Which mechanoreceptor is responsible for detecting changes in muscle length?

The muscle spindle is the mechanoreceptor specifically responsible for detecting changes in muscle length. These specialized sensory receptors are located within the belly of muscles and are sensitive to stretch. When a muscle is stretched, the muscle spindle sends signals to the central nervous system to provide information about the level of muscle stretch and the rate at which the stretch occurs.

This ability to sense changes in muscle length plays a crucial role in muscle coordination and reflex actions. For example, when a muscle is stretched too quickly, the muscle spindle can trigger a reflex response to prevent potential injury by causing the muscle to contract.

The other options refer to different types of mechanoreceptors or sensory receptors with distinct functions. The Golgi tendon organ, for instance, monitors tension within the tendon rather than changes in muscle length. Proprioceptors encompass a broader category that includes various types of receptors that provide body position and movement information but do not specifically relate to muscle length changes. Free nerve endings are involved in sensing pain and temperature rather than muscle length. Thus, the muscle spindle's specific role in detecting changes in muscle length is what makes it the correct answer.