1 00:00:00,250 --> 00:00:01,800 The following content is provided 2 00:00:01,800 --> 00:00:04,030 under a Creative Commons license. 3 00:00:04,030 --> 00:00:06,890 Your support will help MIT OpenCourseWare continue 4 00:00:06,890 --> 00:00:10,740 to offer high-quality educational resources for free. 5 00:00:10,740 --> 00:00:13,360 To make a donation or view additional materials 6 00:00:13,360 --> 00:00:17,239 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,239 --> 00:00:17,864 at ocw.mit.edu. 8 00:00:23,710 --> 00:00:26,570 PROFESSOR: OK, this class, class eight, 9 00:00:26,570 --> 00:00:33,770 we're going to talk about what happens to that cord, 10 00:00:33,770 --> 00:00:36,890 the development of which we were talking about last time. 11 00:00:36,890 --> 00:00:40,510 So we're going to look at the adult spinal cord. 12 00:00:40,510 --> 00:00:44,740 And that will lead to a discussion 13 00:00:44,740 --> 00:00:49,182 of the autonomic nervous for the last part of class. 14 00:00:49,182 --> 00:00:54,020 So we'll look at the different levels, talk 15 00:00:54,020 --> 00:00:59,050 about the sensory channels that we've already introduced-- 16 00:00:59,050 --> 00:01:02,630 reflex, spinocerebellar, spinothalamic tracts, 17 00:01:02,630 --> 00:01:09,650 and another major lemniscal tracts-- 18 00:01:09,650 --> 00:01:13,850 and then you'll see the major descending 19 00:01:13,850 --> 00:01:16,470 pathways in the cord. 20 00:01:16,470 --> 00:01:19,060 At this point, because they originate 21 00:01:19,060 --> 00:01:21,600 from above the cord, areas that we've not 22 00:01:21,600 --> 00:01:24,310 talked about in detail, this is really 23 00:01:24,310 --> 00:01:26,920 just an introduction to those pathways. 24 00:01:32,707 --> 00:01:34,790 But the more you're exposed to it, the more likely 25 00:01:34,790 --> 00:01:36,660 you are to remember it, and we'll 26 00:01:36,660 --> 00:01:39,210 talk about propriospinal fibers. 27 00:01:39,210 --> 00:01:47,460 OK, so first of all, how do the columns of axons of the cord 28 00:01:47,460 --> 00:01:51,440 change as you go from one level to the other? 29 00:01:51,440 --> 00:01:55,880 It's really a very simple question, 30 00:01:55,880 --> 00:01:58,680 and I'm asking why they change. 31 00:01:58,680 --> 00:02:00,900 And the answer there is also pretty simple. 32 00:02:00,900 --> 00:02:03,090 And then I ask one other thing about those 33 00:02:03,090 --> 00:02:06,470 outlines-- what's the lateral horn? 34 00:02:06,470 --> 00:02:08,204 So did you try to figure that out? 35 00:02:11,120 --> 00:02:14,490 The columns of fibers-- if you compare, say, 36 00:02:14,490 --> 00:02:19,120 the cervical level and the lower lumbar level. 37 00:02:19,120 --> 00:02:21,300 Yes? 38 00:02:21,300 --> 00:02:22,152 Sorry? 39 00:02:22,152 --> 00:02:23,610 AUDIENCE: Is it myelination levels? 40 00:02:23,610 --> 00:02:27,195 PROFESSOR: No, the myelin will be there at all levels. 41 00:02:27,195 --> 00:02:29,972 AUDIENCE: [INAUDIBLE] get smaller. 42 00:02:29,972 --> 00:02:31,930 PROFESSOR: Yeah, the relative size will change. 43 00:02:31,930 --> 00:02:34,224 Why? 44 00:02:34,224 --> 00:02:37,020 AUDIENCE: As you go down-- 45 00:02:37,020 --> 00:02:38,340 PROFESSOR: Speak up. 46 00:02:38,340 --> 00:02:42,910 AUDIENCE: --count down the more of those ascending tracts are-- 47 00:02:42,910 --> 00:02:46,105 PROFESSOR: More of them are terminated as you come down. 48 00:02:46,105 --> 00:02:49,305 So there's fewer and fewer axons that haven't terminated yet. 49 00:02:49,305 --> 00:02:52,000 And what about the ones going up? 50 00:02:55,050 --> 00:03:00,660 More and more are being added-- very straightforward. 51 00:03:00,660 --> 00:03:03,900 And do you remember what the lateral horn is? 52 00:03:03,900 --> 00:03:05,990 We talk about dorsal horn and ventral horn 53 00:03:05,990 --> 00:03:09,320 all the time when we talk about spinal cord. 54 00:03:09,320 --> 00:03:10,695 AUDIENCE: So just for the record, 55 00:03:10,695 --> 00:03:13,665 I guess they'll use the [INAUDIBLE]. 56 00:03:13,665 --> 00:03:17,625 The ratio is more in favor of the-- 57 00:03:20,300 --> 00:03:21,425 PROFESSOR: Look at it here. 58 00:03:23,980 --> 00:03:26,100 White matter is in brown here. 59 00:03:29,080 --> 00:03:31,100 Do you remember why we call it white matter? 60 00:03:33,880 --> 00:03:39,790 When it's unstained in the whole brain-- if you do a dissection 61 00:03:39,790 --> 00:03:44,720 or if you do a section, but don't stain it-- 62 00:03:44,720 --> 00:03:49,980 the myelin reflects the light more, so it looks white. 63 00:03:49,980 --> 00:03:52,560 It reflects all the different colors. 64 00:03:52,560 --> 00:03:56,108 And the gray matter here I've shown in a lighter color. 65 00:03:56,108 --> 00:03:58,248 AUDIENCE: I was going to ask-- and this is not 66 00:03:58,248 --> 00:03:59,566 really related to this class. 67 00:03:59,566 --> 00:04:03,024 But this picture, I [INAUDIBLE] for a long time. 68 00:04:03,024 --> 00:04:05,000 So I didn't realize the colors were reversed. 69 00:04:07,970 --> 00:04:11,360 PROFESSOR: This is something I do quite freely, 70 00:04:11,360 --> 00:04:13,020 and you have to get used to it. 71 00:04:13,020 --> 00:04:14,910 You have to get familiar enough with it 72 00:04:14,910 --> 00:04:16,700 so that's never a problem. 73 00:04:16,700 --> 00:04:18,029 And you will. 74 00:04:18,029 --> 00:04:22,380 When we use different stains, it's going to look different. 75 00:04:22,380 --> 00:04:26,980 And almost all stains for axons make them dark. 76 00:04:26,980 --> 00:04:32,580 So in any axon stain, it's going to look more like this. 77 00:04:38,620 --> 00:04:42,760 So you see, the dorsal columns, the lateral columns, 78 00:04:42,760 --> 00:04:45,270 and the ventral columns here, are all 79 00:04:45,270 --> 00:04:48,300 much bigger at the top than at the bottom, 80 00:04:48,300 --> 00:04:51,610 just for the reasons we were just talking about. 81 00:04:51,610 --> 00:04:53,380 And what about that lateral horn? 82 00:04:53,380 --> 00:04:57,390 Just to identify it here, you'll see 83 00:04:57,390 --> 00:05:00,130 that there's a dorsal horn and a ventral horn. 84 00:05:00,130 --> 00:05:04,125 The ventral horn is pretty wide here 85 00:05:04,125 --> 00:05:07,470 because of the cervical enlargement. 86 00:05:07,470 --> 00:05:10,405 But right here, you see on the left side, 87 00:05:10,405 --> 00:05:12,320 there's a little lateral horn. 88 00:05:12,320 --> 00:05:15,380 And here, it's on both sides-- a little bump on the side. 89 00:05:18,090 --> 00:05:22,000 And it's still there in the upper part of the lumbar 90 00:05:22,000 --> 00:05:24,880 coordinate, and it disappears. 91 00:05:24,880 --> 00:05:26,550 So do you remember what that is? 92 00:05:30,680 --> 00:05:38,100 Preganglionic motor neurons of what-- 93 00:05:38,100 --> 00:05:42,490 the sympathetic part, the sympathetic nervous system. 94 00:05:42,490 --> 00:05:43,670 So we'll talk about that. 95 00:05:43,670 --> 00:05:46,400 We'll see it in enlarged figures and so forth, 96 00:05:46,400 --> 00:05:48,821 and we'll see exactly what's there. 97 00:05:48,821 --> 00:05:49,320 All right. 98 00:05:57,440 --> 00:06:01,880 Now, the thing we didn't talk about here, 99 00:06:01,880 --> 00:06:05,166 we talked about gray and white matter. 100 00:06:05,166 --> 00:06:08,220 Of course, grey matter, also in unstained sections 101 00:06:08,220 --> 00:06:12,090 or in a dissection, it looks grayer because of the cells 102 00:06:12,090 --> 00:06:15,500 and fewer myelinated axons. 103 00:06:15,500 --> 00:06:21,620 But look at the ventral horn in particular, 104 00:06:21,620 --> 00:06:24,400 and also just the size of the cord. 105 00:06:24,400 --> 00:06:30,230 The biggest section is here, not just above it. 106 00:06:30,230 --> 00:06:32,490 That's the cervical enlargement. 