CNS is brain and spinal cord. PNS are the sensory and motor neurons. The PNS breaks down further into the autonomic and the sensory-somatic nervous system. The autonomic breaks down further into the sympathetic and parasympathetic nervous systems. It is very complex but to answer your question when you touch a hot stove sensory neurons (PNS) send a message to the brain (CNS) and the brain process that the hand is being injured and reacts by sending motor neurons (PNS) to tell the hand to pull away.