Final answer:
Sensory neurons are responsible for transmitting information about muscle length to the CNS when the muscle spindle detects stretching of the extrafusal fibers.
Step-by-step explanation:
When extrafusal fibers attached to the muscle spindle are stretched, it is the sensory neurons that transmit information to the Central Nervous System (CNS) about the muscle's length. The muscle spindle is a stretch receptor that detects changes in muscle length and sends proprioceptive signals to the spinal cord, which then relays the information to the brain region responsible for coordinating muscle contractions. Muscle spindles, being different from the Golgi tendon organs which detect force, help in maintaining muscle length by initiating stretch reflexes, such as the knee reflex.