Sliding filament theory

Sliding filament theory

• explains how a muscle can shorten from a macroscopic perspective

• sliding filament theory: during contraction the thin filaments slide past the thick filaments so that actin and myosin overlap to a greater degree

• when a muscle shortens it shortens at the sarcomere and within the sarcomere the thin filaments attached to the z discs are sliding over the thick filaments so the Z discs move closer together during muscle contraction

Sarcomere shortening: relaxed

• Z discs: dark ling, easy to distinguish

• H zone is bisected by the M line

• can see I band and A band

• there are regions of the sarcomere where there is either no thick or thin filament

Sarcomere shortening: partially stimulated

• z discs in this image are closer together

• Z discs move towards the center by the thin filaments are sliding over the thick filaments and the thin filaments are attached to the Z discs

• if the thin filaments are sliding past the thick filaments, the distance between the z discs will decrease
  - as contraction continues, the distance between the z discs get even shorter

Sarcomere shortening: fully contracted

• when the sarcomere is fully contracted, you will no longer be able to see the I bands, but may be able to distinguish the M lines