Edexcel International A Level Biology

Revision Notes

7.11 The Process of Skeletal Muscle Contraction

The 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 filaments 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 fibre) 
    • 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
      • Myosin heads bend, pulling the actin filaments towards the centre of the sarcomere and causing the muscle to contract a very small distance; this bending of the myosin heads is known as the power stroke
    • ATP plays an important role in this process
      • The binding of ATP to the myosin heads produces a change in shape of the myosin heads that allows them to detach 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 binding of the myosin heads to their new binding site causes the release of ADP and phosphate and results in a new power stroke
    • The myosin heads move again, pulling the actin filaments even closer to 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

Sliding filament model of muscle contraction (1)_Sliding filament model of muscle contraction (2)_1

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 the 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!

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Author: Marlene

Marlene graduated from Stellenbosch University, South Africa, in 2002 with a degree in Biodiversity and Ecology. After completing a PGCE (Postgraduate certificate in education) in 2003 she taught high school Biology for over 10 years at various schools across South Africa before returning to Stellenbosch University in 2014 to obtain an Honours degree in Biological Sciences. With over 16 years of teaching experience, of which the past 3 years were spent teaching IGCSE and A level Biology, Marlene is passionate about Biology and making it more approachable to her students.