Okay, well, now let's turn our attention to the parasympathetic division of the visceral motor system. And we see that illustrated here, on the left-hand side of figure 20.1. So the parasympathetic division, likewise, is a two neuron connection, between the central nervous system and the viscera. However, the first neuron tends to project a much longer distance than did the first neuron or the preganglionic neuron on the sympathetic side. And the second importance difference about our preganglionic neurons is that in the parasympathetic division those preganglionic neurons are found in a different location. They're not found in the thoracic cord. They're found in the brain stem, and then, in the inferior end of the spinal cord, in the sacral segments of the spinal cord. So the preganglionics are found in different locations in the sympathetic and parasympathetic side. And their axons tend to go quite a different length with the parasympathetic preganglionic axons being distributed typically to the end organ itself. Where the ganglia that receive the synaptic connections are embedded in the end organ itself. Consequently, the ganglionic neuron in this system extends only a short distance to reach the effector tissue itself. So this illustration is not quite anatomically accurate in that respect. These ganglia are often embedded in the tissue itself. so the locations here are meant to be more illustrative of this relationship between pre and postganglionic neurons, rather than describing their actual anatomical location of the ganglia that serve the viscera. Okay, well, with that said, let me just comment about the neurotransmitters, since we're here on this slide. And then we'll move on and see in more detail the locations of these preganglionic neurons. So, the neurotransmitter of the preganglionic neuron, like the sympathetic division is acetylcholine. So, these preganglionic, parasympathetic axons that supply these targets emanating from the brain stem and from the sacral spinal cord, are going to release acetylcholine in their ganglia. Well, the post ganglionic neurotransmitter of the parasympathetic division is also acetylcholine. So, acetylcholine is a prominent neurotransmitter on both sides of this system. Now, I should mention on the sympathetic side, there is one place where we know that acetylcholine is the ganglionic neurotransmitter. And that's right here in our sweat glands. So the sympathetic division is what activates our sweat glands, and it does so via the mechanisms of action of the neurotransmitter acetylcholine. But otherwise, we associate acetylcholine with the parasympathetic ganglionic neurons. And we associate norepinephrine [NOISE] with the ganglionic neurons on the sympathetic side. Okay. Well, let's now have a look at the anatomy of these preganglionic neurons of the parasympathetic division of the visceral motor system, looking first at the brain stem. So, these neurons are found in four pairs of nuclei of the brain stem. And, beginning in the midbrain, we have the Edinger-Westphal nucleus. Now, I hope you'll, you will recall that this is the small parasympathetic nucleus that's associated with the third cranial nerve. This Edinger-Westphal nucleus is found just dorsal and a little bit lateral to the somatic motor neurons of the ocular motor complex. So, we can find that in Sylvius and I'd like to point that out in just a moment. But for now, just know that the Edinger-Westphal nucleus is a parasympathetic nucleus of the midbrain. The remaining parasympathetic nuclei of the brain stem are found in the caudal pons and in the medulla. So in the caudal pons, we have a set of nuclei that we call the salivatory nuclei. So these nuclei, provide for parasympathetic outflow to our tear glands and our saliva glands. Now, just at the junction of the caudal pons and then down into the upper part of the medulla, we have our dorsal motor nucleus of vagus and the nucleus ambiguus. Now the dorsal motor nucleus of vagus is a primary source of axons that supply visceral targets via the vagus nerve. one additional, contribution to that vagus nerve, that is very important in governing the activity of the heart, comes from a set of cells that are anatomically associated with the nucleus ambiguus. So, the nucleus ambiguous really has at least two distinct populations of neurons. There are the brachial motor neurons, that supply the muscles of the larynx and the pharynx. And then there are these parasympathetic preganglionic neurons that are involved in slowing the heart. Okay? So these are the four major sources of, parasympathetic preganglionic neurons in the brain stem. And here they are in cross-sections. So here's our Edinger-Westphal nucleus, up in the midbrain. Somewhere in this location, we don't need to attempt to find it precisely, are the salivatory nuclei. My, my colleague, Mel Cant, who helped me write these tutorial notes for you, often notes that, there are very few people in the world who know where to find the salivatory nuclei in the, caudal pons in the upper medulla. Well, a little bit more prominently and a little bit easier to find is the dorsal motor nucleus of vagus and the nucleus ambiguus which are found in the upper part of the medulla. So here we are in Sylvius and let's just find the structures. So, if we, look at the midbrain section, what you'll see if we follow back the nerve roots of the ocular motor nerve is our ocular motor nucleus. So this is the source of the somatic motor, outflow that governs those, strided muscles that we have in the orbit. And, a levator muscle that opens our eyelids. But just on the dorsal aspect of this oculomotor complex is where we find our Edinger-Westphal nucleus. So again, this nucleus gives rise to parasympathetic preganglionic axons that have their cell bodies here. And then they grow their axon out through the third nerve to constrict the iris in the eye. Alright, so, next let's jump into the caudal part of the pons. And somewhere in this tegmentum, in the, core of the brain stem, is where we would find our salivatory nuclei. So we don't identify them in Sylvius. I don't happen to be among the small number of people in the world that know precisely where to look for these neurons. I suspect I could find them if we were looking at a Nissl stain. But, in a Myelin stain, there just aren't enough cells to produce a distinct cluster of, of, gray matter that we could recognize in a Myelin stain here. So I won't attempt to point out where those salivatory nuclei are. However, if we go into the upper part of the medulla, maybe just go down one more section. I certainly can highlight the location of the dorsal motor nucleus of vagus. So this is a major parasympathetic preganglionic outflow that supplies for much of the innervation of the vagus nerve throughout the viscera of the thorax and the abdomen. Now, in a more anterior position in the tegmentum, we find the location of the nucleus ambiguus. And so with respect to visceral motor control, this is where we find our cardio-inhibitory parasympathetic preganglionic neurons. Well, that's it for the parasympathetic preganglionics of the brain stem. to find the rest, we need to move into the sacral segments of the spinal cord. So, let's do so. And, find our way down to the, most inferior section in our brain stem spinal cord atlas. And, here we are in the sacral level of the spinal cord. I know that because there's a very large ventral horn, and not much white matter associated with the spinal cord. And if we look into the dorsal column region, we find that there is only one division of the dorsal column, a gracile tract, no cuneate tract. So, hopefully those cues are now beginning to register with you, as you look at cross-sections at different levels of the spinal cord. Okay, so we're looking for our preganglionic parasympathetic neurons. And we find them in a location that's somewhat similar to where we found a lateral horn of the thoracic cord. But in the sacral cord, we really don't have a distinct lateral horn. Rather, we just have fairly, a smooth outline of the dorsal horn and the intermediate gray between the dorsal and the ventral horn. Then, of course, a very large bulging out, ventral horn. So, we don't have a lateral horn, but we do have a column of cells that we can recognize as our sacral autonomic nuclei or our preganglionic parasympathetic nuclei. So this is where they're found. Okay? So, let's return to our slides and consider this arrangement in a more, illustrative configuration.