107 00:06:32,490 --> 00:06:36,130 So what's the reason for the cervical enlargement, and then 108 00:06:36,130 --> 00:06:38,460 the smaller one down here, the lumbar enlargement? 109 00:06:42,560 --> 00:06:43,985 You've got to speak up. 110 00:06:43,985 --> 00:06:45,410 AUDIENCE: Arms and legs. 111 00:06:45,410 --> 00:06:46,910 PROFESSOR: Arms and legs. 112 00:06:46,910 --> 00:06:51,480 So that means it should be different in animals 113 00:06:51,480 --> 00:06:53,960 without arms and legs, right? 114 00:06:53,960 --> 00:06:54,790 And that's true. 115 00:07:03,140 --> 00:07:05,820 So if we take the animal that has the largest 116 00:07:05,820 --> 00:07:08,930 legs of any animal that's ever lived, 117 00:07:08,930 --> 00:07:15,260 brontosaurus, where we have a complete skeleton-- 118 00:07:15,260 --> 00:07:17,820 it's on display still at Yale University, 119 00:07:17,820 --> 00:07:23,135 where the curator at Yale discovered 120 00:07:23,135 --> 00:07:26,380 it-- look at the lumbar enlargement 121 00:07:26,380 --> 00:07:29,230 where they're showing this whole central nervous system here, 122 00:07:29,230 --> 00:07:30,951 and compare it to the size of the brain. 123 00:07:30,951 --> 00:07:32,950 You'll see that the lumbar enlargement is bigger 124 00:07:32,950 --> 00:07:41,950 than the brain in brontosaurus, because the huge number 125 00:07:41,950 --> 00:07:48,730 of motor neurons and interneurons 126 00:07:48,730 --> 00:07:52,560 innervating those motor neurons to control these huge legs. 127 00:07:52,560 --> 00:07:56,910 And of course, there's also a lot of skin area there, 128 00:07:56,910 --> 00:07:59,590 a lot of surface area getting sensory input. 129 00:07:59,590 --> 00:08:03,810 Up above, it's illustrating a turtle spinal cord. 130 00:08:03,810 --> 00:08:06,762 Now, what's different about the turtle? 131 00:08:06,762 --> 00:08:07,720 Well, he's got a shell. 132 00:08:07,720 --> 00:08:16,990 So the body surface, except for his legs, 133 00:08:16,990 --> 00:08:18,454 there can't be much innervation. 134 00:08:18,454 --> 00:08:19,620 There's not much to respond. 135 00:08:19,620 --> 00:08:22,450 He doesn't respond from his shell. 136 00:08:22,450 --> 00:08:26,485 So here, in between the cervical and lumbar enlargements, 137 00:08:26,485 --> 00:08:31,240 you do see a lot of motor neurons controlling his limbs. 138 00:08:31,240 --> 00:08:34,539 Cervical enlargement would include this one 139 00:08:34,539 --> 00:08:36,590 prior to the left. 140 00:08:36,590 --> 00:08:39,789 But in between, and below the lumber enlargement, 141 00:08:39,789 --> 00:08:42,150 his cord gets very small. 142 00:08:42,150 --> 00:08:46,850 It's still needed, because it is innervating various bodily 143 00:08:46,850 --> 00:08:49,182 organs at those levels too. 144 00:08:53,610 --> 00:08:56,670 So who is this guy, Bror Rexed? 145 00:08:56,670 --> 00:09:01,630 Notice the pronunciation-- Rex-ed. 146 00:09:01,630 --> 00:09:04,420 Bror Rexed. 147 00:09:04,420 --> 00:09:09,160 He added something to the way we talk about the spinal cord, 148 00:09:09,160 --> 00:09:12,850 especially when we're doing cyto and fiber 149 00:09:12,850 --> 00:09:15,220 architecture of the cord. 150 00:09:15,220 --> 00:09:17,790 What was it he added? 151 00:09:17,790 --> 00:09:20,420 Layers. 152 00:09:20,420 --> 00:09:23,195 He studied the spinal cord of various animals 153 00:09:23,195 --> 00:09:26,880 as well as human and published a series of papers 154 00:09:26,880 --> 00:09:32,040 and was consistently able to define the same layers. 155 00:09:34,560 --> 00:09:41,490 And this is just an example where 156 00:09:41,490 --> 00:09:43,275 you see the Nissl-stained section. 157 00:09:46,870 --> 00:09:50,450 This is the seventh cervical segment. 158 00:09:50,450 --> 00:09:55,570 So these are motor neurons innervating the [INAUDIBLE], so 159 00:09:55,570 --> 00:09:58,616 the arms and hands. 160 00:09:58,616 --> 00:10:00,240 And you can see a lot of motor neurons. 161 00:10:00,240 --> 00:10:03,830 But they're in two groups-- the lateral group that's actually 162 00:10:03,830 --> 00:10:08,580 got several different clumps there, he's outlined it here. 163 00:10:08,580 --> 00:10:10,800 It's layer nine. 164 00:10:10,800 --> 00:10:13,780 And this one he called layer eight in here. 165 00:10:13,780 --> 00:10:15,980 These are so much layers as groupings. 166 00:10:15,980 --> 00:10:20,160 These are the motor neurons innervating the body axons. 167 00:10:20,160 --> 00:10:25,190 And then all of the interneurons that tend to be premotor, 168 00:10:25,190 --> 00:10:30,350 they contact the motor neurons, most of them. 169 00:10:30,350 --> 00:10:33,880 That's layer seven. 170 00:10:33,880 --> 00:10:37,680 He does have a layer 12, this area right 171 00:10:37,680 --> 00:10:41,500 around the ventricle there. 172 00:10:41,500 --> 00:10:45,330 Notice how relatively small the ventricle looks now. 173 00:10:45,330 --> 00:10:48,440 It's not because it got smaller since the embryo. 174 00:10:48,440 --> 00:10:51,790 It's that everything else got so much bigger. 175 00:10:51,790 --> 00:10:53,820 And then you see his other layers. 176 00:10:53,820 --> 00:10:57,730 And the dorsal horn in particular, 177 00:10:57,730 --> 00:11:02,130 it's very easy to identify the layers. 178 00:11:02,130 --> 00:11:05,370 So if I ask you to describe some differences in cytoarchitecture 179 00:11:05,370 --> 00:11:09,760 of the dorsal and ventral horns, you see, 180 00:11:09,760 --> 00:11:11,910 you would mention the motor neurons. 181 00:11:11,910 --> 00:11:17,250 They're the largest and usually deep-staining neurons, 182 00:11:17,250 --> 00:11:20,290 the motor neurons in the ventral horn. 183 00:11:20,290 --> 00:11:23,530 And the dorsal horn has the most clearly defined layers. 184 00:11:23,530 --> 00:11:30,310 You can see it best in a fiber stain, like here. 185 00:11:30,310 --> 00:11:33,260 Here's a human lumbar level, and you can see. 186 00:11:33,260 --> 00:11:37,464 Very light myelin there in layer three. 187 00:11:37,464 --> 00:11:41,120 So you see layers one, two, and three always stand out. 188 00:11:41,120 --> 00:11:42,500 The layers are easy to identify. 189 00:11:45,200 --> 00:11:47,780 There's one of a rat. 190 00:11:47,780 --> 00:11:52,790 Because of myelinated axons you can hardly see-- layer two 191 00:11:52,790 --> 00:11:54,650 doesn't look very light. 192 00:11:54,650 --> 00:11:59,740 And also I've done it with a very high contrast here. 193 00:11:59,740 --> 00:12:04,600 But the upper layers are very, very white. 194 00:12:04,600 --> 00:12:05,110 All right. 195 00:12:08,460 --> 00:12:15,610 So myelin has been called a crucial vertebrate innovation. 196 00:12:15,610 --> 00:12:19,200 That was a quote from Allman's book. 197 00:12:19,200 --> 00:12:24,925 So what is it that made myelin such a crucial innovation 198 00:12:24,925 --> 00:12:26,010 of the vertebrates? 199 00:12:26,010 --> 00:12:30,980 It's not found in invertebrates, and it's actually not found 200 00:12:30,980 --> 00:12:33,930 in the most primitive vertebrates, 201 00:12:33,930 --> 00:12:36,740 the jawless vertebrates, the hagfish and lamprey. 202 00:12:36,740 --> 00:12:41,090 But it is found in all the rest of them. 203 00:12:41,090 --> 00:12:43,690 So why was it so important? 204 00:12:43,690 --> 00:12:45,006 Yes? 205 00:12:45,006 --> 00:12:47,246 AUDIENCE: Does it help with increasing the speed? 206 00:12:49,990 --> 00:12:52,380 PROFESSOR: It allows the increase in speed of the action 207 00:12:52,380 --> 00:12:57,170 potential without increasing the diameter so much. 208 00:12:57,170 --> 00:13:00,840 Invertebrates do have pretty fast conduction, 209 00:13:00,840 --> 00:13:03,350 like in the squid. 210 00:13:03,350 --> 00:13:05,020 They're a pretty big animal. 211 00:13:05,020 --> 00:13:08,620 And to get fast enough conduction for their escape 212 00:13:08,620 --> 00:13:11,810 response, they've developed these giant axons. 