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Anxiety is said to worsen the effect of pain. Humans, along with other vertebrate animals possess benzodiazepine receptors- think of receptors as many tiny "docks" in our body which accept or allow the entry of ships that contain the substance benzodiazepine. Now benzodiazepine is a type of drug that has sleep-inducing, anti-anxiety, anti-convulsant, muscle relaxant characteristics. So these receptors allow the "entry" of benzodiazepene, and thus allow it to exert its effect in the body. The article tells you that even simple animals such as the earthworm possess the capability to accept this anti-anxiety substance through the receptor. From this, we can imply that animals have evolved or developed throughout the years but still keep this anti-anxiety receptor because it protects us from pain.