Fig. 1.

UVR activates neuro-immuno-endocrine pathways leading to the regulation of the local and global homeostasis. UVR energy absorbed by the skin induces functional changes directly (chromophores, ROS/RNS, and physicochemical alterations) or indirectly (cellular production and release of biomolecules) and is coordinated by the local neuro-immuno-endocrine system to regulate homeostasis. Locally released molecules or activated immune cells (cellular messengers) can enter systemic circulation and act on brain, endocrine and internal organs. Furthermore, nerve ending in the skin is stimulated directly or indirectly with further neurotransmission to the brain and central organs. All of these elements act in a coordinated manner to regulate systemic homeostasis. UV: ultraviolet radiation, ROS: reactive oxygen species, RNS: reactive nitrogen species.