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Module 204 Theme 2 > Physiology of skeletal muscle contraction > Flashcards

Physiology of skeletal muscle contraction Flashcards

1
Q

What binds to troponin C in skeletal muscles?

A

4 Ca2+

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2
Q

What binds to troponin C in the heart muscle?

A

3 Ca2+

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3
Q

What happens to troponin after calcium binds?

A

Changes conformation

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4
Q

What does a conformational change in troponin (TnC) lead to?

A

Shuts off TnI
tropomyosin-troponin leaves F-actin groove
Unmasks the myosin binding site on actin

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5
Q

What happens to the next myosin that binds?

A

Next myosin head makes cross bridges (cycling) to actin
Myosin breaks down ATP
Myosin pulls thin filaments

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6
Q

What is total TnI used as a marker of?

A

Total muscle breakdown

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7
Q

What is cardiac TnI used as a marker of?

A

Myocardial infarction

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8
Q

What is cross bridge cycling?

A

Molecular cycle of actin-myosin interaction

Mechanism of contraction at a molecular level

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9
Q

What does contraction through cross bridge cycling depend on?

A

Binding of myosin heads to thin filaments (actin) at specific binding sites

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10
Q

Describe the resting state of a sarcomere

A

Myosin heads are blocked from binding to actin by tropomyosin, which occupies the specific binding sites ( F actin double helical groove)

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11
Q

What can increase the force generated?

A

Increased overlap of thick and thin filaments

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12
Q

When does the greatest force generation occur?

A

When sarcomere is at optimal length

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13
Q

Describe the process of cross bridge cycling in terms of te 4 reactions

A

Myosin releases actin
Myosin head cleaves ATP
Myosin binds actin
Power stroke

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14
Q

Where is creatine found?

A

Muscle fibres

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15
Q

What is creatine phosphorylated to?

A

Creatine phosphate

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16
Q

How is energy stored in muscle?

A

Stored as creatine phosphate in muscle fibres

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17
Q

Describe what happens to ATP during cross bridge cycling

A

ATP is hydrolysed to ADP + Pi , creatine phosphate donates high energy phosphate to ADP restoring it to ATP

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18
Q

How are ATP levels in muscle kept stable?

A

Buffering and regeneration

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19
Q

Which enzyme catalyses the reaction of ATP during cross bridge cycling?

A

Creatine kinase or Creatine phosphokinase (CK, CPK) in both directions

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20
Q

What is creatine?

A

Small molecule that accepts high energy phosphate bonds from ATP

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21
Q

What is plasma creatine phosphokinase a marker of?

A

Muscle destruction

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22
Q

What is creatinine a diagnostic marker of?

A

Kidney function

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23
Q

What is creatinine?

A

Breakdown product of creatine

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24
Q

What are the two calcium gradients?

A

Extracellular vs cytosolic free calcium

SR vs cytosolic free calcium

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25
Q

What provides most of the calcium?

A

Efflux of calcium from the sarcoplasmic reticulum to cytoplasm

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26
Q

What does calcium entering the cell from the outside provide?

A

Only a small fraction of the calcium needed for muscle contraction

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27
Q

What does calcium trigger?

A

Contraction

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28
Q

Which neurotransmitter leads to depolarisation?

A

ACh

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29
Q

What does depolarisation cause?

A

Increase in calcium

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30
Q

Name the receptor where there is a net inward current

A

Active nicotinic AChR

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31
Q

How does depolarisation spread?

A

Through T tubules

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32
Q

What do local action potentials trigger?

A

Ca2+ efflux from terminal cisternae across sarcoplasmic reticulum into the fibre cytoplasm

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33
Q

Where is the ryanodine receptor found?

A

In the sarcoplasmic reticulum

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34
Q

What does the ryanodine receptor do?

A

Releases Ca2+ from SR

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35
Q

What is the ryanodine receptor triggered by?

A

Voltage sensor on Ca2+ channel

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36
Q

Where is SERCA found?

A

In sarcoplasmic reticulum

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37
Q

What does SERCA stand for?

A

Smooth endoplasmic reticulum calcium ATPase

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38
Q

What is the function of SERCA?

A

Pumps Ca2+ back into SR

39
Q

What does SERCA require?

40
Q

What causes a twitch?

A

Single action potential leading to calcium release from sarcoplasmic reticulum

41
Q

What causes a twitch to end?

A

Ca2+ ions are rapidly pumped back into the SR

42
Q

Describe the molecular basis of tetany

A

Frequent APs
Insufficient Ca2+ resequestration
Summation of contraction

43
Q

Name the two types of muscle fibres

A

Slow twitch

Fast twitch

44
Q

Describe the structure of slow twitch fibres

A

High myoglobin

Many mitochondria

45
Q

Give the size, colour and property of slow twitch fibres

A

Red
Small diameter
Oxidative

46
Q

Give the size, colour and property of fast twitch fibres

A

White
Large diameter
Non-oxidative

47
Q

How do the two muscle fibre types differ?

