Which structures are responsible for muscle contractions?

Muscle contractions are primarily facilitated by the sarcomeres within muscle fibers. Sarcomeres are the smallest functional units of muscle tissue, composed of the proteins actin and myosin. These proteins interact in a process known as the sliding filament theory, where myosin heads attach to actin filaments, causing the filaments to slide past each other, resulting in contraction.

When a muscle fiber receives a signal from a neuron, it triggers an electrical impulse that leads to the release of calcium ions, which bind to proteins on the actin filaments. This action exposes binding sites for the myosin heads. Once activated, the myosin heads pull the actin filaments towards the center of the sarcomere, shortening the muscle fiber and thus producing muscle contraction.

While myofibrils are also crucial for muscle contraction, they are structures that contain multiple sarcomeres in sequence. Neurons play a role in transmitting signals that initiate muscle contractions but do not directly cause the contraction themselves. Ligaments, on the other hand, connect bones to bones and do not play a role in muscle contraction. Thus, the proper understanding of sarcomeres' role in the contraction process identifies them as the specific structures responsible for this function.