The Hypothalamus and the Pituitary Gland

Hypothalamus-and-Pituitary

The first aim of this article is to discuss the role of the hypothalamus in the endocrine system. Second, understand the differences between the posterior and the anterior pituitary glands, and finally, explain thyroid regulation pathway, starting with the hypothalamus itself.

Hypothalamus: Where is it?

As far as where it is located, if you take a brain, cut in half, it is right there. The pituitary gland, which we’re going to be talking about, quite a lot in this article, hangs off it.

The Functions

The basic function of the hypothalamus is that it receives signals from the nervous system. Signals will come in, they’ll get processed in the brain, or they will go directly to the hypothalamus. The hypothalamus then works on the endocrine system through the secretion of hormones, or stimulation of the pituitary gland. The hypothalamus is the point in our body that integrates our nervous system with an endocrine response.

Most of that regulation actually happens through the pituitary gland. The hypothalamus secrets some stuff directly into the bloodstream, but most things are done using the pituitary, which is an extension of the hypothalamus.

The Pituitary: Posterior & Interior

The pituitary gland has got a distinct posterior and anterior section. I’m going to talk about each one individually, in a moment.

A couple of other things to note about the pituitary is that it has got neurons hooked to it, so those would be nervous cells that actually synthesize and secrete neurosecretory hormones, into the posterior pituitary.

This would be the integration point, where these nervous cells that receive a nervous signal, secrete things that stimulate the pituitary to jump into action.

There are also a bunch of capillaries wrapped around the pituitary, which the hormones are going to be dumped into, that are going to work on other cells within the body. Let’s take a minute, and talk about the two halves of the pituitary. They are very distinct from each other, and they act differently.

The Posterior Pituitary

This is an extension of the hypothalamus, so it comes right off of it. It has neurosecretory cells, which come in from the hypothalamus and run into the posterior pituitary. They receive signals from the nervous system.

There are two major hormones that are secreted into the posterior pituitary, and those hormones then travel to the rest of the body.

Oxytocin and ADH

Oxytocin is responsible for lactation in females, some sexual behavior, and growth and development. It’s a pretty important hormone.

ADH is an antidiuretic hormone, which is responsible for regulation of blood pressure, osmolarity of blood, the amount of fluids in the blood, and the amount of water that a person excretes.

The posterior pituitary receives signals directly from the nervous system, which is different from the anterior pituitary.

These are the two major hormones that are associated with the posterior pituitary.

The Anterior Pituitary

This gland is capable of producing and sending out tons of different hormones that work on all different parts of the body. Whether the anterior pituitary releases, or doesn’t release a hormone, is up to the hypothalamus.

What the hypothalamus does, is it will secrete, let’s say, a releasing hormone. If the hypothalamus secretes a releasing hormone, then whatever hormone that’s associated with in the anterior pituitary will be released into the bloodstream. For instance, there’s a thyroid releasing hormone, so the hypothalamus would send out that hormone to the anterior pituitary, then the anterior pituitary would send out a hormone that works on the thyroid gland to actually stimulate it into action.

If the hypothalamus was to send out an inhibiting hormone, then it would cause the anterior pituitary to hold off the secretion of anything that would cause action in the body. Hypothalamus basically tells the anterior pituitary, “Send this hormone, and don’t send that hormone,” and the hypothalamus does that by sending hormones itself.

Chemicals pass messages from one organ, to the next, and to the next. A general hormone cascade looks something like this. A signal goes into the brain, once the signal’s into the brain, it stimulates the hypothalamus. Hypothalamus then stimulates the anterior pituitary, which we just talked about, it will give it either an inhibiting or releasing hormone.

Once that anterior pituitary has been stimulated, it will then send a hormone signal out to the target tissue, whatever the target tissue may be. It might be the adrenal glands, it might be the testes, might be the thyroid. A general pathway goes from the brain, to the hypothalamus, into your pituitary, then to the target tissue, and that’s how most of the hormone signaling works within the body.

One specific example of this is the thyroid gland, which is in your neck, wrapped around your trachea, is a pretty important endocrine gland. It does a lot of things, controls metabolism, temperature regulation, growth, and general level of activity, there’s a lot that goes on with it. I want to talk very quickly about how it controls temperature. Basically what you get is a cascade. The cold works on the body, that cold ends up stimulating the hypothalamus saying, “Hey, it’s too cold. We need to speed up metabolism.”

Once the hypothalamus has been stimulated, it sends out a hormone signal to the anterior pituitary, and the anterior pituitary sends out a hormone called Thyroid Stimulating Hormone ( TSH). When that TSH gets to the thyroid, it causes the thyroid to secrete further hormones that result in increased metabolism, so the body’s metabolism rate goes up, you produce more heat, and then it works like a feedback loop.

It’s actually the TSH that shuts down the process, it’s not the warming up of the body, so once enough TSH has collected in the thyroid gland, that shuts down the pathway, and body is now a comfortable temperature, everybody is happy!

The problem with the thyroid gland is, that is prone to disorders.

There are several of them, but I want to focus on just two, hypothyroidism and hyperthyroidism. If you remember your science class, you’ll recall that hypo means under, hyper means above. If you have hypothyroidism, your thyroid is not functioning at full capacity. It’s not receiving enough hormones causing it to give off enough TSH, or to give off whatever it’s going to give off, so you have low thyroid activity.

Results of hypothyroidism are usually weight gain, because your metabolism is lower, you’ll feel lethargic, you could be cold, because your metabolism is not running high, so just a general feeling of tiredness, and being run down.

On the other end of things, you got hyperthyroidism, where the thyroid is being overstimulated, which leads to a really quick metabolism, which is going to lead to weight loss, you can be hot, you can be irritable, probably high blood pressure because your circulatory system is working in overdrive. These are two opposite ends of the spectrum, but both are the result of some sort of disorder within the thyroid gland.

Tropic and Nontropic Hormones

Finally, you need to know the difference between a tropic and a nontropic hormone. It’s very easy. A tropic hormone acts on another endocrine gland. An example of a tropic hormone would be the Thyroid Stimulating Hormone, because it’s a hormone that goes from the pituitary, and then it stimulates another endocrine gland, the thyroid gland.

If you are a nontropic hormone, you work on something else within the body. You send a signal, you work on a cell, you cause some sort of change, but you’re not controlling the function of an endocrine gland.

A tropic hormone actually controls the function of another endocrine gland. The hypothalamus is connected to the posterior pituitary, which is giving off corticotropin, which is going to control the function of the adrenal glands, so that would be an example of a tropic hormone.

This wasn’t meant to be an in depth investigation into all of the ins and outs of the hypothalamus and the pituitary and all of hundreds of hormones that are associated with it. Just going for the basic concept of how the hypothalamus integrates the nervous system, and endocrine system, using the pituitary gland.

If you have any questions about anything in this article, feel free to contact us using the contact page.

References

https://en.wikipedia.org/wiki/Pituitary_gland

http://www.sciencedirect.com/science/article/pii/S0893133X96000966

http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/hypopit/anatomy.html

https://en.wikipedia.org/wiki/Neurosecretion

http://www.medicinenet.com/script/main/art.asp?articlekey=2622

https://en.wikipedia.org/wiki/Corticotropin-releasing_hormone

 

 

 

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