1 00:00:00,250 --> 00:00:01,800 The following content is provided 2 00:00:01,800 --> 00:00:04,040 under a Creative Commons license. 3 00:00:04,040 --> 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 --> 00:00:17,241 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,241 --> 00:00:17,866 at ocw.mit.edu. 8 00:00:22,990 --> 00:00:25,910 PROFESSOR: The main topic today is the methods 9 00:00:25,910 --> 00:00:30,730 we use to study neuroanatomy, in particular connections 10 00:00:30,730 --> 00:00:31,310 in the brain. 11 00:00:37,530 --> 00:00:41,260 We didn't get through the discussion of slides 12 00:00:41,260 --> 00:00:45,000 from the pictures from chapter one of the book. 13 00:00:45,000 --> 00:00:53,270 And I know that interpreting presynaptic inhibition 14 00:00:53,270 --> 00:00:55,600 and presynaptic facilitation that 15 00:00:55,600 --> 00:00:59,140 happens in synapses with this arrangement, which you see 16 00:00:59,140 --> 00:01:02,270 on one of the pictures from last time. 17 00:01:02,270 --> 00:01:06,790 So I'd like to go over that. 18 00:01:06,790 --> 00:01:12,120 And are there other questions from last time? 19 00:01:12,120 --> 00:01:15,830 Because I know if you have questions after a class, 20 00:01:15,830 --> 00:01:18,900 you don't get a chance to answer them, 21 00:01:18,900 --> 00:01:21,124 you can always send them to me by email, 22 00:01:21,124 --> 00:01:23,040 or you can just bring them up at the beginning 23 00:01:23,040 --> 00:01:24,760 of the next class. 24 00:01:24,760 --> 00:01:27,000 I really want to get your questions answered. 25 00:01:31,050 --> 00:01:36,510 And as I mentioned only after the class to some students that 26 00:01:36,510 --> 00:01:42,020 were still up here, if you find terms that you don't think 27 00:01:42,020 --> 00:01:46,160 are adequately defined in the textbook, 28 00:01:46,160 --> 00:01:52,030 or you think should be defined in the glossary, please let me 29 00:01:52,030 --> 00:01:55,250 know, or let Caitlin know, because I 30 00:01:55,250 --> 00:01:58,730 will get them defined, and I will add them to the glossary. 31 00:01:58,730 --> 00:02:03,890 And that will affect everybody reading the book, 32 00:02:03,890 --> 00:02:07,460 because I'm going to post- I haven't posted it yet 33 00:02:07,460 --> 00:02:11,466 on the MIT press website-- but we have a provision for that. 34 00:02:11,466 --> 00:02:12,840 And so when the book comes out, I 35 00:02:12,840 --> 00:02:15,150 want that glossary to be there. 36 00:02:15,150 --> 00:02:18,415 I just want to go over it a little bit more before I post 37 00:02:18,415 --> 00:02:22,705 it, but you already have it because it's on Stellar. 38 00:02:22,705 --> 00:02:27,450 And I'd be happy to answer other questions. 39 00:02:27,450 --> 00:02:32,090 Most of those words come from last year's class. 40 00:02:32,090 --> 00:02:36,830 And I spent a lot of time writing definitions. 41 00:02:36,830 --> 00:02:43,200 And the words were given to me by the teaching assistant, 42 00:02:43,200 --> 00:02:46,100 so I'm sure they were words that she wanted defined. 43 00:02:46,100 --> 00:02:50,510 Maybe you students will find others that should be there. 44 00:02:50,510 --> 00:02:52,400 And there's probably a few defined there 45 00:02:52,400 --> 00:02:55,710 that weren't in the book, or maybe they 46 00:02:55,710 --> 00:02:57,175 were part of a figure or something, 47 00:02:57,175 --> 00:03:00,400 and she wanted a little more detail. 48 00:03:00,400 --> 00:03:00,900 OK. 49 00:03:00,900 --> 00:03:04,650 So consider this kind of connection 50 00:03:04,650 --> 00:03:11,810 that you see right here. 51 00:03:11,810 --> 00:03:14,160 This is a usual kind of synapse. 52 00:03:14,160 --> 00:03:15,950 It's a bouton with only one synapse. 53 00:03:15,950 --> 00:03:18,810 Remember, if a bouton is big enough, 54 00:03:18,810 --> 00:03:23,200 it often makes several synapses with various processes 55 00:03:23,200 --> 00:03:25,880 on different sides of the bouton. 56 00:03:25,880 --> 00:03:29,030 But this is the way it's usually pictured. 57 00:03:29,030 --> 00:03:36,040 This would be probably an excitatory synapse. 58 00:03:36,040 --> 00:03:38,860 They're usually fairly asymmetric. 59 00:03:38,860 --> 00:03:44,619 The presynaptic thickening here is varying in thickness. 60 00:03:44,619 --> 00:03:46,410 And then there's a postsynaptic thickening. 61 00:03:46,410 --> 00:03:47,868 And you always find this connection 62 00:03:47,868 --> 00:03:50,800 with vesicles on the presynaptic side. 63 00:03:50,800 --> 00:03:54,510 We know for sure now that they contain 64 00:03:54,510 --> 00:03:57,300 little packets of neurotransmitters. 65 00:03:57,300 --> 00:03:59,820 And if there are peptide modulators 66 00:03:59,820 --> 00:04:04,420 in the synapse in addition, they're 67 00:04:04,420 --> 00:04:08,030 generally in separate vesicles, but they could also 68 00:04:08,030 --> 00:04:10,410 be released and affect the action of the synapse. 69 00:04:10,410 --> 00:04:14,350 But what about a synapse on the bouton that's 70 00:04:14,350 --> 00:04:16,180 making the synapse? 71 00:04:16,180 --> 00:04:18,690 What is its influence? 72 00:04:18,690 --> 00:04:21,230 So think about it now. 73 00:04:21,230 --> 00:04:26,330 Let's say that just before or during the time 74 00:04:26,330 --> 00:04:28,770 this action potential is arriving 75 00:04:28,770 --> 00:04:35,190 over this axon, the axon making this synapse on cell two here-- 76 00:04:35,190 --> 00:04:41,590 we'll call this cell one, this cell two-- this axon 77 00:04:41,590 --> 00:04:47,060 fires and reduces the membrane potential here. 78 00:04:50,520 --> 00:04:52,940 You can't really call it an-- it's 79 00:04:52,940 --> 00:04:56,000 like an excitatory postsynaptic potential, but there's 80 00:04:56,000 --> 00:04:57,050 nothing too excited. 81 00:04:57,050 --> 00:04:58,630 It's not near an axon hillock. 82 00:04:58,630 --> 00:05:01,950 It can't affect directly the firing 83 00:05:01,950 --> 00:05:03,900 of an action potential here. 84 00:05:03,900 --> 00:05:07,160 What does it do, then? 85 00:05:07,160 --> 00:05:10,610 Well, if it reduces the membrane potential here, 86 00:05:10,610 --> 00:05:15,340 when the action potential arrives here, 87 00:05:15,340 --> 00:05:17,580 it doesn't depolarize the membrane 88 00:05:17,580 --> 00:05:21,550 as much because the membrane potential was lower. 89 00:05:21,550 --> 00:05:25,140 So we call that presynaptic inhibition, 90 00:05:25,140 --> 00:05:28,505 even though it's a depolarization 91 00:05:28,505 --> 00:05:31,700 with that synapse, because it decreases 92 00:05:31,700 --> 00:05:33,650 the amount of neurotransmitter released here, 93 00:05:33,650 --> 00:05:35,910 because the neurotransmitter released depends 94 00:05:35,910 --> 00:05:39,651 on that change in membrane potential. 95 00:05:39,651 --> 00:05:40,150 OK. 96 00:05:40,150 --> 00:05:44,122 So now let's say it causes hyperpolarization 97 00:05:44,122 --> 00:05:48,566 of the membrane, uses a different neurotransmitter, 98 00:05:48,566 --> 00:05:52,290 and different receptors here on the postsynaptic side. 99 00:05:52,290 --> 00:05:54,240 It causes a hyper-polarization. 100 00:05:54,240 --> 00:05:56,350 What will be the effect then? 101 00:05:56,350 --> 00:06:01,230 Then, even more neurotransmitter will be released here. 102 00:06:01,230 --> 00:06:04,360 So we call it presynaptic facilitation. 103 00:06:04,360 --> 00:06:07,470 It increases the effectiveness of that synapse. 104 00:06:10,160 --> 00:06:13,900 That's the basic explanation for presynaptic inhibition 105 00:06:13,900 --> 00:06:15,840 and presynaptic facilitation. 106 00:06:15,840 --> 00:06:22,530 They depend on axoaxonal synapses. 107 00:06:22,530 --> 00:06:26,570 It's common, for example, in synapses of the dorsal root 108 00:06:26,570 --> 00:06:30,110 axons on the secondary sensory cells 109 00:06:30,110 --> 00:06:32,826 in the dorsal horn of the spinal cord. 110 00:06:32,826 --> 00:06:36,750 And those effects were discovered just 111 00:06:36,750 --> 00:06:39,160 with large electrodes by physiologists 112 00:06:39,160 --> 00:06:43,100 who were extremely clever at using electrodes 113 00:06:43,100 --> 00:06:44,350 before we had microelectrodes. 114 00:06:47,140 --> 00:06:49,630 All right. 115 00:06:49,630 --> 00:06:53,870 This is just an explanation for why 116 00:06:53,870 --> 00:06:57,810 all these interconnections in the CNS had to evolve. 