Final answer:
Muscle spindles are the stretch receptors that sense the stretching of muscles and are found parallel to muscle fibers. They differ from Golgi tendon organs which detect the tension of muscle contractions.
Step-by-step explanation:
Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. They sense the stretch in the muscle parallel to the muscle fibers. The sensory receptors that sense how much a muscle is stretched, positioned parallel to muscle fibers, are known as muscle spindles. These are different from Golgi tendon organs, which are tension receptors that detect the force of muscle contraction. The muscle spindle is a key component in the regulation of muscle length, sending proprioceptive signals that run from the spinal cord to the cerebellum, which coordinates muscle contraction. Sarcomeres and myofibrils are related to muscle structure and contraction rather than sensory detection.