Video: Hypothalamus

Did you know that the weight of your average human brain is approximately one point four kilograms? That’s one thousand four hundred grams. What if I told you that one little part of your brain weighing just four grams or just one-third of a percentage of your brain is a vital epicenter for your hormone production, behavioral patterns, and involuntary or subconscious body functions? It's the part of your brain that helps tell you when you're hungry or when you need to stop eating, when you're thirsty or perfectly hydrated, when you're sleepy or perhaps chiefly wide awake, tells you when you're feeling chirpy or just plain grumpy, when you're feeling all loved up or just not in the mood. It’s also responsible for releasing your inner beastly rage that you never knew you had, setting off your internal panic alarms when you're getting stressed and even getting you pumped to fight an approaching enemy.

This structure that I'm talking about here is none other than your hypothalamus – this small structure right here – which packs a mega punch when it comes to controlling your hormone production, bodily functions, and behavior. But don't let the relatively small size of the hypothalamus deceive you. Being involved in so many processes comes with a price and that price is complexity. Yes, the hypothalamus is going to be one of the trickiest topics for you to learn in your neuroanatomy course. But, don't worry, we're Kenhub, which means we love to help.

I hope you're adequately caffeinated right now because you're probably going to need it as we explore the hypothalamus. So let's waste no time and get started with this tutorial.

So we're going to begin with an introduction to some of the basic principles helpful to know when studying the hypothalamus. As a part of this introduction, we're going to be exploring the anatomic landmarks surrounding the area of the hypothalamus and we'll also introduce you to the anatomic subdivisions of the hypothalamus, which is an approach of breaking down the hypothalamus into distinct compartments which will hopefully help us when learning all about its many nuclei in more detail. And, finally, later on, we'll explore some basic information on the white matter tracts associated with the region of the hypothalamus.

So let's begin our discussion of the landmarks in and around the hypothalamus. This will help you better localize the region of the hypothalamus when looking at radiology pictures or gross specimens. First of all, we should point out that we're looking at a midsagittal section of the brain with the hypothalamus highlighted in green. It belongs to a greater region of the brain known as the diencephalon or the interbrain, and this region is highlighted in a different color.

Now for those of you who may be unfamiliar with the major landmarks of the medial view of the brain, don't worry, we're going to be presenting the most important of these here in this section, but if you'd like a more thorough understanding of all the anatomy here, you might like to watch our video on the medial view of the brain.

So to cover these structures briefly, let's group all the surrounding structures into four regions – superior, inferior, anterior, and posterior. And these directions can also be classified as dorsal, ventral, rostral, and caudal respectively. Superior to the hypothalamus, we're going to find the thalamus, the choroid plexus, the fornix, and the corpus callosum. Inferior to the hypothalamus, we're going to find the optic chiasm, the pituitary stalk and gland, and the mammillary body. Anterior to the hypothalamus, we’ll find the lamina terminalis and, further anterior, the paraterminal gyrus. And, finally, posterior to the hypothalamus, we’ll find the floor of the third ventricle and the midbrain.

In order to be able to delve deeper into the hypothalamic anatomy, it's important to understand its anatomic subdivisions.

The hypothalamus can be subdivided into regions from both a mediolateral perspective – that is, on a coronal plane – as well as from an anteroposterior perspective – that is, on a sagittal plane. When we look at the hypothalamus from a coronal view, we can distinguish three subdivisions or zones – the periventricular zone of the hypothalamus, which is the thin region surrounding the third ventricle; the medial zone of the hypothalamus, which is the most nuclei-dense zone; and, finally, the lateral zone of the hypothalamus.

Taking the hypothalamus from a sagittal aspect now, we can alternatively identify four subdivisions or areas, and these are the preoptic area, which is a region of nuclei associated anterior to the optic chiasm; the anterior hypothalamic area, also known as the suprachiasmatic area, which is a region located superior to the optic chiasm; the intermediate hypothalamic area, also known as the tuberal hypothalamic area, which is the region above the tuber cinereum interposed between the mammillary body and the optic chiasm; and, finally, the posterior hypothalamic area, also known as the mammillary hypothalamic area, which contains the nuclei of the mammillary body and the nuclei above it.

