Fig. 1.

The HPA axis and the autonomous nervous system receive circadian and stress-induced input and influence each other. A) The central circadian pacemaker in the suprachiasmatic nucleus (SCN) of the hypothalamus is entrained by light. Neurons from the SCN project to the hypothalamic-pituitary-adrenal (HPA) axis, and induce a rhythmic hormone secretion that ultimately stimulates rhythmic GC production in the adrenal gland, as indicated by the sine waves. At the same time, SCN projections reach the autonomic nervous system (ANS), resulting in rhythmic norepinephrine (NE) and epinephrine (E) output at the adrenal medulla and cortex. In addition, ANS innervation of the adrenal gland influences GC rhythms and the HPA axis also stimulates the release of NE and E. B) Stress activates both the HPA axis and the ANS, stimulating the release of GCs and catecholamines. HPA axis and ANS are interconnected so that both systems interact during the stress response.