Sliding Filament Theory Contraction
Biology: Cambridge International A Level, topic 15.1.11
Sliding Filament Theory
The sliding filament theory explains how muscle contraction is coordinated in myofibrils. An overview of the steps involved are:
Depolarisation of the sarcolemma
- Muscle contraction is initiated when an action potential arrives at the muscle cells.
- The action potential depolarises the sarcolemma.
Contraction of the sarcomeres
- Depolarisation of the sarcolemma causes the myosin and actin filaments to slide over each other.
- The sliding movement causes the sarcomeres to contract.
Muscle contraction
- There are multiple sarcomeres along the length of myofibrils.
- As many sarcomeres contract simultaneously, the muscle fibres contract.
- Contraction of the muscle fibres causes the whole muscle to contract.
Muscle relaxation
- After the muscle has contracted, the sarcomeres relax.
- The filaments slide back over each other and the muscle relaxes.
Myosin Heads
The Sliding Filament Theory takes place due to globular heads on myosin filaments. The globular heads allow myosin and actin filaments to bind together and slide past each other.
Globular head
- Myosin filaments have globular heads.
- Globular heads can move back and forth.
- The movement of the globular heads is what allows actin and myosin filaments to slide past each other in muscle contraction.
Binding sites
- There are two binding sites on every myosin head:
- One site can bind to actin.
- One site can bind to ATP.
- There is also a binding site for the myosin heads on actin filaments. This is called the actin-myosin binding site.
Tropomyosin
- Tropomyosin is a protein that is located on actin filaments.
- Tropomyosin plays an important role in muscle contraction because it blocks the actin-myosin binding site when muscle fibres are at rest.
- When muscle fibres are stimulated, the tropomyosin protein is moved so that myosin heads can bind to the actin-myosin binding site.
- When actin and myosin bind, they can slide past each other to cause muscle contraction.
Test yourself on Sliding Filament Theory Contraction
Stages in Sliding Filament Theory
What causes the myosin and actin filaments to slide over each other?
Which of these do NOT bind to myosin heads?
Which of the following statements about myosin heads are true?