1 00:00:15,579 --> 00:00:16,079 All right. 2 00:00:16,079 --> 00:00:20,410 So let me move onto the last lecture in this module. 3 00:00:20,410 --> 00:00:22,120 Not in the module. 4 00:00:22,120 --> 00:00:24,660 In the last lecture in the nervous system 5 00:00:24,660 --> 00:00:26,365 part of the systems module. 6 00:00:29,550 --> 00:00:33,170 And I want to start by talking briefly 7 00:00:33,170 --> 00:00:40,960 about learning and memory, very briefly 8 00:00:40,960 --> 00:00:43,750 about learning and memory. 9 00:00:43,750 --> 00:00:46,670 We talked last time about setting up synapses, 10 00:00:46,670 --> 00:00:51,300 we talked about making connections between the wires, 11 00:00:51,300 --> 00:00:57,250 and we talked about how changes in membrane potential 12 00:00:57,250 --> 00:01:00,130 could either determine whether a synapse fired 13 00:01:00,130 --> 00:01:02,040 or whether it did not. 14 00:01:02,040 --> 00:01:04,550 However, there's more to it than that. 15 00:01:04,550 --> 00:01:09,600 There is significant data that indicates that the more often 16 00:01:09,600 --> 00:01:15,280 you use a synapse the strong that synapse will become. 17 00:01:15,280 --> 00:01:17,700 What do I mean by stronger? 18 00:01:17,700 --> 00:01:20,770 Well, I'm actually not sure what I mean by stronger, 19 00:01:20,770 --> 00:01:24,180 except in a couple of cases that I'll tell you about. 20 00:01:24,180 --> 00:01:31,940 So the term I want to, so synaptic connections, this 21 00:01:31,940 --> 00:01:33,710 is going to be long. 22 00:01:33,710 --> 00:01:36,770 Synaptic connections strengthen with use. 23 00:01:44,520 --> 00:01:47,060 And the phenomenon I want to tell you about 24 00:01:47,060 --> 00:01:55,720 is long-term potentiation or long-term depression. 25 00:01:55,720 --> 00:02:01,630 And this is a phenomenon that was discovered 26 00:02:01,630 --> 00:02:04,990 in neurons that were studied in isolation. 27 00:02:04,990 --> 00:02:07,810 So it's very difficult, it's impossible to study neurons 28 00:02:07,810 --> 00:02:12,130 within the brain of any complex animal, especially mammals. 29 00:02:12,130 --> 00:02:15,760 And so what's done is to use so-called slice cultures. 30 00:02:15,760 --> 00:02:18,480 Where you take a slice out of the brain, 31 00:02:18,480 --> 00:02:22,310 a very thin slice that's just got a few layers of cells, 32 00:02:22,310 --> 00:02:24,980 and you look and see what these neurons do 33 00:02:24,980 --> 00:02:28,930 in some kind of plastic tissue culture dish with media. 34 00:02:28,930 --> 00:02:31,050 And they actually do quite a bit. 35 00:02:31,050 --> 00:02:34,170 The neurons fire and they make synapses. 36 00:02:34,170 --> 00:02:36,600 And a lot has been learned with them, 37 00:02:36,600 --> 00:02:39,040 with these kind of slice cultures. 38 00:02:39,040 --> 00:02:40,530 So here's the deal. 39 00:02:40,530 --> 00:02:44,660 If one has a cell, if you look at this picture 40 00:02:44,660 --> 00:02:49,120 here, if one has a cell and one stimulates the synapse 41 00:02:49,120 --> 00:02:56,230 you see that there is a response to the first stimulus. 42 00:02:56,230 --> 00:02:58,460 And you're actually measuring change 43 00:02:58,460 --> 00:03:00,990 in potential in the cell. 44 00:03:00,990 --> 00:03:03,100 Let's not worry about how the experiment was 45 00:03:03,100 --> 00:03:05,864 done because we're not going to get through the material 46 00:03:05,864 --> 00:03:06,780 I want to cover today. 