The anteroposterior subdivisions of the hypothalamus are particularly relevant in the context of the medial zone, but we're going to be exploring that in more detail in just a moment.

So having completed our introduction, it's time to focus on the three mediolateral hypothalamic zones we mentioned earlier, and we'll start with the periventricular zone. The name of this periventricular zone is derived from the fact that it surrounds the space of the third ventricle. Compared to its medial and lateral counterparts, this zone is relatively small and contains one primary cluster of neural cells collectively known as the periventricular nucleus.

Some neuroanatomical classifications also place the arcuate nucleus and the suprachiasmatic nucleus into this region, however, we're going to be discussing these two nuclei as elements in the medial zone to better understand their correlation with the other hypothalamic nuclei. So let's explore the primary nucleus of this zone which is the periventricular nucleus.

So as we just mentioned, this nucleus is located in the periventricular zone of the hypothalamus, which is the name of the zone because the nucleus also surrounds a large portion of the lateral wall of the third ventricle. The periventricular nucleus is reported to receive fibers from another structure known as the arcuate nucleus, which we'll meet in a few moments. The periventricular nucleus projects to the anterior portion of the pituitary gland which is also known as the adenohypophysis.

The periventricular nucleus is involved in mood regulation so much so that in the past when neurosurgeons made contact with the floor of the third ventricle, patients were observed to burst into laughter or keep joking until the contact with the floor of the third ventricle was stopped. This nucleus also releases somatostatin, a hormone which is secreted into the hypophyseal portal system, and inhibits thyrotropin-releasing hormone and growth hormone-releasing hormone from the pituitary gland.

Finally, the anterior portion of the periventricular nucleus or the anteroventral periventricular nucleus contains kisspeptin neurons which have a role in kickstarting puberty and stimulating feelings associated with romantic and sexual arousal.

Moving on from the periventricular zone and nucleus, it's time we talked about the medial zone.

So as you'll see in a moment, this is probably the most complex zone of the hypothalamus and to help us remember everything contained within the medial zone, we're going to be subdividing it into four regions based on the anteroposterior axis which we looked at earlier. And these were the preoptic area, the anterior hypothalamic area, the intermediate hypothalamic area, and the posterior hypothalamic area. And as you can see from the illustration, each of these regions contains many distinct nuclei, so let's take some time to find out a little bit more.

So, let's begin with the most anterior region which is the preoptic area, and the preoptic area derives its name from the fact that it is located anterior to the optic chiasm. There are several nuclei contained within this area, however, two of the better-known preoptic nuclei include the medial preoptic nucleus and the lateral preoptic nucleus. As it name suggests, this nucleus is located anterior and medial to the optic chiasm.

So the medial preoptic nucleus is very interesting due to the fact that it is sexually dimorphic. And what does this mean? This means that researchers have documented differences in the size of this nucleus between the male and the female brain. The nucleus is mainly believed to regulate the release of gonadotropic hormones which are the hormones which are destined for your gonads. The medial preoptic nucleus can also be correlated to sexual behavior and, furthermore, if that wasn't enough, researchers also speculate that this nucleus is implicated in the regulation of sleep-wake cycles.

In the past when wacky brain surgeons were allowed to do crazy things, the region which contains the medial preoptic nucleus was targeted in patients diagnosed with abnormal sexual behavior and sexual deviations – i.e. patients with pedophilic disorder or aggressive sexual behavior – and a procedure known as anterior hypothalamotomy was performed in such cases.

So, the results were pretty interesting. It resulted in reduced sexual drive without changing sexual preferences, but there was also the risk for hyperphagia, which involves overeating and obesity. So, such procedures, of course, have no place in the modern world, but recent research has come to validate that this nucleus is implicated in the regulation of sexual behavior. Good to know!

