Final answer:
Intrafusal muscle fibers are located within the muscle spindle capsule, which serves as a sensory receptor within the muscle and is involved in detecting changes in muscle length.
Step-by-step explanation:
Intrafusal muscle fibers lie within the muscle spindle capsule, which is a sensory receptor within the muscle. Muscle spindles are distributed throughout the muscles and are involved in the detection of changes in muscle length, playing a crucial role in the maintenance of muscle tone and the stretch reflex.
The endomysium is a thin layer of connective tissue that individually encases each skeletal muscle fiber but is not directly related to intrafusal muscle fibers or muscle spindles. Intrafusal fibers are distinct from the more abundant extrafusal muscle fibers which are responsible for the active contraction of the muscle.