Final answer:
The three parts of the reflex theory are sensory input, central processing, and motor output, which together allow for reflex actions such as the withdrawal from painful stimuli.
Step-by-step explanation:
The three parts of the reflex theory when considering a stimulation that creates a response are: sensory input, central processing, and motor output. A reflex, such as the withdrawal reflex from a painful stimulus, begins with sensory input, where sensory receptors detect the stimulus and send nerve impulses along sensory nerves to the spinal cord. The second part, central processing, occurs when these impulses are communicated to interneurons in the spinal cord where the information is processed and integrated with other stimuli, memories, or states. The final part is motor output, which involves the activation of motor neurons that cause a muscle to contract and produce a reflex action, like pulling your hand away from a hot surface.