So let's move on now to the lateral preoptic nucleus, and this nucleus is actually part of the lateral zone but I'm adding it here for the sake of coherence. As its name suggests, it's located lateral to the medial preoptic nucleus. The lateral preoptic nucleus is believed to control states of arousal, sleep, and transitions between these two states together with the reticular activating system and the widely projecting orexin neuronal system that originates in the lateral hypothalamus.

This nucleus activates when it is exposed to sleep inducing the release of sleep-related neurotransmitters like serotonin, adenosine, and prostaglandin D2 and it's mostly active during sleep primarily during nonrapid eye movement sleep and it has been recorded to inhibit the neurons of the reticular activating system via the release of inhibitory neurotransmitters like gamma-aminobutyric acid a.k.a. GABA and galanin. On the flipside, arousal-inducing neurotransmitters like norepinephrine and acetylcholine have been recorded to inhibit activity in this nucleus.

So, it's been proposed that people with more inhibitory neurons containing the neurotransmitter galanin in their lateral preoptic nucleus have a better quality more continuous sleep while people with a reduced number of these neurons have more fragmented sleep and, that is, more awakenings throughout the night. Currently, this nucleus is being studied as a potential target for brain stimulation for the treatment of narcolepsy and insomnia.

Having completed our discussion on the preoptic area, we're now going to move a little further back just superior to the optic chiasm to a region known as the anterior hypothalamic area or suprachiasmatic area and we'll discuss four entities in this area, namely the anterior hypothalamic nucleus, the suprachiasmatic nucleus, the supraoptic nucleus, and the paraventricular nucleus. So let's start of course with the anterior hypothalamic nucleus which is located directly behind the preoptic nuclei and above this structure known as the superchiasmatic nucleus.

The connections of this area have not been thoroughly examined, however, we do know some details in regards to its function. So, we think that this nucleus serves two main roles. First of all, it's involved in thermal regulation and particularly in the cooling down of the body or, in technical terms, the dissipation of heat. So when your anterior hypothalamic nucleus senses that you're getting all hot and bothered, it sends signals which tell your body to get rid of heat by means of perspiration or sweating as well as vasodilation which causes your blood vessels to dilate making you a little more red in the face. This can be further underlined by the fact that when this nucleus is damaged or destroyed in lab animals, they suffer from hyperthermia.

The anterior hypothalamic nucleus has been shown to stimulate the parasympathetic nervous system, and remember that the parasympathetic nervous system is responsible for many of the body's unconscious functions implicated in rest/digest and feed/breed activities. The parasympathetic nervous system enables functions such as salivation, secretion of digestive juices, increased GI motility, defecation, sexual arousal, and erection, as well as other functions.

Moving on, we're going to talk about the next the nucleus of interest in the anterior hypothalamic area which is the suprachiasmatic nucleus. This nucleus which we’re currently highlighting for you is situated superior to the optic chiasm and actually sits between the medial and periventricular zones, which is why some classifications may place it in the periventricular zone. The proximity of these suprachiasmatic nucleus enables it to directly receive signals from specialized cells in the retina via a neural pathway known as the retinohypothalamic tract, and this means that activity in the suprachiasmatic nucleus is modulated by activity in the retina which to you and me means that the activity of this nucleus is dependent on the features of environmental light.

The suprachiasmatic nucleus is believed to be the main hypothalamic nucleus regulating the circadian rhythms of the body which, in basic terms, is your body clock. This is supported by both its afferent retinal connections and its projections to other hypothalamic nuclei and the pineal gland, and remember that the pineal gland produces melatonin which is implicated in the regulation of sleep-wake cycles. Moreover, this is increasing evidence that cells in this suprachiasmatic nucleus have an ability for light-induced gene expression, and this means that genes in these cells are expressed in response to changes in environmental light conditions.

Directly behind the suprachiasmatic nucleus and also superoposterior to the optic chiasm lies this supraoptic nucleus. Its main role is to regulate water balance and, to do so, it produces and secretes a hormone known as vasopressin, also known as antidiuretic hormone, in response to concentration of sodium ions in the blood and vasopressin is secreted when serum sodium concentration increases or blood volume or blood pressure decreases. By releasing vasopressin, this nucleus ensures that blood volume pressure and electrolyte consistency will remain within an acceptable range that can sustain life.