A

Aerobic (slow fibres) vs anaerobic respiration
Faster calcium reuptake in fast fibres
Maximum tension produced in fast fibres
Fatigue resistance in slow fibres

48
Q

Name the fibre types in order of slow to fast fibres

A

Type 1
Type IIA
Type IIX
Type IIB

49
Q

What do muscles contain?

A

A mixture of fibre types

50
Q

What does muscle composition depend on?

A

Muscle action

51
Q

Give the composition of soleus

A

80% Type 1 (slow)

20% Type IIA

52
Q

Give the composition of vastus lateralis

A

Mixture of type 1, IIA, IIX

53
Q

Name the different groups of physical fitness

A

Inactive
Moderately active
Endurance athlete
Anaerobic athlete

54
Q

What colour do type 1 fibres stain?

55
Q

What colour do type II fibres stain?

56
Q

List the 3 types of coordination of contraction

A

Motor units
Tetany
Fusion of myocytes into long myofibres

57
Q

Define motor unit

A

A single alpha motor neuron and all muscle fibres that it innervates

58
Q

What do motor units function as?

A

A single contractile unit of skeletal muscle

59
Q

What type are all muscle fibres in a single motor unit?

60
Q

Describe the synapses in large muscles responsible for powerful gross contractions

A

Single motor neuron may synapse on 1000 fibres

61
Q

Describe the synapses in small muscles mediating precision movements

A

Single motor neuron may synapse with as few as 2 - 3 fibres

62
Q

What determines the muscle fibre?

A

Type and function of lower motor neuron

63
Q

Define isometric contraction

A

Generates a variable force while length of the muscle remains unchanged

64
Q

Define isotonic contraction

A

Generates a constant force while the length of the muscle changes

65
Q

Describe the types of force generation when picking up a glass

A

stage 1: isometric – force increases, joint does not move
Muscle Force < force of gravity –> force increases
biceps and brachioradialis generate force by isometric contraction as muscles have not yet shortened

stage 2: isotonic – force remains the same, arm moves
Glass moves upward in response to force
an isotonic contraction starts as the force generated by the muscles overcomes gravitational and inertial forces keeping glass on the table

glass starts to rise as the muscles shorten and the elbow bends and force generated by the muscle is constant as the glass is moving

66
Q

What does muscle contraction not necessarily mean?

A

Muscle shortening

67
Q

What is meant by concentric?

A

Force during contraction

68
Q

Give an example of concentric force

A

Tossing a ball into the air

69
Q

What is meant by eccentric?

A

Force during muscle elongation

70
Q

Give an example of eccentric force

A

When braking

When the weight of the object is overwhelming - catching a ball

71
Q

When can both types of force occur?

A

In one behaviour

72
Q

What does proprioception control?

A

Force generation

73
Q

What is proprioception based on?

A

Length and strength

74
Q

Describe the size principle

A

As the initial isometric contraction occurs more and more motor units are recruited starting with smaller ones and progressively adding larger ones

Allows fine gradation of force for small movements

75
Q

What does lower motor neuron disease cause?

A

Weakness

Muscle atrophy

76
Q

What does upper motor neuron disease cause?

A

Spasticity, hypertonia

77
Q

Describe the stretch reflex

A

Controls muscle length

Increases muscle force

78
Q

What is westphal’s sign?

A

Lack of patellar reflex

79
Q

What is the function of the patellar reflex

A

Posture and balance

80
Q

Name the fibres for sensation in the stretch reflex

A

Intrafusal fibres

81
Q

Name the fibres for contraction in the stretch reflex

A

Extrafusal fibres

82
Q

What does the muscle spindle fibre do?

A

Detects stretch

83
Q

Give the location of the muscle spindle

A

Parallel to muscle fibres

84
Q

What do the muscle spindles contain?

A

3-12 intrafusal fibres

85
Q

What do gamma motor neurons do?

A

Increase sensitivity

Drive contraction of edge of intrafusal fibres

86
Q

What are sensors from muscle spindles called?

A

Type 1a and Type 2

87
Q

Where are sensors from muscle spindles found?

A

Wrap around the intrafusal fibres

88
Q

What do sensors from muscle spindles do?

A

Detect stretch of central non-contacting region using stretch receptors

89
Q

What is the function of the muscle spindle?

A

Like a thermostat that regulates the relationship between the muscle length and muscle contractility
ie. the relationship between neural drive and force generation

90
Q

What can absence of the muscle spindle reflex suggest?

A

Receptor damage
Femoral nerve damage
Peripheral nerve disease eg. peripheral neuropathy

91
Q

What can happen to the muscle spindle reflex in upper motor neuron disease

A

Can lead to hypertonia and spasticity
UMN inhibits normal descending inhibitory input to spinal interneurons
Spindle reflex can become over sensitive - can attempt to contract the muscle all the time

92
Q

What is the function of the tendon reflex?

A

Protects from overloading

Decreases muscle force- dropping the load - sensor firing - decreased contraction

93
Q

Describe the path of the tendon reflex

A

Sensor to spinal cord
Interneuron to motor neuron
Motor neuron inhibited
Motor neuron to muscle

94
Q

What is the sensor of the tendon reflex?

A

Golgi tendon organ - detects tension
in series with muscle
in tendon - near border of muscle

Module 204 Theme 2 (31 decks)

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