47 00:03:06,780 --> 00:03:09,279 You can come see me in office hours and I'll explain to you. 48 00:03:09,279 --> 00:03:12,660 What I want to point out is that after a first stimulus 49 00:03:12,660 --> 00:03:16,860 there is a change in the potential of the responding 50 00:03:16,860 --> 00:03:17,760 cell. 51 00:03:17,760 --> 00:03:21,480 If you continue to give that stimulus, 52 00:03:21,480 --> 00:03:25,790 you get a much stronger response later on. 53 00:03:25,790 --> 00:03:28,920 So the strength of the response, the change 54 00:03:28,920 --> 00:03:32,100 in membrane potential in a responding cell 55 00:03:32,100 --> 00:03:36,920 increases with stimulation of that presynaptic cell, 56 00:03:36,920 --> 00:03:38,060 of that synapse. 57 00:03:38,060 --> 00:03:41,140 And that is what long-term potentiation is. 58 00:03:41,140 --> 00:03:46,190 It increases the chances that the postsynaptic cell 59 00:03:46,190 --> 00:03:47,210 will fire. 60 00:03:47,210 --> 00:03:50,640 And it is believed that long-term potentiation has 61 00:03:50,640 --> 00:03:55,440 got a profound role in memory and in learning. 62 00:03:55,440 --> 00:03:57,820 So how does that actually work? 63 00:03:57,820 --> 00:03:59,440 Well, it's not clear in most cases, 64 00:03:59,440 --> 00:04:01,350 but this is an example from your book 65 00:04:01,350 --> 00:04:06,070 that is the shining example of how learning and memory can 66 00:04:06,070 --> 00:04:08,320 work at the molecular level. 67 00:04:08,320 --> 00:04:10,560 And it has to do with glutamate. 68 00:04:10,560 --> 00:04:12,690 You do not have this on today's handout. 69 00:04:12,690 --> 00:04:15,810 It was on your last lecture's in-class handout. 70 00:04:15,810 --> 00:04:17,790 But you can look up here and then 71 00:04:17,790 --> 00:04:19,760 go back to your last lecture's handout 72 00:04:19,760 --> 00:04:21,010 if you don't have it with you. 73 00:04:21,010 --> 00:04:22,280 It has to do with glutamate. 74 00:04:22,280 --> 00:04:25,470 So glutamate is the major excitatory neurotransmitter 75 00:04:25,470 --> 00:04:27,130 in the central nervous system. 76 00:04:27,130 --> 00:04:32,200 And glutamate activates two different ionotropic receptors. 77 00:04:32,200 --> 00:04:36,560 One of them is called the AMPA or the non-NMDA receptor 78 00:04:36,560 --> 00:04:39,490 and the other is called the NMDA receptor. 79 00:04:39,490 --> 00:04:43,170 With low level of stimulation or with single stimulation 80 00:04:43,170 --> 00:04:49,320 of a presynaptic neuron the AMPA receptors are activated. 81 00:04:49,320 --> 00:04:51,410 And these are sodium channels, so these 82 00:04:51,410 --> 00:04:53,060 are ionotropic channels. 83 00:04:53,060 --> 00:04:57,060 And glutamate is released, binds to the AMPA ion channel, 84 00:04:57,060 --> 00:05:00,360 changes its confirmation, and you get an influx of sodium, 85 00:05:00,360 --> 00:05:03,480 and therefore get closer to threshold potential. 86 00:05:03,480 --> 00:05:09,030 However, on the same synapse, like right next door 87 00:05:09,030 --> 00:05:12,940 are these other receptors called the NMDA receptors. 88 00:05:12,940 --> 00:05:17,750 And they are not used if the presynaptic cell is stimulated 89 00:05:17,750 --> 00:05:20,580 at a low level or infrequently. 90 00:05:20,580 --> 00:05:24,430 But if you keep stimulating that presynaptic cell, 91 00:05:24,430 --> 00:05:30,170 eventually these NMDA receptors will open. 92 00:05:30,170 --> 00:05:33,400 And they will open and they will allow 93 00:05:33,400 --> 00:05:37,020 an influx of both calcium and sodium. 94 00:05:37,020 --> 00:05:41,550 And what happens is that you get a much stronger change 95 00:05:41,550 --> 00:05:45,730 in potential because you've got now two sets of receptors open. 96 00:05:45,730 --> 00:05:48,900 And this postsynaptic cell is much more likely to fire 97 00:05:48,900 --> 00:05:52,750 and is actually much more stable in the synapses 98 00:05:52,750 --> 00:05:54,300 that it has made. 99 00:05:54,300 --> 00:05:59,240 So this NMDA channel only open after repeated stimulation. 100 00:05:59,240 --> 00:06:03,240 So that's an example of what repeated stimulation can 101 00:06:03,240 --> 00:06:05,010 do to changing channels. 