What structure in the muscle detects changes in fiber length and signals these changes to the spinal cord?

Muscle spindles are specialized sensory receptors located within the belly of muscles that play a critical role in proprioception, which is the body's ability to sense its position and movement in space. These structures are sensitive to changes in muscle fiber length and the rate of change in length, which allows them to detect both static and dynamic stretches of the muscle.

When a muscle is stretched, the muscle spindles are activated and send signals to the spinal cord via afferent nerve fibers. This information is essential for reflexive actions, such as the stretch reflex, which helps maintain muscle tone and posture by initiating a contraction in response to the stretch. Hence, the correct answer highlights the key function of muscle spindles in monitoring muscle length and facilitating communication between the muscles and the central nervous system regarding changes in muscle dynamics.

The other structures listed have different functions; the Golgi tendon organ, for instance, is more involved in sensing tension in the muscle tendons, while sarcomeres and myofibrils are basic structural units of muscle tissue involved in contraction.