Edexcel A (SNAB) A Level Biology:复习笔记7.1.3 Muscle Contraction

Sliding Filament Theory

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

Exam Tip

There is a lot to remember here so take some time to go through it and ensure you understand the order of events.

Because muscles require a source of ATP for myosin heads to detach (and the muscle to stop contracting) this explains rigor mortis (stiffening of the joints and muscles of a body a few hours after death) as there is no ATP after death to detach the myosin heads, the muscles remain contracted!