What makes muscles become shorter and bones move?

Muscles become shorter and bones move due to the process of muscle contraction. When a muscle contracts, it generates force that pulls on the tendons, which are then attached to the bones. This force causes the bones to move, resulting in joint movement. The contraction and relaxation of muscles are controlled by the nervous system, which sends signals to the muscles, telling them when to contract and relax.

The process of muscle contraction involves the interaction of several proteins within the muscle fibers. When a nerve impulse reaches the muscle, it causes the release of calcium ions from the sarcoplasmic reticulum, the muscle's internal storage site for calcium. These calcium ions bind to specific proteins called troponin and tropomyosin, which control the access to the myosin-binding sites on the actin filaments.

The binding of calcium ions to troponin and tropomyosin triggers a conformational change in the actin filament, exposing the myosin-binding sites. Myosin heads, which are part of the myosin protein, then bind to these exposed sites on the actin filaments, forming crossbridges. These crossbridges generate force through a mechanism known as the power stroke, which involves the tilting of the myosin head and pulling of the actin filament towards the center of the sarcomere, the basic unit of muscle contraction.

The repeated formation and breaking of these crossbridges, along with the sliding of actin and myosin filaments past each other, result in the shortening of the muscle fibers, generating force that ultimately leads to bone movement. The relaxation of the muscle occurs when the nerve impulse stops, leading to a decrease in calcium ion concentration, which causes the detachment of myosin heads from the actin filaments, allowing the muscle to return to its resting length.

In summary, muscle contraction involves the interaction of calcium ions, troponin, tropomyosin, actin, and myosin proteins, leading to the formation of crossbridges and the sliding of filaments past each other. This process generates force that shortens the muscle fibers and causes bones to move, enabling various types of body movements.