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BIOL 2321 - Cell Biology > Skeletal Muscle > Flashcards

Skeletal Muscle Flashcards

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1
Q

What causes muscle contraction?

A

interactions between actin microfilaments and bipolar myosin filaments

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2
Q

are the skeletal muscles attached to our bones are under voluntary or involuntary control?

A

voluntary

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3
Q

What are the 3 types of muscle tissues?

A

cardiac
skeletal
smooth

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4
Q

what does contraction of the 3 muscle types depend on?

A

ATP-driven sliding of actin against myosin

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5
Q

T or F: skeletal muscles are small with similar organization to other muscle cells

A

FALSE, they are LARGE and UNIQUELY organized

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6
Q

How many nuclei does a single skeletal muscle cell contain?

A

hundreds

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7
Q

Why does a single skeletal cell contain so many nuclei?

A

they are formed by the fusion of many pre-muscle cells in the embryo

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8
Q

How thick are skeletal muscles?

A

10-100 um

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9
Q

How long are skeletal muscles?

A

can be hundreds of mm in length

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10
Q

What fills up most of a skeletal muscle cell?

A

mostly cytoskeleton arrays, some cytoplasm

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11
Q

T or F: skeletal muscle cells are mostly composed of cytoplasm

A

false! mostly cytoskeleton arrays

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12
Q

Briefly describe the structure of skeletal muscles

A

a single cell contains hundreds of nuclei
is very thin (10-100 um)
is very long (hundreds of mm)
contains very little cytoplasm - mostly cytoskeleton arrays

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13
Q

What is a fascicle?

A

a bundle of muscle cells

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14
Q

What is a muscle fibre?

A

an individual muscle cell

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15
Q

What is a bundle of muscle fibres called?

A

a fascicle

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16
Q

What surrounds a single muscle fascicle?

A

a connective tissue sheath

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17
Q

What is a bunch of sheath-coated fascicles called?

A

skeletal muscle tissue

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18
Q

What are myofibrils?

A

long cylindrical strands inside every muscle cell/fibre

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19
Q

What are myofibrils mostly composed of?

A

cytoskeleton components (long stretches of actin and myosin)

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20
Q

What does a bunch of myofibrils make?

A

a single muscle fibre

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21
Q

What does the prefix ‘sarco’ imply?

A

the component is part of a muscle cell

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22
Q

What is the sarcolemma?

A

the plasma membrane of skeletal muscle fibres/cells

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23
Q

What is the sarcoplasm?

A

the cytoplasm of skeletal muscle fibres/cells

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24
Q

What is the sarcoplasmic reticulum?

A

the smooth endoplasmic reticulum of the muscle cells

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25
Q

T or F: sarcoplasmic reticulum is the smooth ER of skeletal muscle cells

A

True

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26
Q

What are T tubules?

A

they are folds made by the sarcolemma invaginating and they surround the myofibrils and contact the sarcoplasmic reticulum at terminal cisternae

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27
Q

What does T tubule stand for?

A

Transverse tubules

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28
Q

What are the contractile units in myofibrils?

A

sarcomeres

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29
Q

What are sarcomeres?

A

the contractile units in each myofibril that shorten when a muscle contracts

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30
Q

How do sarcomeres allow muscle movement?

A

they contract and then return to their original size

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31
Q

T or F: sarcomeres allow muscle movement by twisting, elongating, bending, or stretching

A

FALSE!! they only contract and then return to their normal size

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32
Q

Briefly describe a sarcomere structure

A

each sarcomere is formed from parallel and partly overlapping thick and thin filaments

thick = myosin
thin = actin
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33
Q

Which cytoskeleton component makes the thick filaments of sarcomeres?

A

myosin II

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34
Q

Which cytoskeleton component makes the thin filaments of sarcomeres?

A

actin

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35
Q

Describe the thick filaments of sarcomeres

A

myosin II thick filaments consist of two opposed bundles of myosin with the motor heads jutting outwards in staggered alignment and the tails wind together

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36
Q

What are the 5 density bands/lines in sarcomeres?

