15.1.12 Sliding Filament Model of Muscular Contraction

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