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  University of Idaho
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  Psychology Dept.
  University of Idaho
  Design - P&D  CTI

Before we begin a discussion of the hypothalamus, you need to take out DeArmond et al. and look at slides or figures 73 and 74.  This shows where the hypothalamus is and structures in it.  Thus, you will have an idea of what we’re talking about as we go through this section.

First of all, as you can see here on slide one, the hypothalamus is an extremely important structure.  As we see in slide two, it’s very small and weighs only about 4 grams while the brain weighs approximately 1400 grams.  The hypothalamus also contains a wide variety of specialized structures that are extremely important for a wide variety of different behaviors.  And as we can see in slide three, it regulates things such as blood pressure and electrolyte composition.  It also regulates body temperature, metabolism, reproduction and it also has an extremely important role in emergency responses to stress.  Finally there are other things that it’s involved with in relation to the limbic system (or what is called the Papez Circuit) such as pleasure and other emotional aspects.  So it’s involved with a lot of different things.

In general, the hypothalamus regulates mechanisms by receiving sensory information from all areas of the body.  It then compares the sensory information with biological set points that we have within the system.  Ultimately, what it does is secrete different hormones and other things.  Finally, it adjusts the system to restore the body’s balance.  That occurs when some specific deviation from a biological set point occurs.

So, what are some of these set points?  What do they do?  Well, as we can see in slide five, there’s a wide variety of set points.  There are set points for blood sugar, for hormones, for temperature, for sodium and for a wide variety of different other things.  One of the common ones you might think about is your temperature set point.  It is approximately 98.6.  When you get too cold, your body system does things to warm you up and when you get too hot, your body system does things to cool you down (e.g., sweating).

Well, let’s talk about some structures that are within the hypothalamus.  We begin this in slide six.  As we can see, the hypothalamus can be divided into three regions; the anterior region, middle region and posterior region.  Let’s talk about the anterior region first.  That is called the preoptic nucleus.  The preoptic nucleus is primarily concerned with integration of sensory stimuli that relates to set points.  This is one of the major structures that are involved with regulating a wide variety of different systems within you.  Some preoptic nuclei that control functions are seen in slide eight.  They include blood pressure, blood composition, how many white blood cells you have, hormonal control, reproductive activity, and many, many others as well.

The middle third of the hypothalamus overlays what is called the pituitary stalk.  As we can see here, it contains a wide variety of different nuclei.  Each of these nuclei performs a wide variety of different things.  So let’s talk about a couple of these and start on slide 10 with the paraventricular nucleus.  The paraventricular nucleus includes a couple of different structures.  It includes magnocellular and parvocellular components.  These are systems that are going to control the pituitary gland.  It also contains nerves that intervate sympathetic nervous systems and parasympathetic neurons in both the medulla and in the spinal cord.  Ultimately, these nuclei are going to regulate some very important systems in relation to autonomic control.

The other structures involved with the middle third of the hypothalamus include the dorsalmedial and ventralmedial nuclei.  These structures, regulate things such as growth, feeding, maturation and reproduction and are located, (as we see in slide 12) between the medial forebrain bundle.  The medial forebrain bundle is a series of long pathways and run through the lateral hypothalamus.  They connect the hypothalamus with the brain stem, the amegdela, the cortex, and the basal forebrain.  These structures are extremely important with autonomic functioning.  They are also highly involved with the addictions process and are heavily loaded with dopamine neurons.  We will come back and discuss this in more detail a little bit later.

As we move on to the posterior third of the hypothalamus, we see there are some structures here as well.  The first of these is the mammalary body whose function is unknown.  The other structures or what we call posterior hypothalamic nuclei contain structures such as the tuberomammillary nucleus which regulates arousal and regulates wakefulness.

Now we’ve talked about a variety of different structures that are located within the hypothalamus.  We continue on with the discussion now of some specific systems are regulated by the hypothalamus.  The first of these is what we call the endocrine system.  The endocrine system is regulated from the hypothalamus by two connections.  The first of these is a direct connection, which in essence sends neuroendocrine materials from the posterior pituitary. The second one is a more indirect connection.  Here the hypothalamus is going to send hormones to the anterior pituitary.  These hormones, then, regulate the production and release of pituitary hormones into the circulatory system.

Several of these hormones are located on slide 16.  Some of these are things such as human growth hormone, thyrotrophin releasing factor and a variety of other substances.  Each of these hypothalamic hormones is extremely important for functioning.

The next major system that’s controlled by the hypothalamus is thirst.  Thirst is controlled by two mechanisms, osmolarity and vascular volume.  Both of these systems act on osmoreceptors in the hypothalamus and on the kidney.  Ultimately, they are extremely important for good regulation of fluid content within the body.

The first of these structure controlled by the hypothalamus is the kidney.  The kidney secretes rennin.  Renin basically cleaves a blood substance called angiotensinogen into a compound called angiotensin one.  Angiotensin one (or A-1) is then hydrologized into angiotensin two, angiotensin three and then angiotensin four.  We won’t talk about angiotensin three and four.  Angiotensin 2 does a couple of major things.  The first thing that it does is causes vasoconstriction.  That causes; it also causes the release of aldosterone and the release of vasopressin by the paraventricular nucleus.  Ultimately these systems regulate water retention.

Another structure called subfornical organ is also very sensitive to angiotensin two.

These help with a combination of baroreceptors located in the preoptic area to regulate drinking and fluid levels as well.

Another system that’s regulated by the hypothalamus is hunger.  In the past, it was believed that the hunger was regulated by two major structures within the hypothalamus.  The first of these was the lateral hypothalamus.  If you stick a microelectrode into the lateral hypothalamus and give it a little bit of electricity, the animal starts to eat.  If you destroy this structure, the animal will stop eating and an actually starve.

The other structure is what is called the ventromedial hypothalamus.  When you stimulate this area, the animal stops eating.  If you destroy the structure, the animal will in fact become obese.  So it was believed that we had two systems, a start center where you started eating and a stop center which stopped you from eating.  Today it’s not as clear cut.  For example, if you damage other nearby structures, such as the trigeminal system or damaged fibers within the medial forebrain bundle is also causes a decrease in the amount of eating and may alter set points.

So while the lateral and ventral medial hypothalamus may be extremely important, they probably work with other systems.  So, it all works together to regulate hunger and eating.
The next major system that’s important within the hypothalamus is what is called the pleasure system.  In the past, when you placed a microelectrode into hypothalamic nuclei and you stimulated it, the animal would do anything to get that small microelectric voltage.  For example, it would press bars for hours, and do it more than everything else, including eating and including sexual activity.  However, today it’s not as clear cut.  The reason that it’s not as clear cut is that when we used other pathways into hypothalamic nuclei in similar areas, we didn’t get the same kind of a response.  Today we know the medial forebrain bundle is extremely important in the pleasure system, and involved with other things as well.

So in conclusion, as we see in slide 23, the hypothalamus is an extremely important structure.  It’s involved with a lot of different things.  It’s involved with sexual behavior, temperature, food regulation, and other things.  When you damage this structure, it causes you to have lots and lots of problems.

In the next section, we continue on with a discussion of different brain structures, so until then, we hope that you are progressing well and have a great day.