213 00:13:11,810 --> 00:13:14,880 They're, like, a millimeter in diameter. 214 00:13:14,880 --> 00:13:20,040 But that is a lot of cell to maintain. 215 00:13:20,040 --> 00:13:25,410 If you want very many axons to get finer control, 216 00:13:25,410 --> 00:13:30,260 that wouldn't be very good for the vertebrates, the larger 217 00:13:30,260 --> 00:13:32,910 animals. 218 00:13:32,910 --> 00:13:35,970 Can you imagine a whale trying to control his muscles 219 00:13:35,970 --> 00:13:38,852 with any speed at all if he didn't have myelin? 220 00:13:38,852 --> 00:13:41,110 His movements would be incredibly slow. 221 00:13:41,110 --> 00:13:43,155 His reaction time would be very slow. 222 00:13:46,410 --> 00:13:47,578 Yeah? 223 00:13:47,578 --> 00:13:51,000 AUDIENCE: So myelin is like [INAUDIBLE], right? 224 00:13:51,000 --> 00:13:55,710 But I wasn't sure how, actually, it helps [INAUDIBLE]. 225 00:13:55,710 --> 00:13:57,156 PROFESSOR: OK. 226 00:13:57,156 --> 00:13:59,430 If you want to search in my book, 227 00:13:59,430 --> 00:14:03,060 just search-- you've got the PDF files, 228 00:14:03,060 --> 00:14:07,040 so you can search under myelin. 229 00:14:07,040 --> 00:14:11,170 And you can look for saltatory conduction. 230 00:14:11,170 --> 00:14:13,190 Saltatory means jumping. 231 00:14:16,860 --> 00:14:20,500 I did talk about this briefly in class before. 232 00:14:20,500 --> 00:14:24,940 The action potential jumps from node to node. 233 00:14:24,940 --> 00:14:27,000 A node is where there's no myelin. 234 00:14:27,000 --> 00:14:29,850 The myelin is never continuous. 235 00:14:29,850 --> 00:14:31,810 And where it is present, it insulates. 236 00:14:31,810 --> 00:14:34,880 So you can't get the ion hook through the membrane. 237 00:14:34,880 --> 00:14:38,060 So that means all the ions have to flow at those nodes. 238 00:14:38,060 --> 00:14:40,400 They're close enough together. 239 00:14:40,400 --> 00:14:47,370 So the detrimental conduction down in the membrane, 240 00:14:47,370 --> 00:14:51,030 which is very, very fast, is strong enough, 241 00:14:51,030 --> 00:14:54,140 it won't decay enough, that it will reach the next node 242 00:14:54,140 --> 00:14:56,670 and trigger the next action potential there. 243 00:14:56,670 --> 00:15:01,860 So it literally goes-- you only get the action potential 244 00:15:01,860 --> 00:15:03,010 at the nodes. 245 00:15:03,010 --> 00:15:04,914 AUDIENCE: So it's kind of like dominoes? 246 00:15:04,914 --> 00:15:05,955 PROFESSOR: Yeah, exactly. 247 00:15:05,955 --> 00:15:10,890 It's like a series of dominoes falling. 248 00:15:10,890 --> 00:15:12,610 And of course, if something happens, 249 00:15:12,610 --> 00:15:17,980 if you get an area where the node is too far, 250 00:15:17,980 --> 00:15:20,380 the action potential will just stop. 251 00:15:20,380 --> 00:15:23,270 And this does happen sometimes, especially 252 00:15:23,270 --> 00:15:25,660 in certain pathologies. 253 00:15:25,660 --> 00:15:29,200 So one more question here. 254 00:15:29,200 --> 00:15:31,390 How does the spinal cord of humans 255 00:15:31,390 --> 00:15:34,940 differ from the spinal cords of other mammals? 256 00:15:34,940 --> 00:15:40,540 How are humans different? 257 00:15:40,540 --> 00:15:42,755 What do we do better than other animals? 258 00:15:47,800 --> 00:15:50,870 Well, for one thing, this kind of thing. 259 00:15:50,870 --> 00:15:55,480 That requires a lot of dexterity, tremendous control 260 00:15:55,480 --> 00:15:57,310 of these fingers. 261 00:15:57,310 --> 00:15:59,720 That requires a lot of axons coming 262 00:15:59,720 --> 00:16:04,850 from the brain that controls motor neurons. 263 00:16:04,850 --> 00:16:08,360 So that's going to mean a lot of descending axons. 264 00:16:08,360 --> 00:16:14,010 So the white matter, with those descending myelinated axons, 265 00:16:14,010 --> 00:16:15,182 is going to be big. 266 00:16:15,182 --> 00:16:15,890 Another question? 267 00:16:15,890 --> 00:16:20,995 AUDIENCE: Do other animals have something 268 00:16:20,995 --> 00:16:25,370 to the equivalent of our [INAUDIBLE]? 269 00:16:25,370 --> 00:16:27,850 PROFESSOR: Yeah, all the animals have these same parts. 270 00:16:27,850 --> 00:16:29,730 You know what the cauda equina is? 271 00:16:32,690 --> 00:16:34,870 They're roots. 272 00:16:34,870 --> 00:16:38,980 They're dorsal and ventral roots below the level 273 00:16:38,980 --> 00:16:44,880 of the spinal cord, you see, because our spinal cord doesn't 274 00:16:44,880 --> 00:16:47,930 grow as fast as the rest of our body does. 275 00:16:47,930 --> 00:16:50,362 So the spinal cord ends at the small of your back 276 00:16:50,362 --> 00:16:53,190 here-- fairly high up. 277 00:16:53,190 --> 00:16:57,050 But your spinal column, the bony spinal column, 278 00:16:57,050 --> 00:16:59,470 continues further down. 279 00:16:59,470 --> 00:17:04,670 And in that spinal column, you have the dorsal and ventral 280 00:17:04,670 --> 00:17:09,369 roots for the caudalmost part of the cord. 281 00:17:09,369 --> 00:17:11,950 That will be similar in animals. 282 00:17:11,950 --> 00:17:16,230 It's very similar in other primates and humans. 283 00:17:16,230 --> 00:17:18,849 But it's present in other animals too. 284 00:17:18,849 --> 00:17:24,730 The biggest difference is in sensation and dexterity. 285 00:17:24,730 --> 00:17:30,410 To get that very-- we have a dense innervation 286 00:17:30,410 --> 00:17:33,582 of these fingertips. 287 00:17:33,582 --> 00:17:36,040 Now, for the spider monkey, he's got some dense innervation 288 00:17:36,040 --> 00:17:39,600 of his tail too, and he's very dexterous with his tail. 289 00:17:39,600 --> 00:17:41,960 He uses it a lot. 290 00:17:41,960 --> 00:17:45,720 So he also has a lot of fibers for that. 291 00:17:45,720 --> 00:17:49,850 So that means the dorsal roots, and that leads them 292 00:17:49,850 --> 00:17:54,590 to the dorsal columns for the fast-conducting pathway that's 293 00:17:54,590 --> 00:17:56,884 also bigger in humans. 294 00:17:56,884 --> 00:17:59,850 AUDIENCE: What about our ability to stand upright? 295 00:17:59,850 --> 00:18:02,280 PROFESSOR: The ability to stand upright, 296 00:18:02,280 --> 00:18:06,170 that required evolution of special feet, 297 00:18:06,170 --> 00:18:09,310 special vestibular control, vestibular reflexes, 298 00:18:09,310 --> 00:18:14,050 and somatosensory reflexes from the soles of our feet. 299 00:18:14,050 --> 00:18:16,830 So people that wear thick prosthetics in their feet 300 00:18:16,830 --> 00:18:20,640 don't lose some of that. 301 00:18:20,640 --> 00:18:25,420 But we compensate very well for it in various ways. 302 00:18:25,420 --> 00:18:27,540 So don't worry about it if you do. 303 00:18:30,800 --> 00:18:34,250 All right, so just note here, I'm 304 00:18:34,250 --> 00:18:39,110 going to compare human and rat. 305 00:18:39,110 --> 00:18:41,930 Look at the lateral column here. 306 00:18:41,930 --> 00:18:45,260 You can see it in the drawing more easily. 307 00:18:45,260 --> 00:18:46,975 And now look at it in the rat. 308 00:18:50,380 --> 00:18:52,480 There it is in human. 309 00:18:52,480 --> 00:18:54,680 Why is this so much bigger? 310 00:18:54,680 --> 00:18:58,800 Especially because of the descending axons, 311 00:18:58,800 --> 00:19:01,605 coming from the neocortex. 312 00:19:04,300 --> 00:19:06,240 The dorsal columns are also bigger, 313 00:19:06,240 --> 00:19:10,640 but this is lumbar level, where the difference is great. 314 00:19:10,640 --> 00:19:18,850 But you can see this is not as big as this. 315 00:19:18,850 --> 00:19:22,820 But you notice that difference very much more 316 00:19:22,820 --> 00:19:24,450 at the cervical level. 317 00:19:24,450 --> 00:19:25,780 All right. 318 00:19:25,780 --> 00:19:28,220 So let's look at the sensory channels now. 319 00:19:28,220 --> 00:19:31,110 We'll talk about reflexes, the spinoreticular, 320 00:19:31,110 --> 00:19:35,370 spinothalamic tracts, spinocerebellar, 321 00:19:35,370 --> 00:19:38,860 and how the dorsal column axons get started. 