Sliding Filament Theory

The Sliding Filament Theory is used to describe the mechanism of muscle contraction.

Inside each muscle cells are two key filaments; the thick and thin filaments. The thick filament contains a protein called Myosin, whilst the thin filament is made from a protein called Actin.

Muscle cells can be broken down into sections called Sarcomeres. In one Sarcomere there will be overlapping Myosin and Actin filaments.

The Myosin filament has structures called Myosin heads, which point outwards from the filament like hair on a hairy man’s arm. A molecule called ATP can attach to Myosin heads and this causes them to change their position. When ATP is attached , the Myosin heads are in the right place to attach to Actin.

Once Myosin has attached to Actin the ATP is released. This means that the Myosin head wants to move back to it’s original position. However, because both filaments are attached to each other, the Myosin head ‘pulls’ the Actin filament along with it. The Myosin head can now detach, meaning that the process can occur again.

This process repeated many times over many sarcomeres causes the contraction of a whole muscle. This is as the shortening of all the sarcomeres means that the length of the whole muscle has got shorter too.

Muscle contraction does not occur when it isn’t wanted. This happens as a third protein called Tropomyosin normally blocks the binding site for Myosin on the Actin filament. The only way this can be exposed is if Calcium ions are present in the muscle. When they are, these attach to Tropomyosin, changing it’s shape, and therefore allowing myosin to bind.

Calcium ions are present only when an action potential has occurred. (An action potential is just another way of saying a nerve impulse)

In summary:

  • Muscles contain two key filaments, called Actin and Myosin
  • Each muscle cell can be split up into smaller units called Sarcomeres
  • One Sarcomere is the distance between two Z-discs, which are where the Mysosin filaments are attached
  • Sacrcomere shortening is the basis of muscle contraction
  • Sarcomeres shorten by Myosin heads ‘pulling’ the Actin filaments towards the centre of the Sarcomere
  • In order for this to happen, the Myosin requires ATP
  • Unwanted muscle contraction is prevented as a protein called Tropomyosin blocks the site at which Myosin heads bind to Actin
  • Calcium ions attach to Tropomyosin when muscle contraction is required, which leads to a change in shape. This change in shape allows for the Myosin to attach to the Actin, pulling it along.