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biochemistry > muscle contraction > Flashcards

muscle contraction Flashcards

1
Q

how are myocytes specialised?

A
  • multi- nucleated due to fusion of myoblasts during development
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2
Q

what does the fusion of myoblasts form?

A
  • myotubes or muscle fibres
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3
Q

what are muscle fibres filled with?

A
  • around 1000 parallel rods of contractile material
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4
Q

how are rods packed in muscle fibres?

A
  • cytosol
  • myofibrils
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5
Q

how are filaments arranged? what is the purpose?

A
  • striated pattern
  • deliberate arrangement in order to maximise muscle contraction
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6
Q

what is the main functional unit of the muscle?

A
  • sarcomere
  • between Z lines
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7
Q

what does sarcomere consist of?

A
  • thick and thin filaments overlapping
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8
Q

what does the thick filament consist of?

A
  • myosin
  • highly abundant
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9
Q

what does the thin filament consist of?

A
  • actin, tropomyosin and troponin
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10
Q

what do tropomyosin and troponin act as?

A
  • regulators
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11
Q

what does I band consist of?

A
  • thin filament only
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12
Q

what does H zone consist of?

A
  • thick band only
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13
Q

what does A band consist of?

A
  • overlap of thick and thin filament
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14
Q

what is the m line made up of?

A
  • myomesin and M protein
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15
Q

what does the Z line contain?

A
  • alpha actinin
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16
Q

what are ATPases?

A
  • proteins that catalyse the hydrolysis of ATP to liberate free energy for cellular wok
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17
Q

what does the sliding filament model suggest?

A
  • lengths of thick and thin filaments doesn’t change when the sarcomere contracts instead they overlap
  • contraction caused by active sliding of thick and thin filaments past each other
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18
Q

what is the specialised protein in contraction and what is it effectively?

A
  • myosin
  • acts as an enzyme as it breaks ATP down so acts like ATPase
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19
Q

describe the structure of myosin

A
  • very large (520kda)
  • abundant in the muscle
  • consists of two large heavy chains and small light chains
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20
Q

describe what the light and heavy meromyosin chains do

A
  • light meromyosin chain forms filaments spontaneously (self- assembly)
  • heavy meromyosin forms cross bridges and S1 sub fragment hydrolyses ATP and binds actin
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21
Q

describe the S1 fragment

A
  • fragment on myosin has an ATP binding site which combines actin
  • allow the conversion of ATP into movement via hydrolysis
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22
Q

how many gene products are labelled as ATPase?

A
  • 430 gene products
  • most processes unlock energy via enzymes
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23
Q

what forms does actin exist in?

A
  • G- actin (globular)
  • F- actin (fibrous)
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24
Q

describe what F actin monomers do and what they form

A
  • monomers intertwine and form the trunk of thin filaments to which tropomyosin and troponin attach
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25
Q

what does F actin increase?

A
  • ATPase activity of myosin by increasing the rate at which ADP and Pi are released from the active site
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26
Q

what happens at rest to myosin and actin?

A
  • they are close together but there is no binding of myosin and actin
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27
Q

at rest does ATP hydrolysis happen?

A
  • it does happen but it is hydrolysed by myosin slowly since actin is not involved to help release ADP+ Pi
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28
Q

what happens when the muscle is excited?

A
  • myosin head binds to actin and ADP and Pi is released from the active site
  • causes a conformational change in S1 to create a lever arm and power stroke
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29
Q

what is the power stroke?

A
  • main generator of force that stimulates the muscle
  • causes ATPase to break down ATP which allows attachment of myosin to actin
30
Q

what bit of myosin uses energy that’s liberated and why?

A
  • little cross bridge utilises energy in order to switch as conformational change
31
Q

what happens when the active site is empty of ADP and Pi?

A
  • ATP returns causing detachment of actin and myosin
32
Q

what is the cycle dependent on?

A
  • availability of ATP
33
Q

what does myosin convert free energy to?

A
  • converts free energy of ATP hydrolysis to kinetic energy
34
Q

what are myosin isoforms?

A
  • genes encode proteins but variants in proteins can arise from one gene via alternative splicing or RNA editing
  • adult humans have myosin heavy chain (MHC), 1, 11A and 11X
35
Q

describe type 1 fibres

A
  • slow twitch fibres
  • low myosin ATPase activity
  • low shortening velocity
  • high economy
  • high resistance to fatigue
36
Q

describe type IIa fibres

A
  • fast twitch fibre
  • intermediate myosin ATPase activity, shortening velocity, economy and resitsance to fatigue
37
Q

describe type IIx fibres

A
  • fast twitch fibre
  • high myosin ATPase activity
  • high shortening velocity
  • low economy
  • low resistance to fatigue
38
Q

describe the experimental assessment of myotube contraction

A
  • create muscles in a dish ‘ in culture’ and contract the myotubes without a nervous system
  • contraction is enabled via electrical stimulation as it releases calcium ions
39
Q

what is calcium?

