First let us see what Z line, H zone, I band and A band are.



A band -

It is a dark band present on myofibril. It is also known as anisotropic band.



I band

It is a light band present on myofibril. It is also known as isotropic band.



H band

It is a ligher area present at the centre of A band. It also known as Hensen's zone.



Z line

It is a dark line that passes through I band. It is also known as Zwischenscheibe line.



During muscle contracting, muscle fibres shorten, hence, -



1. Z line - pulled inwards (hence sarcomere shortens)

2. H zone - narrows

3. I band - length gets reduced

4. A band - length remains unchanged

Hloo





Upon muscle contraction, the A-bandsdo not change their length (1.85 micrometerinmammalian skeletalmuscle), whereas the I-bandsand theH-zoneshorten. This causes the Z lines to come closer together. The protein [tropomyosin] covers the myosin binding sites of the actin moleculesinthemusclecell.



Good luck!