Final answer:
Z discs define the boundaries of sarcomeres. Each sarcomere is the space between two consecutive Z discs. Sarcomeres are the functional units of skeletal muscle.
Step-by-step explanation:
The features that define the boundaries of sarcomeres are Z discs. Each sarcomere is the space between two consecutive Z discs. Z discs mark the border of sarcomeres, which are the functional units of skeletal muscle. A myofibril is composed of many sarcomeres running along its length.