reflex
n. any of a number of automatic, unlearned, relatively fixed responses to stimuli that do not require conscious effort and that often involve a faster response than might be possible if a conscious evaluation of the input were required. An example is the pupillary reflex. The concept of a reflex was first proposed by René Descartes, who supposed, before the distinction between sensory and motor nerves had been made, that the sensory impulses flowed to the spinal cord, where they were reflected back down to the muscles.