Final answer:
The structure that detects and responds to the stretch of a muscle during a rapid arm movement, like throwing a baseball, is the muscle spindle, a type of stretch receptor. Muscle spindles trigger a reflexive muscle contraction to prevent damage and maintain muscle tone. Golgi tendon organs, motor endplates, and synaptic clefts have different roles in muscle and nerve function.
Step-by-step explanation:
When an athlete's arm is rapidly stretched just prior to throwing a baseball, the structure that detects and responds to that stretch by reflexively increasing muscle activity is the muscle spindle. Muscle spindles are stretch receptors located within the muscle that detect changes in muscle length. If a muscle is suddenly stretched, the muscle spindles are activated, which sends a signal to the spinal cord and triggers a reflex that leads to the contraction of the muscle being stretched. This mechanism helps to prevent muscle damage from overstretching and maintains muscle tone.
Contrary to muscle spindles, Golgi tendon organs are involved in detecting the force of muscle contractions and prevent excessive contractions by causing the muscle to relax when force is too high, thus protecting the muscle from potential injury.
The motor endplate and synaptic cleft are involved in the transmission of signals from nerve to muscle but do not directly detect muscle stretch.