322 00:19:43,290 --> 00:19:45,825 We'll start with this question-- where 323 00:19:45,825 --> 00:19:51,322 do the largest axons in the dorsal roots originate? 324 00:19:51,322 --> 00:19:52,780 So now we're going to start looking 325 00:19:52,780 --> 00:19:54,510 at cross-sections of the cord. 326 00:19:54,510 --> 00:19:57,590 We're going to look at the axons coming in 327 00:19:57,590 --> 00:19:59,990 and the circuits they form. 328 00:19:59,990 --> 00:20:03,300 But where do the largest axons arise? 329 00:20:03,300 --> 00:20:05,880 I talked about it twice in the class already, 330 00:20:05,880 --> 00:20:07,900 but you may not remember. 331 00:20:07,900 --> 00:20:11,340 That's why I repeat myself a lot-- 332 00:20:11,340 --> 00:20:16,010 because I know that when you're learning neuroanatomy, 333 00:20:16,010 --> 00:20:18,082 you're hit with a lot of these things. 334 00:20:18,082 --> 00:20:20,100 And you have to hear it again and again. 335 00:20:20,100 --> 00:20:23,080 So I often tell you, just understand me. 336 00:20:23,080 --> 00:20:25,470 Don't worry about writing it all down. 337 00:20:25,470 --> 00:20:27,170 Just understand me. 338 00:20:27,170 --> 00:20:29,860 If you understand and you keep encountering it, 339 00:20:29,860 --> 00:20:33,300 you just start to remember. 340 00:20:33,300 --> 00:20:35,980 That's also why it's useful to come to class-- those 341 00:20:35,980 --> 00:20:42,640 of you who are not here-- because going over it 342 00:20:42,640 --> 00:20:45,620 is actually very helpful for learning. 343 00:20:45,620 --> 00:20:48,440 Some of you, in some of your classes, can read it once 344 00:20:48,440 --> 00:20:49,630 and you'll be fine. 345 00:20:49,630 --> 00:20:51,640 If you do that in neuroanatomy, you 346 00:20:51,640 --> 00:20:55,747 won't be fine, unless you are-- maybe Sheldon could do it. 347 00:20:58,669 --> 00:20:59,643 All right. 348 00:21:04,040 --> 00:21:06,640 In case you don't know what I'm talking about, 349 00:21:06,640 --> 00:21:10,910 you don't want watch TV in the evenings, The Big Bang Theory. 350 00:21:10,910 --> 00:21:11,410 OK. 351 00:21:11,410 --> 00:21:14,395 Here is a section of the cord, and this 352 00:21:14,395 --> 00:21:15,870 is from a medical book. 353 00:21:15,870 --> 00:21:18,330 And it's illustrating different sizes 354 00:21:18,330 --> 00:21:20,270 of axons in the dorsal root. 355 00:21:20,270 --> 00:21:27,370 It just gives one example of the largest axons here. 356 00:21:27,370 --> 00:21:30,690 They're called A alpha axons. 357 00:21:30,690 --> 00:21:34,098 Do you remember where they're coming from? 358 00:21:34,098 --> 00:21:39,380 They're coming from the stretch receptors in muscles. 359 00:21:39,380 --> 00:21:42,215 Where are the stretch receptors the densest? 360 00:21:46,725 --> 00:21:50,030 These muscles doing this. 361 00:21:50,030 --> 00:21:52,740 Any place we have a really good control-- 362 00:21:52,740 --> 00:21:55,007 where would another good example be? 363 00:21:55,007 --> 00:21:56,001 AUDIENCE: The tongue? 364 00:21:56,001 --> 00:21:57,780 PROFESSOR: The tongue, exactly. 365 00:21:57,780 --> 00:22:01,260 We have to have extremely precise control of the tongue 366 00:22:01,260 --> 00:22:03,970 in order to talk. 367 00:22:03,970 --> 00:22:06,080 And we're also pretty good at eating. 368 00:22:06,080 --> 00:22:08,040 All right, so here they are, big axons. 369 00:22:08,040 --> 00:22:10,620 And there's the branch going into the dorsal column. 370 00:22:10,620 --> 00:22:12,190 And here it comes into the cord. 371 00:22:12,190 --> 00:22:16,000 And it's terminating here in layer six, 372 00:22:16,000 --> 00:22:20,170 including some very-- those are ones destined 373 00:22:20,170 --> 00:22:24,000 for the cerebellum, and we'll see that in a minute. 374 00:22:24,000 --> 00:22:25,710 But the most important thing here 375 00:22:25,710 --> 00:22:28,920 is they go directly to motor neurons. 376 00:22:28,920 --> 00:22:31,160 Some of them contact the motor neurons, 377 00:22:31,160 --> 00:22:35,860 so it's a monosynaptic reflex, the stretch reflex. 378 00:22:35,860 --> 00:22:42,370 Whenever a muscle stretches, it triggers those receptors, 379 00:22:42,370 --> 00:22:44,890 and it tends to cause the same muscle to contract. 380 00:22:44,890 --> 00:22:47,240 You say, well, that's counterproductive. 381 00:22:47,240 --> 00:22:48,485 No, it isn't. 382 00:22:48,485 --> 00:22:53,150 It's a perfect way to maintain good muscle 383 00:22:53,150 --> 00:22:56,730 tone so you're ready to move. 384 00:22:56,730 --> 00:22:59,890 If you don't have good stretch reflexes, 385 00:22:59,890 --> 00:23:02,220 so you don't have some tension in those muscles, 386 00:23:02,220 --> 00:23:03,920 you can't respond quickly. 387 00:23:06,690 --> 00:23:10,750 And if you wonder, when you're moving around rapidly, 388 00:23:10,750 --> 00:23:12,770 if I'm moving my arm like this, you 389 00:23:12,770 --> 00:23:16,090 might be surprised if you do this. 390 00:23:16,090 --> 00:23:19,770 Feel the muscle on the other side. 391 00:23:19,770 --> 00:23:22,350 It should just be the biceps here, right? 392 00:23:22,350 --> 00:23:26,830 But you'll see that the triceps contract also when I do that. 393 00:23:26,830 --> 00:23:28,885 Any rapid movement, that's true. 394 00:23:28,885 --> 00:23:35,370 All right, so what else did I ask you? 395 00:23:35,370 --> 00:23:37,700 We already talked about that. 396 00:23:37,700 --> 00:23:39,380 Describe the axons and connections 397 00:23:39,380 --> 00:23:42,340 of a withdrawal reflex-- the flexion reflex. 398 00:23:42,340 --> 00:23:44,040 This came up before. 399 00:23:44,040 --> 00:23:47,720 And then describe the origins of the spinothalamic tract 400 00:23:47,720 --> 00:23:49,520 within the spinal cord. 401 00:23:49,520 --> 00:23:53,190 And we'll contrast that with the spinoreticular tract. 402 00:23:53,190 --> 00:23:55,200 So first of all, what was the characteristic 403 00:23:55,200 --> 00:23:58,390 of the withdrawal reflex? 404 00:23:58,390 --> 00:24:02,390 How is it different from the stretch reflex? 405 00:24:05,380 --> 00:24:08,680 What would be the difference? 406 00:24:08,680 --> 00:24:12,420 The first synapse would not be on the motor neuron. 407 00:24:12,420 --> 00:24:17,690 It always involves at least two synapses in the cord. 408 00:24:17,690 --> 00:24:22,940 So here, in this little sketch, I show, first of all, 409 00:24:22,940 --> 00:24:25,980 they're not the largest axons, but they're 410 00:24:25,980 --> 00:24:30,780 axons that terminate in the dorsal horn 411 00:24:30,780 --> 00:24:35,460 or in the intermediate layer. 412 00:24:35,460 --> 00:24:41,980 And those cells, like this one, can contact a motor neuron. 413 00:24:41,980 --> 00:24:47,000 That would be the shortest pathway to a flexor muscle 414 00:24:47,000 --> 00:24:49,590 to get the withdrawal reflex. 415 00:24:49,590 --> 00:24:51,960 The fibers carrying that input tend 416 00:24:51,960 --> 00:24:56,105 to be small, carrying input from pain sensations. 417 00:24:59,610 --> 00:25:02,731 It also happens with extreme pressure and so forth. 418 00:25:05,620 --> 00:25:07,610 Note, there are similar connections here. 419 00:25:07,610 --> 00:25:11,190 I show same axon with a branch that 420 00:25:11,190 --> 00:25:13,170 gives rise to a flexor reflex. 421 00:25:13,170 --> 00:25:14,885 And there's a branch here contacting 422 00:25:14,885 --> 00:25:20,330 a neuron that sends its axon across to the other side, 423 00:25:20,330 --> 00:25:23,390 becomes a spinothalamic tract axon. 424 00:25:23,390 --> 00:25:28,980 That's the typical origin of the spinothalamic tract. 425 00:25:28,980 --> 00:25:35,170 Remember, the spinothalamic, no more than 20% 426 00:25:35,170 --> 00:25:38,380 of the axons, even in the higher primates, 427 00:25:38,380 --> 00:25:40,412 actually reach the thalamus. 