102 00:06:05,010 --> 00:06:06,640 And there is some molecular data, 103 00:06:06,640 --> 00:06:08,110 although it's really not complete, 104 00:06:08,110 --> 00:06:10,700 as to why you switch this channel usage. 105 00:06:10,700 --> 00:06:13,920 One thing, however, that's very interesting about long-term 106 00:06:13,920 --> 00:06:17,890 potentiation, and indeed about learning and memory anyway, 107 00:06:17,890 --> 00:06:21,870 is that you need -- 108 00:06:21,870 --> 00:06:29,910 -- protein synthesis for learning especially. 109 00:06:29,910 --> 00:06:34,990 For classical learning, the kind of learning 110 00:06:34,990 --> 00:06:37,920 that you do before a test, the kind of learning 111 00:06:37,920 --> 00:06:40,470 that a mouse does to find its way through a maze 112 00:06:40,470 --> 00:06:43,530 repeatedly requires protein synthesis. 113 00:06:43,530 --> 00:06:45,510 And this is very interesting because what 114 00:06:45,510 --> 00:06:49,230 I've been telling you about in this module is stuff that 115 00:06:49,230 --> 00:06:52,770 happens in the neuron that is actually independent 116 00:06:52,770 --> 00:06:54,880 of transcription or protein synthesis. 117 00:06:54,880 --> 00:06:58,160 It involves changes in gene activity. 118 00:06:58,160 --> 00:07:00,210 It can involve changes in gene regulation, 119 00:07:00,210 --> 00:07:03,000 as I'll talk about in a moment, but it does not actually 120 00:07:03,000 --> 00:07:07,050 require any transcription or protein synthesis per se. 121 00:07:07,050 --> 00:07:09,970 Learning and memory of the classical kind 122 00:07:09,970 --> 00:07:11,480 that we think about does. 123 00:07:11,480 --> 00:07:13,100 And it's one of the reasons that you 124 00:07:13,100 --> 00:07:16,960 need to get a few hours sleep before you take a test. 125 00:07:16,960 --> 00:07:19,460 It's one of the reasons if you pull an all-nighter, 126 00:07:19,460 --> 00:07:22,370 and there are many, you do not do that well the next day 127 00:07:22,370 --> 00:07:25,160 because you need this round of protein synthesis 128 00:07:25,160 --> 00:07:29,390 that does stuff that probably synthesizes new receptors that 129 00:07:29,390 --> 00:07:31,200 allows the synaptic connections you've 130 00:07:31,200 --> 00:07:33,332 made through your learning to be strengthened. 131 00:07:33,332 --> 00:07:34,790 And that's still a mystery, really, 132 00:07:34,790 --> 00:07:37,420 as to what the protein synthesis is required for. 133 00:07:37,420 --> 00:07:40,080 But if you're interested, this work from [Ed Plezia?] that I 134 00:07:40,080 --> 00:07:42,830 mentioned at the beginning of class has shed some really 135 00:07:42,830 --> 00:07:43,830 fascinating light on it. 136 00:07:43,830 --> 00:07:45,550 So come see me if you want to. 137 00:07:45,550 --> 00:07:46,320 All right. 138 00:07:46,320 --> 00:07:50,331 So I want to move on now to circuitry. 139 00:07:50,331 --> 00:07:50,830 All right. 140 00:07:50,830 --> 00:07:52,788 I'll show you this slide because it's up there. 141 00:07:52,788 --> 00:07:55,860 This is a piece of data that I pulled down 142 00:07:55,860 --> 00:07:57,440 that's kind of interesting. 143 00:07:57,440 --> 00:08:00,320 This looks at the AMPA NMDA ratio, 144 00:08:00,320 --> 00:08:02,460 receptor ratio with cocaine. 145 00:08:02,460 --> 00:08:06,140 So in a control there are about the same number 146 00:08:06,140 --> 00:08:11,130 of AMPA and NMDA receptors on a particular synapse. 147 00:08:11,130 --> 00:08:15,090 It's very interesting in some studies with cocaine use, 148 00:08:15,090 --> 00:08:18,180 you see the amount of the AMPA receptor. 149 00:08:18,180 --> 00:08:21,350 That is the receptor that is the short-term receptor 150 00:08:21,350 --> 00:08:29,540 increases relative to the NMDA ratio, an NMDA receptor level, 151 00:08:29,540 --> 00:08:32,760 indicating that with cocaine you might not 152 00:08:32,760 --> 00:08:35,980 be as good at learning as without cocaine. 