Muscle contractile mechanisms Flashcards

- Outline anatomical differences between striated and smooth muscle, including organisation of actin, myosin, tropomyosin - Describe the different phases of the sliding filament hypothesis - Outline the process of rigor mortis, and how it relates to the sliding filament hypothesis

1
Q

What is each sarcomere composed of?

A
  • Actin AND myosin, the proteins responsible for muscular contraction
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1
Q

What is a sarcomere?

A

The basic contractile unit of muscle fibre

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2
Q

What is the function of muscle?

A
  • Provides movement to skeleton + hollow organs e.g. heart, blood vessels, GI tract
  • Provides structure to skeleton and hollow organs when under pressure e.g. chambers of heart, circulation through vessels, bladder
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3
Q

What are the 2 types of muscle?

A
  • Striated- skeletal and cardiac muscle
  • Smooth - blood vessels, GI tract, internal organs bar heart
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4
Q

What 2 proteins are essential for contraction of both types of muscle?

A
  • Actin
  • Myosin
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5
Q

Describe skeletal muscle

A
  • Voluntary
  • Single, very long cylindrical, multinucleate cells with obvious striations (stripes)
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6
Q

Describe cardiac muscle

A
  • Involuntary
  • Striated
  • Branching chains of cells, uni or binucleate; less organised striations than skeletal
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7
Q

Describe smooth muscle

A
  • Involuntary
  • Single, fusiform, uninucleate, NO striations
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8
Q

Why are muscles termed striated or smooth?

A
  • Due to the structure of the myosin or actin filaments
  • Striated muscles are striated as the A and I bands are aligned
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9
Q

What are voluntary contractions controlled by?

A
  • Motor nerves
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10
Q

What are involuntary contractions controlled by?

A
  • ANS (autonomic nervous system)
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11
Q

How is striated muscle organised?

Describe it in terms of the Z, A, I and H bands

A
  • Unit of striation- sarcomere
  • Z band- attachment sites for actin, separate each sarcomere
  • Light or I band- non- superimposed length of actin,
  • Dark or A band- entire length of myosin, and shows slight overlap of actin with myosin
  • H band - only myosin component of sarcomere, middle of A band
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12
Q

What happens to the striated sarcomere during contraction?

A
  • Myosin length stays the same, but actin moves across myosin, so the contraction occurs due to the increased overlap, whereas when the muscle is relaxed, there’s only a very small overlap
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13
Q

How is smooth muscle organised?

A
  • Myosin and actin filaments are disorganised interaction at dense bodies
  • This disorganisation allows for a more 3D contraction e.g. in hollow organs
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14
Q

What are the 2 types of actin?

A
  • G actin
  • F actin
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15
Q

Describe G actin

A
  • Globular protein, binds to ATP, contains ATPase activity
16
Q

Describe F actin

A
  • Helical protein, uses ATP to make filaments
17
Q

What do the actin filaments contain?

A
  • Active actin binding sites- allow interactions with myosin
18
Q

What do striated muscles contain that smooth muscle does not?

A
  • Tropomyosin
19
Q

What does tropomyosin do?

A
  • Covers the actin binding sites at rest, preventing actin-myosin interactions
20
Q

What does myosin II contain?

A
  • 2 myosin HEAVY chains- intertwined, forms head domain, binds ATP and ADP, ATPase activity, binds to active actin binding site
  • 4 myosin LIGHT chains- 2 per head- especially in smooth muscle, modulates myosin-actin interactions
21
Q

What is the sliding filament hypothesis?

A

Cycling of 90 degrees to 45 degrees motion of myosin head-actin interactions, rowing action

22
Q

Describe the different phases of the sliding filament hypothesis

A

1 - Myosin heads hydrolyse ATP and become reoriented and energised
2 - ADP/Pi + myosin head have a high affinity for actin, the myosin head binds to actin, forming crossbridges at 90 degrees, releasing the ADP, which ultimately causes the contraction
3 - Myosin crossbridges rotate toward centre of the sarcomere (power stroke) at 45 degrees
4 - ATP + myosin head have low affinity for actin- as myosin heads bind to ATP, the crossbridges detach from actin

23
Q

What are the consequences of the sliding filament hypothesis?

A
  • Myosin heads must be ‘primed’ with ATP for muscular contraction + movement, hence need plentiful sources of ATP
24
Q

What are the main sources of ATP for muscle contraction?

A
  • Aerobic respiration - glycolysis and oxidative phosphorylation
  • Anaerobic respiration - production of lactate into ATP in absence of O2
  • Phosphocreatine- source of ATP
25
Q

What initiates striated muscle contraction?

A
  • Rise in Ca2+ leads to removal of tropomyosin from actin actin binding sites, allowing myosin heads to interact with actin
  • Rise is produced by influx of Ca2+ and Ca2+ release from sarcoplasmic reticulum (SR)
26
Q

What happens to the muscle if there’s a decrease in Ca2+?

A
  • Muscle relaxation
27
Q

What is needed for the detachment of myosin-actin?

A
  • Need ATP-myosin head binding
28
Q

Outline the process of rigor mortis following death

A
  • Rise in ca2+, removal of tropomyosin from actin-myosin binding sites
  • Loss of ATP production (due to no longer respiring aerobically), preventing detachment of actin-myosin filaments, this is what causes the stiffness of muscles known as ‘rigor’
  • This starts 2-6 hours after death (anaerobic respiration still produces some ATP, so stops the process from beginning immediately), peaks after approx 12 hours
  • Rigor stops after 24-48 hours due to decomposition of myosin/actin