6-3-4-muscle-contraction

The sliding filament theory of muscle contraction

• During muscle contraction, actin and myosin filaments slide over each other, causing sarcomeres to shorten

• The structure of actin and myosin proteins enables this process:

  • Myosin filaments are fibrous proteins with a globular head

  • Actin filaments have:

    • actin-myosin binding sites to which the myosin heads can bind

    • associations with another protein, tropomyosin, which blocks the binding sites when the muscle is relaxed

• The process by which actin and myosin cause muscle contraction is as follows:

  1. An action potential arrives at a neuromuscular junction, and calcium ions are released from the sarcoplasmic reticulum

  2. Calcium ions cause tropomyosin proteins to change position on the actin filaments, exposing the actin-myosin binding sites

  3. Myosin heads bind to actin, forming cross-bridges between the two types of filament

  4. The myosin heads bend, releasing ADP and inorganic phosphate and pulling the actin filaments towards the centre of the sarcomere

     • This is known as the power stroke

     • Note that this bending occurs spontaneously with the release of ADP and Pi, and does not occur as the direct result of ATP hydrolysis

  5. ATP can now bind to the myosin heads, causing them to release from actin

  6. ATP hydrolase enzyme hydrolyses the ATP, providing energy that allows the myosin heads to reset to their original positions

  7. The myosin heads are then able to bind to new binding sites on the actin filaments, and the process repeats

• The process described above continues for as long as the muscle fibre is stimulated by nerve impulses

• When a muscle is no longer stimulated:

  • calcium ions are taken up again by the sarcoplasmic reticulum,

  • the actin-myosin binding sites are blocked and myosin can no longer bind to actin

  • the filaments can be pulled apart by the action of an antagonistic muscle

ATP and phosphocreatine

• A supply of ATP is required for muscle contraction

  • Energy is needed to reset the position of myosin heads for the contraction process to repeat

  • Calcium ions return to the sarcoplasmic reticulum after contraction via active transport

• ATP for muscle contraction can come from:

  • aerobic respiration; this requires a sufficient oxygen supply, so it can only take place fast enough to sustain low-intensity exercise

  • anaerobic respiration; this can produce small amounts of ATP very quickly, but can only be sustained for short periods due to the build-up of lactate

  • phosphocreatine; a molecule stored by muscles that can be used for the rapid production of ATP over short periods

    • A phosphate ion from phosphocreatine is transferred to ADP

ADP + phosphocreatine → ATP + creatine