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BS22001 Human physiology and pharmacology > 13. E-C coupling and skeletal muscle contraction > Flashcards

13. E-C coupling and skeletal muscle contraction Flashcards

1
Q

How much body weight is skeletal, smooth and cardiac muscle?

A
  • skeletal; 40%

- smooth and cardiac; 10%

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2
Q

What triggers contraction?

A

an increase calcium ion concentration

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3
Q

What are the steps in skeletal muscle contraction?

A
  • Ach receptors are concentrated in the NMJ
  • Ach is released from the pre-synaptic nerve terminal and bind to nicotinic acetylcholine receptors at the NMJ
  • These receptors are non-selective cation channels that open in response to Ach binding, causing depolarisation of the end-plate potential
  • If EPP exceeds threshold, an action potential is generated (activated by opening of voltage gated sodium channels)
  • Generation of an AP initiates a sequence of events leading to contraction
  • Ach is rapidly inactivated by Acetylcholine-esterase
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4
Q

The reaction mechanism for the breakdown of acetylcholine when inactivated

A

Ach to Acetate and choline (inactive states) using acetylcholine esterase

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5
Q

Describe the structure and size of a muscle fibre

A

elongated and cylinder shaped, 10-100 micrometers in diameter

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6
Q

How are muscle fibres formed?

A

through the fusion of myoblasts (smaller cells)

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7
Q

What are features of a muscle fibre?

A
  • multiple nuclei
  • abundance of mitochondria
  • striated
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8
Q

Myosin

A
  • thick filaments
  • A / dark band
  • arranged in a circle (surrounded by 6 actin molecules)
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9
Q

Actin

A
  • thin filaments
  • I / light band
  • arranged in a triangle (surrounded by 3 myosin molecules)
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10
Q

Myofibril

A
  • 80% volume of a muscle fibre
  • extend the entire length of a muscle fibre
  • consist of thin and thick filaments
  • 1 micrometer in diameter
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11
Q

Name the cytoskeletal components of myofibrils

A
  • m line
  • z line
  • h zone
  • sarcomere
  • thin and thick filaments
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12
Q

M line

A
  • centre of the sarcomere

- keeps A bands together

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13
Q

Z lines

A
  • either side of the sarcomere (joins adjacent sarcomeres)

- keep I bands together

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14
Q

H zone

A
  • centre of an A band
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15
Q

Sarcomere

A

smallest component of a muscle fibre that can be stimulated to contract

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16
Q

Cross-bridge section

A
  • 3 dimensions
    1. actin surrounded by 3 myosin
    2. myosin surrounded by 6 actin
    3. cross-bridge
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17
Q

Describe the levels of organisation in a skeletal muscle

A
  • whole skeletal muscle (an organ)
  • muscle fibre ( a single cell)
  • myofibril (a specialised intracellular structure)
  • thin and thick filaments (cytoskeletal elements)
  • myosin and actin (protein molecules)
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18
Q

What are the 3 components of thin filaments?

A
  • actin molecules
  • troponin
  • tropomypson
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19
Q

What are actin molecules?

A

binding sites for attachment of myosin to form cross-bridges

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20
Q

What is the main component of thin filaments?

A

double stranded alpha helix polymer of actin (F actin molecules)

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21
Q

What are the 2 regulatory protein molecules?

A
  • tropomyosin and troponin
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22
Q

What are the 2 types of troponin and what do they do?

A
  • Troponin c - binds to calcium ions

- Troponin I - bonds to actin and inhibits contraction

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23
Q

What happens when troponin c is not bound to calcium?

A

it stabilises tropomyosin in its blocking site over the actins cross-bridge binding site

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24
Q

What are the 2 components of myosin?

A
  • heads of myosin heavy chain (hinge region)

- tail (tail region)

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25
Q

What are the binding sites on the heads of myosin?

A
  • actin-binding site

- myosin ATPase site

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26
Q

What are the two chains on the myosin heads?

A
  • alkali light chain

- regulatory light chain

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27
Q

What do the myosin heads form when bound to actin?

A

cross-bridges

28
Q

How is activity of the chain regulated?

A

by phosphorylation of kinases

29
Q

What site does the myosin head bind to the actin filament at?

A

the myosin binding site

30
Q

Where does energy for the cycle come from?

A

through hydrolysis of ATP

31
Q

Describe the cross-bridge cycle

A

myosin-II head binds to actin, the cross-bridge becomes distorted and myosin heads detach from actin

32
Q

What happens when there is a decrease in calcium ion concentration?

A

it is a signal to cease cross-bridge cycling and relax

33
Q

What happens when calcium levels are low/ high?

