Final answer:
The muscle spindle is the sensory receptor in muscles that detects changes in length to help regulate contraction, preventing muscle damage by initiating protective reflexes.
Step-by-step explanation:
The sensory receptor within a muscle that detects changes in length and helps regulate contraction is the muscle spindle. Muscle spindles are proprioceptive stretch receptors that lie within the muscle and are involved in sensing the amount of stretch or lengthening of muscles.
They are crucial for maintaining optimal muscle length and preventing muscle damage by initiating reflexes that can prevent overstretching or over-contraction.