What are “Nervous System” and “Reactions?”
The central nervous system (CNS) is the part of human and animal nervous systems that consists of clusters of nervous cells (neurons) and their interactions; The CSN in invertebrates is a system of tightly connected nerve knots (ganglions) whereas in vertebrates (including humans) the CNS is made up of the brain and spinal cord.
Reaction is a voluntary (i.e. conscious) action initiated by certain stimuli. Reaction is determined by your psychology and helps you to perform certain actions faster and more effectively, e.g. win sport competitions, cope with dangerous work, be good at games, or work as a flight operations officer.