Myofibril contractile subunits,the sarcomeres,are bounded on each end by a disc of protein called the

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The Z line, also known as the Z disc, is a protein band that marks the boundary of each sarcomere, the smallest contractile unit of muscle fibers. The actin filaments from opposite sides of the sarcomere extend towards the Z line and are anchored there by specialized proteins, while the myosin filaments remain in the center of the sarcomere, causing contraction when they interact with the actin filaments. The other choices, H line, I line, I zone, and Z frame, are either incorrect or do not exist in muscle anatomy.

Myofibril Contractile Subunits,the Sarcomeres,are Bounded on Each End by a Disc