A 
Z-lines/discs
M-line
A-band/dark band
I-band/light band
H zone
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37
Q

Describe the Z-lines/discs

A

the two lines formed by accessory proteins on the boundaries of a single sarcomere

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38
Q

Describe the M-line

A

a dense line at the center of thick myosin filament

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39
Q

Describe the A-band/dark band

A

the entire length of the myosin thick filament

40
Q

Describe the I-band/light band

A

where the thin actin is present without myosin

41
Q

Describe the H zone

A

the zone where only thick myosin is present without actin

42
Q

What are the 5 proteins outside of myosin and actin that help form the sacromere?

A

CapZ

Tropomodulin

Titin

Myomesin

Nebulin

43
Q

What is the function of CapZ?

A

A protein outside of myosin and actin that help form the sarcomere

it helps form the Z disc and keeps the actin plus end anchored to it

44
Q

What is the function of tropomodulin?

A

A protein outside of myosin and actin that help form the sacromere that caps the actin minus end

45
Q

What is the function of titin?

A

A protein outside of myosin and actin that help form the sacromere

it positions the myosin equidistant between the Z discs by connecting myosin filaments to the Z discs and acts as a spring to prevent over-extension during sarcomere relaxtion

46
Q

What is the function of myomesin?

A

A protein outside of myosin and actin that help form the sacromere

it joins the myosin tails together at the M line

47
Q

What is the function of nebulin?

A

A protein outside of myosin and actin that help form the sacromere

acts a ‘molecular ruler’ to control the length of the actin thin filaments and prevents too many monomers from assembling

48
Q

What 2 accessory proteins are critical for proper sarcomere contraction?

A

tropomyosin

troponin

49
Q

What is the function of tropomyosin?

A

it is an accessory protein that wraps around the actin filament

in resting position, it blocks the binding site for myosin on actin in order to block contraction

50
Q

What does tropomyosin do in its resting position?

A

it blocks the binding site for myosin on actin and blocks contraction

51
Q

What is the function of troponin?

A

it is an accessory protein that regulates the position of the tropomyosin

it is bound to both actin and tropomyosin

52
Q

What is troponin bound to?

A

both actin and tropomyosin

53
Q

What happens to actin and myosin filaments during contraction? What causes this?

A

they slide past each other caused by the myosin heads binding to actin and ‘walking’ toward the + ends of the microfilaments

54
Q

What changes length during contraction?

A

the filaments remain the same length but the sarcomere shortens as the filaments overlap more

55
Q

In a resting state, what is going on in your skeletal muscle cells?

A

tropomyosin is coiled around actin to block the binding site on actin for myosin = contraction is blocked

56
Q

What is required to initiate contraction?

A

calcium needs to bind to troponin

57
Q

Describe the structure of troponin

A

a heterotrimer

subunits are (I,T,C)

Inhibitory

Calcium-binding

Tropomyosin-binding

58
Q

What are the 3 subunits of troponin?

A

I C T

Inhibitory
Calcium-binding
Tropomyosin-binding

59
Q

What does the binding of Ca2+ to troponin cause?

A

when Ca2+ binds to the C subunit it alters the conformation of the Inhibitory subunit

the conformational change pulls the troponin T subunit into a different position, dragging tropomyosin with it

60
Q

What does the conformational change in the I subunit of troponin cause?

A

the change pulls the troponin T subunit into a different position, dragging tropomyosin with it, exposing the myosin binding sites on actin

61
Q

What happens when the tropomyosin is moved out of the way by troponin?

A

the myosin binding site on actin is exposed and contraction is possible

62
Q

Where in muscle cells is calcium sequestered?

A

in the sarcoplasmic reticulum

63
Q

How is Ca2+ released from the sarcoplasmic reticulum?

A

the arrival of an action potential to the synapse of a motor neuron (a stimulus) releases a neurotransmitter (Ach) into the synaptic cleft and binds to the muscle cell receptors to release Ca2+

64
Q

Where is Ca2+ released from? Where does it go to?

A

it is released from the sarcoplasmic reticulum into the sarcoplasm

65
Q

What is the concentration of Ca2+ in the cytosol?

A

Very low

66
Q

What are neuromuscular junctions?

A

The sites where motor neurons reach a muscle fibre

67
Q

What is the motor plate?

A

the area of the sarcolemma in the neuromuscular junction

68
Q

Where does the depolarization travel from and to once the Ca2+ has been released into the sarcoplasm?