This nucleus also synthesizes and secretes another hormone known as oxytocin which is a substance known to assist labor by causing contractions of the uterus, but lately has also proven to be implicated in the emotion of love. Clinically, significant lesions in this nucleus are very often accompanied by a condition known as diabetes insipidus as without antidiuretic hormone, the kidneys cannot reabsorb water which ultimately leads to loss of great quantities of water during urination.

Next up is the paraventricular nucleus, and this nucleus is situated justo superior or dorsal to the anterior hypothalamic nucleus. Be careful not to confuse this nucleus – the paraventricular nucleus – with the one examined a little earlier, which was the periventricular nucleus. And this nucleus has three core functions. The first is that it helps regulate blood pressure and electrolyte composition, so just like the supraoptic nucleus, this nucleus produces vasopressin.

The next function is that this nucleus is involved in energy metabolism as it is implicated in the control of hunger and feeding as well as autonomic control of digestion. And, finally, this nucleus is also involved in defensive behavior. The paraventricular nucleus interacts with other hypothalamic nuclei to regulate stress responses and the fight-or-flight response to environmental threats, and damage to this nucleus has also been recorded to cause diabetes insipidus.

So we're now ready to move on to the intermediate hypothalamic area of the hypothalamus, and this area is also known as the tuberal hypothalamic area due to the fact that it is located just superior to this structure here, which is the tuber cinereum – a region of gray matter extending from the optic chiasm anteriorly to the mammillary bodies posteriorly, and we'll examine three nuclei in this region namely the arcuate nucleus, the ventromedial hypothalamic nucleus, and the dorsomedial hypothalamic nucleus.

So let's start with the arcuate nucleus which is located just here, and this is a nucleus that is located in the anteroinferior part of the intermediohypothalamic area posterior to the optic chiasm and it's located very close to the third ventricle and median eminence and forms the posterior wall of the pituitary stalk. This medial portion of this nucleus lies adjacent to the third ventricle and subsequently some classifications place it in the periventricular zone. But this nucleus is not a superficial sheet of neural cell nuclei like the periventricular nucleus, but rather it reaches deeper into the medial zone of the hypothalamus which is why we have added it here.

The arcuate nucleus receives input from the amygdala complex, the hippocampus, and the enterorhinal cortex. It then projects to most other hypothalamic nuclei leading neuroscientists to believe that this nucleus is implicated in neuroendocrine regulation and research has shown that this nucleus is responsible for the regulation of appetite and the monitoring of adipose tissue fat, the regulation of the cardiovascular system, as well as the regulation of sexual behavior. Its connection with the medial preoptic nucleus seems to be more relevant to the regulation of sexual behavior. Moreover, this nucleus contains dopaminergic neurons which negatively control prolactin release.

Just superior to the arcuate nucleus and still within the intermediate hypothalamic region, we will find the ventromedial hypothalamic nucleus, and this nucleus is shown to function as a satiety regulation center, which basically means that it tells us when we're feeling satisfied or promotes the feeling of fullness. It also is center for responses to stress which involves the regulation of panic-related behavior and fear. In addition, the ventromedial nucleus also has influence over processes of thermoregulation and sexual activity.

So let's just take a couple minutes to talk about some clinical correlations to do with this nucleus. So, lesions in the ventromedial nucleus have been correlated with overeating, weight gain, and obesity as well as sexual deviations and drug dependence while stimulation of the same nucleus has been shown to lead rejection of food and weight loss in both animals and humans. Finally, surgical destruction of the nucleus has shown to cause persistent impotence and loss of libido in female and male patients regardless of the levels of their sexual hormones in their blood.