117 00:06:57,810 --> 00:07:00,350 There had to be communication between different parts 118 00:07:00,350 --> 00:07:03,010 of the organization because the organism is big. 119 00:07:03,010 --> 00:07:06,230 So it's like the question of how does the right hand know 120 00:07:06,230 --> 00:07:08,440 what the left hand is doing? 121 00:07:08,440 --> 00:07:09,940 You've got to have interconnections. 122 00:07:09,940 --> 00:07:14,460 How does the right hand know what the left hand is doing? 123 00:07:14,460 --> 00:07:15,465 Connections up here. 124 00:07:19,030 --> 00:07:24,020 And how do we study connections? 125 00:07:24,020 --> 00:07:26,310 Well, the methods of neuroanatomy. 126 00:07:26,310 --> 00:07:29,280 Sometimes we call it neuromorphology. 127 00:07:29,280 --> 00:07:30,950 But there are other methods that can 128 00:07:30,950 --> 00:07:34,030 be used-- electrophysiological methods in particular. 129 00:07:34,030 --> 00:07:36,010 We'll mention one of them. 130 00:07:36,010 --> 00:07:40,890 It's been known since Sherrington's time. 131 00:07:40,890 --> 00:07:43,240 But also, there are chemical means. 132 00:07:43,240 --> 00:07:46,580 There are behavioral studies that play a role. 133 00:07:46,580 --> 00:07:50,820 And now, of course, various imaging methods. 134 00:07:50,820 --> 00:07:55,410 But I just want you to pay attention to this remark here. 135 00:07:55,410 --> 00:07:57,780 The imaging methods are very limited 136 00:07:57,780 --> 00:08:00,880 for the study of pathways and connections. 137 00:08:00,880 --> 00:08:03,070 They do show you pathways now, and you'll 138 00:08:03,070 --> 00:08:05,270 see a picture of that. 139 00:08:05,270 --> 00:08:09,760 But they are not-- they don't give you 140 00:08:09,760 --> 00:08:13,800 certain knowledge of an actual connection. 141 00:08:13,800 --> 00:08:16,510 So let's look at neuroanatomical methods. 142 00:08:16,510 --> 00:08:19,760 They start with fixing the tissue. 143 00:08:19,760 --> 00:08:22,310 That means both preserving the tissue 144 00:08:22,310 --> 00:08:28,170 and fixing it, because the tissue is very gelatinous. 145 00:08:28,170 --> 00:08:29,880 You need some way to make it firmer. 146 00:08:29,880 --> 00:08:33,169 And that's what fixatives do. 147 00:08:33,169 --> 00:08:36,990 The most common fixatives are the aldehydes-- formaldehyde, 148 00:08:36,990 --> 00:08:38,580 gluteraldehyde. 149 00:08:38,580 --> 00:08:43,450 Gluteraldehyde makes the tissue so hard the stains don't 150 00:08:43,450 --> 00:08:45,840 penetrate very well, but it's very 151 00:08:45,840 --> 00:08:47,370 useful for electron microscopy. 152 00:08:50,710 --> 00:08:52,580 And then, of course, you cut the brain. 153 00:08:52,580 --> 00:08:56,400 It shows a little cartoon here that's, I think, in the book. 154 00:08:56,400 --> 00:08:58,120 You make a thin section. 155 00:08:58,120 --> 00:09:00,510 You mount the section on the slides. 156 00:09:00,510 --> 00:09:04,050 And in some stains, you do the stain before you mount them. 157 00:09:04,050 --> 00:09:07,120 But usually you're mounting them on slides-- often many sections 158 00:09:07,120 --> 00:09:10,520 per slide if it's a small brain. 159 00:09:10,520 --> 00:09:15,600 And then you dry them, go through a set procedure, 160 00:09:15,600 --> 00:09:20,210 and put them through the staining solutions 161 00:09:20,210 --> 00:09:23,740 in order to get the structures visible, 162 00:09:23,740 --> 00:09:25,115 the structures you need to see. 163 00:09:28,130 --> 00:09:29,650 So what are some of the-- I think 164 00:09:29,650 --> 00:09:31,667 we talked about this last time. 165 00:09:31,667 --> 00:09:33,750 Can you just tell me what some of those techniques 166 00:09:33,750 --> 00:09:36,500 are if we want to do cytoarchitecture, 167 00:09:36,500 --> 00:09:40,100 we want to see the arrangement of cell in the brain? 168 00:09:40,100 --> 00:09:41,940 What do we do? 169 00:09:41,940 --> 00:09:42,440 Yes? 170 00:09:42,440 --> 00:09:43,370 AUDIENCE: [INAUDIBLE]. 171 00:09:47,090 --> 00:09:49,890 PROFESSOR: Yes, that's a good one. 172 00:09:49,890 --> 00:09:54,560 We can get very nice pictures of fiber architecture that way. 173 00:09:54,560 --> 00:09:56,036 Yes? 174 00:09:56,036 --> 00:09:58,530 AUDIENCE: [INAUDIBLE]. 175 00:09:58,530 --> 00:10:02,710 PROFESSOR: Nissl stains for cell bodies. 176 00:10:02,710 --> 00:10:05,730 That's the most common one. 177 00:10:05,730 --> 00:10:12,605 OK, and for fibers, if you want to do fiber architecture, 178 00:10:12,605 --> 00:10:14,700 I actually have a separate question here. 179 00:10:14,700 --> 00:10:17,610 What besides the silver stains do we stain for? 180 00:10:21,640 --> 00:10:24,240 What's the usual black-looking stain, 181 00:10:24,240 --> 00:10:27,110 where fibers are black or blue? 182 00:10:27,110 --> 00:10:30,040 Myelin-- they're myelin stains. 183 00:10:30,040 --> 00:10:33,950 Sure, that misses a lot of the small fibers, of course. 184 00:10:33,950 --> 00:10:35,940 But that's been a very important method. 185 00:10:35,940 --> 00:10:40,170 And myelin stains and certain cell 186 00:10:40,170 --> 00:10:45,760 stains-- not the cresyl violet that we use in the laboratory 187 00:10:45,760 --> 00:10:51,140 but [INAUDIBLE] stains are used by pathologists in human tissue 188 00:10:51,140 --> 00:10:53,420 when they're evaluating the tissue. 189 00:10:53,420 --> 00:10:55,900 And so it's very common in the pathology literature 190 00:10:55,900 --> 00:10:59,860 to read, for example, hematoxylin and eosin stains, 191 00:10:59,860 --> 00:11:03,670 because eosin is a rapidly applied cell stain-- not as 192 00:11:03,670 --> 00:11:07,830 good as with cytoarchitecture, but useful for interpreting 193 00:11:07,830 --> 00:11:09,850 damaged areas in the brain. 194 00:11:09,850 --> 00:11:12,380 And iron hematoxylin stains the fibers. 195 00:11:12,380 --> 00:11:13,191 Yes? 196 00:11:13,191 --> 00:11:16,800 AUDIENCE: What about tracers? 197 00:11:16,800 --> 00:11:19,680 PROFESSOR: Tracers-- here we're only 198 00:11:19,680 --> 00:11:24,000 talking about cytoarchitecture and fiber architecture. 199 00:11:24,000 --> 00:11:28,740 You need tracers if you want to follow a specific axon pathway 200 00:11:28,740 --> 00:11:30,760 and find out where it goes. 201 00:11:30,760 --> 00:11:33,290 So that's the main topic today. 202 00:11:33,290 --> 00:11:35,960 But here's an example of cytoarchitecture 203 00:11:35,960 --> 00:11:39,310 in an experimental animal. 204 00:11:39,310 --> 00:11:40,700 This is from a rat. 205 00:11:40,700 --> 00:11:46,760 And it's from one of Larry Swanson's beautiful atlases. 206 00:11:46,760 --> 00:11:52,010 And it's a section through the tween brain of a rat. 207 00:11:52,010 --> 00:11:57,125 And you can see here that I've labeled a really obvious cell 208 00:11:57,125 --> 00:12:00,710 group pair in the hypothalamus. 209 00:12:00,710 --> 00:12:03,690 It's the ventromedial hypothalamic nucleus. 210 00:12:03,690 --> 00:12:07,790 And you can see some evidence that there are differences 211 00:12:07,790 --> 00:12:09,110 from one region to the other. 212 00:12:09,110 --> 00:12:12,480 Like right here, you can see the medial part of this area 213 00:12:12,480 --> 00:12:14,290 is very different from the lateral part. 214 00:12:14,290 --> 00:12:15,940 And that whole region is a little bit 215 00:12:15,940 --> 00:12:17,540 different from the area around it. 216 00:12:17,540 --> 00:12:18,540 That's cytoarchitecture. 217 00:12:21,080 --> 00:12:24,045 Anatomists try to mark boundaries 218 00:12:24,045 --> 00:12:27,650 of these different cell groups if they can consistently see, 219 00:12:27,650 --> 00:12:31,310 often using more than one method to make them up. 220 00:12:31,310 --> 00:12:33,860 These are the cortical areas. 221 00:12:33,860 --> 00:12:35,880 Remember, I said the hemispheres sort 222 00:12:35,880 --> 00:12:37,480 of grow out of the diencephalon. 223 00:12:37,480 --> 00:12:39,532 You see that very clearly here. 224 00:12:39,532 --> 00:12:40,990 The connection's rather quick here, 225 00:12:40,990 --> 00:12:45,020 but there's a lot of axons in here. 226 00:12:45,020 --> 00:12:48,060 And you say, well, actually there's a lot of cells there. 227 00:12:50,710 --> 00:12:56,240 Well, look at this. 228 00:12:56,240 --> 00:13:01,360 Right there, right here, right here, right here-- 229 00:13:01,360 --> 00:13:03,100 these are all axons. 