Remember, I described the hypothalamus and pituitary gland as the "master endocrine glands." That's because you will see that their actions mainly regulate the secretion of other hormones by other glands. In this web page, you will first review a bit of hypothalamus and pituitary gland anatomy, and then you will continue on to investigate each master gland in turn.
General Layout of the hypothalamus and its connection to the pituitary gland
This image below is from your textbook. It shows the close juxtaposition of the hypothalamus with the pituitary. In fact, it shows how the two are actually physically connected, since the hypothalamus drops down through the infundibulum to form the posterior pituitary. Do you see that? Take a close look.
As the hypothalamus secretes its hormones, it secretes them into the blood. This is the same with the secretions of hormones from the anterior pituitary or from the posterior pituitary.
But, since the hypothalamus is so intimately associated with the pituitary, it should make sense to you that the hypothalamus can particularly affect the pituitary gland. It has a kind of bizarre way to accomplish that... especially if you consider the anterior pituitary:
Basically, what I've told you is that there is a direct line of communication between the hypothalamus and the anterior pituitary, based on that description of blood flow. I also pointed out that the hypothalamus extends to form the posterior pituitary gland. That means that there is a direct line of communication between the hypothalamus and the posterior pituitary, too.
(when we cover the circulatory system I'll point out why the hypothalamus-to-anterior-pituitary blood connection is so bizarre).
There are two portions to any description of the hypothalamic hormones:
Let's consider each of these in turn.
Seven tropic hormones are produced in the hypothalamus:
Note that all of these seven hormones are tropic hormones. Even the last one, SS, used to be called GHIH for growth hormone release-inhibiting hormone. Every single one of these hormones exerts an effect directly upon the anterior pituitary.
Two Hormones are Produced in the Hypothalamus for Posterior Pituitary Secretion
The hypothalamus makes two additional hormones: OT (oxytocin) and ADH (antidiuretic hormone). The hypothalamus does NOT secrete these hormones, however. Instead, the neurons that make the hormones transport the hormones to their axonal terminals. These axonal terminals are located in the posterior pituitary.
I edited the image from above and put it here to highlight this function. Can you imagine that one of the three neurons shown here makes OT? The other two, let's say, make ADH. After the cell bodies of these neurons make the hormones, they get transported down their axons to be released from their axonal terminals.
The axonal terminals simply release the hormones into the blood, rather than onto a postsynaptic neuron. It's that easy. And this is how the hormones OT and ADH are released by the posterior pituitary into the blood.
Take a look through the PowerPoint presentation and your textbook to get a feel for what these hormones do. Oxytocin is the same hormone that is injected into a woman to induce labor, but it also has some other functions. ADH may require a bit more explanation...
ADH is involved in water retention in your body. ADH signals the kidneys to not allow any excess water leave the body. How does the hypothalamus know when we need to retain water? There are sensory neurons detecting visceral things, as well as sensory neurons that detect our obvious senses like touch and vision. Some visceral sensory neurons detect blood gas levels, others detect changing concentrations of fluids; the ones of this second type are called osmoreceptors (do you recognize "osmo-" from osmosis, another term about water?). If our blood starts getting to concentrated, we have to retain some more water to dilute it back out. The osmoreceptors signal the hypothalamus to secrete ADH, and our goal is accomplished.
I have already explained what happens in the posterior pituitary gland above, but the anterior pituitary remains for explanation.
The anterior pituitary secretes four tropic hormones and two regular hormones
Here are the hormones, starting with the tropic hormones, and finishing with the regular ones:
The functions of the last two merit a bit more description. PRL is pretty straightforward in females, promoting milk production by the mammary glands. In males, PRL has the function of actually decreasing LH secretion. Therefore, in males, PRL actually acts like a tropic hormone, more specifically as an inhibitory tropic hormone.
GH stimulates cells to grow in size, speed up their mitotic cycle (dividing faster), increases protein synthesis, and more. These functions of GH lead to growth! Because its effects are so dramatic, problems in GH secretion are also dramatic. For example, dwarfism and acromegaly are both cases of altered GH secretion. Take a look at your book and the PowerPoint presentation for more on this.
© 2011 STCC Foundation Press