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 theory

• Muscles cause movement by?contracting
• During muscle contraction,?sarcomeres?within myofibrils?shorten?as the?Z discs are pulled closer together
• It is?not?the myofilaments that contract as the myosin and actin molecules remain the?same length
• Myosin and actin filaments?slide over one another
• This is known as the?sliding filament theory of muscle contraction?and occurs via the following process:
  • An?action potential arrives?at the?neuromuscular junction?(a specialised synapse between a motor neuron nerve terminal and its muscle fiber)
  • 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 filaments
  • 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?ATPase?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 theory of muscle contraction

• Once muscle stimulation stops,?calcium ions leave?their binding sites on troponin molecules
• They are?actively transported?back to SR
• Without calcium ions bound to them, the?troponin molecules return?to their original shape
• This pulls the?tropomyosin?molecules in a position that?blocks the actin-myosin binding sites
• Since?no cross bridges?can form between actin and myosin,?no muscle contraction?can occur
• The?sarcomere will lengthen?again as actin filaments slide back to their relaxed position