428 00:25:40,412 --> 00:25:43,820 They go to the reticular formation, the hindbrain, 429 00:25:43,820 --> 00:25:47,290 the midbrain, they go to the older parts of the thalamus, 430 00:25:47,290 --> 00:25:51,760 and then some of them go to the new parts 431 00:25:51,760 --> 00:25:55,380 of the thalamus that project the neocortex. 432 00:25:55,380 --> 00:26:03,120 So how is the spinal reticular pathway different? 433 00:26:03,120 --> 00:26:05,290 We call this spinothalamic, even though it 434 00:26:05,290 --> 00:26:07,980 does go to reticular formation too. 435 00:26:07,980 --> 00:26:10,680 But when we call it spinoreticular, 436 00:26:10,680 --> 00:26:12,930 we're talking about an even older pathway 437 00:26:12,930 --> 00:26:15,030 before there was any crossing. 438 00:26:15,030 --> 00:26:18,540 So a pathway you find in the most ancient 439 00:26:18,540 --> 00:26:22,250 of the vertebrates, like hagfish and lamprey. 440 00:26:22,250 --> 00:26:27,190 And even amphioxus has pathways like the spinal reticular. 441 00:26:27,190 --> 00:26:29,850 There they are. 442 00:26:29,850 --> 00:26:34,610 They come from neurons here to get input in the dorsal roots. 443 00:26:34,610 --> 00:26:40,350 The axon enters the lateral columns on the same site, 444 00:26:40,350 --> 00:26:43,180 and the axons ascend. 445 00:26:43,180 --> 00:26:45,401 Some of them actually terminate in the spinal cord, 446 00:26:45,401 --> 00:26:47,067 but many of them are reaching the brain. 447 00:26:47,067 --> 00:26:51,030 They're called spinoreticular because they terminate mainly 448 00:26:51,030 --> 00:26:57,260 in reticular formation at all levels of the brain stem. 449 00:26:57,260 --> 00:27:01,430 The longest ones do get into the thalamus, but most of them 450 00:27:01,430 --> 00:27:07,760 terminate in the older parts of the thalamus and subthalamus. 451 00:27:07,760 --> 00:27:10,155 That is, the parts that don't project 452 00:27:10,155 --> 00:27:12,460 the neocortex, or at least that's 453 00:27:12,460 --> 00:27:15,650 not their only projection. 454 00:27:15,650 --> 00:27:17,620 This is just for those of you who 455 00:27:17,620 --> 00:27:21,100 are curious what you might encounter in medical school. 456 00:27:21,100 --> 00:27:24,180 You will get pictures like this where they show in blue here, 457 00:27:24,180 --> 00:27:28,080 spinoreticular pathways in a bunch of sections 458 00:27:28,080 --> 00:27:29,230 of the human brain. 459 00:27:29,230 --> 00:27:32,295 This is midbrain up here, caudal midbrain here, 460 00:27:32,295 --> 00:27:35,427 and there's the pons in human. 461 00:27:35,427 --> 00:27:36,926 You can see how much I've simplified 462 00:27:36,926 --> 00:27:42,550 when I present this introduction to neuroanatomy. 463 00:27:42,550 --> 00:27:46,520 Understanding that concepts is the most important thing. 464 00:27:46,520 --> 00:27:48,960 Once you understand them well and get that organization 465 00:27:48,960 --> 00:27:53,430 in your mind, then figures like this won't confuse you so much. 466 00:27:53,430 --> 00:27:58,670 And they do show-- some of the axons right here-- going 467 00:27:58,670 --> 00:28:03,400 above the midbrain to intralaminar 468 00:28:03,400 --> 00:28:07,560 thalamic nuclei in hypothalamus. 469 00:28:07,560 --> 00:28:11,090 We haven't talked about what are the intralaminar thalamic 470 00:28:11,090 --> 00:28:13,100 nuclei, but that's a name for the older 471 00:28:13,100 --> 00:28:15,430 parts of the thalamus, parts that 472 00:28:15,430 --> 00:28:18,620 survived complete removal of the neocortex. 473 00:28:18,620 --> 00:28:21,540 All right, so where do the longest 474 00:28:21,540 --> 00:28:24,770 axons of the dorsal columns originate? 475 00:28:24,770 --> 00:28:26,900 And where do they terminate? 476 00:28:26,900 --> 00:28:31,580 We introduced the dorsal column, medial lemniscus pathway, 477 00:28:31,580 --> 00:28:34,560 less time. 478 00:28:34,560 --> 00:28:37,000 It's different from the spinothalamic tract, 479 00:28:37,000 --> 00:28:39,710 but like the spinothalamic tract, 480 00:28:39,710 --> 00:28:42,675 it's a pathway that carries input 481 00:28:42,675 --> 00:28:45,605 to the opposite side of the forebrain. 482 00:28:49,570 --> 00:28:52,230 So where do the longest axons of that pathway originate? 483 00:28:57,680 --> 00:29:00,010 Well, we know where they terminate. 484 00:29:00,010 --> 00:29:04,095 The dorsal columns are primary sensory axons. 485 00:29:04,095 --> 00:29:07,285 They're these axons we're talking about, up here, 486 00:29:07,285 --> 00:29:09,670 the dorsal columns. 487 00:29:09,670 --> 00:29:14,970 And as I show here, the primary sensory axon 488 00:29:14,970 --> 00:29:17,650 doesn't terminate on anything there. 489 00:29:17,650 --> 00:29:20,520 It just turns rostrally. 490 00:29:20,520 --> 00:29:22,755 It often has a caudal branch too, 491 00:29:22,755 --> 00:29:26,280 but we're concerned about the rostral branch. 492 00:29:26,280 --> 00:29:29,870 The primary sensory axon goes all the way up 493 00:29:29,870 --> 00:29:37,580 to the top of the spinal cord, to the dorsal column nuclei. 494 00:29:37,580 --> 00:29:41,780 They have names, dorsal column nuclei. 495 00:29:41,780 --> 00:29:45,325 I don't care if you even remember that right now. 496 00:29:45,325 --> 00:29:48,500 They're called the slender nucleus 497 00:29:48,500 --> 00:29:50,700 and the wedge-shaped nucleus. 498 00:29:50,700 --> 00:29:54,360 The gracile and cuneate nuclei-- nucleus 499 00:29:54,360 --> 00:29:58,230 gracillus, nucleus cuneatus. 500 00:29:58,230 --> 00:30:00,940 Whenever you learn medical school anatomy, 501 00:30:00,940 --> 00:30:03,024 you learn these names. 502 00:30:03,024 --> 00:30:07,970 But remember them just as the dorsal column nuclei for now, 503 00:30:07,970 --> 00:30:11,800 and they are the secondary sensory neurons. 504 00:30:15,000 --> 00:30:20,300 So where would the longest axons originate? 505 00:30:20,300 --> 00:30:24,150 We know the nuclei are way up at the top of the spinal cord. 506 00:30:24,150 --> 00:30:30,480 So the longest axons originate from the most caudal part 507 00:30:30,480 --> 00:30:32,580 of the body. 508 00:30:32,580 --> 00:30:35,710 That's not the feet. 509 00:30:35,710 --> 00:30:37,350 Remember, the dermatome map. 510 00:30:41,460 --> 00:30:45,620 They originate in the caudalmost dermatomes-- right 511 00:30:45,620 --> 00:30:51,700 around your tailbone in the [INAUDIBLE] of your tail. 512 00:30:51,700 --> 00:30:54,540 That's where they originate, and they go all the way up 513 00:30:54,540 --> 00:30:56,000 to the top of the spinal cord. 514 00:30:56,000 --> 00:31:00,030 And they're primary sensory axons-- rapidly conducting. 515 00:31:00,030 --> 00:31:03,505 They tend to be larger than spinothalamic tract axons, 516 00:31:03,505 --> 00:31:05,000 so they conduct more rapidly. 517 00:31:08,320 --> 00:31:15,160 Those longest axons terminate in the most medial nuclei. 518 00:31:15,160 --> 00:31:17,365 It's characteristic of dorsal columns 519 00:31:17,365 --> 00:31:20,340 that the axons of the dorsal roots 520 00:31:20,340 --> 00:31:24,650 enter, join the dorsal columns from lateral side. 521 00:31:24,650 --> 00:31:30,290 So the further up you go, the more lateral the axons are. 522 00:31:30,290 --> 00:31:33,850 And the ones that originate more caudally are more medial. 523 00:31:36,380 --> 00:31:40,160 And when you get, then, to the dorsal column nuclei, 524 00:31:40,160 --> 00:31:41,910 they have an organization there. 525 00:31:41,910 --> 00:31:44,500 It's quite topographic. 526 00:31:44,500 --> 00:31:46,510 And then once you understand that, 527 00:31:46,510 --> 00:31:48,360 you can answer this question. 528 00:31:48,360 --> 00:31:51,780 Explain how there's an organized representation of the surface 529 00:31:51,780 --> 00:31:57,150 of the entire body in cell groups at the dorsalmost end 530 00:31:57,150 --> 00:31:59,850 of the spinal cord, those dorsal column nuclei. 