A

it travels along the T tubules to reach the sarcoplasmic reticulum

69
Q

What does a depolarization in the sarcolemma cause?

A

a voltage-gated Ca2+ channel to open in the T tubules

70
Q

What opens the Ca2+ channels in the sarcoplasmic reticulum membrane?

A

When the T tubule protein’s voltage gated Ca2+ channel opens, it causes a conformational change that opens the Ca2+ channels in the SR membrane

71
Q

What two components have Ca2+ channels?

A

T tubule membranes

Sarcoplasmic reticulum membranes

72
Q

Briefly describe the steps in Ca2+ binding to troponin

A

Action potential travels across the synaptic cleft of a motor neuron to an NMJ on a muscle cell

T tubules depolarize and voltage gated Ca2+ channels open

conformational change in sarcoplasmic reticulum membrane Ca2+ channels open

Ca2+ is released from SR into the sarcoplasm

Ca2+ binds to troponin

73
Q

How come contraction is a concerted action?

A

Once Ca2+ enters the sarcoplasm, every myofibril and sarcomere has Ca2+ available to initiate contraction all at once

74
Q

What happens to Ca2+ that doesn’t bind to troponin?

A

it is quickly pumped back into the sarcoplasmic reticulum by a Ca2+ ATPase

75
Q

How are muscle fibers arranged?

A

into motor units

76
Q

Describe a motor unit

A

A group of cells that are all innervated by one nerve

77
Q

What is the function of motor units?

A

they ensure more contraction occurs when necessary or less contraction occurs when necessary

ex. lifting heavy objects vs. light objects doesn’t require the same amount of contraction

78
Q

What is a crossbridge?

A

what forms when a myosin head binds to actin

79
Q

T or F: there will be multiple crossbridges - why/why not?

A

true because there are multiple myosin heads which will bind to actin

80
Q

What do the crossbridges do?

A

they work together to repeatedly bind and release actin to pull actin filaments closer to the M line

81
Q

What 3 steps are involved in a cross bridge cycle?

A

ATP attachment

hydrolysis

release

82
Q

How does the sliding filament model begin?

A

myosin head is attached to the binding site on actin but not yet bound to ATP/ADP

83
Q

What happens when ATP binds to the myosin head?

A

a conformational change occurs in the head which pulls it away from the binding site on actin (ie., myosin releases actin)

84
Q

What happens once the myosin head releases actin?

A

the myosin head hydrolyzes ATP

85
Q

What does ATP hydrolysis by the myosin head cause?

A

causes the head to undergo a second conformational change into a ‘cocked’ position which displaces the head along the actin filament + rebinds to actin

86
Q

After the second conformational change, is the myosin still bound to the actin?

A

Yes, the myosin with ADP + Pi is weakly bound to actin in its new position

87
Q

What does the weak binding of myosin to actin cause?

A

the Pi to release from myosin

88
Q

What does the release of Pi cause?

A

causes another conformational change aka the power stroke

89
Q

What is the power stroke? How is it caused?

A

the power stroke is caused by the release of Pi causing a third conformational change in the myosin

it moves the myosin head back to its original position which pulls the actin filament closer to the M line at the middle of the sarcomere (bc they are attached)

causes the sarcomere to shrink

90
Q

When will muscle relaxation occur?

A

when nerve stimulation stops and acetylcholine is no longer present at the neuromuscular junction

91
Q

What happens when nerve stimulation stops and Ach is not present at the NMJ?

A

the sarcolemma repolarizes

92
Q

What does the repolarization of the sarcolemma cause?

A

voltage-gated Ca2+ channels in the T tubules close

93
Q

What happens when the Ca2+ voltage gated channels in the T tubules close?

A

Ca2+ ATPase pumps Ca2+ back into the sarcoplasmic reticulum

94
Q

What does the lack of Ca2+ in the sarcoplasm cause?

A

troponin shifts back to its original conformation where tropomyosin blocks the myosin binding sites on actin

95
Q

What happens when tropomyosin re-blocks the myosin binding sites on actin?

A

myosin no longer pulls and titin ‘spring’ pushes back so the sarcomere can relax

BIOL 2321 - Cell Biology (26 decks)

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