HUBS 191 Lecture 9: Skeletal Muscle Contraction

• Skeletal Muscle: Contraction, Tension, and Fibre Types are discussed in Lecture 9 of HUBS 191

Learning Objectives:

• Describe the sequence of events that occur during a cross-bridge cycle.
• Explain the two determinants of skeletal muscle force generation.
• Describe the key differences between fast and slow muscle fibres.

Myofilaments

• The myofilament is primarily composed of actin and myosin.
• Actin forms the thin filament, a structural scaffold that runs along the myofilament.
• Myosin forms the thick filament and acts as a motor molecule.
• Myosin attaches to actin and generates force to pull.
• Actin and myosin bind together when calcium is present. This forms cross-bridges and allows contraction to occur.

Cross-Bridge Cycling

• A molecule of ATP binds to the myosin to prepare for releasing some energy to prime the myosin head.
• Burning ATP makes energy and some ADP waste. Energy is stored by changing the shape of the myosin, getting it ready to pull on actin again.
• The binding of ATP causes the myosin head to release actin, breaking the old cross-bridge.
• If calcium is present and bound to the myofilament, it is now time for the next contraction!
• The energized myosin head can now bind to actin, forming a cross-bridge.
• Myofilaments finish a power stroke (pulling on actin), so the myofilament has just finished a power stroke. The actin/myosin cross-bridges are still present.
• The energized myosin head attaches to actin, signifying the start of a new cross-bridge.
• Myosin uses stored energy to pull, causing the actin filament to slide.
• The sarcomere shortens and thus contraction occurs.

Muscle Tension

• Tension, or force, depends on two things: the number of muscle fibres recruited and the rate at which the muscle is stimulated.
• The number of fibres activated are is regulated by how many neurons are active at one time.
• A small number of active neurons produces low force from the muscle. The amount of force generally increases as more neurons are activated.
• Activating more fibres to make more force is called recruitment.
• A single action potential yields a pulse of Ca2+ release and contraction, called a twitch.
• Many action potentials fired in rapid sequence causes release of Ca2+ from the SR, actin-myosin interaction, and contraction.
• Maximal signaling and contraction capability is eventually reached, at which point the force plateaus, called tetanus.

Length-Tension Relationship

• Each muscle has an optimal length where it will be strongest. It will be weaker when either longer or shorter than that length.
• This is a result of the changing overlap between the actin and myosin filaments.
• Not enough overlap will not get many cross-bridges, too much and there's no space left to contract.

Muscle Fiber Types: Fast vs. Slow Fibres

• Fast fibres are mighty but fatigue quickly, while slow fibres supply steady force and are slow to tire.
• Fast fibers in cross section have a bigger cell size to produce more contractile power.
• Slow fibers in cross section are smaller cells not allowed to get tired, with more blood vessels.
• A comparison of fast and slow fibres:
  • Fibre Diameter: Large vs. Small.
  • Capillary Supply: Few vs. Many- to keep O2 + ATP Supply up.
  • Mitochondria Supply: Few (for storing energy) vs Many keeping constant energy up.
  • Colour: White vs Red.
  • Fatigue Resistance: Low vs High.
  • Time to Peak Tension: Fast! vs Slow...

Summary

• The myofilament proteins actin and myosin bind together when cellular calcium is high, creating cross-bridges.
• Myosin then uses the energy liberated from ATP to pull on the actin filament, causing a contraction.
• Skeletal muscle force generation depends on the number of fibres that contract (recruitment) and the frequency of stimulation, up to a maximum possible force (tetanus).
• Muscles rich in fast fibres produce lots of force quickly thanks to their energy reserves, but they tire out quickly too! Slow fibres are able to generate a lot of energy even while working, so they are difficult to fatigue.