What is the basic unit of a myofibril that contains myosin and actin?

The basic unit of a myofibril that contains the proteins myosin and actin is known as a sarcomere. Sarcomeres are the repeating structural units within myofibrils and are responsible for muscle contraction. They contain thick filaments made of myosin and thin filaments made of actin. When a muscle contracts, the sarcomeres shorten, pulling the Z-discs closer together, which ultimately leads to the whole muscle shortening or contracting.

Each sarcomere is delineated by Z-lines, and the arrangement of these sarcomeres creates the striated appearance characteristic of skeletal muscle. The coordinated contraction of numerous sarcomeres within a myofibril generates the mechanical force necessary for movement.

In contrast, myofibrils are the larger structures that are comprised of many sarcomeres aligned end to end, while fascicles refer to bundles of muscle fibers, and muscle bundles encompass groups of fascicles. Understanding the hierarchical structure of muscle composition helps clarify the role of each component in muscle function.