Welcome back to the next part of our tutorial, on the Visceral Motor System. In this session I want to focus on the hypothalamus. So again, our discussion relates to the complexity of the brain and some of the circuitory of the brain that is genetically determined. And it provides the foundation for the operations of the, of the elements of the nervous system that are the focus of this session. And for us it's going to be the circuitry of the hypothalamus. So my learning objective for you in this session is that I want you to be able to describe the organization of the hypothalamus. And identify several functions associated with key hypothalamic nuclei. Well, we concluded the first part of our visceral motor tutorial by focusing in on this central autonomic network and we talked about the Ascending aspects of sensory signals that are derived from the viscera, and how they are integrated within this central automomic network. And the outflow back down to our visceral motor end organs is coordinated in a large measure by this key structure in the ventral part of the diencephalon called the hypothalamus. So I think it will be well worth your time to just think we me for a few minutes about the hypothalamus structure. And then the myriad functions associated with the circuits that are localized to different structures that are part of the hypothalamus. Okay, so with that as our context, let's look at the anatomy of the hypothalamus. So I'll just remind you with this midsagittal view of the human brain that the hypothalamus is found in the ventral aspect of the diencephalon. And in a somewhat anterior position. Now I've drawn a triangle here because this is roughly the region where we find the hypothalamus. So this triangle is essentially the third ventricle, and the hypothalamus forms the walls of the third ventricle. And obvisously extends from the ventricular surface into the depth of the brain. and this ventral part of the diencephalon. So this triangle is really formed by three structures that I think you can see quite plainly here in this image. So one structure is found right here. And this is a white matter structure that crosses the midline. Now, it's not the corpus callosum, the corpus callosum is up here. It is another commissure. So perhaps you can think of what that structure is. I'll give you just a moment and everyone shout out together. It's the anterior commisure, right here. So that anterior commisure is abit of a visual landmark that defines one point of this triangle that can help bound your recognition of the hypothalamus. Okay so the low point of the triangle is right here. And again, does anyone know what this is? this is a structure associated with the visual pathway, connecting the retina to the diencephalon, and the pretectal region of the upper part of the brain stem. This is actually the optic chiasm, okay? So hopefully you can recognize that. So those are two points of the triangle. The third point is actually a part of the hypothalamus itself. And that is this medial ball like structure that sits very close to the mid-line. There's a pair of them, one on either side. And again who knows what this is? This is one of the two mamillary bodies. Okay, so these are the three visual landmarks that form this triangular region that allows us to recognize the location of the hypothalamus. Well this is the gross view of the hypothalamus. A more detailed anatomous view of the hypothalamus would really be quite different. So, if we were to look now at this triangular region in some detail, I, here again is our triangle, connecting our anterior commissure, our optic chiasm, and our mammillary body. What we find is that the hypothalamus is not just one, uniform structure, but it really is a region that consists of many different nuclear divisions. In this respect, the hypothalamus is quite like the thalamus. Where in the thalamus as hopefully you are now quite comfortable with this idea, there are a number of discrete Nuclei, discreet clusters of cells, that have distinct input and output connections. And the information that's conveyed via those inputs and outputs, helps to define, the functional distinctions of each of those divisions. Well the very same concept applies to the hypothalamus. So what we find in this figure, is a rendering of just a subset of the nuclear divisions of the hypothalamus that might be recognized. So the color code just organizes these nuclei in a rostral to caudal, or an anterior to posterior, progression. But there's also a medial to lateral progression of nuclei that I want to highlight for you in just a moment. Well, the big picture here is that the hypothalamus is comprised of a number of discrete nuclei that maintain distinct patterns of input and output connections. And many of these nuclei are associated with very particular functions. And I'd like to highlight a few of them for you in just a little bit. But, before we get there, what I'd like to be able to do is to give you a bit of a broader picture of what the hypothalamus and it's many nuclei are involved in. I'd like to give you a sense of the general mechanism of hypothalamic function in each of these circuits. And then we'll talk through some particular examples, that will give you a sense of. What kinds of functions might be associated with hypothalamic nuclei? Okay, well thinking very broadly, I would assert that the hypothalamus regulates five basic functions. It regulates blood pressure and electrolyte balance. And it does so by modifying a variety of behaviors and physiological functions that can impact blood pressure and electrolyte balance, such as, seeking out fluids. Drinking generating an appetite for salts. Adjusting the osmolarity of our blood. And adjusting our vasomotor tone, which, as you know, has an impact on our blood pressure. The hypothalamus can regulate