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5.7.2 – Explain the contraction of skeletal muscle in terms of the sliding filament theory (including the role of actin, myosin, troponin, tropomyosin, Ca2+, ATP).

Muscles are made from muscle fibres arranged into bundles. Each fibre is made from bundles of myofibrils, which are extremely long, cylindrical muscle cells.

The functional unit of contraction is the sarcomere. Muscle cells contain many sarcomeres arranged in parallel. The muscle cell takes on a characteristic banded appearance because of the regular arrangement of the sarcomeres. This is called striation.


A sacromere. Note the striated appearance of the muscle

The sarcomere contains overlapping actin and myosin. The myosin is often called the thick filament because the myosin heads make it appear thick. The actin is, therefore, the thin filament

 The process by which the thin filaments are pulled in towards each other by the myosin is called

cross-bridge cycling. It is how muscles contract.

CROSS-BRIDGE CYCLING:

  1. A nerve impulse arrives at the neuromuscular junction.
  2. The muscle cell is depolarized.
  3. Ca2+ is released from the sarcoplasmic reticulum inside muscle cells.
  4. Ca2+ bids to Troponin protein in the thin
  5. Troponin protein and Tropomyosin protein move position in the thin filament.
  6. Myosin binding sites are exposed on the thin
  7. Myosin heads of the thick filament stick to
  8. ATP (already bound to the myosin head) is hydrolysed causing the myosin head to pivot forwards in the
  9. As the head pivots the thick filament moves across the thin filament – muscle contraction
  10. ADP diffuses away from the myosin head leaving the ATP-binding site empty.
  11. New ATP binds & the myosin head & causes the myosin head to detach from the
  12. The myosin head re-cocks.
  13. The head rebinds further up the myosin.
  1. Repeat stages 7 to 13 until the [Ca2+] falls too low, when contraction
Key Point: ATP is required to release myosin  from  actin. If ATP levels drop (assuming Ca2+ is present) the myosin stays attached to the actin and the  muscle  stays permanently contracted. This is what causes rigor mortis