230 00:13:03,100 --> 00:13:05,550 So what are the cells? 231 00:13:05,550 --> 00:13:09,180 What is the cell making the myelin? 232 00:13:09,180 --> 00:13:12,830 Oligodendrocytes, remember? 233 00:13:12,830 --> 00:13:18,910 So you stain the Nissl substance in the oligodendrocytes. 234 00:13:18,910 --> 00:13:22,960 And for example, here the fiber is coming from the hippocampus. 235 00:13:22,960 --> 00:13:26,070 You see a lot of oligodendrocytes there too. 236 00:13:26,070 --> 00:13:28,470 The neurons are bigger. 237 00:13:28,470 --> 00:13:30,560 We're normally looking at it in lower power. 238 00:13:30,560 --> 00:13:34,190 But when you blow it up, you can easily see those differences. 239 00:13:34,190 --> 00:13:35,770 And then look at the cortex. 240 00:13:35,770 --> 00:13:38,020 What's the most characteristic feature 241 00:13:38,020 --> 00:13:39,230 of these cortical layers? 242 00:13:39,230 --> 00:13:41,360 Here's neocortex up here. 243 00:13:41,360 --> 00:13:43,890 Here's olfactory cortex. 244 00:13:43,890 --> 00:13:46,700 Layers, lamination. 245 00:13:46,700 --> 00:13:50,710 And you can see very clear evidence of layers here, 246 00:13:50,710 --> 00:13:53,442 and we'll be talking a lot more about that. 247 00:13:53,442 --> 00:13:56,340 AUDIENCE: [INAUDIBLE]. 248 00:13:56,340 --> 00:13:58,139 PROFESSOR: This is the habenula, yes. 249 00:13:58,139 --> 00:13:59,180 AUDIENCE: Why is it dark? 250 00:13:59,180 --> 00:14:01,580 PROFESSOR: The medial habenular nucleus is much more 251 00:14:01,580 --> 00:14:03,992 densely packed and stains more darkly. 252 00:14:03,992 --> 00:14:05,200 There's more Nissl substance. 253 00:14:10,440 --> 00:14:14,920 This is a more magnified view to show 254 00:14:14,920 --> 00:14:17,730 you another example of cytoarchitecture. 255 00:14:17,730 --> 00:14:19,940 This is a frequently used example, 256 00:14:19,940 --> 00:14:23,170 because the boundary is so clear. 257 00:14:23,170 --> 00:14:25,530 The boundaries aren't always this clear. 258 00:14:25,530 --> 00:14:28,680 But look at the difference between the cortex 259 00:14:28,680 --> 00:14:32,120 on the right and the cortex on the left. 260 00:14:32,120 --> 00:14:37,040 It's area 17 on the right here and area 18 261 00:14:37,040 --> 00:14:41,010 on the left-- really clear. 262 00:14:41,010 --> 00:14:49,850 Area four here goes all the way from here down to-- it's right 263 00:14:49,850 --> 00:14:51,730 here, actually. 264 00:14:51,730 --> 00:14:57,170 This is because of the sub-layers in layer four. 265 00:14:57,170 --> 00:14:59,670 OK, I've shown it over here. 266 00:14:59,670 --> 00:15:02,110 And you can see the differentiation 267 00:15:02,110 --> 00:15:03,110 of the different layers. 268 00:15:03,110 --> 00:15:07,520 And you see that some of the layers have sublayers. 269 00:15:07,520 --> 00:15:11,600 Layer six, even in the rat and mouse, very clear sub-layer, 270 00:15:11,600 --> 00:15:13,390 same as in human. 271 00:15:13,390 --> 00:15:16,450 Layer one, by the way, has very few neurons. 272 00:15:16,450 --> 00:15:17,960 We call it a zonal layer. 273 00:15:21,670 --> 00:15:26,140 It does have a few neurons, but very few. 274 00:15:26,140 --> 00:15:27,577 What is there? 275 00:15:27,577 --> 00:15:28,410 [AUDIO OUT] neurons. 276 00:15:31,100 --> 00:15:34,920 It's the same thickness as other layers. 277 00:15:34,920 --> 00:15:36,360 What's in all that tissue? 278 00:15:36,360 --> 00:15:38,975 What's in the neuropil between the cell bodies? 279 00:15:42,230 --> 00:15:46,340 Dendrites and axons, and of course glial cells. 280 00:15:46,340 --> 00:15:49,330 But it's mostly dendrites and axons. 281 00:15:49,330 --> 00:15:52,570 The pyramidal cells down here send their dendrite, 282 00:15:52,570 --> 00:15:53,685 many of them. 283 00:15:53,685 --> 00:15:57,090 The pyramidal cell has an apical dendrite, goes right up 284 00:15:57,090 --> 00:16:01,160 and arborizes at the surface. 285 00:16:01,160 --> 00:16:02,600 And we'll see pictures of that. 286 00:16:02,600 --> 00:16:07,045 But even these cells down here, a pyramidal cell 287 00:16:07,045 --> 00:16:11,571 will send its dendrite up like this and branch. 288 00:16:11,571 --> 00:16:14,610 Well, what's it doing that for? 289 00:16:14,610 --> 00:16:17,780 It has other branches down here and down here. 290 00:16:17,780 --> 00:16:19,630 What's it doing that for? 291 00:16:19,630 --> 00:16:20,750 Because they're axons. 292 00:16:20,750 --> 00:16:22,890 They're terminating there. 293 00:16:22,890 --> 00:16:26,070 Axons from other cells in the cortex and axons 294 00:16:26,070 --> 00:16:26,865 from the thalamus. 295 00:16:30,270 --> 00:16:32,190 Here's an example of fiber architecture. 296 00:16:34,760 --> 00:16:37,820 Here the myelin stain is not super-sensitive. 297 00:16:37,820 --> 00:16:41,730 It is missing the smaller myelinated axons. 298 00:16:41,730 --> 00:16:43,450 This is a human brain, but you do 299 00:16:43,450 --> 00:16:48,360 see the big bundles of myelinated axons very clearly. 300 00:16:48,360 --> 00:16:51,610 What is that huge bundle at the base? 301 00:16:51,610 --> 00:16:55,220 Cerebral peduncle coming from neocortex. 302 00:16:55,220 --> 00:16:56,862 And I've labeled it there for you. 303 00:17:00,170 --> 00:17:03,170 And here's another myelin stain, but this one 304 00:17:03,170 --> 00:17:05,470 is a super-sensitive one. 305 00:17:05,470 --> 00:17:08,329 Because here, I've actually prepared this in my lab. 306 00:17:08,329 --> 00:17:11,869 This was done on a developing hamster. 307 00:17:11,869 --> 00:17:20,030 We were using at that time a newly discovered molecule 308 00:17:20,030 --> 00:17:24,619 discovered in oligodendrocyte membranes. 309 00:17:24,619 --> 00:17:26,910 And an antibody to it was developed. 310 00:17:26,910 --> 00:17:28,440 And when that became available, we 311 00:17:28,440 --> 00:17:30,150 applied it to developing animals. 312 00:17:30,150 --> 00:17:33,080 And we could see, even before the myelin stains 313 00:17:33,080 --> 00:17:35,425 could stain the myelin, we would see 314 00:17:35,425 --> 00:17:39,270 these oligodendrocyte membranes. 315 00:17:39,270 --> 00:17:41,260 This is just a blow-up where you can actually 316 00:17:41,260 --> 00:17:43,970 see single cells there. 317 00:17:43,970 --> 00:17:47,950 So it gives you a very sensitive picture of the myelin. 318 00:17:47,950 --> 00:17:49,970 So you can see here, you don't see 319 00:17:49,970 --> 00:17:54,890 any-- this is the area we're dealing with in the human. 320 00:17:54,890 --> 00:17:59,820 There, you'll see there's actually a lot of myelin there. 321 00:17:59,820 --> 00:18:05,370 It's just a little harder to stain than in those larger 322 00:18:05,370 --> 00:18:07,472 axons in the peduncle. 323 00:18:07,472 --> 00:18:10,990 Now, here's another use of the myelin stain. 324 00:18:10,990 --> 00:18:17,740 This is from a developing human, seven-week human. 325 00:18:17,740 --> 00:18:20,190 It's a horizontal section. 326 00:18:20,190 --> 00:18:24,330 So this is caudal back here, where you see the cerebellum. 327 00:18:24,330 --> 00:18:26,180 And there's the hemisphere. 328 00:18:26,180 --> 00:18:30,835 And there's the thalamus and some midbrain in between. 329 00:18:34,090 --> 00:18:36,320 So what is interesting here? 330 00:18:36,320 --> 00:18:38,130 Well, in the hemisphere, you'll note 331 00:18:38,130 --> 00:18:39,830 there's a lot of myelin in the brain 332 00:18:39,830 --> 00:18:44,270 stem-- thalamus, midbrain, and hindbrain. 333 00:18:44,270 --> 00:18:47,080 Cerebellum is part of the hindbrain. 334 00:18:47,080 --> 00:18:50,880 But in the cortex, just a few fiber pathways 335 00:18:50,880 --> 00:18:53,370 are starting to myelinate. 336 00:18:53,370 --> 00:18:55,960 The earliest ones to myelinate are 337 00:18:55,960 --> 00:19:01,630 going to-- somatosensory areas, the primary auditory areas, 338 00:19:01,630 --> 00:19:03,250 the primary visual areas. 339 00:19:03,250 --> 00:19:07,240 They're very early to myelinate. 340 00:19:07,240 --> 00:19:09,160 And that's from the work of Paul Flechsig. 341 00:19:09,160 --> 00:19:14,460 He was very well known for these studies of myelination, 342 00:19:14,460 --> 00:19:15,710 or myelinization. 343 00:19:15,710 --> 00:19:18,010 They're words that mean the same thing. 344 00:19:18,010 --> 00:19:21,210 Myelination, or myelinization. 