Okay then, well let's consider the systems within the central nervous system that organize our visceral motor output. So first speaking very broadly, and then we'll begin to look at each division in terms of structure-function relations. So beginning up in the forebrain. We will talk about higher brain centers that are principally in the ventral and medial aspects of the forebrain, those parts of the brain that we associate with the limbic system. Or, as you'll hear me talk later, I prefer the term limbic forebrain, rather than limbic system, because there's really not one system associated with this set of structures, but probably multiple systems that have different kinds of functions. Nevertheless, these structures form a limbus, or rim, around the corpus callosum in the diencephalon, and extending into the medial parts of the temporal lobe. Well, with respect to visceral motor control, the relevant aspects of the limbic forebrain are the cingulate gyrus, the orbital and medial division of the prefrontal cortex, and the amygdala. So these structures are telencephalic. They integrate a tremendous amount of information coming from just about every source that one can think of. And the result of this integration is some sort of governance of the output of our visceral motor system. Among other possible consequences of integration in the limbic forebrain. Well one key structure that mediates the effect of these telencephalic structures on our motor elements in the brain stem of the spinal cord is the hypothalamus. And working closely with the hypothalamus, are networks of neurons in the reticular formation of the brain stem. So, these together we might call higher brain centers interact with our lower motor neuronal pools, that ultimately convey the signals out to the vector structures in the viscera. And so, the connections, then, between the higher brain centers and the lower motor neural pools become important. So there are, of course then, descending projections, that connect structures like the amygdala. The hypothalamus, the reticular formation with circuits of lower motor neurons, more closely associated with the relevant cranial nerve nuclei and the neurons that exit the spinal cord interact with our ganglionic neurons that govern sympathetic and parasympathetic overflow via the spinal cord. So, indeed, we're going to want to roughly localize these descending projections, but primarily focus on the distribution of our preganglionic and ganglionic neurons, when we talk about the visceral motor outflow. So one point of emphasis in your study will be to understand. How the organization of the sympathetic and parasympathetic divisions differ not only functionally but also anatomically. Well let's begin to look at that and consider just very broadly the distribution of elements that convey the outflow from the central nervous system to the viscera. So this is a figure from the textbook that I know is small, and it may be hard to appreciate in this view. but I'll try to highlight just a few features here for you that might, might make it more obvious to you what the points are that I want you to take from this. And you'll probably find some versions of this information in various sources that are more readily accessible to you. Well, lemme just state that, while we're looking at the impact of visceral motor outflow on a variety of effector systems, I, by no means, intend for you to learn or commit to memory each single action on each of the end organs that are illustrated here. what's more important to me is that you understand the basic functional distinctions between the sympathetic and parasympathetic divisions, and that you know something about the organization from an anatomical perspective, especially how preganglionic neurons connect to ganglionic neurons. Okay, well let, let's step through this. And highlight a few salient features here. So let's begin on the right side of this figure. And on the right side we have a depiction of the sympathetic division of the nervous system. and what I want you to recognize is that to get from the spinal cord to the viscera there is a two-neuron chain. The first neuron is distributed in the thoracic levels of the spinal cord. And so we see these representations of these neurons, here in a column of cells that we call the intermediolateral cell column. This is where we find the neuron that connects the central nervous system to the neuron that ultimately supplies the viscera. So, because it's a two-neuron chain, this neuron that we find in the thoracic spinal cord, we call a preganglionic neuron. So this preganglionic neuron sends an axon a short distance. Out through the ventral route to the spinal chord. And it connects to a ganglion. Now this term ganglion, as I think you now appreciate, is a word that we use to describe a collection of cell bodies outside the central nervous system. we'll talk. As we have already about the basal ganglia as being really an exceptional use of this term ganglia. so putting basal ganglia aside, the usual use of the term ganglia is to refer to these collections of neurons that sit outside the central nerovus system. Typically associated with a nerve. A nerve route, in this case, a chain of ganglia that run alongside the spinal cord just outside the vertebral bodies. So, we have this chain of elements that run alongside the spinal cord just outside the vertebral body. And it is within these ganglia that we have the cell bodies of the neurons that actually then