Final answer:
The sarcomere changes during muscle contraction by the slipping of actin towards the M line, bringing Z discs closer, diminishing the H zone and narrowing the I band, while the A band's width stays the same.
Step-by-step explanation:
The question is asking to describe the structure of a sarcomere in both its relaxed and contracted states, with emphasis on the changes that occur during muscle contraction according to the Sliding Filament Model. In a relaxed sarcomere, the key components include the Z discs, I band, H zone, A band, and M line. The A band, identified by overlapping thick (myosin) and thin (actin) filaments, maintains its width, while the H zone and I band, which are lighter regions, appear more pronounced. However, during contraction, the Z discs move closer together as the actin filaments slide towards the M line pulling the Z discs inward. This movement causes a reduction in the distance between the Z discs, essentially shortening the sarcomere. The H zone all but disappears and the I band becomes narrower, while the A band remains consistent in width. This is because while the actin and myosin filaments overlap more, they do not change in length; instead, they slide past each other due to the myosin heads binding to the actin and pulling inward, a process that requires ATP for energy.