Just Another Teachin’ Tuesday: The HPA Axis

I have featured it several times in the past on chipur, but I wanted to do another run-through. And that’s because my mood and anxiety research always references it as a major contributor. Ladies and gentlemen, I bring you the HPA Axis.

The HPA axis involves the interaction of the brain structure known as the hypothalamus, the pituitary gland (just below the hypothalamus), and the adrenal glands (at the top of the kidneys). These three pieces of anatomy work together to regulate functions and states such as stress response, mood, digestion, immune response, sexuality, and energy usage.

Now, to really capture the purpose and functioning of the HPA axis one needs to become familiar with a little bit of physiology. It may appear complicated, but hang in there because it’s really important.

Corticotropin-Releasing Hormone (CRH) – also referred to as corticotropin-releasing factor (CRF), it’s produced and secreted by the hypothalamus and deeply involved with our autonomic and behavioral responses to stress. CRH stimulates the pituitary gland to secrete adrenocorticotropic hormone (ACTH).

Adrenocorticotropic Hormone (ACTH) – secreted by the pituitary gland, it stimulates the adrenal glands to ramp-up production of the mineralocorticoids and, of particular interest to us, the glucocorticoids.

Glucocorticoids – produced in the adrenal glands, this family of steroids is necessary for the regulation of energy metabolism, as well as immune and inflammatory responses. Cortisol is responsible for the vast majority of glucocorticoid activity.

Cortisol – produced in the adrenal glands and referred to as the “stress hormone,” this glucocorticoid is best known for ramping-up our physical response to stress. Motivated by stressors such as distressing life-events, trauma, excessive exercise, anxiety, and depression, cortisol preps us for action by stimulating norepinephrine (noradrenaline) to flip the switch on our fight/flight response. Obviously, situational secretion of cortisol is natural and necessary; however, when it’s secreted in the presence of chronic stress, all sorts of icky consequences may occur – high blood pressure, hardening of the arteries, diabetes, immune system inhibition, muscle atrophy, and osteoporosis.

Feedback Loops – most any system, biological or otherwise, has input and output capabilities. When the output of a system in some manner loops back to the system as input, and influences its functioning, a feedback loop has been established. A positive feedback loop increases system output, a negative feedback loop decreases it.

As it pertains to the functioning of the HPA axis, just as cortisol stimulates the activation of our fight/flight response, it also initiates a negative feedback loop, sending a signal to the hypothalamus and the pituitary gland to chill-out and relax. Well, as we consider the flow of the HPA axis, this serves to inhibit the production of CRH in the hypothalamus and the production of ACTH in the pituitary gland, leading to stable levels of cortisol. It also reduces noradrenergic (having to do with norepinephrine) activity, positively impacting levels of mood and anxiety.

So really, this is all about a cortisol checks-and-balances system, and given the negative impact of chronically hypersecreted cortisol, that’s a good thing. But never ever forget about the positive feedback loop created as cortisol signals norepinephrine and epinephrine (adrenaline) to get busy, because the result will be increased production of CRH, ACTH, and yet more cortisol. Just what the doctor ordered, right? Oh, our fight/flight response will be switched on as well.

So there it is – a bit of education on Just Another Teachin’ Tuesday. I’d love to read your comments. Won’t you?