531 00:31:59,850 --> 00:32:04,620 But wait a minute-- if it's the entire body, 532 00:32:04,620 --> 00:32:06,920 it has to include the face and head, right? 533 00:32:10,030 --> 00:32:12,270 So why is that represented up there? 534 00:32:12,270 --> 00:32:13,310 I say the whole body. 535 00:32:16,070 --> 00:32:17,780 So here's my picture of it. 536 00:32:17,780 --> 00:32:19,490 Here's the dorsal columns. 537 00:32:19,490 --> 00:32:22,090 And as you get up to the top of the cord, 538 00:32:22,090 --> 00:32:28,870 cells appear, right among those axons of the dorsal columns. 539 00:32:28,870 --> 00:32:32,660 One group here, medially, that's the more slender nucleus. 540 00:32:32,660 --> 00:32:37,390 It represents the lower part of the body, 541 00:32:37,390 --> 00:32:40,910 the tail region, the lower legs. 542 00:32:40,910 --> 00:32:42,940 And then the nucleus cuneatus tends 543 00:32:42,940 --> 00:32:46,620 to be larger because the forelimbs-- 544 00:32:46,620 --> 00:32:49,375 certainly in humans, especially, the arms and hands-- 545 00:32:49,375 --> 00:32:50,570 are highly innervated. 546 00:32:50,570 --> 00:32:56,060 So you have many more axons terminating. 547 00:32:56,060 --> 00:32:58,880 That's where the upper body is represented. 548 00:32:58,880 --> 00:33:03,610 But out here in the dorsal columns of the spinal cord, 549 00:33:03,610 --> 00:33:06,820 the face is represented. 550 00:33:06,820 --> 00:33:10,470 That seems really odd when you first encounter it, 551 00:33:10,470 --> 00:33:15,040 but it's because axons that come in through the trigeminal 552 00:33:15,040 --> 00:33:21,930 nerve, some of them descend and run along, 553 00:33:21,930 --> 00:33:24,910 through the hindbrain and into the top of the spinal cord. 554 00:33:24,910 --> 00:33:30,230 They're found right here at this level, the upper cervical cord. 555 00:33:30,230 --> 00:33:32,130 They are terminating in those cells 556 00:33:32,130 --> 00:33:37,550 there of the dorsal columns representing the face. 557 00:33:37,550 --> 00:33:45,252 The sensation that they are responsible for, 558 00:33:45,252 --> 00:33:48,390 those ones that descent that far, are pain and temperature. 559 00:33:51,080 --> 00:33:53,490 And that's why a surgical operation 560 00:33:53,490 --> 00:33:57,180 for very difficult-to-control sometimes 561 00:33:57,180 --> 00:34:00,000 consists in going to the very top of the spinal cord. 562 00:34:00,000 --> 00:34:05,100 It's dangerous surgery because you can't do much extra damage. 563 00:34:05,100 --> 00:34:08,739 They will actually cut these axons to reduce the pain. 564 00:34:08,739 --> 00:34:14,179 The only trouble is, surgery to control pain 565 00:34:14,179 --> 00:34:18,130 is almost never permanently successful. 566 00:34:18,130 --> 00:34:20,400 Pain always tends to come back, and there's 567 00:34:20,400 --> 00:34:22,630 various reasons for that, but it's still 568 00:34:22,630 --> 00:34:28,109 an area of active research-- representation 569 00:34:28,109 --> 00:34:30,440 of pain in the body. 570 00:34:30,440 --> 00:34:33,630 OK, so the entire body here represented 571 00:34:33,630 --> 00:34:36,770 in a topographic way, from the caudal end, 572 00:34:36,770 --> 00:34:42,570 medially, to the face area laterally. 573 00:34:42,570 --> 00:34:46,000 At the level of the dorsal column nuclei. 574 00:34:46,000 --> 00:34:47,690 So now let's talk about spinocerebellar. 575 00:34:51,199 --> 00:34:55,120 The source of axons terminating in this column of cells 576 00:34:55,120 --> 00:34:57,139 we call Clark's column-- sometimes 577 00:34:57,139 --> 00:35:01,280 just called nucleus dorsalis, the spinal cord-- 578 00:35:01,280 --> 00:35:03,170 from the lower limbs. 579 00:35:03,170 --> 00:35:05,760 And the descending fiber tract that 580 00:35:05,760 --> 00:35:09,620 originates in Clark's column is one of the spinocerebellar 581 00:35:09,620 --> 00:35:11,040 tracts. 582 00:35:11,040 --> 00:35:14,140 This one's called the dorsal spinocerebellar tract. 583 00:35:14,140 --> 00:35:16,825 It travels like this. 584 00:35:16,825 --> 00:35:26,150 So here we are, axons coming in, terminating 585 00:35:26,150 --> 00:35:31,190 in nucleus dorsalis, or Clark's column, right there. 586 00:35:31,190 --> 00:35:33,170 Those are lateral horns, so you know 587 00:35:33,170 --> 00:35:37,970 you're in most likely the thoracic spinal cord. 588 00:35:37,970 --> 00:35:40,660 It could be even upper lumbar. 589 00:35:40,660 --> 00:35:43,710 And the cells there send their axon 590 00:35:43,710 --> 00:35:48,230 into the more dorsal parts of the lateral column 591 00:35:48,230 --> 00:35:50,050 and then turn rostrally. 592 00:35:50,050 --> 00:35:52,970 And they ascend all the way to the cerebellum. 593 00:35:52,970 --> 00:35:57,210 That's one of two or three pathways, 594 00:35:57,210 --> 00:35:59,890 depending on the animal. 595 00:35:59,890 --> 00:36:04,650 But most of them have at this level two different pathways-- 596 00:36:04,650 --> 00:36:08,270 an uncrossed pathways like this and a crossed pathway 597 00:36:08,270 --> 00:36:10,710 where the axons crossover and are 598 00:36:10,710 --> 00:36:12,470 much more like a spinothalamic tract, 599 00:36:12,470 --> 00:36:13,750 but also go to the cerebellum. 600 00:36:13,750 --> 00:36:16,750 So where are they originating? 601 00:36:16,750 --> 00:36:23,720 What kind of information is important for the cerebellum? 602 00:36:23,720 --> 00:36:27,190 They carry information on joint movements, 603 00:36:27,190 --> 00:36:31,852 primarily from the lower limbs and the trunk. 604 00:36:31,852 --> 00:36:35,830 So whenever you're moving, you are generating sensations that 605 00:36:35,830 --> 00:36:39,990 you can't-- we don't have this much conscious sensation, 606 00:36:39,990 --> 00:36:44,710 although we do have some sense of where our arm is positioned 607 00:36:44,710 --> 00:36:47,880 because we do some of that, but it doesn't go directly 608 00:36:47,880 --> 00:36:49,552 to the cerebellum. 609 00:36:49,552 --> 00:36:53,830 It goes also to the cortex. 610 00:36:56,335 --> 00:36:59,274 But that's the dorsal spinocerebellar tract. 611 00:36:59,274 --> 00:37:02,350 OK, another group of axons. 612 00:37:02,350 --> 00:37:09,180 Propriospinal-- it literally means 613 00:37:09,180 --> 00:37:11,790 proper to the spinal cord. 614 00:37:11,790 --> 00:37:14,876 In other words, they never leave the spinal cord. 615 00:37:14,876 --> 00:37:17,860 There's a lot of axons like that. 616 00:37:17,860 --> 00:37:25,720 So for example, if we transect the spinal cord in an animal, 617 00:37:25,720 --> 00:37:29,020 keep him alive-- if we make the cut low enough, 618 00:37:29,020 --> 00:37:33,600 he will live even without artificial respiration. 619 00:37:33,600 --> 00:37:36,340 Below the level of C4, you'll still be able to breathe. 620 00:37:39,790 --> 00:37:44,660 But if we really completely transected the spinal cord, 621 00:37:44,660 --> 00:37:48,750 the movements of the entire body caudal to that level 622 00:37:48,750 --> 00:37:52,150 will have to be controlled only by the spinal cord. 623 00:37:52,150 --> 00:37:58,075 And after a period of recovery, you can get different gaits. 624 00:37:58,075 --> 00:38:02,130 You can get locomotor patterns because 625 00:38:02,130 --> 00:38:05,170 of propriospinal connections, which 626 00:38:05,170 --> 00:38:07,605 are responsible for fixed action patterns-- that 627 00:38:07,605 --> 00:38:09,870 is, they're under genetic control. 628 00:38:16,140 --> 00:38:20,840 Let's look at this picture on the left side, the orange ones, 629 00:38:20,840 --> 00:38:24,830 this sketch propriospinal axons, they 630 00:38:24,830 --> 00:38:31,590 tend to run in the columns of axons nearest the gray matter. 