345 00:19:21,210 --> 00:19:22,610 The development of myelin. 346 00:19:25,350 --> 00:19:30,640 Now, if we wait a lot longer, we can't discriminate these three 347 00:19:30,640 --> 00:19:32,670 pathways from all the others. 348 00:19:32,670 --> 00:19:35,560 There would be so much myelin. 349 00:19:35,560 --> 00:19:39,720 Any large brain where axons are long, there's 350 00:19:39,720 --> 00:19:42,020 a lot of myelin on the axons. 351 00:19:42,020 --> 00:19:44,190 It's the only way the conduction can be fast enough 352 00:19:44,190 --> 00:19:48,790 to get efficient information processing. 353 00:19:48,790 --> 00:19:50,880 This is the same as one I have in the book, 354 00:19:50,880 --> 00:19:54,185 except I think I took out all these German labels. 355 00:19:58,780 --> 00:19:59,280 OK. 356 00:19:59,280 --> 00:20:01,720 Now, what other methods do we have? 357 00:20:01,720 --> 00:20:06,180 You can use immunohistochemistry for your anatomical studies. 358 00:20:08,980 --> 00:20:12,510 What do we mean by immunohistochemistry? 359 00:20:12,510 --> 00:20:16,410 First of all, just say histochemical methods. 360 00:20:16,410 --> 00:20:19,580 Any method for specifically staining a specific chemical 361 00:20:19,580 --> 00:20:22,370 in the brain, in the tissue. 362 00:20:22,370 --> 00:20:26,820 If it's immunohistochemistry, you use an antibody to bind 363 00:20:26,820 --> 00:20:29,860 to a specific chemical or a specific molecule type. 364 00:20:32,985 --> 00:20:36,440 And this just shows an example of immunohistochemistry. 365 00:20:39,700 --> 00:20:44,690 This is a section stained by a former MIT graduate 366 00:20:44,690 --> 00:20:48,640 student, Miles Herkenham, who's worked at the NIH laboratories 367 00:20:48,640 --> 00:20:50,660 for a long time. 368 00:20:50,660 --> 00:20:55,850 And he's converted the intensity of staining two colors here, 369 00:20:55,850 --> 00:20:58,650 just to make it more dramatic. 370 00:20:58,650 --> 00:21:04,480 He's binding to opioid receptors with his antibody and showing 371 00:21:04,480 --> 00:21:07,260 that the opioid receptors are much 372 00:21:07,260 --> 00:21:09,520 denser in some areas than others. 373 00:21:13,970 --> 00:21:16,110 And then I want to ask a question here. 374 00:21:16,110 --> 00:21:23,530 How was histochemistry used for comparing forebrain structures 375 00:21:23,530 --> 00:21:26,410 in mammals and birds? 376 00:21:26,410 --> 00:21:31,000 It was really important study because it raised a big issue. 377 00:21:31,000 --> 00:21:34,840 This is the picture-- sorry, this one. 378 00:21:34,840 --> 00:21:37,210 Here's a mammal. 379 00:21:37,210 --> 00:21:39,270 And note that the cortical areas there 380 00:21:39,270 --> 00:21:43,300 don't have a lot of acetylcholine esterase stained 381 00:21:43,300 --> 00:21:46,750 with the histochemical method for that molecule. 382 00:21:46,750 --> 00:21:49,430 Whereas the subcortical regions, that's 383 00:21:49,430 --> 00:21:53,090 all corpus striatal region, the corpus striatum, 384 00:21:53,090 --> 00:21:55,870 has a lot of acetylcholine esterase. 385 00:21:55,870 --> 00:21:59,220 So then, you look at the pigeon and you 386 00:21:59,220 --> 00:22:03,380 see these areas down here have all the acetylcholine esterase. 387 00:22:03,380 --> 00:22:06,946 But there's this really large area 388 00:22:06,946 --> 00:22:10,860 up above that doesn't appear to be cortex. 389 00:22:10,860 --> 00:22:13,260 They have a little cortex visible here, 390 00:22:13,260 --> 00:22:15,710 and there's a little bit of cortex out here. 391 00:22:15,710 --> 00:22:18,190 You can't see the lamination very well. 392 00:22:18,190 --> 00:22:20,940 But what is all that? 393 00:22:20,940 --> 00:22:22,720 You see, it raised this big question 394 00:22:22,720 --> 00:22:25,910 about comparative anatomy. 395 00:22:25,910 --> 00:22:31,680 We were thinking before then that birds and reptiles, 396 00:22:31,680 --> 00:22:33,310 they have this huge corpus striatum. 397 00:22:36,830 --> 00:22:38,930 This raised the question that maybe it's 398 00:22:38,930 --> 00:22:40,340 not really corpus striatum. 399 00:22:40,340 --> 00:22:44,620 Maybe this is the only real corpus striatum in the pigeon. 400 00:22:44,620 --> 00:22:46,440 That turned out to be true. 401 00:22:46,440 --> 00:22:50,340 This is just another example in the mammal of the same stain, 402 00:22:50,340 --> 00:22:52,640 staining for acetylcholine esterase. 403 00:22:52,640 --> 00:22:54,175 This is from Ann Graybiel's work, 404 00:22:54,175 --> 00:23:01,620 where she showed this patchiness of acetylcholine axons 405 00:23:01,620 --> 00:23:05,510 in the deeper, just below the optic fiber layer coming 406 00:23:05,510 --> 00:23:07,100 into the superior colliculus. 407 00:23:07,100 --> 00:23:10,000 And a lot there in the superficial gray. 408 00:23:10,000 --> 00:23:16,030 So histochemistry is more specific than the general cell 409 00:23:16,030 --> 00:23:20,180 and myelin and other axon stains that we're talking about, 410 00:23:20,180 --> 00:23:21,090 much more specific. 411 00:23:21,090 --> 00:23:23,480 And of course, there may be other molecules 412 00:23:23,480 --> 00:23:26,330 that you can stain for besides acetylcholine esterase. 413 00:23:26,330 --> 00:23:30,870 But that's been a common one in neuroanatomical studies. 414 00:23:30,870 --> 00:23:33,700 And then we have newer technology. 415 00:23:33,700 --> 00:23:39,810 Now we can look for gene expression patterns in the CNS. 416 00:23:39,810 --> 00:23:43,900 And you can see when I did this, I called it recent. 417 00:23:43,900 --> 00:23:49,680 That was 2007. 418 00:23:49,680 --> 00:23:54,020 I point out here you can find online expression patterns 419 00:23:54,020 --> 00:23:56,149 of about 200,000 genes. 420 00:23:56,149 --> 00:23:57,690 I don't know if that's actually true, 421 00:23:57,690 --> 00:24:00,850 but that's what I have been told. 422 00:24:00,850 --> 00:24:04,810 And here's a Nissl stain of a little piece 423 00:24:04,810 --> 00:24:07,840 of cortex in the upper left. 424 00:24:07,840 --> 00:24:15,770 And here's one that's showing cells 425 00:24:15,770 --> 00:24:18,950 expressing a particular molecule, 426 00:24:18,950 --> 00:24:21,420 but they're only in layer two. 427 00:24:21,420 --> 00:24:23,610 Here's one that's in layer two and three. 428 00:24:23,610 --> 00:24:26,710 Here's one that's only in the deeper part of layer three. 429 00:24:26,710 --> 00:24:29,020 Here's one in layer four. 430 00:24:29,020 --> 00:24:31,270 Here's one in layer five. 431 00:24:31,270 --> 00:24:34,790 Here's one in layer six, all the parts of layer six. 432 00:24:34,790 --> 00:24:38,750 And here's only layer six B. 433 00:24:38,750 --> 00:24:42,340 So you see how specific they can be. 434 00:24:42,340 --> 00:24:47,030 And I've just given a reference where that was published. 435 00:24:47,030 --> 00:24:51,555 And it was a huge amount of work, getting all this to work. 436 00:24:51,555 --> 00:24:54,190 There was 108 authors of that paper. 437 00:24:59,990 --> 00:25:04,320 Now, I don't think we talked about this-- the advantages 438 00:25:04,320 --> 00:25:08,260 and disadvantages of using the Golgi method for tracing 439 00:25:08,260 --> 00:25:11,670 interconnections of structures in the central nervous system. 440 00:25:11,670 --> 00:25:16,960 We know the Golgi method has been available for a long time. 441 00:25:16,960 --> 00:25:22,700 It was discovered before-- it was 442 00:25:22,700 --> 00:25:27,576 the end of the 19th century. 443 00:25:27,576 --> 00:25:30,020 It was developed by Camillo Golgi 444 00:25:30,020 --> 00:25:34,730 but put to great used by the Spanish neuroanatomist Ramon y 445 00:25:34,730 --> 00:25:35,230 Cajal. 446 00:25:38,280 --> 00:25:40,790 What were the advantages and disadvantages? 447 00:25:40,790 --> 00:25:45,830 Well, you also should know who Ramon y Cajal was. 448 00:25:45,830 --> 00:25:48,740 In many ways, he's the father of modern neuroanatomy, 449 00:25:48,740 --> 00:25:52,040 but there were plenty of neuroanatomists already doing 450 00:25:52,040 --> 00:25:53,325 pretty interesting work. 451 00:25:53,325 --> 00:25:59,300 But he put this one method to incredible use 452 00:25:59,300 --> 00:26:03,340 because of the advantages of the Golgi method. 453 00:26:03,340 --> 00:26:10,005 This is just one picture of a brain stain with the Golgi Cox 454 00:26:10,005 --> 00:26:10,505 method. 