What structure of the sarcomere shortens during muscle contraction?

The Z lines and H- zone shortens during the muscle contraction.

Explanation:

Sarcomere is the area between the two Z lines. The Z- lines are present in the centre of I - band and H - zone is present in the A - band. The actin filaments which are thin filaments present in I- band and contracts, slide over the myosin filament during contraction.

The I - band, H- zones are became shortens as they are thin than the A- band. It is anisotropic band having thick myosin filaments. They are not flexible and remain in its constant shape during the muscle contraction.

After the muscle contraction, the A- band and the H- zone comes to their original shape. In other words, the sarcomere shortens and comes back during muscle contraction, and relaxation.

the dark variety, as they are not visible on the dark tree barks

explanation:

endo:

regulates body hormones, aldosterone, vasopressin

neural:

signal, regulates body using electric signals, reflex

endo has to do with hormones; aldosterone and vasopressin are both hormones. i also had this question and got it right : d