Sliding filament theory is the mechanism by which muscles are thought to contract at a cellular level. A good understanding of skeletal muscle structure is useful when learning how sliding filament theory works.
What is sliding filament theory?
At a very basic level, each muscle fibre is made up of smaller fibres called myofibrils. These contain even smaller structures called actin and myosin filaments. These filaments slide in and out between each other to form a muscle contraction hence called the sliding filament theory!
The diagram above shows part a myofibril called a sarcomere. This is the smallest unit of skeletal muscle that can contract. Sarcomeres repeat themselves over and over along the length of the myofibril.
Here is a quick reminder of all the muscle structures involved:
- Myofibril: A cylindrical organelle running the length of the muscle fibre, containing Actin and Myosin filaments.
- Sarcomere: The functional unit of the Myofibril, divided into I, A and H bands.
- Actin: A thin, contractile protein filament, containing ‘active’ or ‘binding’ sites.
- Myosin: A thick, contractile protein filament, with protrusions known as Myosin Heads.
- Tropomyosin: An actin-binding protein that regulates muscle contraction.
- Troponin: A complex of three proteins, attached to Tropomyosin.
Here is what happens in detail. The process of a muscle contracting can be divided into 5 sections:
- A nerve impulse arrives at the neuromuscular junction, which causes a release of a chemical called Acetylcholine. The presence of Acetylcholine causes the depolarisation of the motor endplate which travels throughout the muscle by the transverse tubules, causing Calcium (Ca+) to be released from the sarcoplasmic reticulum.
- In the presence of high concentrations of Ca+, the Ca+ binds to Troponin, changing its shape and so moving Tropomyosin from the active site of the Actin. The Myosin filaments can now attach to the Actin, forming a cross-bridge.
- The breakdown of ATP releases energy which enables the Myosin to pull the Actin filaments inwards and so shortening the muscle. This occurs along the entire length of every myofibril in the muscle cell.
- The Myosin detaches from the Actin and the cross-bridge is broken when an ATP molecule binds to the Myosin head. When the ATP is then broken down the Myosin head can again attach to an Actin binding site further along the Actin filament and repeat the ‘power stroke’. This repeated pulling of the Actin over the myosin is often known as the ratchet mechanism.
- This process of muscular contraction can last for as long as there are adequate ATP and Ca+ stores. Once the impulse stops the Ca+ is pumped back to the Sarcoplasmic Reticulum and the Actin returns to its resting position causing the muscle to lengthen and relax.
It is important to realise that a single power stroke results in only a shortening of approximately 1% of the entire muscle. Therefore to achieve an overall shortening of up to 35% the whole process must be repeated many times. It is thought that whilst half of the cross-bridges are active in pulling the Actin over the Myosin, the other half are looking for their next binding site.
Looking at the diagram above again shows a stretched muscle where the I – bands and the H – zone is elongated due to reduced overlapping of the myosin and actin filaments. There would be reduced muscle strength because few cross-bridges can form between the actin and myosin.
Partially Contracted Muscle
The diagram above shows a partially contracted muscle where there is more overlapping of the myosin and actin with lots of potential for cross bridges to form. The I – bands and H – zone is shortened.
Fully Contracted Muscle
The diagram above shows a fully contracted muscle with lots of overlap between the actin and myosin. Because the thin actin filaments have overlapped there is a reduced potential for cross bridges to form again. Therefore, there will be low force production from the muscle.