sarcolemma receptors, and acetylcholinesterase degrades ACh –> acetyl + choline; Na+ channels open; Na+ enters sarcoplasm; local current is estabilshed between the depolarised motor end plate and the adjacent sarcolemma; muscle fibre action potential is initiated; wave of depolarisation is conducted along and down t-tubules, into the muscle fibre; Ca2+ is released from SR