A

high - myosin can bind to actin

low - actin is blocked by troponin/ tropomyosin (actin-myosin interactions are blocked)

34
Q

Describe the 1st step of cross-bridge cycling

A

ATP binding - ATP binds to myosin head, causing dissociation of the actin-myosin complex

35
Q

Describe the 2nd step of cross-bridge cycling

A

ATP hydrolysis - ATP is hydrolysed, causing the myosin head to return to resting conformation

36
Q

Describe the 3rd step of cross-bridge cycling

A

Cross-bridge formation - a cross-bridge forms and the myosin head binds to the new position on actin

37
Q

Describe the 4th step of cross-bridge cycling

A

A phosphate is released, myosin heads change conformation, resulting in the power-stroke

38
Q

Describe the 5th step of cross-bridge cycling

A

ADP is released

39
Q

How does the sliding filament mechanism bring about contraction?

A

cross-bridge interaction between actin and myosin brings about contraction

40
Q

What happens during contraction?

A

due to interactions between the thick and thin filaments, the filaments slide over each other but the length of the filaments themselves do not shorten

41
Q

What happens to the A bands during contraction?

A

stay the same width

42
Q

What happens to the I bands during contraction?

A

Thin filaments not overlapping thick so width decreases

43
Q

What happens to the H zone during contraction?

A

Within A band, thick filaments not overlapping thin so width decreases

44
Q

What happens to the distance between z lines during contraction?

A

decreases

45
Q

What becomes shorter during contraction?

A

the H zone and I bands

46
Q

What is ‘Rigor mortis’? and how long does it occur?

A

stiffness of death which begins 3-4 hours after death and completes in 12 hours

47
Q

What happens following this death?

A
  • Calcium ion concentration rises allowing the regulatory proteins to move aside, letting myosin heads bind to actin to form cross-bridges.
  • dead cells cannot produce ATP and so actin and myosin, once bound, cannot detach
  • after several days, the proteins involved start to degrade as rigorous mortis subsides
48
Q

What is E-C coupling?

A

when excitation triggers an increase in calcium ion concentration which triggers contraction

49
Q

Describe the structure of the sarcoplasmic reticulum

A

TRIAD;

  1. sarcoplasmic reticulum cistema
  2. Transverse tubule
  3. sarcoplasmic reticulum ecistema
50
Q

What forms transverse tubules?

A

invaginations in the plasma membrane

51
Q

What is the role of the sarcoplasmic reticulum?

A

to store intracellular calcium ions

52
Q

What happens in the skeletal muscle when the T tubule depolarises?

A

leads to calcium ion release from the sarcoplasmic reticulum at the triad

53
Q

Describe the 4 steps involved in a single fibre triad

A
  1. membrane depolarisation opens the L-type calcium channel
  2. mechanical coupling between the L-type calcium channel and the calcium release channel causes the calcium ion release channel to open
  3. calcium ions exit the SR via the calcium ion release channel and activate Troponin C, leading to muscle contraction/ cross-bridge cycling
54
Q

What are present on each of the 4 calcium release channels on the post-synaptic membrane?

A

Feet projecting into the cytosol

55
Q

What drugs block muscle contraction

A

Dihydropyridines (DHP) and ryanodine

56
Q

Describe how calcium ions are removed from the cytoplasm and their uptake into the SR

A
  • Na-Ca exchanger and Ca2+ pump in the plasma membrane both extrude CA2+ from the cell
  • Ca2+ pumps sequesters Ca2+ within the SR
  • Ca2+ is bound in the SR by calreticulin and calsequestrin
57
Q

How is contraction terminated?

A

by uptake of calcium ions into the SR

58
Q

How does muscle contraction continue?

A

it will continue as long as there is a high concentration of calcium ions in the cytosol

59
Q

What is calreticulin present in?

A

smooth muscle

60
Q

What is calsequestrin present in?

A

smooth, skeletal and cardiac

61
Q

Excitatory “pulse of calcium” steps

A
  1. initiation (full excitation of t-tubules/ SR system causes the release of calcium ions
  2. increase in calcium levels
  3. immediately after, the SERCA-calcium depletes, causing a pump of calcium and reuptake of calcium into SR, contraction occurs
62
Q

What is the latent period?

A

short period when stimulation of onset of contraction occurs

63
Q

What is the contractile period/ how is it turned off

?

A

the re-uptake of calcium ions into the SR

64
Q

What is the relaxation period?

A

the time from peak tension to relaxation

65
Q

How does the SR take calcium ions into the cytosol?

A

it uses calcium-ATPase pumps, which actively transport calcium

66
Q

wWhat filaments return to resting conformation after contraction?

A

thin

BS22001 Human physiology and pharmacology (9 decks)

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