MITOCW | MIT9_14S14_lec09.mp3 The following content is provided under a Creative Commons license. Your support will help MIT OpenCourseWare continue to offer high-quality educational resources for free. To make a donation or view additional materials from hundreds of MIT courses, visit MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: This is the ninth session, but we have a little bit from the last session to finish. I want to say just a little more about the autonomic nervous system. And this is where we ended last time. And let's go back to when we talked about the early development of the spinal cord and the formation of the sympathetic ganglia. This was the picture we used. You see the thickening of the neural plate there, formation of the neural groove, and then the neural tube, formation of the dorsal root ganglia. But then, from those neural crest cells, we also have the formation-- some of these cells migrate and form the paravertibral ganglia, a series of interconnected ganglia alongside, just in front and to the side of the spinal cord. They're outside the vertebrae. In between the paravertibral ganglia and the neural tube there are the spinal vertebrae. And then there's also the prevertibral ganglia. One is the largest, cilia ganglion, innervating the gut. So we're going to talk about that innervation, sympathetic innervation pattern, first. The preganglionic motor neurons that innervate these three ganglia you see hear, sympathetic ganglia, are found from the first thoracic segment in the cord to about the second lumbar segment. And if you look at this series of sections that we saw before in slightly different colors, you'll see the little lateral horn in these three segments. This is L1 and to of the thoracic segments. There you see the little protrusion of the lateral horn. And if we take just a section-- this would be one of these sections-- there's the lateral horn, and I'm showing some of the preganglionic motor neurons. I will also show an axon descending from the brain that's connecting there directly to the lateral horn neurons. 1 So these are preganglionic of the sympathetic system. So they're sending their axon out to a ganglia, either the paravertibral ganglion here at that level or a prevertibral ganglion, like this cilia ganglion, which I show here sending an axon to the smooth muscle of the gut. And notice that axons of the paravertibral ganglion cells send their axon back into the ventral root. These are the communicating nerves that connect the paravertibral ganglia, each of them, to the ventral roots. So the axon that goes back into the ventral root, besides to some of the intestinal tract, it's going to smooth muscles, glands, blood vessels, the arrector pili muscles, the muscles that make the hair fluff up, and sweat glands. And the upper thoracic, some of those nerves go to the cardiac muscles. This is a typical textbook illustration. This is one, I think, from the earlier Brodal volume. This is Per Brodal. And it shows the spinal cord. These would be lateral horn neurons sending their axons out to terminate. The paravertibral ganglia-- he just shows the chain of ganglia on one side. And then he shows a prevertibral ganglion here. There's also plexi. A plexus is just a tangle of nerve fibers. It may or may not have ganglion cells in it. The cilia ganglion, like you see here, has cells of the synthetic nervous system in it. And the axons of those cells then go out to the end organs. Now, for the parasympathetic, it's quite different in the nature of the innervation. Because now the preganglionic motor neurons are up in the brain. They're in the dorsal motor nucleus of the vagus nerve. They're in salivatory nuclei, very tiny nuclei in the upper hindbrain, the rostral hindbrain, or in a little part of the third nerve nucleus, the Edinger-Westphal nucleus, the nucleus that controls the pupus. OK, so now in the case of a parasympathetic system, the postsynaptic cell here, it gets the synapses from the brain, the axon coming from the brain. It's always near the end organ. So for the eye, it's just a ganglion behind the eye. And the axons go to the iris. 2 With the gut, it's cells right in the wall of the gut. And as we will see, they form an entire nervous system by themselves. But there you see the axon of the postganglionic cell innervating the smooth muscle of the gut. And the same is true for pelvic organs here. This is, you can see, another way textbooks sometimes illustrate it here. They're showing three sections of the brain, midbrain, and two of the hindbrain, showing how the axons of the parasympathetic system go out to a ganglion. And they're just sketching some of those ganglia that are near the organs being innervated. And then you have the same thing with the [INAUDIBLE] for the innervation of the organs of lumenation and the sexual organs. OK. So what about the neurotransmitters? How are the neurotransmitters different? Remember when we encountered this before? We talked about the discovery of Otto Loewi? What was his discovery? What was he studying the innervation of? The heart, exactly. And he found that it was a chemical released by the neurons that was causing the heart to speed up or slow down when the accelerator nerve or the decelerator nerve were activated. But Loewi didn't actually discovered the neurotransmitters of the autonomic innervation of the heart. What are the neurotransmitters? Acetylcholine for both. It actually is true for both if we're talking about the neurotransmitter release in the autonomic ganglia. It's always acetylcholine-- sympathetic and parasympathetic. But when the organ itself is innervated by the ganglion cell axon, then the neurotransmitters are different. So then what you were about to say is correct, they are different. Acetylcholine for the parasympathetic system in almost all cases and noradrenaline, or norepinephrine is the technical name, for-- that's the one that speeds the heart, norepinephrine. Norepinephrine or noradrenaline is very similar. Adrenaline is actually a trade name, but it's often used. The molecule is very similar to that of adrenaline, released by the adrenal glands. And they have similar effects on the postsynaptic side. So here I've just shown the innervation of the heart and the innervation of the gut. 3 If you look at the gut here, I show acetylcholine being released in the cilia ganglion. And then the axon that innervates the gut is releasing norepinephrine, or is often just N-E in my abbreviations, or A-C-H for acetylcholine if it's parasympathetic. And notice the preganglionic cell for the-- you could be way down in the lower gut. But the parasympathetic axon originates here in the dorsal motor nucleus, the vagus nerve in the hindbrain. So the reason it's called the vagus nerve, it's a wandering nerve. It goes through a lot of the body, from all the way up here to all the way down to the body. So vagus means the wanderer, the vagus nerve, the 10th cranial nerve. So what about the enteric nervous system? I just mentioned it. Why do you think it's considered to be a separate system? It's it just the ganglionic neurons of the parasympathetic? It actually isn't quite that way. It is that, but it's much more. First of all, it's semi- autonomous. That means the neurons connect with each other and interact. Peristalsis in the gut is regulated by it. That will go on even if the gut loses its innervation in the brain. And there may be as many neurons in that system as in the entire spinal cord. This was all discovered in the last 20 years with advances in neuroanatomical methods. So most of these neurons are in the walls of the intestine. It's a network of multiple plexi. And here I list the different layers of the plexus, each containing neurons. It's innervated by the vagus nerve. It's similar to the cardiac ganglion, but we think the cardiac ganglion also has some interconnections within itself. So maybe the heart has a brain too, but the heart definitely has, and it's called the enteric nervous system. And I also point out here that various neurotransmitters are used in that system, various peptides, various neurotransmitters, not only acetylcholine. So that was a major discovery, very important in modern medicine, and of course opened up all 4 kinds of new treatments with greater knowledge of the neurotransmitters there. I want to discuss just briefly the hierarchy of central control in the autonomic nervous system. And the best way to illustrate that is to talk about control of body temperature. We know body temperature is regulated by autonomic nervous system. And you could say in general for autonomic system, there's different levels of control, spinal level, hindbrain level, midbrain level, and forebrain level. Temperature regulation is the thing that's been studied most thoroughly by this woman, Evelyn Satinoff. She's written some very nice review papers. I think I cite a major review she wrote in the book as a suggested reading. So what does this mean, that there's these multiple levels? It means that if you have an animal in which the spinal cord has been disconnected from the brain-- so the brain can influence only the head region, no longer influences the body-- you still have temperature regulation of the body.