CIE A Level Biology (9700) 2019-2021

Revision Notes

15.1.11 Sliding Filament Model of Muscular Contraction

Sliding Filament Model of 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 myosin heads move and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
    • ATP hydrolysis occurs at the myosin heads, providing the energy required for the myosin heads to release the actin filaments
    • The myosin heads move back to their original positions and 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
    • The myosin heads hydrolyse ATP once more in order 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

Exam Tip

The sliding filament model can be difficult to visualise fully with diagrams. To help you more clearly understand the steps involved, try to find some animations or videos of the sliding filament model online to see the movement of the myosin heads and thin (actin) filaments during muscle contraction!

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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