FNDC5 and Cognitive impairment: The signalling pathways through which irisin improves cognitive impairment are complex and highly regulated processes, involving multiple signalling pathways such as the adenosine monophosphate-activated protein kinase (AMPK) signalling pathway, mitogen-activated protein kinase (MAPK) signalling pathway, nuclear factor-κB (NF-κB) signalling pathway, ERK-STAT3 signalling pathway, cAMP/PKA/CREB signalling pathway and Nrf2/HO-1 signalling pathway.