go and project out to our end organ. Okay. So for example, let's look at the function of the sympathetic division with respect to the heart. Well, what happens when there's activation of these preganglionic neurons and the postganglionic elements is that the beating of the heart accelerates, and the contraction, or the contractility, of the left ventricle increases, thereby increasing the cardiac output. So the sympathetic system is associated with supplying a greater volume of blood to the body for those times when it is especially in need of extra energy in the form of oxygen and glucose. We'll spend a little bit of time talking about the governance of the bladder in the third part of this tutorial, so I won't say anymore about that now. Okay, so here's the important take home message that I want you to understand about the sympathetic division. it is a two-neuron chain. So two neurons from CNS to the peripheral organ, and the sympathetic division is preparing the body for action. So it's involved in mobilizing the resources of, of the body. And if one were to look across this entire set of effects that we see here on the right side of this figure. We could understand how the body is getting ready for action in various ways. Now lets have a, little bit of a closer look at this relationship of preganglionic to postganglionic neuron, in the sympathetic division of the visceral motor system. So, here's a bit of a closer look, now, at the thoracic spinal cord. And if we look in the thoracic spinal cord, I hope you'll recall that there's a distinctive feature of the grey matter of the thoracic spinal cord, that we find on the lateral aspect of that grey matter. In fact we call this feature the Lateral horn, which is one way that, if you're looking at cross sections of the spinal cord, you know you're in the thoracic region. If you see that distinctive lateral horn. So, this lateral horn is present because there is a column of neurons. And that column of neurons is called the intermediolateral cell column, and we find it right here. And these are the preganglionic neurons that send their axons out from the spinal cord, to supply the ganglionic neurons, which in turn innervate the viscera. So here's a look at this organization. So again we have the lateral horn of the spinal cord, we have a cell body that grows an axon that exits through a ventral root, and it's now entering the spinal nerve. But it exits that spinal nerve to join the sympathetic trunk. And we call this little branch here the white communicating ramus. white because, these axons are, well myelinated. And once these axons enter the sympathetic trunk they may make a synapse, with a ganglion at the same level of that spinal nerve. some of these axons will actually run some distance to reach more distant ganglia. Some might run right through the ganglion, in order to reach a more distant target. there are some ganglia that are not right along the sides of the vertebral bodies, but are set in a more interior location. At a somewhat greater distance, these are called prevertebral ganglia. So, those tend to be more common in the lower part of the viscera in the abdominal cavity. Well, okay. So these preganglionic axons then interact with a postganglionic neuron or a ganglionic neuron. That we find here in one of these, collections of cell bodies. And these ganglionic neurons then give rise to an axon that leaves the sympathetic trunk, joins up again with the spinal nerve, and then goes out to supply some visceral target. Now we call this little branch that reconnects the ganglion to the spinal nerve, a gray communicating ramus, because these axons, for the most part are unmyelinated or only very poorly myelinated. So they don't have quite the same glistening appearance as does the white communicating ramus. Well, just a word about the neurochemistry here. so, the transmitters of the ganglionic neurons and the preganglionic neurons are, are quite rich. And, and quite diverse. In fact, there are probably multiple neurotransmitters being released at these synaptic connections. Nevertheless, there, there are some dominant transmitters that you should know about. The neurotransmitter of the preganglionic neuron, and, this was actually highlighted in the previous slide here. So, the neurotransmitter of the preganglionic neuron. That connects the lateral horn of the thoracic core to the sympathetic truck, trunk, is the neurotransmitter acetylcholine. Now the neurotransmitter of the postganglionic neuron, that is the neuron that connects the ganglion itself to the end target here. this neuron is releasing the neurotransmitter norepinephrine. So, the modifier that we use to describe that is noradrenergic. And, as we'll come to see, these neurotransmitters interact with adrenergic receptors, the alpha and the beta receptors for norepinephrine. Where as the acetylcholine neurons, they interact with a variety of muscarinic acetylcholine receptors. And all of these neurotransmitter receptors in our visceral motor system are of the G protein coupled seven-transmembrane receptor family. So as you will recall, these are the receptors that mediate a broad diversity of post synaptic effects depending upon the second messenger systems that are activated by those G proteins. So these effects tend to be a little bit slower to develop than would be the case in our fast ionotropic receptors. but as I mentioned, the diversity effects can, can be quite remarkable, and in some cases quite long lasting.
