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Myofibrils are the fundamental contractile units of muscle cells, also known as muscle fibers. They are rod-like structures that run parallel to the long axis of the muscle fiber and are composed of repeating units called sarcomeres. These sarcomeres are the smallest functional units of muscle contraction.

The internal structure of a myofibril is highly organized and responsible for the striated appearance of skeletal and cardiac muscle. This striation arises from the alternating arrangement of two primary protein filaments: actin and myosin. Actin filaments are thinner and lighter, while myosin filaments are thicker and darker. The precise overlap and interaction of these filaments within the sarcomere generate force.

Within a sarcomere, the actin filaments are anchored at the Z-lines, which define the boundaries of each sarcomere. The myosin filaments are located in the center of the sarcomere. The region where only myosin is present is called the H-zone, and the region where both actin and myosin overlap is the A-band. The I-band contains only actin filaments.

Muscle contraction occurs through the sliding filament theory. When a muscle is stimulated, myosin heads bind to actin filaments and pull them towards the center of the sarcomere, shortening the sarcomere and thus the entire myofibril. This process requires energy in the form of ATP and is regulated by calcium ions. The coordinated contraction of millions of myofibrils results in the overall movement of a muscle. Myofibrils are essential for all forms of muscle movement, from voluntary actions to the involuntary beating of the heart.