Modulation of pain occurs by different ways in order to inhibit the pain sensation. First, the endogenous opiates system, the naturally occurring opioids present throughout the central nervous system (CNS). Second, the descending system which is a pathway from the brain for natural pain modulation (inhibition) at the spinal cord, consist of descending neurons that synapse with the ascending neuron lead to releasing of the neurotransmitter such as endogenous opiates, serotonin, and gamma amino butyric acid (GABA) that suppress the pain signals and this explain the use of antidepressants and opioids in the treatment of chronic pain. Under normal condition, balance presents between the excitatory (glutametergic) system and the inhibitory (GABAergic)
