Fig. (2).

The dorsal horn is the area that receives the painful stimuli; it is divided into five Rexed laminae. The 5-HT1, 5-HT2, 5-HT3 and 5-HT7 receptors are mainly located in the lamina I and II. The receptor involved in antinociception (5-HT1A, 5-HT2 and 5-HT7) receptors are mainly expressed in the lamina I, also known as marginal zone. This lamina is mostly innervated by A-δ fibers. In the lamina II, substantia gelatinosa, the receptors involved in pronociception (5-HT1B, 5-HT3 and 5-HT7) are expressed. This lamina is innervated by C fibers.