Final answer:
Muscle spindles are composed of encapsulated nerve endings, whereas free nerve endings, tactile discs, and hair follicle receptors are not encapsulated.
Step-by-step explanation:
Within the options provided, muscle spindles are composed of encapsulated nerve endings. Muscle spindles are sensory receptors within the belly of a muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via sensory neurons.
This information can be used to determine the position of body parts and is integral to proprioception and the stretch reflex. In contrast, free nerve endings are unencapsulated and are sensitive to pain, temperature, and light touch. Tactile discs, also known as Merkel cells, are slow-adapting, unencapsulated receptors that respond to light touch. Hair follicle receptors are rapidly adapting, unencapsulated nerve endings wrapped around hair follicles detecting movement and deflection of the skin.