Moving superiorly, let's now look at the dorsomedial hypothalamic nucleus which you can now see highlighted in green, and it's also located posterior to the anterior hypothalamic nucleus and superior to the ventromedial nucleus. The dorsomedial hypothalamic nucleus receives input from the suprachiasmatic nucleus whose fibers tune the dorsal medial nucleus to circadian rhythms. Almost all other hypothalamic nuclei feeds the dorsomedial nucleus with information, but the extent to which they regulate its output is still unknown.

Now, the dorsomedial nucleus itself integrates all the information it receives and sends out fibers to the ventrolateral preoptic area and the locus coeruleus of the brainstem. This connection to the locus coeruleus suggests a correlation between the function of this nucleus and the regulation of wakefulness. Indeed, this is the nucleus that sets your daytime feeding schedule, but in addition to that, it also affects emotional responses to stress, panic-related behavior, and libido. And stimulation of this nucleus has been shown to lead to obesity and even outbursts of savage behavior.

Studies have shown that lesions in the dorsomedial nucleus of rats prevented food gathering during wakefulness, locomotor activity, and core body temperature, and such findings further underline that this nucleus integrates feeding behaviors with circadian activity.

So, we're now transitioning to the posterior hypothalamic area of the hypothalamus which is also known as the mammillary hypothalamic area. So this area became of some interest to the Romanian neuropsychiatrist and renowned neuropathologist, Constantin von Economo, when he was looking for pathological patterns in the brains of deceased patients having suffered from an atypical form of encephalitis, which left many of its victims speechless and motionless and their bodies perpetually paralyzed in statue-like states. So those of you who've watched the 1990s film Awakenings or read the book the film was based upon, might be familiar with this rare condition.

Anyway, von Economo identified lesions in the posterior hypothalamus and adjoining brainstem regions and proposed that these areas are critical for behavioral wakefulness.

So we're now going to read the nuclei in this region and try to figure out why this hypothalamic area came to be associated with behavioral wakefulness.

So there are three nuclei in this area and these are mammillary complex, the tuberomammillary nucleus, and the posterior hypothalamic nucleus, and we're going to be first looking at the mammillary complex.

So as its name suggests, this structure is a collection of nuclei located in the mammillary body which is a landmark of the base of the brain that can be found in gross neuroanatomic specimens. The mammillary nucleus receives direct hippocampal input via the fornix and it integrates this information and then projects to the anterior thalamic nucleus by the mammillothalamic tract which we'll see later.

The mammillary nucleus is a very important structure for episodic memory encoding. Episodic memory is a cognitive neuroscience term that actually refers to those memories that we can remember as episodes or events in our lifetime such as what you ate for breakfast last Sunday, what you did for your ninth birthday, or maybe even your first kiss. And these memories allow an individual to figuratively travel back into their past and re-experience an event that has actually taken place. The mammillary nuclei helps organize or encode new experiences into episodic memories.

The mammillary nuclei can be damaged due to thiamine deficiency resulting from chronic malnourishment or even alcoholism. So thiamine is a substance more commonly known as vitamin B1 and people who suffer from chronic alcoholism and thiamine deficiency may develop difficulties forming new memories and accessing old memories, and these symptoms, in addition to many other symptoms, constitute a condition known as Korsakoff's syndrome.

So, now we're going to have a look at our tuberomammillary nucleus, and this nucleus is located in the posterior hypothalamic area just above the mammillary complex. So, many neurons in this nucleus contain histamine vesicles and you may have heard from immunology lectures that histamine is a substance implicated in allergic responses in the body and this is true for the rest of the body, but here, histamine is a neurotransmitter in the brain. Neurons containing histamine are known as histaminergic neurons.

The histaminergic neurons from this nucleus projects to many other regions of the brain including other hypothalamic nuclei. In the peripheral regions of the body, histamine would trigger immune reactions, however, in the CNS and particularly in this CNS segment, it is implicated in the promotion of wakefulness collaborating with norepinephrine secreted from the neurons of the locus coeruleus which is a major nucleus in the brainstem promoting wakefulness, and this nucleus is also believed to play a role in motivated behaviors related to food, water, sex, and addiction.