153 00:08:35,980 --> 00:08:36,970 It is not clear. 154 00:08:36,970 --> 00:08:38,510 There are a lot of studies like this 155 00:08:38,510 --> 00:08:41,110 and the interpretations of them are complicated, 156 00:08:41,110 --> 00:08:43,240 but this is a solid piece of data 157 00:08:43,240 --> 00:08:47,920 that you do change the ratio of the non-NMDA and the NMDA 158 00:08:47,920 --> 00:08:49,770 receptors. 159 00:08:49,770 --> 00:08:50,270 All right. 160 00:08:50,270 --> 00:08:51,710 So let's move on. 161 00:08:51,710 --> 00:08:54,390 So let's talk about circuitry. 162 00:08:54,390 --> 00:08:58,340 We've talked about the wires, the passage of current 163 00:08:58,340 --> 00:09:00,680 along the wires, we've talked about the connections 164 00:09:00,680 --> 00:09:02,100 between the wires, and now I want 165 00:09:02,100 --> 00:09:03,700 to talk about the circuits. 166 00:09:03,700 --> 00:09:07,120 And I want to start by asking you a question. 167 00:09:07,120 --> 00:09:10,760 In thinking about your brain and how much of your brain 168 00:09:10,760 --> 00:09:14,220 you used in circuitry, how many of you 169 00:09:14,220 --> 00:09:18,410 were taught that you use one-tenth of your brain? 170 00:09:18,410 --> 00:09:19,270 Raise your hands. 171 00:09:19,270 --> 00:09:21,311 I want to see what you've been taught at schools. 172 00:09:21,311 --> 00:09:22,830 It's no reflection on your teachers. 173 00:09:22,830 --> 00:09:23,330 It's OK. 174 00:09:23,330 --> 00:09:23,670 Thank you. 175 00:09:23,670 --> 00:09:24,950 It's a substantial portion. 176 00:09:24,950 --> 00:09:28,390 I feel that it is my duty as your professor 177 00:09:28,390 --> 00:09:32,380 to debunk that myth once and for all forever. 178 00:09:32,380 --> 00:09:33,027 It's not true. 179 00:09:33,027 --> 00:09:35,610 And you might want to actually email your high school teachers 180 00:09:35,610 --> 00:09:37,690 and tell them it's not true. 181 00:09:37,690 --> 00:09:38,270 OK. 182 00:09:38,270 --> 00:09:40,370 And you perhaps even learned about the boy who 183 00:09:40,370 --> 00:09:42,920 used, you know, one-fifth of his brain 184 00:09:42,920 --> 00:09:46,140 and was an unbelievable genius. 185 00:09:46,140 --> 00:09:48,310 There is, none of this, I was taught that as well, 186 00:09:48,310 --> 00:09:49,720 and it is absolutely untrue. 187 00:09:49,720 --> 00:09:51,840 And the data for it is very solid. 188 00:09:51,840 --> 00:09:55,510 For example, any neurosurgeon who goes into your brain 189 00:09:55,510 --> 00:09:58,310 knows that no matter what part of the brain 190 00:09:58,310 --> 00:10:01,290 he or she goes through to remove your brain tumor 191 00:10:01,290 --> 00:10:03,800 or do whatever it is, is going to damage 192 00:10:03,800 --> 00:10:06,200 some aspect of your brain function. 193 00:10:06,200 --> 00:10:09,210 Anyone who has any kind of stroke, 194 00:10:09,210 --> 00:10:11,800 any kind of brain hemorrhage, any kind of leakage of blood 195 00:10:11,800 --> 00:10:15,110 into the brain is going to have some kind of deficit. 196 00:10:15,110 --> 00:10:17,200 It might be a small one or not. 197 00:10:17,200 --> 00:10:20,130 But there is ample evidence that you use all your brain. 198 00:10:20,130 --> 00:10:23,210 And, indeed, why would one go to the trouble of making a brain 199 00:10:23,210 --> 00:10:25,590 that is so large if one was not using it? 200 00:10:25,590 --> 00:10:27,130 So there we go. 201 00:10:27,130 --> 00:10:29,650 Let me talk a little bit about circuitry. 202 00:10:29,650 --> 00:10:33,980 To indicate the complexity of circuitry in your brain 203 00:10:33,980 --> 00:10:37,860 let me point out what parts of your brain 204 00:10:37,860 --> 00:10:40,730 it takes to put together language. 