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Wize University Physiology Textbook > Skeletal Muscle Physiology

Skeletal Muscle Physiology [more detailed]

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Physiology of Muscle Contraction


Types of muscle:
  1. Cardiac muscle - heart contraction
  2. Smooth muscle - involuntary contraction such as in gut, uterus, blood vessels
  3. Skeletal muscle - stripped and with many nuclei

Structure of Skeletal Muscle:
  • 1 muscle = several fiber bundles
  • 1 bundle = several muscle fibers (cells)
  • 1 muscle fiber = several myofibrils
  • 1 myofibril has several actin & myosin filaments arranged in end-to-end sarcomeres


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commons.wikipedia.org

Muscle Contraction
  • Sliding-filament model = thick filaments (myosin) move past thin filaments (actin)
  • Sarcomeres shorten while filaments remain same length
  • Requires actin-myosin binding sites, ATP and Ca2+Ca^{ 2+}



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Skeletal Muscle Contraction: The Big Picture

  • Caused by changes in intracellular calcium concentration
  • Each muscle fiber (cell) connects with only one motor neuron but each motor neuron can control many muscle fibers
  • Calcium levels rise --> muscle contracts
  • Calcium levels drop --> muscle relaxes

Contraction Cycle:

1. At rest – myosin binding sites on actin are blocked by tropomyosin

2. Action potential in motor neuron – induces release of acetylcholine into neuromuscular junction

3. ACh binds to nicotinic acetylcholine receptors on the muscle cell - induces influx of Na+ and action potential propagates

4. Voltage-gated Ca2+Ca^{ 2+}channels (called DHP receptor) open to allow calcium influx

5. This triggers the opening of Ryanodine receptors, allowing calcium to exit the sarcoplasmic reticulum into cytosol

6. Calcium binds troponin and pull off tropomyosin - myosin can bind to actin (cross-bridge formation)

7. Reset:Ca2+Ca^ {2+} pumped back into sarcoplasmic reticulum






commons.wikipedia.org

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Cross-Bridge Cycle:




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Length-Tension Relationship:

  • There's an optimal amount of overlapping between actin and myosin heads to generate the most muscle tension (force generated by a muscle)
  • Too much overlap --> not enough room for sliding
  • Too little overlap --> not enough cross-bridges can form









  • Summation: several action potential bursts add up to generate sustained contraction
  • Recruitment: recruitment of additional motor neuron-muscle fiber pairs helps increase muscle tension

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Muscle Energetics:

  • ATP needed – during contraction cycle (cross-bridge formation) and to pump Ca2+ from cytosol back into SR
  • ATP pool in fiber only enough for a few contractions
  • Creatine phosphate supplies a phosphate group to ATP


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Types of Skeletal Muscle Fibers:

  • Classified by ATP source and speed of muscle contraction:
  • Slow twitch fibers = contract slower but sustained longer, all oxidative
  • Fast twitch fibers = rapid but shorter contractions, glycolytic or oxidative

Muscle Fatigue

  • Protection from damage due to overuse
  • Several factors come into play for this and is not fully understood
  • Lactic acid buildup
  • Changes in ion gradients
  • CNS-mediated
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You have the chance to visit a cadaver lab in order to see human organs up close. You notice that some of the cadavers have their muscles tensed up, which is called rigor mortis. Why does this occur?

Rigor mortis occurs because of the lack of ATP that ensues after death. ATP is required to break the cross-bridges between actin and myosin, which allows the muscles to relax. In the absence of ATP, the muscles remain in contraction.
Which of the following are true regarding skeletal muscle?
What is incorrect about different skeletal muscle fibers?
Select the correct statement: