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Stress axis

October 15, 2019

In this video we’re going to talk about the
stress axis which is an example of a tripartite axis meaning it has three parts and the
three parts are three different endocrine glands. The
first one is the hypothalamus which is labeled as HT and the second one is the anterior pituitary gland, APG, and the third is the adrenal cortex right here which is the outer part of the adrenal gland and the adrenal glands are located just on top each kidney. So we have the adrenal gland. The final hormone secreted from third part of this tripartite axis, cortisol, is
controlled by two other hormones that are upstream in the pathway, and what starts this whole pathway to the secretion of cortisol is stress. so stress in many different types of forms activates this process and stress is integrated in the hypothalamus here and when the hypothalamus perceives different types of stress such
as it could be physical stress emotional stress then it starts secreting a hormone called corticotropin releasing hormone CRH travels through a specialized part of the circulatory system that I will try and draw here called the
hypothalamic-anterior pituitary portal system and it’s a network of capillaries that connect the hypothalamus to the anterior
pituitary gland so the CRH travels through here to the anterior pituitary where it finds its receptors on specialized cells in the anterior pituitary called corticotropes and when CRH binds with those receptors the anterior
pituitary secretes a hormone called adrenocorticotropic hormone (ACTH) so that comes from the anterior pituitary. ACTH is secreted
into the blood- just the regular systemic blood stream
where it travels all over the body and where happens to find receptors are in the adrenal cortex and when ACTH binds with receptors in the adrenal cortex those cells secrete the final hormone which is cortisol. Cortisol is a steroid so when ACTH binds with receptors on those cortisol producing cells the cortisol is actually synthesized and released because cortisol being steroid or fat-soluble hormone can’t be stored cells well then cortisol
goes into the systemic circulation and it has receptors on many different cell
types including immune system cells and so it actually works to suppress the immune system which is why if you’ve ever had
inflammation you might have had injection of prednisone which is a synthetic form of cortisol. There are receptors in the liver, receptors in muscles and also in adipose tissue. Receptors for cortisol are collectively called glucocorticoid receptors and we see catabolic types of reactions happening when cortisol binds with these tissues so gluconeogenesis, or making new glucose happens in the liver. We break down protein in the muscles and we break down fats in the adipose tissue and liberate nutrients The other thing we need to add on to this is negative feedback. This tripartite axis is a nice example of negative feedback. So cortisol when it’s released it goes throughout
the circulatory system as I said but it not only has receptors in the
target tissues but there are also receptors in the APG and in the HT This line with the hatches on it indicates negative feedback and what happens is when cortisol binds with
receptors in the APG and in the the HT it actually supresses the secretion of ACTH and CRH respectively so that controls the release of cortisol so even if the stimulus for secretion cortisol is really high there is a limit to how much cortisol will be secreted because of this phenomenon of negative feedback so you should print out your notes
template you’ve got a blank maybe prettier version this picture and
you should challenge yourself to labeling it and
explain what’s going on and if you can do that without looking
at your notes then you have mastered the stress axis

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