What constitutes each myofibril?

Myofibrils are the long, thread-like structures within muscle cells that are essential for muscle contraction. Each myofibril is composed of repeating units called sarcomeres, which contain the filaments necessary for contraction. Sarcomeres themselves are made up of myofilaments, specifically the thick myofilaments composed of myosin and the thin myofilaments primarily made of actin.

When considering what constitutes each myofibril, the answer revolves around the broader structure of myofibrils and their essential components. The myofilaments within the sarcomeres interact with each other to facilitate contraction, making myofilaments the defining elements that compose the functional units of myofibrils. Therefore, myofibrils can be understood as being constructed from these myofilaments that work together in the sarcomeres to produce muscle contractions, embodying the core functional units of muscle fibers. This highlights the importance of myofilaments in understanding muscle physiology and contraction mechanics.