Illustration shows part of a tubular myofibril, which consists of many sarcomeres. Zigzagging lines, called Z lines, run perpendicular to the fiber. Each sarcomere starts at one Z line and ends at the next. A straight perpendicular line, called an M line, exists halfway between each Z line. Thick filaments extend out from the M lines, parallel to the length of the myofibril. Thin filaments extend from the Z lines, and extend into the space between the thick filaments.