Structure of thick & thin filaments in a myofibril

• The?thick filaments?within a myofibril are made up of?myosin molecules
  • These are?fibrous protein molecules?with a?globular head
  • The?fibrous part?of the myosin molecule?anchors?the molecule into the thick filament
  • In the thick filament, many myosin molecules lie next to each other with their?globular heads all pointing away from the M line
• The?thin filaments?within a myofibril are made up of?actin molecules
  • These are?globular protein molecules
  • Many actin molecules link together to form a?chain
  • Two?actin chains?twist?together to form one?thin filament
  • A?fibrous?protein known as?tropomyosin?is twisted around the two actin chains
  • Another protein known as?troponin?is attached to the actin chains at regular intervals

How muscles contract – the sliding filament model

• Muscles cause movement by?contracting
• During muscle contraction,?sarcomeres within myofibrils shorten as the Z discs are pulled closer together
• This is known as the?sliding filament model of muscle contraction?and occurs via the following process:
  • An?action potential arrives?at the neuromuscular junction
  • Calcium ions?are?released?from the?sarcoplasmic reticulum?(SR)
  • Calcium ions?bind to troponin molecules, stimulating them to?change shape
  • This causes?troponin?and?tropomyosin?proteins to?change position?on the actin (thin) filaments
  • Myosin binding sites are exposed?on the actin molecules
  • The?globular heads of the myosin molecules bind?with these sites, forming cross-bridges between the two types of filament
  • The?myosin heads move and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
  • ATP hydrolysis?occurs at the myosin heads, providing the?energy?required for the myosin heads to?release?the actin filaments
  • The myosin heads move back to their?original positions?and bind to?new binding sites?on the actin filaments,?closer to the Z disc
  • The myosin heads move again, pulling the actin filaments?even closer the centre of the sarcomere, causing the sarcomere to?shorten?once more and pulling the Z discs closer together
  • The myosin heads hydrolyse ATP once more in order to?detach?again
  • As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP, this process?repeats?until the muscle is?fully contracted

The sliding filament model of muscle contraction