MICROANATOMY

Detailed explanation-1: -Muscle contraction is brought about by sliding movement of actin filaments over myosin filaments. When a muscle fibril contracts, it’s A band remains constant and I band shortens. H – Zone disappears. M – line and Z – line also come closer.

Detailed explanation-2: -During muscle contraction, the I band reduces in size as the overlap between the thick filaments and the thin filaments increases. The H zone also decreases in length because of the same reason.

Detailed explanation-3: -The H zone-the central region of the A zone-contains only thick filaments (myosin) and is shortened during contraction. The H zone becomes smaller and smaller due to the increasing overlap of actin and myosin filaments, and the muscle shortens. Thus when the muscle is fully contracted, the H zone is no longer visible.

Detailed explanation-4: -Each sarcomere has a central A band made of thick myosin filaments and two half I bands made of thin actin filaments on either side of the thick filaments leaving the central part of the thick filaments. This central part of thick filament, not overlapped by thin filament is called H zone.

Detailed explanation-5: -Within the A band, there is a relatively brighter central region, called the H-zone in which there is no actin / myosin overlap, when the muscle is in a relaxed state.