So we’ll now be moving on to the posterior hypothalamic nucleus and the anterior border of this region is set by the mammillothalamic tract. This area is located above the tuberomammillary nucleus and the function of this nucleus is less well known than other nuclei although it has proven to be associated with sympathetic responses and defensive and aggressive behaviors, which can actually be thought of as an extent to the general concept of fight-or-flight responses enabled by the activation of the sympathetic nervous system.

So we'll now be looking at the lateral zone of the hypothalamus. And there is only one area in this region and this one is known as the lateral hypothalamic area. So the lateral hypothalamic area lies laterally to almost all the medial zone nuclei, and this nucleus is functionally related to most other hypothalamic nuclei. Specifically, it projects to the posterior hypothalamic nucleus, the tuberomammillary nucleus which we’ve explained to be a major source of histaminergic neurons, the arcuate nucleus, and the paraventricular hypothalamic nucleus.

In addition to its hypothalamic projections, this area also sends neural fibers to the ventral tegmental area, dopamine nucleus, the locus coeruleus noradrenergic nucleus, the serotonergic raphe nuclei, the cholinergic pedunculopontine nucleus, and laterodorsal tegmental nucleus, and finally, the ascending reticular activating system.

As you can see, this area is indirectly connected to every major neurotransmitter center in the central nervous system. Its specific function has not been yet determined, but has been clinically implicated in a disease known as narcolepsy, which is a chronic sleep disorder that causes overwhelming daytime drowsiness.

Among its other neurons, the lateral hypothalamic area contains orexinergic neurons which are involved with wakefulness. Reduction in the number of these orexinergic neurons is becoming a popular explanation for the pathophysiology of narcolepsy.

With the lateral hypothalamic area, our discussion on the hypothalamic nuclei is over, however, before you take a big sigh of relief, we're not quite finished yet. You see, the hypothalamus contains more than just nuclei and we also have to take some white matter tracts into consideration. So, let's have a look and find out a little bit more.

So, now as you may remember, we can identify tracts based on their origin and destination to afferent tracts and efferent tracts and, of course, we're going to start with afferent tracts. Afferent tracts start distally and terminate at their point of reference, which in our case, is the hypothalamus. So, in simple terms, afferent tracts are in the incoming pathways to the hypothalamus and we'll examine one major afferent tract to the hypothalamus which is the fornix.

So the fornix is a C-shaped bundle of nerve axons which originates from the hippocampus which is not visible in this illustration and extends to the mammillary bodies as well as several other structures in this region. And the fornix is a major communication pathway of the Papez circuit of the limbic system which is involved in motivation, emotion, learning, memory, as well as other cognitive functions.

So moving swiftly onto the efferent tracts of the hypothalamus, those are the tracts that begin in the hypothalamus and in other locations of the central nervous system, a.k.a., the outgoing tracts of the hypothalamus, and we're going to be looking at four of these. Firstly, we're going to look at the supraopticohypophyseal tract then the dorsal longitudinal fasciculus then the mammillothalamic tract, and finally the descending hypothalamic connection.

The supraopticohypophyseal tract, as its name suggests, connects the supraoptic nucleus with the hypophysis which is the Greek name for the pituitary gland, and we can also see fibers of this tract arising from the arcuate nucleus, and these fibers transfer vasopressin vesicles to the posterior pituitary to be secreted there.

Next, we're looking at the dorsal longitudinal fasciculus, and the origins of this white matter tract can be traced back to many nuclei of the hypothalamus, but particularly to the paraventricular nucleus, the supraoptic nucleus, and the periventricular nucleus. And this tract travels down the spinal cord where it synapses with preganglionic autonomic neurons, and this is one of the pathways through which the hypothalamus controls the autonomic nervous system.

The next tract we're going to be looking at is called the mammillothalamic tract and, as its name suggests, it connects the mammillary body to the thalamus and specifically to the anterior thalamic nucleus. This is also a part of the Papez circuit we talked about earlier. Remember that the limbic circuit that regulates emotion, learning, and memory which starts from the hippocampus and continues to the mammillary body via the fornix is the same circuit that reaches the anterior thalamus via the mammillothalamic tract.

