Thin filament is firmly attached to the Z-line of the myofibril.

The myofibril is the basic contractile unit of a muscle cell. It is composed of repeating units called sarcomeres, which are responsible for muscle contraction. The sarcomere is made up of thick and thin filaments, which are organized in a highly organized pattern.

Z-lines are structures that run perpendicular to the long axis of the myofibril and serve as anchoring points for the thin filaments. They mark the boundaries of each sarcomere and provide structural stability to the muscle fiber.

Thin filaments, also known as actin filaments, are composed of the protein actin. These filaments extend from the Z-line towards the center of the sarcomere. They are attached to the Z-line by a protein complex called the Z-disc, which connects the actin filaments to the Z-line.

The thick filaments, on the other hand, are composed of the protein myosin. They are located in the center of the sarcomere and partially overlap with the thin filaments. The myosin heads of the thick filaments interact with the actin filaments during muscle contraction.

While the thick filaments do interact with the Z-line through other proteins, they are not directly attached to it. The Z-line primarily serves as an anchor for the thin filaments and provides the structural framework for the sarcomere.

In summary, the thin filaments are firmly attached to the Z-line of the myofibril. This attachment allows for the organized movement of the actin filaments during muscle contraction. The Z-line plays a crucial role in maintaining the structural integrity of the sarcomere and ensuring the efficient transmission of force during muscle contraction.