Question

During contraction, what changes would you expect to see in the various lines, bands, and zones of the sarcomere, on the basis of the sliding filament hypothesis?

Answer 1

• The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins(actin and myosin) that slide past each other to generate movement.
• Each myofibril(muscle filament) has alternate dark and light bands on it.
• In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line.
• The thick filaments in the A’ band are also held together in the middle of this band by a thin fibrous membrane called ‘M’ line.
• During contraction, Utilising the energy from ATP hydrolysis, the myosin head binds to the exposed active sites on actin to form a cross bridge. This pulls the attached actin filaments towards the centre of ‘A’ band. The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction
• It is clear from the above steps, that during shortening of the muscle, i.e., contraction, the A-bands do not change their length (1.85 micrometres in mammalian skeletal muscle), whereas the I-bands and the H-zone shorten. This causes the Z lines to come closer together.