Cannabinoids are a class of chemicals, with structure or activity similar to those found naturally in the cannabis plant. Humans have endogenous (naturally occuring) cannabinoid receptors throughout our bodies and brains, which were discovered after those in the plant. Thus the human receptors are named after the psychoactive principles in marijuana. Novel synthetic cannabinoids and cannabinoid receptor agonists gained popularity in the late 2000s when they began being sold in herbal smoking blends as cannabis replacements.