455 00:26:10,505 --> 00:26:13,900 It does not get every cell. 456 00:26:13,900 --> 00:26:17,810 It seems to randomly stain certain cells and not others. 457 00:26:17,810 --> 00:26:19,330 I played with that a little bit. 458 00:26:19,330 --> 00:26:24,030 I found out that just jostling the tissue a little bit 459 00:26:24,030 --> 00:26:26,690 when it was lightly fixed or hardly 460 00:26:26,690 --> 00:26:30,380 fixed at all altered the staining patterns. 461 00:26:30,380 --> 00:26:32,970 I could sometimes get many more cells, 462 00:26:32,970 --> 00:26:35,290 get so many cells stained that it wasn't useful. 463 00:26:35,290 --> 00:26:38,010 Because the most useful part of Golgi is just that 464 00:26:38,010 --> 00:26:39,720 it doesn't stain everything. 465 00:26:39,720 --> 00:26:42,950 Because when it does stain, it tends to fill the whole cell-- 466 00:26:42,950 --> 00:26:47,730 dendrites, and in many cases even the axon. 467 00:26:47,730 --> 00:26:50,950 And here in the background, a Nissl stain 468 00:26:50,950 --> 00:26:53,720 has been used, with only partial success, 469 00:26:53,720 --> 00:26:58,070 to show all the other cells. 470 00:26:58,070 --> 00:26:59,530 And that's what you see here too, 471 00:26:59,530 --> 00:27:02,920 but this is a more magnified. 472 00:27:02,920 --> 00:27:06,730 It shows some stellate neurons in the cortex surrounded 473 00:27:06,730 --> 00:27:12,120 by many other neurons that are not stained. 474 00:27:12,120 --> 00:27:16,170 And here's a picture of Ramon y Cajal himself. 475 00:27:16,170 --> 00:27:18,630 See, it's a very simple microscope, a monocular 476 00:27:18,630 --> 00:27:19,270 microscope. 477 00:27:19,270 --> 00:27:21,720 Now we usually use binoculars scopes. 478 00:27:21,720 --> 00:27:25,910 But you notice, he appears to be doodling. 479 00:27:25,910 --> 00:27:27,250 He's not even looking here. 480 00:27:29,840 --> 00:27:30,570 What is he doing? 481 00:27:30,570 --> 00:27:32,080 Is he doodling? 482 00:27:32,080 --> 00:27:33,520 No. 483 00:27:33,520 --> 00:27:38,990 He studied through the microscope, memorized 484 00:27:38,990 --> 00:27:42,470 what he was looking at, then he did his drawing, completely 485 00:27:42,470 --> 00:27:44,910 from memory. 486 00:27:44,910 --> 00:27:48,160 He had incredible visual memory. 487 00:27:48,160 --> 00:27:52,800 And now people generally use tracing methods 488 00:27:52,800 --> 00:27:55,640 because the optics is more advanced. 489 00:27:55,640 --> 00:28:02,020 And we're finding that Cajal was almost always correct. 490 00:28:02,020 --> 00:28:05,300 So we know his memory was accurate. 491 00:28:05,300 --> 00:28:10,760 OK, these are just some examples of his beautiful pictures. 492 00:28:10,760 --> 00:28:12,770 This is a section of the spinal cord, 493 00:28:12,770 --> 00:28:15,600 and he's showing axons there, coming-- these are dorsal root 494 00:28:15,600 --> 00:28:19,940 axons, terminating in the upper layers of the dorsal horn 495 00:28:19,940 --> 00:28:22,780 and then in the lower part of the dorsal horn. 496 00:28:22,780 --> 00:28:25,614 And here you see-- we call these 1A axons. 497 00:28:25,614 --> 00:28:27,030 They're coming from muscle fibers, 498 00:28:27,030 --> 00:28:28,990 carrying sensory input from muscle fibers, 499 00:28:28,990 --> 00:28:31,480 and terminating right in the ventral horn, some of them 500 00:28:31,480 --> 00:28:32,910 directly on motor neurons. 501 00:28:35,489 --> 00:28:37,030 And here, on the other side, he shows 502 00:28:37,030 --> 00:28:40,700 some terminating in the medial part 503 00:28:40,700 --> 00:28:43,290 of the gray matter of the [INAUDIBLE]. 504 00:28:43,290 --> 00:28:46,700 That's in an area that is carrying input 505 00:28:46,700 --> 00:28:50,740 on proprioception, and the cells there send their axons up 506 00:28:50,740 --> 00:28:51,490 to the cerebellum. 507 00:28:54,090 --> 00:28:55,370 This is an interesting one. 508 00:28:55,370 --> 00:29:00,760 He's just showing the ventral part of the spinal cord. 509 00:29:00,760 --> 00:29:03,810 And here, he's done something a little different. 510 00:29:03,810 --> 00:29:07,600 He's drawn all the dendrites and the cell bodies in black. 511 00:29:07,600 --> 00:29:10,520 Notice some of the dendrites go right across midline. 512 00:29:10,520 --> 00:29:15,950 And he's drawn the axons in red. 513 00:29:15,950 --> 00:29:19,240 So when you see an axon going out of the cord, 514 00:29:19,240 --> 00:29:21,500 you know that's from a motor neuron. 515 00:29:21,500 --> 00:29:23,880 That's the definition of a motor neuron. 516 00:29:23,880 --> 00:29:25,115 It sends its axon out. 517 00:29:28,650 --> 00:29:32,380 He was very careful about figuring out 518 00:29:32,380 --> 00:29:37,610 the differences in detail of axons and dendrites seen 519 00:29:37,610 --> 00:29:41,140 with the Golgi method. 520 00:29:41,140 --> 00:29:42,840 And you can see, with this method, 521 00:29:42,840 --> 00:29:47,000 he could see connections. 522 00:29:47,000 --> 00:29:49,210 The big disadvantage of the Golgi method 523 00:29:49,210 --> 00:29:52,360 is it's almost impossible to follow 524 00:29:52,360 --> 00:29:55,190 the axon for very big distances. 525 00:29:55,190 --> 00:29:59,760 So if you're trying to find the connection, say, of brain 526 00:29:59,760 --> 00:30:02,410 cells with spinal cord cells, you'll 527 00:30:02,410 --> 00:30:05,960 never be able to follow it all the way with Golgi. 528 00:30:05,960 --> 00:30:09,770 You can do it with painstaking reconstructions, 529 00:30:09,770 --> 00:30:15,490 but we don't have a month to study one connection, 530 00:30:15,490 --> 00:30:18,780 so very few people have done it. 531 00:30:18,780 --> 00:30:23,970 So I like to point that he was the first anatomist 532 00:30:23,970 --> 00:30:26,600 to see something that people had postulated-- 533 00:30:26,600 --> 00:30:29,070 and Sherrington knew, had physiological data 534 00:30:29,070 --> 00:30:33,170 for-- that some connections went directly from the sensory side 535 00:30:33,170 --> 00:30:34,900 to the motor side. 536 00:30:34,900 --> 00:30:37,560 Reflex-type connections, sometimes 537 00:30:37,560 --> 00:30:41,080 with even a single synapse, the monosynaptic reflex. 538 00:30:41,080 --> 00:30:43,300 Cajal, he draws them here. 539 00:30:43,300 --> 00:30:45,450 He thought those were coming from the skin. 540 00:30:45,450 --> 00:30:49,370 Because again, this is a long axon coming in here. 541 00:30:49,370 --> 00:30:51,510 And that was a mistake. 542 00:30:51,510 --> 00:30:55,870 They come from stretch receptors in the muscle. 543 00:30:55,870 --> 00:30:57,940 But other than that, he can't always 544 00:30:57,940 --> 00:31:01,700 tell where the lung connections are coming from or going to, 545 00:31:01,700 --> 00:31:06,330 but he can see the short connections in one region. 546 00:31:06,330 --> 00:31:09,270 And he can see these axons connecting directly 547 00:31:09,270 --> 00:31:10,830 with motor neurons. 548 00:31:10,830 --> 00:31:14,450 So that's why I say he was the first to see this SR 549 00:31:14,450 --> 00:31:14,950 connection. 550 00:31:17,480 --> 00:31:19,410 And people love that because it was 551 00:31:19,410 --> 00:31:22,875 support for a model that had become a model of how behavior 552 00:31:22,875 --> 00:31:26,160 is controlled, developed much earlier. 553 00:31:26,160 --> 00:31:28,870 It's been around-- the idea of SR connections-- 554 00:31:28,870 --> 00:31:33,210 it's been around since Descartes, 17th century. 555 00:31:33,210 --> 00:31:37,220 And it became very popular in the next century. 556 00:31:37,220 --> 00:31:43,840 Whole philosophies were developed based on this idea. 557 00:31:43,840 --> 00:31:46,390 And then Pavlov came along, and what did he 558 00:31:46,390 --> 00:31:51,460 show that made the SR model seem so much more comprehensive, 559 00:31:51,460 --> 00:31:54,605 even for explaining human behavior? 560 00:31:54,605 --> 00:31:55,605 What did Cajal discover? 561 00:32:00,160 --> 00:32:02,150 They could be plastic. 562 00:32:02,150 --> 00:32:05,752 They could change with learning. 563 00:32:05,752 --> 00:32:06,835 He called it conditioning. 564 00:32:10,220 --> 00:32:11,175 Conditioned reflexes. 565 00:32:16,810 --> 00:32:20,670 The reflexologists were so happy after Cajal's discovery. 