body temperature, and it can do so, again, through various physiological and behavioral means. the hypothalamus has governance over thermogenesis in the physiological systems of our body. The hypothalamus can also motivate behaviors that result in seeking, warmer clothing or warmer environments, or the removal of clothing and the seeking of cooler environments. The hypothalamus can regulate energy metabolism by coordinating our feeding behavior. This is a huge topic. I wish we had more time in medical neuroscience to really impact the role of the hypothalamus in the feeding system. but perhaps in future editions of the course we'll try to create some additional time for that. Well, the hypothalamus helps to regulate the intake of food and its digestion. And it also helps to establish a sense of satiety when our intake of food should cease. And as you might imagine, there are a variety of systems, that can become disregulated, and can result perhaps in a variety of eating disorders. including obesity which is a huge problem in many parts of the world and including here the United States. So we definitely need to direct additional attention in understanding the governance of our feeding system and the various disfunction or dis regulations of this part of the brain. Well the hypothalamus is also involved in adjusting metabolic rate. Which also obviously has a major impact on energy metabolism in the body. The hypothalamus has really a fascinating role to play in governing reproduction. And this involves a variety of different aspects of reproductive activity. It involves the physical behaviors that are associated with sex and mating activities. It involves a sense of sexual identity and sexual orientation can be influenced by hypothalamic circuits. There are hypothalamic circuits that operate in females to coordinate the various physiological changes that are necessary to support pregnancy. And then in the postpartum period, lactation and the neurishment of the infant. And then lastly. the hypothalamus is very important for coordinating the responses that support what we call allostasis. That is changing the internal environment to the body in order to respond to emergency conditions. So, the hypothalamus is involved in coordinating the distribution of blood flow. To where it's needed most given the contingency that we're faced with. It is involved with coordinating the release of hormones, governing the neuro endricine relation. And, in this case of emergency response, is increasing the release of stress hormones that help to mobilize the body for action. And the hypothalamus even has interactions with our immune system that likewise are important for mediating how we respond to emergency conditions. Well this list could be longer than five items as I'm sure you can imagine. And perhaps we could kick the, to a discussion forum. a richer discussion of the various dimensions of hypothalamic coordination of homeostasis and allostasis but I think I'll leave it here for now. And now let's talk about the general mechanisms by which the hypothalamus can control these five basic functions. Well I see the hypothalamus as sitting in a key position in the nervous system anatomically and physiologically. It's getting sensory inputs from visceral and somatic sensory pathways. It is sensitive to chemosensory signals. in some parts of the hypothalamus directly sensitive to the chemistry of the blood and the extracellular fluids. in other parts receiving ascending sensory information from the nucleus of the solitary tract and it's relays into the hypothalamus regarding the chemical environment of the body. The hypothalamus is also sensitive to the signaling derived from humoral factors that are circulating in the blood. While all of this sensory information is integrated with contextual information that's derived from the telencephalon. Largely the orbital and medial parts of the prefrontal cortex. Other parts of the cerebral cortex project to the hypothalamus as well, as does the amygdala and the hippocampal formation in the medial parts of the temporal lobe. So the hypothalamus is integrating context, it's integrating sensory signals. And it's giving rise to output signals that are coordinating our visceral motor, our somatic motor, and our neuroendocrine systems. And the result is the production of appropriate behavioral responses, that reflect the coordinated activity of all these different dimensions of movement to produce behavior. One important concept in hypothalamic control that we're still trying to understand is this notion of biological set points. So this integration of context and sensory signal is referenced to some measure of homeostatic set point. Now we don't really understand this concept that as it's played out in neuro biological terms for very many of the systems that are coordinated in the hypothalamus. But we suspect it's a common mechanism and the implementation of this mechanism undoubtably gets us deep into moleculer biology of this circuitry. And how molecular mechanisms are mediating these set points. And how signaling within the neuron and perhaps even within the nucleus is producing patterns of gene expression in space. In particular, in circuits and in time in different conditions that demand different sorts of responses. Well, we'll learn a little bit more about this when we talk about the regulation of sleep and wakefulness. there are circuits in the hypothalamus for which the molecular set points are reasonably well understood. So I'm going to unpack a little bit of that in a later session. But for now I'll just broadly emphasize this general mechanism of hypothalamic function of integrating context and sensory signals and referencing against a set point. And, with deviation from that set point we have implementation of some kind of behavioral change.
