6.3.4 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 formation of the cross-bridges causes the myosin heads to spontaneously bend (releasing ADP and inorganic phosphate), pulling the actin filaments towards the centre of the sarcomere and causing the muscle to contract a very small distance
  • ATP binds to the myosin heads producing a change in shape that causes the myosin heads to release from the actin filaments
  • The enzyme ATP hydrolase hydrolyses ATP into ADP and inorganic phosphate which causes the myosin heads to move back to their original positions (this is known as the recovery stroke)
  • The myosin heads are then able to 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
  • ATP binds to the myosin heads once more in order for them 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

The role of ATP and phosphocreatine

• A supply of ATP is required for muscle contraction
  • Energy is needed for the return movement of myosin heads that causes the actin filaments to slide
  • The return of calcium ions back into 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 aerobically respire 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