566 00:32:20,670 --> 00:32:23,800 Now they could explain everything. 567 00:32:23,800 --> 00:32:27,420 So then, my next question is, describe the argument made 568 00:32:27,420 --> 00:32:31,100 by Karl Lashley against the adequacy of the SR model 569 00:32:31,100 --> 00:32:34,290 for explaining all human behavior. 570 00:32:34,290 --> 00:32:35,650 I summarized it in the chapter. 571 00:32:41,610 --> 00:32:43,540 And I also point out, he actually 572 00:32:43,540 --> 00:32:45,870 believed that when he was a student, 573 00:32:45,870 --> 00:32:49,090 he was studying connections and realized that, oh, 574 00:32:49,090 --> 00:32:52,030 if he could just get all the details of every connection 575 00:32:52,030 --> 00:32:55,850 in the frog, he could explain the frog's behavior. 576 00:32:55,850 --> 00:32:59,460 He didn't believe that anymore later. 577 00:32:59,460 --> 00:33:01,900 And you might wonder, well, why isn't that true? 578 00:33:05,310 --> 00:33:09,570 One of the big reasons is one of those primitive cellular 579 00:33:09,570 --> 00:33:11,800 mechanisms we mentioned last time. 580 00:33:11,800 --> 00:33:14,030 Cells can have endogenous activity. 581 00:33:14,030 --> 00:33:16,670 They can generate their own activity. 582 00:33:16,670 --> 00:33:18,660 In other words, their activity isn't only 583 00:33:18,660 --> 00:33:20,815 determined by stimuli coming from the outside. 584 00:33:23,510 --> 00:33:26,760 But then Lashley made another claim-- 585 00:33:26,760 --> 00:33:30,390 that a lot of human behavior, for example he 586 00:33:30,390 --> 00:33:32,960 used the example of playing the piano, 587 00:33:32,960 --> 00:33:39,130 he said the movements were so fast that reaction times simply 588 00:33:39,130 --> 00:33:41,550 weren't fast enough to explain them. 589 00:33:41,550 --> 00:33:43,420 There couldn't be a chain of reflexes 590 00:33:43,420 --> 00:33:47,930 to explain pattern behavior that's rapid. 591 00:33:47,930 --> 00:33:51,240 Well, then what else can explain it? 592 00:33:51,240 --> 00:33:53,820 It had to come from a program up here that 593 00:33:53,820 --> 00:33:57,590 was acquired and learned in some way. 594 00:33:57,590 --> 00:34:00,375 Well, developing those programs, like playing the piano, 595 00:34:00,375 --> 00:34:05,030 it takes a lot of effort, as you know. 596 00:34:05,030 --> 00:34:07,430 But we know that most of those connections, 597 00:34:07,430 --> 00:34:11,638 they involve both neocortex and corpus striatum, especially 598 00:34:11,638 --> 00:34:12,179 the striatum. 599 00:34:18,159 --> 00:34:21,980 So anyway, it's still a common assumption. 600 00:34:21,980 --> 00:34:25,115 neuroanatomists still like the SR model, 601 00:34:25,115 --> 00:34:26,115 even if it's inadequate. 602 00:34:30,100 --> 00:34:31,929 Get back to neuroanatomy here. 603 00:34:31,929 --> 00:34:36,290 How is the phenomenon of a retrograde degeneration 604 00:34:36,290 --> 00:34:39,489 used in experiments on animals to establish 605 00:34:39,489 --> 00:34:43,070 the existence of a major pathway taken by visual information 606 00:34:43,070 --> 00:34:44,400 to the neocortex? 607 00:34:44,400 --> 00:34:48,130 And what belief was destroyed by those-- 608 00:34:48,130 --> 00:34:50,850 remember what I said in there in that chapter? 609 00:34:50,850 --> 00:34:52,650 This is in chapter two. 610 00:34:55,449 --> 00:34:56,449 I want you to know this. 611 00:34:56,449 --> 00:34:59,120 These have played major roles. 612 00:34:59,120 --> 00:35:02,100 I was reading in the 19th century literature 613 00:35:02,100 --> 00:35:05,380 many years ago-- I was still a graduate student-- 614 00:35:05,380 --> 00:35:08,200 and I read there the belief, the common belief, 615 00:35:08,200 --> 00:35:10,740 I was reading it, they thought the pathway 616 00:35:10,740 --> 00:35:15,380 from the eye-- they knew the posterior 617 00:35:15,380 --> 00:35:17,730 cortex was important in vision. 618 00:35:17,730 --> 00:35:20,090 And they knew the input came in through the eye. 619 00:35:20,090 --> 00:35:23,469 They traced the pathway using just dissection methods, 620 00:35:23,469 --> 00:35:25,385 and it seemed to go to the superior colliculus 621 00:35:25,385 --> 00:35:26,940 in the midbrain. 622 00:35:26,940 --> 00:35:28,700 So they said, then the pathway has 623 00:35:28,700 --> 00:35:32,555 to go from retina to superior colliculus, and from there 624 00:35:32,555 --> 00:35:33,855 to the posterior cortex. 625 00:35:36,580 --> 00:35:41,770 Retrograde degeneration experiments 626 00:35:41,770 --> 00:35:45,530 later showed that's not how the visual cortex got 627 00:35:45,530 --> 00:35:47,410 its information. 628 00:35:47,410 --> 00:35:50,550 What is retrograde degeneration? 629 00:35:50,550 --> 00:35:54,330 Destroy the area where a pathway terminates, 630 00:35:54,330 --> 00:36:00,750 and the cells where that pathway originates shrivel. 631 00:36:00,750 --> 00:36:03,850 They die, in some cases, or at least they shrivel. 632 00:36:03,850 --> 00:36:09,490 That's called retrograde atrophy or retrograde degeneration. 633 00:36:09,490 --> 00:36:11,310 Keep in mind, they don't always degenerate. 634 00:36:11,310 --> 00:36:14,470 They often just lose weight. 635 00:36:14,470 --> 00:36:17,910 Sometimes they actually die over time. 636 00:36:17,910 --> 00:36:20,300 But where was the retrograde degeneration? 637 00:36:20,300 --> 00:36:22,770 It wasn't in the superior colliculus. 638 00:36:22,770 --> 00:36:25,430 It was in the thalamus. 639 00:36:25,430 --> 00:36:29,720 It was in the lateral geniculate body of the thalamus. 640 00:36:29,720 --> 00:36:32,830 So that was the real root, the most direct route 641 00:36:32,830 --> 00:36:36,863 from retina to the visual cortex, through the thalamus. 642 00:36:40,000 --> 00:36:42,160 Neuro-electrophysiological methods, 643 00:36:42,160 --> 00:36:49,140 you should know that antidromic conduction 644 00:36:49,140 --> 00:36:50,660 was the major method. 645 00:36:54,460 --> 00:36:58,730 This is the stuff we've talked about here. 646 00:36:58,730 --> 00:37:01,990 What is antidromic stimulation? 647 00:37:01,990 --> 00:37:07,390 If you already know where the pathway probably originates-- 648 00:37:07,390 --> 00:37:09,160 and this is the big problem of the method, 649 00:37:09,160 --> 00:37:10,690 you've got to know where to look-- 650 00:37:10,690 --> 00:37:14,190 you record from the cells and stimulate where 651 00:37:14,190 --> 00:37:16,920 you think their axons are ending. 652 00:37:16,920 --> 00:37:20,700 And when you stimulate, you can record action potentials 653 00:37:20,700 --> 00:37:28,330 from cell bodies, you can tell that you're 654 00:37:28,330 --> 00:37:31,390 recording from a cell whose axon ends where you're 655 00:37:31,390 --> 00:37:35,630 stimulating, because the waveform doesn't show 656 00:37:35,630 --> 00:37:38,670 any evidence of the synaptic delay, 657 00:37:38,670 --> 00:37:43,400 or the waveform of a postsynaptic discharge. 658 00:37:43,400 --> 00:37:46,620 The action potential is directly invading the cell body 659 00:37:46,620 --> 00:37:48,930 from where you're stimulating. 660 00:37:48,930 --> 00:37:50,375 That's antidromic stimulation. 661 00:37:50,375 --> 00:37:55,550 It was used by Sherrington and has been an important method. 662 00:37:55,550 --> 00:37:59,760 Even now you see physiologists using this, 663 00:37:59,760 --> 00:38:01,360 because they want to know something 664 00:38:01,360 --> 00:38:05,370 about exactly where the cells are that give rise, say, 665 00:38:05,370 --> 00:38:09,350 to pathways to the spinal cord. 666 00:38:09,350 --> 00:38:10,005 All right. 667 00:38:13,030 --> 00:38:18,880 So now, tract-tracing, in answer to your question finally. 668 00:38:18,880 --> 00:38:20,710 What was the major difference between 669 00:38:20,710 --> 00:38:24,330 the tract-tracing methods of Markey, 670 00:38:24,330 --> 00:38:30,850 the earlier one, and Nauta, the neuroanatomist who 671 00:38:30,850 --> 00:38:32,825 had the last part of his career here at MIT? 672 00:38:35,800 --> 00:38:37,530 What was Markey's method for? 673 00:38:40,040 --> 00:38:42,960 It was very different from Nauta's. 674 00:38:42,960 --> 00:38:45,280 Nauta would've used Markey's method 675 00:38:45,280 --> 00:38:50,960 if it has satisfied what he really wanted to do. 676 00:38:50,960 --> 00:38:56,690 The problem with it was it was only for degenerating myelin, 677 00:38:56,690 --> 00:38:58,380 so you could follow pathways. 