The descending hypothalamic fibers comprise the rest of the efferent fibers leaving the hypothalamus and these include circuits belonging to the limbic system; for example, those terminating at the locus coeruleus in the brainstem. Other tracts are involved in the autonomic output of the hypothalamus; for example, the hypothalamospinal tract and this contains fibers mainly originating from the paraventricular nucleus, however, also receives fibers from the dorsomedial, ventromedial, and posterior nuclei, and these fibers descend within the brainstem and the spinal cord where they eventually synapse with cell bodies of preganglionic neurons.

And with that, we have finally covered all you need to know about the hypothalamus. Congratulations! You've survived it all and hopefully you've mastered this complex structure.

Before I let you go, of course, I want to quickly summarize what we've learned in this tutorial. So, bear with me. We first started with an introduction on anatomic landmarks of the medial side of the brain, remembering how to identify the corpus callosum, the fornix, and the thalamus. We then talked about the anatomic subdivisions of the hypothalamus and we learned that the hypothalamus is organized into three zones mediolaterally – the periventricular zone, the medial zone, and the lateral zone.

In the periventricular zone, we identified the periventricular nucleus which we discovered to be associated with mood regulation and, in the medial zone, we distinguished four regions anteroposteriorly and we specifically talked about the preoptic region, the anterior hypothalamic area, the intermediate hypothalamic area, and the posterior hypothalamic area.

More specifically, in the preoptic region of the medial zone, we identified two nuclei – the medial preoptic nucleus, a sexually dimorphic structure which regulates sexual drive and feeding, and the lateral preoptic nucleus, a structure which inhibits awakening during periods of deep sleep.

In the anterior hypothalamic area of the medial zone, we identified four nuclei. First of these was the anterior hypothalamic nucleus which we saw to be associated with thermal regulation and parasympathetic nervous system stimulation. The second nucleus was the suprachiasmatic nucleus which is a cluster of neurons receiving information on environmental light conditions from the retina and setting the body clock for the circadian rhythms and the sleep-wake cycles. And, finally, the two other nuclei in this area – the supraoptic nucleus and the paraventricular nucleus are associated with the regulation of water balance and blood volume.

We then proceeded to examine the intermediate hypothalamic area of the medial zone and we only found three nuclei there – the arcuate nucleus which regulates appetite, adipose tissue consistency, and sexual behavior; and the ventromedial and dorsomedial nuclei which regulate day feeding and responses to stress and panic.

We finished our discussion on the medial zone nuclei with the nuclei of the posterior hypothalamic area, and in this section, we discussed the mammillary complex which is important for episodic memory coding. We talked about the tuberomammillary nucleus – a structure which contains histaminergic neurons and regulates wakefulness and motivated behaviors. We then proceeded to examine the posterior hypothalamic nucleus – a nucleus which controls sympathetic responses and, as an extension, aggressive and defensive behaviors and behavioral wakefulness.

We then moved on to discuss the lateral zone of the hypothalamus, a region connected to brainstem regions regulating wakefulness and levels of consciousness, and this area is also believed to be involved in these functions.

And, finally, we discussed the white matter tracts associated with the hypothalamus. We diversified these into afferent and efferent depending on whether they terminate or originate in the hypothalamus. In our discussion on afferent hypothalamic tracts, we reviewed the fornix and, in our discussion on efferent hypothalamic tracts, we talked about the supraopticohypophyseal tract which connects the supraoptic nucleus to the pituitary.

We also discussed the dorsal longitudinal fasciculus, a tract which connects the paraventricular, supraoptic, and periventricular nuclei to autonomic centers in the brainstem and spinal cord, then we presented the mammillothalamic tract, a white matter tract connecting the mammillary body to the anterior hypothalamic nucleus. And, finally, we briefly talked about the descending hypothalamic fibers which contain tracts pertaining to limbic and autonomic nervous systems.

And with that, we’ve finished our tutorial. I think you've totally earned a break right now. Thanks for watching and happy studying!