Endocrine system and hormonal regulation
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The hypothalamus and pituitary gland
OK, so today what I want to talk about is endocrine control. And in order to talk about endocrine control, I need to talk about two major glands. First, the hypothalamus-- I'm going to draw that in here-- and then in this enlarged image right here, this is just a blown-up view of the hypothalamus. And then the next major gland that we need is the pituitary gland. And the pituitary gland is the gland that dangles right below the hypothalamus. And you can see that the hypothalamus is a structure right here in the forebrain and the pituitary dangles right beneath it. And as a member of the brain, the hypothalamus receives neural signals from the brain and from the peripheral nervous system, and it funnels those signals to the pituitary gland, which ultimately controls the other endocrine glands and our body's hormonal response to the environment. And there are two different parts to the pituitary gland. You have the anterior pituitary gland, and then you have the posterior pituitary gland. And the hypothalamus interacts with the anterior and posterior part in two different ways. And so it interacts with the anterior pituitary gland, primarily through the hypophyseal portal system which, I've kind of drawn in here. And the hypophyseal portal system is a capillary system, so little blood vessels that flow between the hypothalamus and the pituitary gland. And the hypothalamus secretes hormones into this little system, and they go down and they signal the pituitary gland, and so that would be an example of a paracrine signal, or a really regionally-acting signal. And so one example of the hypothalamus hormones that signal the pituitary gland is gonadotropin-releasing hormone, or GnRH. And gonadotropin-releasing hormone is going to go down to the anterior pituitary, and it's going to stimulate the release of follicle-stimulating hormone and luteinizing hormone-- so FSH and LH. And these hormones are going to travel down to the gonads-- in the male, the testes, and in the female, the ovaries-- and they're going to stimulate the gonads to release their hormones. Another example of how the hypothalamus controls the pituitary gland, which ultimately controls the endocrine glands, is corticotropin-releasing hormone. And corticotropin-releasing hormone stimulates the anterior pituitary's release of adrenocorticotropic hormone-- ACTH. Adrenocorticotropic hormone goes down to the adrenal glands, and it stimulates the adrenal gland's release of its hormones. And so moving along, the hypothalamus also releases thyroid-releasing hormone, or TRH. And thyroid-releasing hormone goes down to the anterior pituitary, and it stimulates the anterior pituitary's release of thyroid-stimulating hormone, TSH. And thyroid-stimulating hormone goes down, and it stimulates the thyroid gland to release its hormones, thyroxine and triiodothyronine. And so continuing in that list, you have growth-hormone-releasing hormone. And growth-hormone-releasing hormone is the hypothalamus's signal to the pituitary gland to release its hormone, growth hormone. And growth hormone goes to the long bones and the big muscles in our body, and it stimulates growth. And then last but not least, we have prolactin inhibitory factor, PIF. And prolactin inhibitory factor is a little bit different, because it's constantly being released. And when it stops being released, that's when the pituitary gland is signaled to release prolactin, and prolactin is a hormone involved in milk production in moms. And so some of the anterior pituitary hormones go down and directly stimulate other endocrine glands, like FSH and LH, but some of the anterior pituitary glands directly affect parts of the body, like growth hormone and prolactin. And so there's a nice mnemonic, FLAT PEG, which helps me remember which one's which. Because FSH, LH, ACTH, and TSH, or the FLAT hormones, are called tropic hormones. Let me get a different color for this. And the tropic hormones are hormones that stimulate other endocrine glands. PEG hormones are the direct hormones, and they include prolactin and growth hormone. And the E is for endorphins, which I haven't included in this list because the anterior pituitary does release endorphins, but so do a lot of other parts of the body. So E is for endorphins, but the PEG hormones are direct hormones. And direct hormones stimulate a part of the body directly. So growth hormone directly stimulates the bones and the muscles, and prolactin directly stimulates lactation. And so the hypothalamus signals the anterior pituitary's release of its hormones through the hypophyseal portal system, or this little capillary bed. And then the hypothalamus also communicates with the posterior pituitary, and it does that through stimulation of nerves which run down that pituitary stalk right here. And the hypothalamus sends a signal down those nerves to the posterior pituitary and causes the posterior pituitary to release a couple hormones, too. And I want to make a point, because the posterior pituitary releases these hormones, but they're actually made in the hypothalamus and stored in the posterior pituitary. And the two major hormones are ADH and oxytocin. And ADH is antidiuretic hormone. It stimulates the collecting ducts in the kidneys to retain water. And oxytocin is a hormone involved in uterine contractions in women. And so you see that a lot of the endocrine control in the body ultimately comes back to the pituitary gland, which is then controlled by the hypothalamus, which forms the bridge between the nervous system and the endocrine system. And so as big as the names of the hormones get, the idea behind them is pretty manageable. It's really just one part of the body stimulating another part of the body through chemical messages.