Module 4 Section 4

Question 1

Describe what is meant by fatigue as it relates to skeletal muscle.

Answer 1

Fatigue is when contractile activity cannot be maintained and tension in a muscle declines.

Central fatigue

• Occurs when the CNS decreases its activation of motor neurons
• May be psychological in origin
• Characterized by slowing down and cessation of activity even tho the muscle fibres themselves aren’t fatigued
• Can be b/c the person is bored, tired or lack the motivation to continue to exercise.

Muscle fatigue
- It’s a mechanism used to protect the muscle cells
- Fatigue reduces contractile activity before ATP supplies run out, otherwise rigor mortis would occur
- It may occur due to
• local accumulation of ADP and Pi from ATP hydrolysis (when ATP metabolite concentrations get too high, they interfere w/ cross-bridge cycling)
• accumulation of lactic acid (this inhibits the enzymes of glycolysis, reducing ATP production and can also interfere with excitation-contraction coupling)
• accumulation of extracellular K (it causes membrane depolarization that makes muscle fibres less excitable)
• depletion of glycogen (muscle glycogen stores can become depleted during extreme exercising leading to fatigue)

Question 2

Explain why some muscle is dark and some muscle is pale.

Answer 2

The colour of muscle fibres is determined by how they produce their energy.

Red fibres, such as slow oxidative and fast oxidative glycolytic fibres, contain numerous mitochondria and are highly vascularized. They also contain a lot of myoglobin to help support the high use of O2. The myoglobin gives it its red colour. In short, it’s dense in mitochondria and myoglobin content.

White fibres, such as fast glycolytic fibres rely mainly on anaerobic metabolism. It has a few mitochondria and no myoglobin, resulting in its pale colour. In short, it’s less dense in mitochondria and myoglobin content.

Question 3

Compare and contrast the properties of the different muscle fiber types. Describe the innervation of the different muscle fiber types.

Answer 3

Slow twitch (type 1 muscle fibres)
- Speed of contraction
• contract and relax at slower rates than fast twitch fibres
- Innervation
• innervated by a2 motor neurons, whereas fast twitch = a1 motor neurons
• since a2 motor neurons are smaller than a1, they have a lower activation threshold and a slower conduction speed, thus causing slower twitches
• the ATPase on the myosin head is also slow (slow rate of cross-bridge cycling)
- Metabolic properties
• it’s slow oxidative b/c they produce their ATP by aerobic processes

Fast twitch (type II)
- Speed of conduction
• faster than slow twitch fibres
- Innervation
• innervated by a1 motor neurons
• a1 = larger whch means they have a higher activation threshold and faster conduction speeds, thus causing faster twitching
• the ATPase is fast (faster rate of cross-bridge cycling)
- Metabolic properties
• fast oxidative glycolytic (FOG): fibres produce ATP by both aerobic and anaerobic metabolism
• fast glycolytic (FG): fibre produces ATP by anerobic means

Question 4

Recall the 3 ways ATP is important in the contraction-relaxation process.

Answer 4

1) Splitting of ATP to provide energy for power stroke.
2) Binding of new ATP to myosin head to release the cross-bridge.
3) Active transport of Ca back into the SR.

Question 5

Fill in the chart:

Slow twitch 
- Slow oxidative 
• innervation 
• muscle fibre diameter
• motor neuron conduction 
• speed of contraction/relaxation 
• force production 
• fatigue 
• mitochondrial density 
• colour

Fast twitch
- Fast oxidative glycolytic 
• innervation 
• muscle fibre diameter
• motor neuron conduction 
• speed of contraction/relaxation 
• force production 
• fatigue 
• mitochondrial density 
• colour
- Fast glycolytic 
• innervation 
• muscle fibre diameter
• motor neuron conduction 
• speed of contraction/relaxation 
• force production 
• fatigue 
• mitochondrial density 
• colour

Answer 5

Slow twitch 
- Slow oxidative 
• innervation: a2 motor neurons 
• muscle fibre diameter: small
• motor neuron conduction: slow
• speed of contraction/relaxation: slow contraction and relaxation
• force production: low
• fatigue: resistant to fatigue
• mitochondrial density: high 
• colour: red

Fast twitch
- Fast oxidative glycolytic 
• innervation: a1 motor neurons
• muscle fibre diameter: largest
• motor neuron conduction: intermediate
• speed of contraction/relaxation: fast contraction, intermediate relaxation
• force production: intermediate
• fatigue: fatigable
• mitochondrial density: high
• colour: red
- Fast glycolytic 
• innervation: a1 motor neurons
• muscle fibre diameter: large
• motor neuron conduction: fast
• speed of contraction/relaxation: fast contraction and relaxation
• force production: high
• fatigue: fatigable
• mitochondrial density: low 
• colour: pale

Question 6

If you compared the muscle in a long distant marathon runner, a middle distance runner and a 100m sprinter, what would their muscle fibre composition look like? Mostly slow-twitch, half of both and mostly fast-twitch.

Answer 6

Mostly slow twitch
- Long-distance running

Half of both
- middle distance

Mostly fast twitch
- sprint

Question 7

Explain why different athletes would have muscles with greater proportions of either slow oxidative fibres or fast glycolytic fibres

Answer 7

In muscles specialized for maintaining low-intensity contractions for a long time w/o fatigue (marathon runner), there would be a higher proportion of slow oxidative fibred. Ex: muscles on the back and legs since they need to support the weight of the body.

In contrast, muscles adapted for performing rapid or foreceful movements do fatigue (leg muscle for sprinting). These muscles would have more fast glycolytic fibres.