ABSTRACT
This chapter explores the feedback in the human stress-response pathway whose output is the hormone cortisol. The feedback loop also can act as a seasonal oscillator, providing a hormonal set-point for each season. The chapter also explores the two-gland oscillator as a possible origin for the timescale of mood fluctuations in bipolar disorder. The two-gland oscillator allows the HPA stress pathway to synchronize with the seasons providing hormonal set-points for different times of the year. The hormone circuit that controls cortisol is composed of a series of three glands, which we will denote H, P, and A, explaining its name, the HPA axis. H is activated by emergencies and stresses, including pain, low glucose, low blood pressure, inflammation, and psychological stresses. Like many negative feedback loops in which both arms have similar timescales, the HPA circuit can show damped oscillations.