678 00:38:58,380 --> 00:39:03,450 I read a study by Karl Lashley on the retinal projections 679 00:39:03,450 --> 00:39:06,290 in the rad done with the Markey method. 680 00:39:06,290 --> 00:39:09,310 He got most of them right, but he missed all the smaller ones. 681 00:39:12,090 --> 00:39:15,670 Nauta wanted a more sensitive method, using silver methods, 682 00:39:15,670 --> 00:39:18,920 because he knew from a Russian technique-- [INAUDIBLE], 683 00:39:18,920 --> 00:39:24,030 it was his technique-- could stain even really fine axons, 684 00:39:24,030 --> 00:39:29,022 in even a terminal region, and even unmyelinated axons. 685 00:39:29,022 --> 00:39:29,938 AUDIENCE: [INAUDIBLE]. 686 00:39:35,900 --> 00:39:37,417 PROFESSOR: Sorry? 687 00:39:37,417 --> 00:39:39,208 AUDIENCE: How did he determine from looking 688 00:39:39,208 --> 00:39:41,136 at the Nauta tracing that there [INAUDIBLE]? 689 00:39:44,030 --> 00:39:46,150 PROFESSOR: Oh, he didn't always get it right. 690 00:39:46,150 --> 00:39:48,030 But no, you're making a big assumption. 691 00:39:48,030 --> 00:39:51,150 But you can follow myelin fairly close to the terminals. 692 00:39:51,150 --> 00:39:55,530 And if he sees them going all the way 693 00:39:55,530 --> 00:39:57,580 and entering the superior colliculus, 694 00:39:57,580 --> 00:40:00,470 and that's as far as he can follow, he assumed, 695 00:40:00,470 --> 00:40:02,650 OK, they're terminating in the superior colliculus. 696 00:40:02,650 --> 00:40:04,400 And he could even see part of the layer 697 00:40:04,400 --> 00:40:06,600 in the superior colliculus where they would travel. 698 00:40:06,600 --> 00:40:09,183 He just couldn't see that they were turning up and terminating 699 00:40:09,183 --> 00:40:11,270 in the top layers. 700 00:40:11,270 --> 00:40:13,820 But he still got it right, the main projection. 701 00:40:13,820 --> 00:40:18,810 Same for the geniculate body, same for the pretectal area. 702 00:40:18,810 --> 00:40:22,760 Totally missed the one to the hypothalamus. 703 00:40:22,760 --> 00:40:25,575 That was even missed with most of the Nauta methods. 704 00:40:28,820 --> 00:40:31,560 And then Nauta developed his method. 705 00:40:31,560 --> 00:40:35,070 It's a great method because it suppressed 706 00:40:35,070 --> 00:40:40,500 the staining of the normal axons and left 707 00:40:40,500 --> 00:40:42,180 the staining of the degenerating ones. 708 00:40:42,180 --> 00:40:45,580 He used an oxidizing agent. 709 00:40:45,580 --> 00:40:49,230 And the normal tissue was oxidized faster 710 00:40:49,230 --> 00:40:51,850 than the degenerating tissue. 711 00:40:51,850 --> 00:40:52,880 OK. 712 00:40:52,880 --> 00:40:57,340 And then, when he came to MIT, he 713 00:40:57,340 --> 00:41:02,850 had a research associate named Lennart Heimer from Sweden. 714 00:41:02,850 --> 00:41:06,170 And he had a technical assistant, Robert Fink. 715 00:41:06,170 --> 00:41:10,350 The two of them each developed a more sensitive method 716 00:41:10,350 --> 00:41:15,550 that could stain the axons all the way to the boutons. 717 00:41:15,550 --> 00:41:18,070 When Nauta first saw it, he said, 718 00:41:18,070 --> 00:41:20,580 this looks like measles in the brain. 719 00:41:20,580 --> 00:41:22,360 He saw those little dots. 720 00:41:22,360 --> 00:41:23,735 Could these really be boutons? 721 00:41:23,735 --> 00:41:25,460 I remember him saying it. 722 00:41:25,460 --> 00:41:28,570 Well, they were boutons, and that was verified by Heimer 723 00:41:28,570 --> 00:41:30,365 in the electron microscope studies. 724 00:41:33,180 --> 00:41:34,990 So this is just a little about Nauta. 725 00:41:34,990 --> 00:41:38,020 You can read about his history and how 726 00:41:38,020 --> 00:41:42,075 he came to MIT as the first anatomist in a psychology 727 00:41:42,075 --> 00:41:42,575 department. 728 00:41:46,390 --> 00:41:48,210 All his students worship him. 729 00:41:48,210 --> 00:41:48,980 I'm one of them. 730 00:41:51,790 --> 00:41:53,580 He actually signed my PhD thesis. 731 00:41:53,580 --> 00:41:55,470 He had just come here. 732 00:41:55,470 --> 00:41:57,590 But he gave me some crucial help and let 733 00:41:57,590 --> 00:42:00,620 me do some things in his lab, and then I became his post-doc 734 00:42:00,620 --> 00:42:03,680 for two years afterwards. 735 00:42:03,680 --> 00:42:06,570 This is an example of the Fink-Heimer method. 736 00:42:06,570 --> 00:42:09,090 These are retinal fibers. 737 00:42:09,090 --> 00:42:17,069 And this brain has been damaged right after birth on one side. 738 00:42:17,069 --> 00:42:18,110 And look what's happened. 739 00:42:18,110 --> 00:42:24,120 You see these-- I'm just pointing to areas where 740 00:42:24,120 --> 00:42:28,320 you can see the terminations. 741 00:42:28,320 --> 00:42:31,430 The axon bundles are very heavily stained. 742 00:42:31,430 --> 00:42:33,180 Here you see some of them crossing 743 00:42:33,180 --> 00:42:34,450 the midline abnormally. 744 00:42:34,450 --> 00:42:36,720 You don't see this in the normal lining. 745 00:42:36,720 --> 00:42:40,320 So right away, we use this method to make discoveries 746 00:42:40,320 --> 00:42:42,970 about what happens after early brain damage. 747 00:42:42,970 --> 00:42:44,842 Think of cerebral palsy kids. 748 00:42:44,842 --> 00:42:46,050 They have early brain damage. 749 00:42:46,050 --> 00:42:47,300 What's happening to them? 750 00:42:47,300 --> 00:42:49,590 Why is some of their behavior so abnormal? 751 00:42:49,590 --> 00:42:51,950 They have abnormal brain connections. 752 00:42:51,950 --> 00:42:54,780 We discovered them in the visual system of the hamster 753 00:42:54,780 --> 00:42:56,110 initially. 754 00:42:56,110 --> 00:42:59,831 They've been discovered in many other animals since. 755 00:42:59,831 --> 00:43:02,470 Here you see some of them are terminating actually 756 00:43:02,470 --> 00:43:05,110 on the wrong side of the brain. 757 00:43:05,110 --> 00:43:09,390 And actually, that causes abnormal behavior. 758 00:43:09,390 --> 00:43:13,820 Now, what about HRP, another early method? 759 00:43:13,820 --> 00:43:15,360 What does it mean? 760 00:43:15,360 --> 00:43:17,090 Horseradish peroxidase. 761 00:43:17,090 --> 00:43:21,410 It's a plant enzyme that when you 762 00:43:21,410 --> 00:43:25,910 inject, you inject into an area of axons. 763 00:43:25,910 --> 00:43:30,910 It's taken up by the axonal endings, endocytosis we call 764 00:43:30,910 --> 00:43:33,010 that, cell drinking. 765 00:43:33,010 --> 00:43:38,050 They take it up, and they get encapsulated in little vesicles 766 00:43:38,050 --> 00:43:41,140 and transported back to the cell body. 767 00:43:41,140 --> 00:43:44,410 That's retrograde transport. 768 00:43:44,410 --> 00:43:48,280 If you inject the HRP in a region of cell bodies, 769 00:43:48,280 --> 00:43:52,820 it gets taken up by endocytosis in the cell bodies. 770 00:43:52,820 --> 00:43:55,410 It gets incorporated into vesicles and gets 771 00:43:55,410 --> 00:43:58,180 transported all the way down to the endings. 772 00:43:58,180 --> 00:44:02,010 That's anterograde transport. 773 00:44:02,010 --> 00:44:05,510 So you have to learn to separate those two directions, 774 00:44:05,510 --> 00:44:09,140 but it's a wonderful tracer. 775 00:44:09,140 --> 00:44:12,760 Now we're using more fluorescent molecules 776 00:44:12,760 --> 00:44:15,410 that have the big advantage. 777 00:44:15,410 --> 00:44:17,490 You don't have to stain them. 778 00:44:17,490 --> 00:44:21,360 You just prepare it properly, put them on slides, 779 00:44:21,360 --> 00:44:23,320 and look at a good fluorescence microscope, 780 00:44:23,320 --> 00:44:25,219 and you can see the fluorescent molecules. 781 00:44:25,219 --> 00:44:26,510 So let's show you those things. 782 00:44:26,510 --> 00:44:30,720 Here's an HRP method used in a developing hamster. 783 00:44:30,720 --> 00:44:33,860 I'm using dark field microscopy here. 784 00:44:33,860 --> 00:44:36,610 And so the HRP is showing up. 785 00:44:36,610 --> 00:44:40,010 It's very bright on a darker background. 786 00:44:40,010 --> 00:44:42,050 And here you see something unusual. 787 00:44:42,050 --> 00:44:45,140 You don't see this in the older brain. 788 00:44:45,140 --> 00:44:47,090 Some of the axons are going right 789 00:44:47,090 --> 00:44:51,166 into the somatosensory nucleus more immediately. 