631 00:38:31,590 --> 00:38:34,990 And I'm showing cells in the gray matter sending axons 632 00:38:34,990 --> 00:38:37,140 into that goop. 633 00:38:37,140 --> 00:38:39,510 They go to another level and then they terminate. 634 00:38:39,510 --> 00:38:47,145 So it goes from one part, mostly intersegmental axons, 635 00:38:47,145 --> 00:38:51,880 passing from one segment to the other. 636 00:38:51,880 --> 00:38:57,200 And then I'm asking about the descending axons that 637 00:38:57,200 --> 00:39:03,030 influence movements of the limbs and other movements. 638 00:39:06,460 --> 00:39:09,160 Those movements are heavily influenced 639 00:39:09,160 --> 00:39:12,090 by pathways from the brain. 640 00:39:12,090 --> 00:39:18,030 And at this point, just read the names, 641 00:39:18,030 --> 00:39:19,882 get an idea of where they're coming from. 642 00:39:19,882 --> 00:39:24,690 The big ones here in human, and another 643 00:39:24,690 --> 00:39:27,380 primates-- especially the large primates-- 644 00:39:27,380 --> 00:39:29,450 coming from the neocortex. 645 00:39:29,450 --> 00:39:32,970 This is the position of the corticospinal tract, 646 00:39:32,970 --> 00:39:39,120 the blue dots here, in humans and monkeys and apes. 647 00:39:39,120 --> 00:39:40,870 And I'm showing how the axons tend 648 00:39:40,870 --> 00:39:47,330 to distribute throughout the spinal gray matter. 649 00:39:47,330 --> 00:39:49,320 I show another pathway there that 650 00:39:49,320 --> 00:39:53,256 has a fairly similar trajectory coming from the midbrain. 651 00:39:53,256 --> 00:39:55,230 Call it the rubrospinal tract. 652 00:39:55,230 --> 00:39:57,840 "Ruber" means red, and they come from the red nucleus 653 00:39:57,840 --> 00:39:58,635 in the midbrain. 654 00:39:58,635 --> 00:40:02,330 We'll be talking about that when we talk about midbrain. 655 00:40:02,330 --> 00:40:07,320 They have a little bit more limited distribution, 656 00:40:07,320 --> 00:40:09,930 and they terminate mainly in the enlargements, 657 00:40:09,930 --> 00:40:14,226 not in between the enlargements, whereas corticospinal 658 00:40:14,226 --> 00:40:15,600 tends to terminate on everything. 659 00:40:19,820 --> 00:40:23,740 And then-- oh, I should mention one more thing 660 00:40:23,740 --> 00:40:26,260 about the corticospinal. 661 00:40:26,260 --> 00:40:29,171 It's not always in the lateral columns. 662 00:40:29,171 --> 00:40:32,220 When you're a comparative anatomist like I am, 663 00:40:32,220 --> 00:40:34,190 you really have to pay attention to this, 664 00:40:34,190 --> 00:40:39,070 especially if you're using the animals to study the pathways. 665 00:40:39,070 --> 00:40:40,820 Like, I did a study with a student 666 00:40:40,820 --> 00:40:45,400 of the corticospinal pathways in the hamster and their role 667 00:40:45,400 --> 00:40:46,860 in the behavior of the animal. 668 00:40:46,860 --> 00:40:51,100 Well, it turns out we wanted to cut the pathway. 669 00:40:51,100 --> 00:40:53,240 They don't travel here. 670 00:40:53,240 --> 00:40:57,800 They travel in the ventral part of the dorsal columns. 671 00:40:57,800 --> 00:40:59,870 That's true of the rat too. 672 00:40:59,870 --> 00:41:01,080 It's true of the tree shrew. 673 00:41:01,080 --> 00:41:02,910 It's true of many animals. 674 00:41:02,910 --> 00:41:06,000 For a while, we thought the tree shrew was a primitive primate 675 00:41:06,000 --> 00:41:08,480 until somebody looked at the corticospinal pathways 676 00:41:08,480 --> 00:41:12,410 and found they weren't in the position of primates. 677 00:41:12,410 --> 00:41:16,480 They were in the position of insectivores and rodents-- 678 00:41:16,480 --> 00:41:18,190 here. 679 00:41:18,190 --> 00:41:19,300 OK. 680 00:41:19,300 --> 00:41:23,670 The ventral columns here, which in relative terms 681 00:41:23,670 --> 00:41:27,350 are similar throughout most mammals, 682 00:41:27,350 --> 00:41:31,240 they contain the axons that control 683 00:41:31,240 --> 00:41:35,670 the body axis, of posture, vestibular reflexes, 684 00:41:35,670 --> 00:41:40,990 writing reflexes, also orienting movements 685 00:41:40,990 --> 00:41:43,500 if they're very far rostral. 686 00:41:43,500 --> 00:41:47,130 So I've listed the main ones here-- reticulospinal. 687 00:41:47,130 --> 00:41:49,225 A lot of axons in the reticular formation 688 00:41:49,225 --> 00:41:51,560 that control fixed action patterns. 689 00:41:51,560 --> 00:41:53,630 They send their axons-- most of those 690 00:41:53,630 --> 00:41:59,170 involve axial muscle control, or at least whole body movements. 691 00:41:59,170 --> 00:42:04,510 Some of them might go to the motor neurons controlling 692 00:42:04,510 --> 00:42:06,610 the limbs as well. 693 00:42:06,610 --> 00:42:10,500 But always, whole body movements or axial muscles. 694 00:42:10,500 --> 00:42:15,740 And then vestibulospinal-- fastigiospinal, sorry. 695 00:42:15,740 --> 00:42:19,439 But nucleus fastigius is a deep nucleus of the cerebellum. 696 00:42:19,439 --> 00:42:21,980 So you can think of it at this point as just cerebellospinal. 697 00:42:24,880 --> 00:42:27,760 And then tectospinal, from the tectum of the midbrain, 698 00:42:27,760 --> 00:42:29,960 for turning movements. 699 00:42:29,960 --> 00:42:33,790 They only go to the cervical part of the cord. 700 00:42:33,790 --> 00:42:38,050 There are pustule adjustments whenever the animal makes 701 00:42:38,050 --> 00:42:45,700 a head movement like that, but that involves other neurons, 702 00:42:45,700 --> 00:42:50,756 usually from the reticular formation, reticulospinal. 703 00:42:50,756 --> 00:42:52,890 All right. 704 00:42:52,890 --> 00:42:55,200 And here I just have the same picture, 705 00:42:55,200 --> 00:42:58,960 and I'm just showing that these are almost all ascending 706 00:42:58,960 --> 00:43:03,475 axons in humans and other primates in the dorsal columns. 707 00:43:07,445 --> 00:43:11,000 If you're a rat or a tree shrew, then you'll 708 00:43:11,000 --> 00:43:12,800 find some corticospinals here. 709 00:43:12,800 --> 00:43:16,320 But these are otherwise in us all ascending axons. 710 00:43:16,320 --> 00:43:21,280 The lateral columns are ascending and descending. 711 00:43:21,280 --> 00:43:24,980 And by the way, I'm not including here propriospinal. 712 00:43:24,980 --> 00:43:27,250 They're always both ascending and descending. 713 00:43:27,250 --> 00:43:30,240 They just don't leave the spinal cord. 714 00:43:30,240 --> 00:43:32,260 And in the ventral columns here, they're 715 00:43:32,260 --> 00:43:37,432 all descending axons, almost all. 716 00:43:37,432 --> 00:43:40,620 All right. 717 00:43:40,620 --> 00:43:42,530 We have only a little bit of time 718 00:43:42,530 --> 00:43:45,610 to introduce the autonomic nervous system, 719 00:43:45,610 --> 00:43:48,305 the peripheral nervous system components that innervates 720 00:43:48,305 --> 00:43:50,250 the glands and smooth muscle, and also 721 00:43:50,250 --> 00:43:52,730 the cardiac muscle, which is another type 722 00:43:52,730 --> 00:43:54,690 of striated muscle. 723 00:43:54,690 --> 00:43:57,340 But it's innervated by the autonomic nervous system. 724 00:44:02,620 --> 00:44:05,370 And so I'm giving you the answer or talking about-- sometimes 725 00:44:05,370 --> 00:44:08,060 it's called visceral nervous system. 726 00:44:08,060 --> 00:44:09,850 It innervates the viscera. 727 00:44:09,850 --> 00:44:10,940 But we call it autonomic. 728 00:44:15,460 --> 00:44:18,760 And it contains these two components, as most of you 729 00:44:18,760 --> 00:44:21,530 already know-- sympathetic and parasympathetic. 730 00:44:21,530 --> 00:44:23,230 There's a third component that we 731 00:44:23,230 --> 00:44:25,560 can think of as more like parasympathetic, 732 00:44:25,560 --> 00:44:29,300 but we mention that as well. 733 00:44:29,300 --> 00:44:31,650 We want to contrast the functions. 