Final answer:
Muscle spindles are sensory receptors that detect changes in muscle length and help maintain optimal muscle function through the stretch reflex.
Step-by-step explanation:
Muscle spindles are a type of sensory receptor that detects changes in muscle length. They are proprioceptive stretch receptors that reside within muscles and are responsible for eliciting the stretch reflex when a muscle is stretched. In addition to preventing muscle tearing and over-contraction, muscle spindles initiate a neurological response that results in a slight contraction to maintain optimal muscle length for efficient movement. These receptors play a critical role, especially during movement and posture adjustments, by sending information to the spinal cord and cerebellum without involving conscious awareness.