Fig. 2. The sympathetic nervous system (SNS) and pain. Inflammation activates immune dendritic cells. β-2 receptors are downregulated and α-1 receptors are up-regulated on these immune cells. Following nerve injury, functional adrenoreceptors are expressed on peripheral nociceptors. Activation of the SNS increases the level of norepinephrine (NE), which activates α-adrenoreceptors on the afferent fibers, and releases nerve growth factor (NGF). NGF sensitizes peripheral nociceptors through trk A receptors.