734 00:44:31,650 --> 00:44:34,800 How do people usually summarize the functions, 735 00:44:34,800 --> 00:44:38,110 functional differences between sympathetic and parasympathetic 736 00:44:38,110 --> 00:44:39,230 nervous system? 737 00:44:39,230 --> 00:44:41,360 Take the heart, for example. 738 00:44:41,360 --> 00:44:45,910 Sympathetic nervous system, what does it do to the heart? 739 00:44:45,910 --> 00:44:48,020 It speeds it up. 740 00:44:48,020 --> 00:44:50,440 So parasympathetic slows it down. 741 00:44:53,310 --> 00:44:58,280 What about the gut, peristalsis and all that? 742 00:44:58,280 --> 00:45:00,540 Most of that's controlled locally, 743 00:45:00,540 --> 00:45:04,840 but it's also affected by autonomic nervous system. 744 00:45:04,840 --> 00:45:12,440 Sympathetic nervous system tends to stop that 745 00:45:12,440 --> 00:45:16,740 and tends to increase blood flow to the muscles. 746 00:45:16,740 --> 00:45:18,670 We call the sympathetic nervous system 747 00:45:18,670 --> 00:45:21,270 the fight-or-flight system, because those 748 00:45:21,270 --> 00:45:23,780 are two situations where the sympathetic nervous system 749 00:45:23,780 --> 00:45:25,850 becomes totally dominant. 750 00:45:25,850 --> 00:45:29,560 The autonomic system is always active at rest, 751 00:45:29,560 --> 00:45:33,160 and it's the normal functions of organs controlled 752 00:45:33,160 --> 00:45:36,020 by the parasympathetic nervous system, 753 00:45:36,020 --> 00:45:43,090 not involved in emergencies and fighting and fleeing. 754 00:45:43,090 --> 00:45:45,602 This table, you should read. 755 00:45:45,602 --> 00:45:48,510 Get familiar with-- I'm just listing 756 00:45:48,510 --> 00:45:53,580 some of the major organs, describe some of the things 757 00:45:53,580 --> 00:45:54,880 we've just talked about. 758 00:45:54,880 --> 00:45:58,920 There are some areas which are a little harder to figure out, 759 00:45:58,920 --> 00:46:02,290 just from that fight-or-flight analogy like the sex organs. 760 00:46:02,290 --> 00:46:04,540 You can read that. 761 00:46:04,540 --> 00:46:06,170 That involves also both systems. 762 00:46:06,170 --> 00:46:09,090 Almost all the bodily organs are innervated 763 00:46:09,090 --> 00:46:11,985 by both sympathetic and parasympathetic. 764 00:46:15,410 --> 00:46:17,360 And the innervation is a little bit 765 00:46:17,360 --> 00:46:21,093 different from the innervation of striated muscles. 766 00:46:21,093 --> 00:46:25,295 If you look at the motor system most broadly-- this 767 00:46:25,295 --> 00:46:32,450 was from Swanson-- we know striated muscles 768 00:46:32,450 --> 00:46:34,270 are innervated. 769 00:46:34,270 --> 00:46:38,620 This is the somatic muscles, innervated synaptically, 770 00:46:38,620 --> 00:46:40,770 local synapses that act locally. 771 00:46:40,770 --> 00:46:45,000 But the autonomic is innervated differently. 772 00:46:45,000 --> 00:46:47,430 We call it paracrine innervation. 773 00:46:47,430 --> 00:46:49,490 The neurotransmitter is released, 774 00:46:49,490 --> 00:46:51,860 and it's sort of spritzed out and affects 775 00:46:51,860 --> 00:46:53,956 a bunch of nearby cells. 776 00:46:53,956 --> 00:47:00,740 So you don't have the tight synaptic context that striated 777 00:47:00,740 --> 00:47:03,645 muscles have with the muscle [? in ?] [? place. ?] And then 778 00:47:03,645 --> 00:47:05,520 of course you have the neuroendocrine system, 779 00:47:05,520 --> 00:47:08,907 which is really a third motor system where the secretion is 780 00:47:08,907 --> 00:47:10,820 in the bloodstream. 781 00:47:10,820 --> 00:47:15,800 So then it's like apocrine to an extreme. 782 00:47:15,800 --> 00:47:19,290 The whole body is affected. 783 00:47:19,290 --> 00:47:21,660 Now, the two divisions, sympathetic and 784 00:47:21,660 --> 00:47:25,365 parasympathetic, they both have apocrine innervation 785 00:47:25,365 --> 00:47:27,390 of the end organ. 786 00:47:27,390 --> 00:47:31,940 But the innervation is through ganglion cells 787 00:47:31,940 --> 00:47:33,740 in the periphery. 788 00:47:33,740 --> 00:47:39,980 So here the dashed line represents the division 789 00:47:39,980 --> 00:47:44,987 between CNS here and peripheral nervous system here. 790 00:47:44,987 --> 00:47:47,070 But in each case, sympathetic and parasympathetic, 791 00:47:47,070 --> 00:47:50,410 there's a ganglion cell. 792 00:47:50,410 --> 00:47:52,600 They're located near the CNS if they're 793 00:47:52,600 --> 00:47:54,270 sympathetic nervous system. 794 00:47:54,270 --> 00:47:56,990 And they're located near the end organ 795 00:47:56,990 --> 00:47:59,950 in the case of the parasympathetic nervous system. 796 00:47:59,950 --> 00:48:06,270 So just one example here, my schematic view, 797 00:48:06,270 --> 00:48:09,100 I've only showed the innervation of the iris 798 00:48:09,100 --> 00:48:12,720 of the eye and the gut here. 799 00:48:12,720 --> 00:48:15,110 And here, I've just called the pelvic organs. 800 00:48:15,110 --> 00:48:17,620 I've not drawn any pictures. 801 00:48:17,620 --> 00:48:20,510 So we just take these three. 802 00:48:20,510 --> 00:48:22,900 Look at the iris. 803 00:48:22,900 --> 00:48:25,580 Sympathetic nervous system, the ganglion 804 00:48:25,580 --> 00:48:30,040 cells are always near the spinal cord. 805 00:48:30,040 --> 00:48:34,300 They extend from the superior cervical ganglion in the neck 806 00:48:34,300 --> 00:48:36,595 all the way down to below the cord. 807 00:48:36,595 --> 00:48:39,050 They're always innervated by axons 808 00:48:39,050 --> 00:48:43,880 coming from somewhere between T1 and L2 and 3. 809 00:48:43,880 --> 00:48:47,020 That's where the preganglionic motor neurons are. 810 00:48:47,020 --> 00:48:49,720 But this rostralmost ganglion here, 811 00:48:49,720 --> 00:48:52,330 superior cervical ganglion, sends axons 812 00:48:52,330 --> 00:48:55,130 into the head region. 813 00:48:55,130 --> 00:48:57,730 And some of them go directly to the iris. 814 00:48:57,730 --> 00:49:00,790 So when we get pupillary dilation, when 815 00:49:00,790 --> 00:49:03,080 our sympathetic nervous system is activated, 816 00:49:03,080 --> 00:49:05,860 like if we're angry or we're frightened, 817 00:49:05,860 --> 00:49:09,410 pupils dilate, bring in more light, 818 00:49:09,410 --> 00:49:10,930 reduce our resolution a bit. 819 00:49:13,820 --> 00:49:16,440 That comes from here. 820 00:49:16,440 --> 00:49:21,020 The parasympathetic system, that's the craniosacral system. 821 00:49:21,020 --> 00:49:26,456 It comes from the third nerve nucleus here. 822 00:49:26,456 --> 00:49:29,110 It's a little sub-component of the third nerve 823 00:49:29,110 --> 00:49:31,530 nucleus that's parasympathetic. 824 00:49:31,530 --> 00:49:33,790 It's called the Edinger-Westphal nucleus. 825 00:49:33,790 --> 00:49:35,875 So I've written there, the EW nucleus. 826 00:49:35,875 --> 00:49:37,680 It's in the midbrain. 827 00:49:37,680 --> 00:49:39,380 The axons go out. 828 00:49:39,380 --> 00:49:41,350 They don't go directly to the iris. 829 00:49:41,350 --> 00:49:47,763 They go to a ganglion behind the eye-- the ciliary ganglion. 830 00:49:47,763 --> 00:49:53,650 And the axons of those ciliary ganglion cells, 831 00:49:53,650 --> 00:49:57,830 they're the ones that innervate the iris. 832 00:49:57,830 --> 00:50:01,100 And they use different neurotransmitters. 833 00:50:01,100 --> 00:50:03,980 The neurotransmitter in the ganglion is the same. 834 00:50:03,980 --> 00:50:06,040 It's always acetylcholine. 835 00:50:06,040 --> 00:50:10,520 And then the purple cell, the final cell there, 836 00:50:10,520 --> 00:50:13,860 is acetylcholine again for the parasympathetic, 837 00:50:13,860 --> 00:50:17,730 but it's norepinephrine for the sympathetic. 838 00:50:17,730 --> 00:50:19,590 And we're going to stop here. 839 00:50:19,590 --> 00:50:21,845 And I will put all these slides online. 840 00:50:21,845 --> 00:50:24,390 I might review a few of them at the beginning 841 00:50:24,390 --> 00:50:26,370 of the next class.