Fatigue
When contractile activity cannot be maintained and tension in a muscle declines
Central fatigue
- Occurs when the CNS decreases its activation of motor neurons
- Characterized by a slowing down of activity even though the muscle fibres are not fatigued
Why does central fatigue occur?
- Person feels board with the activity
- They are tired
- They lack motivation to continue exercise
Muscle fatigue
- Occurs at the level of the muscle and is a mechanism used to protect the muscle cells
- It reduces contractile activity before ATP supplies run out otherwise rigor mortis would occur
4 causes of muscle fatigue
- Local accumulation of ADP and Pi from ATP hydrolysis
- Accumulation of lactic acid
- Accumulation of extracellular K+
- Depletion of glycogen
Local accumulation of ADP and Pi from ATP hydrolysis
When ATP metabolite concentrations get too high, they interfere with cross-bridge cycling
Accumulation of lactic acid
- This inhibits the enzymes of glycolysis reducing ATP production
- Can also interfere with excitation-contraction coupling
Accumulation of extracellular K+
- Without ATP the Na-K pump cannot function to restore Na+ and K+ ion gradients
- Accumulation of extracellular K+ causes membrane depolarization that makes muscle fibres less excitable
Depletion of glycogen
Muscle glycogen stores can become depleted during extreme exercise leading to fatigue
Muscle fibre classification
- Slow twitch (type 1)
- Fast twitch (type 2)
Motor units and muscle fibres
- All muscle fibres in a motor unit are the same
- There are different types of muscle fibres so there are different types of motor units
Slow twitch muscle fibres - Speed of contraction
Slow twitch fibres contract and relax at slower rates
Slow twitch muscle fibres - Innervation
- Innervated by a2 motor neurons
- a2 are smaller than a1 so they have a lower activation threshold and slower conduction speeds
- ATPase located at the myosin head is slow which causes a slower rate of cross-bridge cycling
Slow twitch muscle fibres - Metabolic properties
- Also called slow oxidative
- They produce their ATP by aerobic processes
Fast twitch muscle fibres - Speed of contraction
Contract and relax at faster rates
Fast twitch muscle fibres - Innervation
- Innervated by a1 motor neurons
- a1 motor neurons are larger so they have a higher activation threshold and faster conduction speeds
- ATPase located at the myosin head is fast which correlates with a faster rate of cross-bridge cycling
Fast twitch muscle fibres - Metabolic properties
- 2 types of fast twitch muscle fibres which use different metabolic activity
- Fast oxidative glycolytic fibres and fast glycolytic fibres
Fast oxidative glycolytic fibres
Produce ATP by both aerobic and anaerobic metabolism
Fast glycolytic fibres
Produce ATP by anaerobic means
How is the colour of muscle fibre determined?
By how fast they produce their energy
Red fibres
- FOG fibres and slow oxidative fibres
- Contain numerous mitochondria and are highly vascularized
- Contain a lot of myoglobin to support the high use of oxygen
What gives fibres the red colour?
Myoglobin
White fibres
- Fast glycolytic fibres
- Rely on anaerobic metabolism
- Have few mitochondria and no myoglobin
Muscle fibre properties - Slow twitch (slow oxidative)
- Innervation: a2 motor neurons
- Muscle fibre diameter: small
- Motor neuron conduction: slow
- Speed of contraction/relaxation: slow
- Force production: slow
- Fatigue: resistant to fatigue
- Mitochondrial density: high
Colour: red
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Module 1: Section 1 + 256
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Module 1: Section 316
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Module 1: Section 413
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Module 1: Section 531
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Module 2: Section 120
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Module 2: Section 216
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Module 2: Section 332
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Module 2: Section 451
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Module 2: Section 511
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Module 4: Section 127
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Module 4: Section 235
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Module 4: Section 333
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Module 4: Section 426
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Module 4: Section 59
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Module 4: Section 628
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Module 5: Section 115
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Module 5: Section 234
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Module 5: Section 352
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Module 5: Section 47
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Module 5: Section 533
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Module 5: Section 67
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Module 6: Section 121
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Module 6: Section 223
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Module 6: Section 344
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Module 6: Section 428
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Module 6: Section 512
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Module 6: Section 613
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Module 6: Section 743