205 00:10:40,730 --> 00:10:43,280 If you're looking at words, it takes this area right 206 00:10:43,280 --> 00:10:46,230 at the back of your brain, the base of your brain. 207 00:10:46,230 --> 00:10:48,640 Listening to words takes a different region. 208 00:10:48,640 --> 00:10:50,920 Speaking words takes a different region. 209 00:10:50,920 --> 00:10:53,430 Generating words take different regions. 210 00:10:53,430 --> 00:10:55,647 And these are very, these are PET scans. 211 00:10:55,647 --> 00:10:56,980 We can talk about what they are. 212 00:10:56,980 --> 00:10:59,500 They measure the energy usage in your brain. 213 00:10:59,500 --> 00:11:03,510 And these are very, very large parts of your brain. 214 00:11:03,510 --> 00:11:04,170 OK? 215 00:11:04,170 --> 00:11:08,590 These are millions, close to billions of neurons involved 216 00:11:08,590 --> 00:11:13,970 in each part of working with language. 217 00:11:13,970 --> 00:11:14,470 OK? 218 00:11:14,470 --> 00:11:16,553 And you can go to your brain and you can actually, 219 00:11:16,553 --> 00:11:18,420 people have mapped these quite finely. 220 00:11:18,420 --> 00:11:20,850 So the circuitry in your brain is enormous, 221 00:11:20,850 --> 00:11:24,120 and the effort to both set up and maintain the circuitry 222 00:11:24,120 --> 00:11:24,980 is enormous. 223 00:11:24,980 --> 00:11:27,500 Here is another interesting piece of information. 224 00:11:27,500 --> 00:11:32,690 About 20% to 25% of all the energy that you use 225 00:11:32,690 --> 00:11:37,110 is used by your brain, which is one of the reasons you probably 226 00:11:37,110 --> 00:11:37,750 need to sleep. 227 00:11:37,750 --> 00:11:39,590 It's not clear why we need to sleep. 228 00:11:39,590 --> 00:11:42,180 But it is a tremendous, there takes a tremendous amount 229 00:11:42,180 --> 00:11:44,590 of effort to get the wiring set up 230 00:11:44,590 --> 00:11:46,950 and to keep the wiring set up and to keep 231 00:11:46,950 --> 00:11:49,760 the circuitry running. 232 00:11:49,760 --> 00:11:51,559 Here's another example of circuitry 233 00:11:51,559 --> 00:11:52,850 that I'm going to come back to. 234 00:11:52,850 --> 00:11:57,310 It's called a retinotopic map. 235 00:11:57,310 --> 00:12:01,360 And I'm going to start talking about it by addressing 236 00:12:01,360 --> 00:12:05,140 the question of how the circuitry 237 00:12:05,140 --> 00:12:08,330 in your nervous system is set up in the first place. 238 00:12:08,330 --> 00:12:10,330 And I'm going to use this as an example 239 00:12:10,330 --> 00:12:14,420 throughout the rest of the class. 240 00:12:14,420 --> 00:12:17,630 So when people realized way back when that there 241 00:12:17,630 --> 00:12:20,620 were circuits set up in the nervous system, 242 00:12:20,620 --> 00:12:24,800 the question was how did these circuits get set up? 243 00:12:24,800 --> 00:12:28,820 How did neurons know which connections to make? 244 00:12:28,820 --> 00:12:30,720 And there were two competing theories. 245 00:12:39,560 --> 00:12:45,870 One I'm going to call the random/survival theory. 246 00:12:45,870 --> 00:12:47,780 They've got lots of names in the literature, 247 00:12:47,780 --> 00:12:50,220 but I call it the random/survival theory. 248 00:12:50,220 --> 00:12:52,960 And the notion here was that neurons 249 00:12:52,960 --> 00:12:56,810 grow all over the place, and if they make connections 250 00:12:56,810 --> 00:12:59,370 that work, if they're productive connections, 251 00:12:59,370 --> 00:13:01,310 those connections will be maintained, 252 00:13:01,310 --> 00:13:03,780 the neurons will survive, and there you go. 253 00:13:03,780 --> 00:13:05,080 You have your circuit. 