Welcome to a set of tutorials on the visceral motor system. I'd like to take this set of tutorials in, in three parts. this first part, I'd like to give you a bit of an overview including some discussion of the functional divisions of the visceral motor system. And then we'll have session on the hypothalamus, a very important part of the brain's means of controlling visceral motor activity. And then lastly, I'd like to illustrate some of the principles of how visceral motor activity is integrated with sensory input, as well as somatic motor output, in the governance of one important function, and that's the function of controlling the bladder. Well, this topic of visceral motor integration and visceral motor control relates to some of our core concepts in the field of neuroscience. You're very used to hearing me assert that the brain is the body's most complex organ. But that is a fact that I hope is not lost on any of us as we pursue a deeper understanding of how the brain governs the body within which it resides. We'll also be talking more about circuitry today, that is genetically determined, but still subject to life experiences and the modifications that use can feed back into a functioning nervous system. Alright, so in this first part of our tutorial on the visceral motor system, I have three learning objectives for you. First, I want you to be able to describe the anatomical organization of the sympathetic and parasympathetic divisions of the visceral motor system, including the sources of preganglionic innervation and the location of the postganglionic visceral motor neurons. So, as this objective implies, there are important differences between these two major divisions of the visceral motor system that I want you to be able to have a discussion about, perhaps with a friend, even a family member who may be curious as to what you're learning in Medical Neuroscience. I want to you to be able to characterize the major functions of the sympathetic and parasympathetic divisions. Now, that's not to say that I want you to account for sympathetic and parasympathetic governance of each visceral-motor effector system in the body. that would be beyond the scope of this course. But rather, I want you to be able to have a discussion that highlights the general characteristics of sympathetic activation, parasympathetic activation. And one great way to do that is to pick out a subset of those visceral motor effectors that are particularly instructive for you, and think about how do these two systems interact to govern the activity of that system? And then finally, I want you to be able to identify and discuss the neural centers in the central nervous system that regulate the outflow of activity in the preganglionic fibers of the visceral motor system. So we want to back up to higher centers in the brain and understand, how does the brain govern the coordinated output of these two divisions of our visceral motor system? Well, let's now begin by considering in broad overview the functions of the visceral motor system. Well, what the visceral motor system does is it maintains the internal environment of the body, and it regulates the activity of the visceral organs, the glands and the blood vessels. The visceral motor system governs the circulation, respiration, digestion, metabolism, bodily secretions, thermal regulation and reproduction. Elements of this system, together with components of the somatic motor system, are involved in the experience and expression of emotional behavior, and indeed other dimensions of cognition that have an impact on the internal workings of our body. Well, I think you're familiar with some of the basic functional divisions of the visceral motor system. and perhaps I should pause here and just state that I've been using the term visceral motor system. You may be wondering, well aren't we talking about the autonomic nervous system and in-, indeed we are. these terms for the most part are synonymous. I prefer visceral motor systems because it allows us to focus on the effector systems that we're concerned about in this particular session. That is our visceral organs. There's also an element of autonomic nervous system function that involves the somatic motor system. So, I don't want us to conflate thinking about the viscera with thinking about somatic motor effectors. That's a point we'll come back to in the third part of this tutorial. Just so that you're not thrown off by my terminology, so from here on out I'll be referring to the visceral motor system as those aspects of autonomic function that pertain to the governance of visceral effector systems. Okay, with that said, we basically have three functional divisions of our visceral motor system. We have a sympathetic division which is involved in energy mobilization. There is a parasympathetic division, which is involved in energy conservation. So, already you get a feel for the complimentary actions of these two principal divisions of the visceral motor outflow. But there's also a third division that I'll just touch on very briefly. and that has to do with the neural networks that govern the activities of our gastrointestinal tract. We call this the enteric division. So this is a system of neurons that function in something of a semi-autonomous way. I'll say more about this, this system in just a minute. And what this system does, is it coordinates our digestive reflexes. Well again to think in very broad big picture terms, the coordinated activity, of these three divisions of the visceral motor system, is really all about two principle kinds of, of functions. one is homeostasis, so this is a, a wonderful term coined back in the early part of the 20th Century by a great physiologist by the name of Walter Cannon. And what Cannon had in mind with homeostasis, is the constancy of the internal environment. That is working to keep the body more or less at a set point of some sort. Well, homeostasis is what our bodies are about arguably most of the time. But we couldn't survive very long if this was the only mode of action of our visceral motor system. indeed, there are times when in order to restore that proper balance, we have to change, we have to push the body into rather emergency action. And so this concept is what we might have in mind with this term allostasis which was coined more recently by a contemporary neuroscientist name Bruce McEwen. An allostasis refers to imposing a change of the internal environment, so that we can eventually cycle back and re-establish homeostasis. And I think these two terms, homeostasis and allostasis, they capture a sense of what the visceral motor system is all about. It's about trying to maintain the constancy of the internal environment, and the capacity to radically alter the state of the internal environment when the conditions demand for such action. So, we'll see some examples of that as we go, and talk about how these two aspects of this remoter function are, are played out. Well, before we get into more detail of the anatomy and the function of our two principle divisions, our sympathetic and parasympathetic divisions, I just want to mention very briefly the enteric division. So the enteric division really refers to a network of neurons that are found in the lining of the gut. So, here's an illustration that reveals just a small piece of representative tissue from the lining of our gastrointestional system. for example this could very well be the lining of the small intestine. And what I want you to notice are these networks of neural processes that run through particular layers of this tissue. So they are actually two networks we, we call each network a plexus. So in close association with the muscular layers of the gastrointestinal system, especially the longitudinal muscles that are, are responsible for moving the contents of our gastrointestinal tract along through a pattern of muscle contraction that will propel in a wave-like fashion. The contents of the gut along the GI tract. this plexus is called the myenteric plexus, or Auerbach's plexus. So it's responsible for coordinating this rhythmical contraction of our longitudinal muscle. Well, there is a deeper plexus, and this plexus is found in close association with the mucosa that lines the gastrointestinal system, this is called the submucous plexus. And this submucous plexus also known as Meissner's plexus, this is a network of neurons that coordinates the secretions of the various glands that are very important in mediating digestion, within the gastrointestinal system. Now these two networks of neurons, they can function in a semi-autonomous way, that is, if one were to have damage to the spinal cord or the nerves that supply, this network, then this network can still function, however the functions are not very well coordinated with respect to the needs of digestion. So there definitely is input to this enteric division of the visceral motor system from both sympathetic and parasympathetic sources. never the less, there's a certain degree of autonomy that we recognize in the functions of this enteric division. Okay well that's all I'm going to say about the GI system. we certainly could say more. There is a, a rich degree of, of neurobiology in just about every way associated with the activities of neurons in these networks. but we'll turn our attention then to the sympathetic and parasympathetic divisions and then the means by which the brain regulates these two sets of outflows from our visceral motor centers within the brain stem and the spinal cord.
