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