Which of the following structures is primarily responsible for muscle contraction?

The correct choice is the sarcomere, which is the fundamental unit of muscle contraction within myofibrils. Sarcomeres are organized structures that consist of actin (thin) and myosin (thick) filaments. The interaction between these filaments during the contraction process is what enables muscle movement.

When a muscle receives a signal to contract, calcium ions release within the muscle cells. This release enables the myosin heads to bind to the actin filaments, initiating a series of movements that pull the actin filaments closer together, thus shortening the sarcomere. This shortening translates to overall muscle contraction, as multiple sarcomeres within myofibrils contract in unison during this process.

In contrast, while myofibrils are composed of multiple sarcomeres and facilitate the contraction through these units, they themselves are not the primary structures responsible for contraction; rather, they function as the larger organizational structures that encompass many sarcomeres. Myocytes, or muscle cells, house myofibrils and sarcomeres but are not directly responsible for contraction mechanisms. Tendons, on the other hand, serve to connect muscle to bone and transmit the force generated during contraction, but they do not play a role in