The functional unit of a muscle fibril, lying between two successive Z-lines, is the

1. A muscle fibril is built of repeating contractile units. 2. The portion between two successive Z-lines is one such unit. 3. That unit is called a sarcomere, and it is the functional unit of contraction. 4. Sarcolemma is the membrane and sarcoplasm the cytoplasm of the muscle fibre. _Source: syllabus Biology Ch17 'Locomotion and Movement'_