790 00:44:51,166 --> 00:44:54,860 And those disappear with development. 791 00:44:54,860 --> 00:44:56,625 Here is a retrograde tracer. 792 00:44:56,625 --> 00:45:01,500 And here, we put the tracer in the superior colliculus 793 00:45:01,500 --> 00:45:05,390 of the midbrain, waited a little while 794 00:45:05,390 --> 00:45:09,320 for the retrograde transport to take place. 795 00:45:09,320 --> 00:45:12,200 And you should know the molecule involved here. 796 00:45:12,200 --> 00:45:15,110 We used kinesin and dynein. 797 00:45:15,110 --> 00:45:18,340 For the two directions of transport. 798 00:45:18,340 --> 00:45:26,760 There are molecules that-- they can grab the protein and latch 799 00:45:26,760 --> 00:45:30,780 onto the structures of the microtubules 800 00:45:30,780 --> 00:45:33,749 and move the substance along. 801 00:45:33,749 --> 00:45:35,915 But here they've reached the retinal ganglion cells. 802 00:45:35,915 --> 00:45:38,360 We've flat-mounted the retina here. 803 00:45:38,360 --> 00:45:42,116 And here you see these bright cells in the retina. 804 00:45:42,116 --> 00:45:44,530 We even see some of the axon coming in 805 00:45:44,530 --> 00:45:47,510 and assume their dendrites. 806 00:45:47,510 --> 00:45:52,090 Those are the cells that are giving rise to the axons 807 00:45:52,090 --> 00:45:57,030 we've labeled back in the midbrain, 17 millimeters away, 808 00:45:57,030 --> 00:46:00,780 in the hands of the adult hamster. 809 00:46:00,780 --> 00:46:03,890 Here you see the same thing in the retina, 810 00:46:03,890 --> 00:46:05,080 but with a different label. 811 00:46:05,080 --> 00:46:07,860 It's called nuclear yellow. 812 00:46:07,860 --> 00:46:09,320 We didn't have to do any staining, 813 00:46:09,320 --> 00:46:12,490 but we did for these cells. 814 00:46:12,490 --> 00:46:14,970 That's HRP. 815 00:46:14,970 --> 00:46:17,880 We've used two different retrograde labels 816 00:46:17,880 --> 00:46:21,865 that we've injected the label on two different sides 817 00:46:21,865 --> 00:46:25,640 of the brain and shown that there 818 00:46:25,640 --> 00:46:29,550 are a few cells in the retina whose axon branches, one goes 819 00:46:29,550 --> 00:46:31,760 to one side, one goes to the other side. 820 00:46:36,360 --> 00:46:40,530 And here, another double labeling experiment. 821 00:46:40,530 --> 00:46:43,320 Here, there's a bunch of cells with fluorogold. 822 00:46:47,430 --> 00:46:51,283 The label was put in the superior colliculus, 823 00:46:51,283 --> 00:46:59,700 and we put the other label, little plastic beads containing 824 00:46:59,700 --> 00:47:05,200 a substance, we call them the red beads, 825 00:47:05,200 --> 00:47:11,070 they also get taken up, and they fluoresce-- red, 826 00:47:11,070 --> 00:47:12,260 as you can see. 827 00:47:12,260 --> 00:47:14,860 So with a different wavelength of illumination, 828 00:47:14,860 --> 00:47:17,090 we can see that label. 829 00:47:17,090 --> 00:47:19,970 And you see that these two cells are double labeled. 830 00:47:19,970 --> 00:47:21,990 I'm taking the same field of view 831 00:47:21,990 --> 00:47:24,530 with two different wavelengths of light. 832 00:47:24,530 --> 00:47:27,450 Beautiful advantage of the fluorescent methods 833 00:47:27,450 --> 00:47:29,930 for retrograde tracer. 834 00:47:29,930 --> 00:47:33,970 So these two cells project to both the ventral geniculate 835 00:47:33,970 --> 00:47:36,280 body and the superior colliculus, the rest of them 836 00:47:36,280 --> 00:47:37,902 only to the superior colliculus. 837 00:47:40,610 --> 00:47:44,380 And then we use cholera toxin as this-- 838 00:47:44,380 --> 00:47:46,640 you say, why would you use such a poisonous thing? 839 00:47:46,640 --> 00:47:49,860 Well, it's a modification of cholera toxin, 840 00:47:49,860 --> 00:47:53,120 so it's fairly safe. 841 00:47:53,120 --> 00:47:57,640 I used to tell my students that half the substance you 842 00:47:57,640 --> 00:48:01,970 see on these shelves are toxic, so I want you to wear gloves, 843 00:48:01,970 --> 00:48:05,700 wear a mask, and don't spill things. 844 00:48:05,700 --> 00:48:09,120 And if you do, clean it up properly. 845 00:48:09,120 --> 00:48:12,990 But anyway, cholera toxin, we only used a subunit of it. 846 00:48:12,990 --> 00:48:16,300 But when it works-- not all cells 847 00:48:16,300 --> 00:48:22,160 will take it up and transport it as well as other axons. 848 00:48:22,160 --> 00:48:25,010 But when they do, you get this beautiful picture. 849 00:48:25,010 --> 00:48:27,280 You get axons that look like they've 850 00:48:27,280 --> 00:48:29,090 been stained with the Golgi method, 851 00:48:29,090 --> 00:48:32,470 but you can trace them over long distances. 852 00:48:32,470 --> 00:48:34,760 And here, I'm just showing it low power. 853 00:48:34,760 --> 00:48:39,070 From one part of the retina, you can see them reaching-- 854 00:48:39,070 --> 00:48:41,240 and here it is in dark field and light 855 00:48:41,240 --> 00:48:44,720 field, two different microscopic methods. 856 00:48:44,720 --> 00:48:49,860 And just one more method here-- a modern thing, Recent Science 857 00:48:49,860 --> 00:48:51,820 has a beautiful picture on the cover showing 858 00:48:51,820 --> 00:48:53,260 callosal axons in humans. 859 00:48:56,090 --> 00:48:57,280 These are pictures. 860 00:48:57,280 --> 00:49:01,340 Here's a dissected brain showing fibers 861 00:49:01,340 --> 00:49:03,500 coming in and out of the neocortex. 862 00:49:03,500 --> 00:49:06,140 You can see the little bundles there. 863 00:49:06,140 --> 00:49:10,700 And you see some of them going right down into the brain stem. 864 00:49:10,700 --> 00:49:14,860 Well, here they are in a living human brain. 865 00:49:14,860 --> 00:49:16,475 They didn't inject any tracers. 866 00:49:19,010 --> 00:49:26,680 They've worked out the methods to map the major directions 867 00:49:26,680 --> 00:49:30,240 of water diffusion-- where the water is moving. 868 00:49:32,800 --> 00:49:35,390 It tends to move, of course, down axons and not 869 00:49:35,390 --> 00:49:37,260 across axons. 870 00:49:37,260 --> 00:49:42,050 So you set the computer, say, to start here. 871 00:49:42,050 --> 00:49:48,820 And it will follow the bundles of axons from that region 872 00:49:48,820 --> 00:49:50,450 as far as you want. 873 00:49:50,450 --> 00:49:54,910 So they traced them here through the-- we 874 00:49:54,910 --> 00:49:56,620 call it the internal capsule-- you'll 875 00:49:56,620 --> 00:50:00,410 learn all about this soon-- and into the brain stem. 876 00:50:00,410 --> 00:50:03,610 And then you can see, from different areas of the cortex, 877 00:50:03,610 --> 00:50:06,200 they traced them going different ways. 878 00:50:06,200 --> 00:50:07,660 It made a mistake here. 879 00:50:07,660 --> 00:50:09,125 It traced them into the cerebellum, 880 00:50:09,125 --> 00:50:11,450 and there aren't any connections like that. 881 00:50:11,450 --> 00:50:14,470 So depending on how the software is working, 882 00:50:14,470 --> 00:50:18,610 it can jump to the wrong bundle. 883 00:50:18,610 --> 00:50:19,950 That's one of the problems. 884 00:50:19,950 --> 00:50:25,070 The other problem is it's not actually tracing connections. 885 00:50:25,070 --> 00:50:26,070 Remember that. 886 00:50:26,070 --> 00:50:28,780 It's just tracing the large axon bundles. 887 00:50:31,450 --> 00:50:36,660 So when you see a study advertising, new connections 888 00:50:36,660 --> 00:50:40,360 seen in the human brain found with diffusion tensor imaging 889 00:50:40,360 --> 00:50:43,220 you know that it's a great overstatement. 890 00:50:43,220 --> 00:50:45,140 They've not seen connections. 891 00:50:45,140 --> 00:50:48,430 But it is a beautiful method to see the major pathways. 892 00:50:48,430 --> 00:50:52,740 And we've been able to confirm major pathways seen 893 00:50:52,740 --> 00:50:54,640 in the monkey and experimental methods. 894 00:50:54,640 --> 00:50:56,410 We now can see them in humans. 895 00:50:56,410 --> 00:50:58,390 And sorry I took a little extra time, 896 00:50:58,390 --> 00:51:02,760 but we got through all these pictures. 897 00:51:02,760 --> 00:51:04,410 So think about it. 898 00:51:04,410 --> 00:51:05,820 Look at the chapter. 899 00:51:05,820 --> 00:51:09,030 Come up with your questions if you have any more questions, 900 00:51:09,030 --> 00:51:15,410 because we'll be going on to chapter three next time.