Hypothalamic–Pituitary–Adrenal Cortex Axis

00:00 Adrenal hormones.

00:02 There are four main ones we need to think about or classifications.

00:06 Aldosterone, androgen, cortisol and epinephrine.

00:10 We'll specifically discuss the adrenal cortex hormones different than the adrenal medulla hormones.

00:17 The adrenal cortex hormones involves the hypothalamic pituitary adrenal cortex axis.

00:25 So in the hypothalamus where we consider what's the most important, it's the paraventicular nucleus.

00:32 The paraventicular nucleus is what is going to release CRH.

00:36 That will then be released to the anterior pituitary to stimulate corticotropes.

00:43 These corticotropes will then release another hormone which is called ACTH.

00:54 How is this process regulated? The hypothalamus releases the hormones CRH, stimulating the anterior pituitary to release ACTH.

01:09 ACTH has some negative feedback capabilities on it's own.

01:14 It can feedback to the paraventicular nucleus and decrease CRH release.

01:20 The adrenal cortex hormones, such as cortisol will also have negative feedback components.

01:26 That will feedback both to the anterior pituitary to decrease ACTH as well as CRH from the hypothalamus.

01:37 So we have a multiple layered negative feedback system for adrenal cortex hormones.

01:43 Also we have a couple other stimulatory events.

01:50 Stresses such as physical stress, mental stress and even some biological stresses such as low glucose can feed into the system and drive it forward.

02:03 Meaning that despite negative feedback control, you can override this process by the stresses stimulating the release of CRH.

02:16 Which will increase the release of ACTH.

02:20 Which will then increase cortisol.

02:23 There are also lot of effects of biological rhythms.

02:27 And there are number of biological rhythms that we need to concern ourselves with.

02:32 There are those that last about a day which are circadian rhythms.

02:37 There are ones that are associated with longer cycle such as in growth and development.

02:43 Even some cycles that will last less than a day called ultradian rhythm.

02:47 But even if we think about a whole day response, circadian rhythm, there are sometimes in which cortisol release is higher than others, simply because of the time of day.

02:59 This biological rhythm needs to be taken into account when you are measuring cortisol levels in the blood.

03:05 You need to know at what point of the day you're in.

03:08 And you need to know if you are associate with a growth spurt or not, to know what a normal value looks like.

03:14 All of the cortex hormones, aldosterone is controlled a little bit differently.

03:23 Most all the other hormones are primarily controlled by ACTH.

03:29 Aldosterone though has a couple of additional regulatory factors.

03:35 It also is in impartial control by ACTH.

03:38 But aldosterone synthesis is also important.

03:42 So in this last step to from aldosterone, two other items are important.

03:49 The first is Ang II or Angiotensin II.

03:52 And that's stimulated through the renin angiostenin system.

03:56 So in response to something like low blood pressure or low blood volume sensed in the kidney, you have an up regulation and an increase in renin release.

04:06 This will increase angiotension II which stimulates this enzyme aldosterone synthase to produce more aldosterone.

04:16 Other item that becomes important is the level of potassium.

04:23 If you have high levels of potassium, it will also stimulate aldosterone synthase to convert in more aldosterone.

04:32 Low levels of potassium inhibit this enzyme.

04:36 So you can see here we have a number of extra regulatory steps for aldosterone that we don't have for other cortex hormones such as cortisol.

04:47 So what does aldosterone do that effects both of these two additional regulatory factors.

04:54 Well, aldosterone causes fluid retention.

04:57 And therefore, blood volume becomes important.

05:01 Aldosterone also causes potassium secretion and excretion.

05:07 Therefore, potassium becomes an important regulatory step, in its overall synthesis pathway.

05:13 The control of epinephrine is different than the other adrenal hormones.

05:22 So the cortex hormones were primarily regulated by what, ACTH and then aldosterone, by aldosterone synthesis.

05:31 Epinephrine is a medulla hormone which means it synthesizes in a little different spot.

05:38 It's primarily controlled by the sympathetic nervous system.

05:41 So let's go through the classic sympathetic nervous system.

05:45 And I'll use a compare and contrast approach to show you where there is a little bit different with producing epinephrine versus a normal nervous system response.

05:57 So in the sympathetic nervous system, you have a pre-gangilonic nerve fiber.

06:02 With synapse within a sympathetic chain ganglion, releasing the acetylcholine to bind to a nicotinic type II receptor.

06:12 This part is the same.

06:14 You'll have a similar neuron postganglionic fiber.

06:19 But now it will synapse in the adrenal medulla rather than a ganglion.

06:24 And it will directly simulate a chromatin cell.

06:28 The chromattin cell has the same nicotinic type II receptor.

06:32 Both of these acetylcholine responses are stimulatory.

06:37 If we go back to our traditional sympathetic nerve, What happens? You then stimulate a postganglionic fiber, that then travels releasing another neurotransmitter, norepinephrine in this case to bind to alpha and beta adrenergic receptors is on a target tissue.

06:57 The adrenal medulla is a little different.

07:02 Chromaffin cells will produce hormones primarily epinephrine, a little bit of norepinephrine, release it into the blood and then it travels through the blood to target tissues.

07:14 And the epinephrine, norepinephrine then can also bind to alpha and beta adrenergic receptors.

07:20 So it utilizes just a little different route to get to some of the same places.