254 00:13:05,080 --> 00:13:12,970 The other theory is the guidance theory, 255 00:13:12,970 --> 00:13:16,480 that there's actually something telling the neurons where 256 00:13:16,480 --> 00:13:18,540 to go. 257 00:13:18,540 --> 00:13:20,260 And I'll show you a piece of data 258 00:13:20,260 --> 00:13:22,250 that suggested that this guidance 259 00:13:22,250 --> 00:13:24,610 theory was really correct. 260 00:13:24,610 --> 00:13:30,170 But today, in fact, both of these theories 261 00:13:30,170 --> 00:13:32,770 seem to be correct when put together 262 00:13:32,770 --> 00:13:35,290 in a way I'll try to build up for you. 263 00:13:35,290 --> 00:13:40,550 So your eye, your eye and your visual system are very complex. 264 00:13:40,550 --> 00:13:44,070 Your eye starts off sort of like a camera, but very rapidly 265 00:13:44,070 --> 00:13:45,830 that analogy breaks down. 266 00:13:45,830 --> 00:13:51,090 And light falls onto your retina and the photoreceptors 267 00:13:51,090 --> 00:13:56,670 in your retina, and from your retina a series of axons 268 00:13:56,670 --> 00:14:01,360 are bundled together to make the optic nerve, which 269 00:14:01,360 --> 00:14:03,080 goes to your brain. 270 00:14:03,080 --> 00:14:04,500 And, in fact, I should point out, 271 00:14:04,500 --> 00:14:06,110 I'm not going to talk more about this, 272 00:14:06,110 --> 00:14:10,440 but the image that your retina sees we know quite clearly, 273 00:14:10,440 --> 00:14:13,450 a Nobel Prize was given for it, is not the image 274 00:14:13,450 --> 00:14:16,800 that you see when you look at me or you look around the room. 275 00:14:16,800 --> 00:14:20,480 It's a series of shadows and bright points and so on. 276 00:14:20,480 --> 00:14:23,160 It's really not a direct camera image 277 00:14:23,160 --> 00:14:24,920 as the camera does it for us. 278 00:14:24,920 --> 00:14:26,390 But that is no matter now. 279 00:14:26,390 --> 00:14:29,630 What is important is that some set of stimuli 280 00:14:29,630 --> 00:14:35,460 go from your retina through the optic nerve back to your brain. 281 00:14:35,460 --> 00:14:38,400 And they go to two places in your brain. 282 00:14:38,400 --> 00:14:40,150 So here are some diagrams. 283 00:14:40,150 --> 00:14:42,750 Let's look at the very left most one. 284 00:14:42,750 --> 00:14:47,620 This isn't on your handout but look at it up here. 285 00:14:47,620 --> 00:14:49,470 I've got an equivalent one later on. 286 00:14:49,470 --> 00:14:59,000 So in your eye, from your eye the axons from the eye 287 00:14:59,000 --> 00:15:02,920 are sent back, project back to a region in your brain 288 00:15:02,920 --> 00:15:03,980 called the tectum. 289 00:15:03,980 --> 00:15:06,350 It's got a lot of different names, lateral geniculate 290 00:15:06,350 --> 00:15:09,250 nucleus or the thalamus or the tectum. 291 00:15:09,250 --> 00:15:09,750 OK. 292 00:15:09,750 --> 00:15:12,610 We'll call it the, I think I'm calling it 293 00:15:12,610 --> 00:15:14,980 the tectum in this lecture. 294 00:15:14,980 --> 00:15:15,570 Let me see. 295 00:15:15,570 --> 00:15:18,380 Let me make sure I've got it correct. 296 00:15:18,380 --> 00:15:20,100 Yeah, I'm going to call it the tectum. 297 00:15:20,100 --> 00:15:20,370 OK? 298 00:15:20,370 --> 00:15:21,661 So we'll talk about the tectum. 299 00:15:21,661 --> 00:15:24,930 And from the tectum connect synapses are made, 300 00:15:24,930 --> 00:15:27,770 another set of neurons go back to another region 301 00:15:27,770 --> 00:15:29,710 of your brain called the visual cortex. 302 00:15:29,710 --> 00:15:33,130 And it's the summation of all these different connections 303 00:15:33,130 --> 00:15:35,680 that somehow allows you to interpret what you see and see 304 00:15:35,680 --> 00:15:36,180 it. 305 00:15:36,180 --> 00:15:37,760 It's extremely complicated. 306 00:15:37,760 --> 00:15:40,310 What I want to concentrate on are the connections 307 00:15:40,310 --> 00:15:42,550 between the retina and the tectum. 