5.5.16 The Sliding Filament Model

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 actin and myosin filaments move past each other
• 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 into the sarcoplasm by diffusion
  • Calcium ions bind to troponin molecules, stimulating them to change shape
  • This causes troponin and tropomyosin proteins to change position on the actin 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 bend and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
    • The movement of the myosin heads is known as the power-stroke
    • When the myosin heads bend, it releases a molecule of ADP
  • ATP binds to the myosin head, allowing it to detach from actin
  • The myosin head acts as an ATPase enzme, hydrolysing ATP into ADP and Pi; the energy released during this reaction allows the myosin head to return to its original position
  • The myosin head can now bind to a new binding site on the actin filaments
  • The myosin heads move again, pulling the actin filaments even closer to the centre of the sarcomere and causing the sarcomere to shorten further
  • 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

The role of ATP and phosphocreatine

• A supply of ATP is required for muscle contraction
  • ATP binding allows myosin to detach from actin and ATP hydrolysis allows the myosin heads to return to their original shape; both of these processes are essential to allow the process described above to repeat
  • The return of calcium ions to the sarcoplasmic reticulum occurs via active transport
• Resting muscles have a small amount of ATP stored that will only last for 3-4 seconds of intense exercise
• The mitochondria present in the muscles fibres are able to respire aerobically and produce ATP but this is slow and can take a considerable amount of time
  • Anaerobic respiration, which is faster than aerobic, still takes 10 seconds before it even begins to produce any ATP
• Phosphocreatine is a molecule stored by muscles that can be used for the rapid production of ATP
  • A phosphate ion from phosphocreatine is transferred to ADP
  • ADP + phosphocreatine → ATP + creatine
• Different muscle fibre types contain different limited amounts of phosphocreatine
• It allows for muscles to continue contracting for a short period of time until the mitochondria are able to supply ATP
  • For example, it would be utilised by the muscles of a 100m sprinter as sprinting involves an intense level of muscle contraction
• For prolonged activity, once the supply of phosphocreatine has been used up then the rate of muscle contraction must equal the rate of ATP production from both aerobic and anaerobic respiration