The HPA (hypothalamic-pituitary-adrenal) axis is an endocrine system that controls the secretion of stress hormones (glucocorticoids) including cortisol. Exposure to high levels of glucocorticoids can be particularly detrimental for the aging hippocampus and is associated with decrements in learning and memory. Among the things that psychotherapy is best at, stress reduction is one of the most important things you can do for your overall health.