Pain is one of the three existing physiologic types of somatic senses, the other two being the mechanoreceptive somatic sense and the thermo-receptive sense. The pain sense is activated by tissue damage, and therefore constitutes an important protective mechanism of tissue integrity. According to pain intensity, pain is classified as acute pain, such as that caused by cuts or burns, or chronic pain, usually caused by progressive tissue destruction, such as chronic inflammation or infections.

Skin and other tissues contain embedded free nerve ends, known as pain receptors, which are excited by three different types of stimuli: thermal, mechanical, and chemical. Acute pain is usually caused by thermal stimuli (i.e., burns) or mechanical stimuli (bruises, cuts, etc), whereas the chronic pain may be caused by any of the three types of stimuli. Pain receptors conduct pain signal through two different pathways into the central nervous system, one...