7.1.3 Muscle 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 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