A
  • non- energy nutrient that controls muscle contraction by permitting the binding of myosin to F actin
  • via troponin and tropomyosin
40
Q

what does the tropomyosin do at rest?

A
  • blocks the binding site of actin
41
Q

what does TnI bind to?

A
  • actin
42
Q

what does TnC bind to?

A
  • Ca2+
43
Q

what does TnT bind to?

A
  • tropomyosin
44
Q

what happens when the nerve impulse arrives at the muscle?

A
  • calcium is released from the sarcoplasmic reticulum
45
Q

what does calcium bind to once released? what does this allow?

A
  • binds to troponin C to cause a conformational change
  • causes detachment of troponin L from actin so tropomyosin moves
46
Q

what happens to tropomyosin after calcium binds to tnC?

A
  • tropomyosin moves to expose the binding site and hence permits binding
47
Q

what happens after the power stroke has occurred?

A
  • sequestration of calcium in the sarcoplasmic reticulum so calcium withdraws
  • tropomyosin blocks active site again
48
Q

how is neural stimulation linked to calcium?

A
  • via signals from neurons to muscles that are carried out chemically in motor units
49
Q

what are motor units?

A
  • motor neurons and the muscle cells that they innervate
50
Q

what happens as the motor neuron approaches the muscle?

A
  • it splits into hundreds of branches ending at the neuromuscular junction
51
Q

what is the biochemical process and when does it occur?

A
  • occurs when nerve impulse reaches the junction that allows downstream effect of calcium release
52
Q

what does each junction contain? what NT is discharged?

A
  • contains many synapses where acetylcholine is discharged when action potentials arrive at pre- synaptic membrane
53
Q

how is the signal carried in a muscle?

A
  • via acetylcholine receptor
54
Q

what happens to the neurotransmitter and what allows transmission after?

A
  • NT released into the synaptic cleft
  • ligand receptor allows transmission of impulse to result in calcium release and hence contraction
55
Q

what does acetylcholine cause?

A
  • a conformational change that causes many sodium ions to flow into cytosol with fewer leaving
  • leads to depolarisation (postsynaptic potential)
56
Q

what is the process at the neuromuscular junction known as?

A
  • feedforward mechanism as postsynaptic potential is aided by voltage gated Na+ channels in plasma membrane
  • facilitates Na+ entry after depolarisation
57
Q

why do voltage gated K+ channels open?

A
  • to let K+ out of the cytosol and resting membrane potential is resumed
  • maximises the signal for calcium release
58
Q

describe the ligand receptor job

A
  • ligand gated receptor changes it conformation only when interacting with its ligand (Ach)
  • which is the opposite to Na+ and K+ voltage gated channels
59
Q

what happens to the free acetylcholine after excitation has passed?

A
  • acetylcholine is hydrolysed in synaptic cleft by acetylcholinesterase
  • receptor returns to its original conformation
60
Q

what links neurons to muscle contraction?

A
  • excitation contraction coupling
61
Q

what does acetylcholine mediated depolarisation stimulate? what is the definition?

A
  • transverse tubules
  • extension of the plasma membrane closely appose to Ca2+ containing sacs called sarcoplasmic reticulum
62
Q

how is reservoir of calcium maintained?

A
  • by Ca2+ ATPase pump which creates steep gradient across the membrane
63
Q

what does transmission of action potential across transverse tubules cause?

A
  • opening of Ca2+ channel called ryanodine receptor
64
Q

how is the opening of the ryanodine receptor stimulated?

A
  • via conformation change of the dihydropyridine receptor
65
Q

how many folds does Ca2+ rise?

A
  • approx. 100 fold
66
Q

what are the three disruptions in calcium handling?

A
  • ryanodine is a poison that binds with high affinity to ryanodine receptor
  • blocks calcium release from SR, which causes paralysis
  • lack of activity of Ca2+ ATPase means resting gradient cannot be stored causes rigor mortis
67
Q

describe rigor mortis

A
  • no ATP pumping calcium back so no relaxation and hence muscle in cross bridge state
68
Q

what does passing of Ca2+ to TnC activate? what happens when action potential passes?

A
  • muscle contraction
  • when AP passes, ryanodine receptor closes to prevent Ca2+ efflux from SR, Ca2+ ATPase returns resting gradient
69
Q

what are the major energy consumers in muscles?

A
  • myosin ATPase and Ca2+ ATPase
70
Q

describe how large ATP is

A
  • 1 ATP used for one power stroke; 100’s myosin head in sarcomere; 100s of thousands of sarcomeres in each muscle fibre; 250,000 fibres in muscle
71
Q

assuming all fibres are engaged, how many molecules are required for a single muscle twitch?

A
  • 7.5 billion molecules
72
Q

how high can demand for ATP hydrolysis during strenuous exercise be?

A
  • 12 hexillion molecules of ATP per minute

biochemistry (15 decks)

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