muscle contraction

• myofribrils are the contractile unites of skeletal muscle and contain 2 types of protein filament:

  thin filaments, which are aligned to make up the light band and are held together by the z line

  thick filaments that make up the dark band

  
  

  the thick and thin filaments overlap, but in the middle of the dark band there is no overlap and and this is called the H zone. The distance between the two z lines is called a sarcomere, this is the functional unit of the muscle.
• the thin filaments are actin, each filament consists of two chains of actin subunits twisted around eachother. wound around the actin is a molecule of tropomyosin to which are attached globular troponin. each troponin complex consists of three polypeptides; one binds to actin, one to tropomyosin and the third to calcium when available. tropomyosin and toponin are part of the mechanism that controls muscular contraction. at rest these molecules cover binding sites to which the thick filaments can bind.
• each thick filament consists of a bundle of myosin molecules. each myosin molecule has 2 protruding heads that stick out at the end of each molecule. these heads are mobile and can bind to the actin when binding sites are exposed.
• During contracting, the light band and H band get shorter- ths, the Z lines move closer together and the sarcomere gets shorter. making the thick and thin filaments slide past eachother.
• the thick and thin filaments slide past eachother during a muscle contraction; the movement of myosin heads causes this. when the muscle is stimulated, the tropomyosin is moved aside, this exposes the binding sites on the actin. the myosin heads attach to the actin and move, thus causing the actin to slide past the myosin