Sliding Filament Theory

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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.

ATP and Phosphocreatine

Muscle contraction is a very energetically demanding process so ATP needs to be made rapidly. This is done in the following ways:

Aerobic respiration

Aerobic respiration makes ATP through oxidative phosphorylation.
Aerobic respiration requires oxygen. It is mainly used for extended periods of low-intensity muscle use (e.g. jogging 5km).

Anaerobic respiration

Anaerobic respiration makes ATP by glycolysis and lactate fermentation.
Lactate is produced by lactate fermentation.
The build-up of lactate in the muscles can cause fatigue.
Anaerobic respiration is mainly used short periods of high-intensity muscle use (e.g. sprinting 100m).

Phosphocreatine

Phosphocreatine is a molecule that can supply ATP for muscle contraction.
During intense muscular effort, phosphocreatine donates phosphate to ADP to produce ATP. The ATP produced is used to sustain muscle contraction.
During low periods of muscle activity, ATP can be used to phosphorylate creatine back to phosphocreatine.
This process is anaerobic and produces no lactate but phosphocreatine is in short supply.