308 00:15:42,550 --> 00:15:44,660 And what's interesting in ourselves 309 00:15:44,660 --> 00:15:47,040 and any animals that see binocularly, 310 00:15:47,040 --> 00:15:50,490 that can see 3-dimensionally, is that the reason you can see 311 00:15:50,490 --> 00:15:53,460 in 3D is that some of the neurons that 312 00:15:53,460 --> 00:15:56,920 come from your right eye go to the right side of the brain 313 00:15:56,920 --> 00:15:59,961 and some of them go to the left side of the brain and vise 314 00:15:59,961 --> 00:16:00,460 versa. 315 00:16:00,460 --> 00:16:03,300 Some of the neurons on the left side of your brain 316 00:16:03,300 --> 00:16:06,210 go to the left side, yeah, and some of them 317 00:16:06,210 --> 00:16:07,530 go to the right side. 318 00:16:07,530 --> 00:16:12,340 And so if you look in some, just look at the diagram 319 00:16:12,340 --> 00:16:13,700 so you're with me. 320 00:16:13,700 --> 00:16:17,000 If you look in some, you'll see that some of the neurons 321 00:16:17,000 --> 00:16:19,430 go to the same side and some cross over 322 00:16:19,430 --> 00:16:21,180 and go to the other side. 323 00:16:21,180 --> 00:16:21,810 OK. 324 00:16:21,810 --> 00:16:26,880 So you can look at this in a different way. 325 00:16:26,880 --> 00:16:29,930 This very beautifully labeling experiment 326 00:16:29,930 --> 00:16:32,850 where you can actually label neurons 327 00:16:32,850 --> 00:16:35,530 in different parts of the retina, 328 00:16:35,530 --> 00:16:37,350 and you can follow where they go. 329 00:16:37,350 --> 00:16:40,590 The label is maintained in their axons. 330 00:16:40,590 --> 00:16:43,250 And as the axons, well, I'll talk about in a moment. 331 00:16:43,250 --> 00:16:47,190 As the axons send out their processes back into, 332 00:16:47,190 --> 00:16:50,910 oh, I've called it the thalamus here, back into the thalamus 333 00:16:50,910 --> 00:16:54,310 then you can see those same axons. 334 00:16:54,310 --> 00:16:55,650 So let's look here. 335 00:16:55,650 --> 00:17:01,350 This is the left eye and the right eye. 336 00:17:01,350 --> 00:17:01,910 What? 337 00:17:01,910 --> 00:17:03,560 The left eye and the right eye. 338 00:17:03,560 --> 00:17:04,170 OK. 339 00:17:04,170 --> 00:17:07,490 And let's look here at the images 340 00:17:07,490 --> 00:17:11,667 that you're going to get here on the left side, the connections 341 00:17:11,667 --> 00:17:13,750 you're going to get on the left side of your brain 342 00:17:13,750 --> 00:17:15,960 that will give you binocular vision 343 00:17:15,960 --> 00:17:18,859 in your right visual field. 344 00:17:18,859 --> 00:17:20,540 Here are a set of neurons. 345 00:17:20,540 --> 00:17:22,690 They're called the nasal neurons because they're 346 00:17:22,690 --> 00:17:25,690 on the side of your retina that's next to your nose. 347 00:17:25,690 --> 00:17:29,350 And these red neurons are going to send their axons back 348 00:17:29,350 --> 00:17:30,890 into the left side of the brain. 349 00:17:30,890 --> 00:17:32,540 Here they are. 350 00:17:32,540 --> 00:17:37,450 And the same time the neurons from the left eye, 351 00:17:37,450 --> 00:17:40,760 from the side of your eye nearest the outside called 352 00:17:40,760 --> 00:17:44,210 the temporal, are going to send their axons back 353 00:17:44,210 --> 00:17:48,120 to the left side or to a different side 354 00:17:48,120 --> 00:17:50,960 of this thalamus or this tectum. 355 00:17:50,960 --> 00:17:51,460 OK? 356 00:17:51,460 --> 00:17:55,010 So what I want you to see is red can go to one side. 357 00:17:55,010 --> 00:17:55,510 Excuse me. 358 00:17:55,510 --> 00:17:57,200 The red here cross over when they 359 00:17:57,200 --> 00:18:00,410 go to that side and the green cross over and go to that side. 360 00:18:00,410 --> 00:18:03,530 So let me diagram this more clearly 361 00:18:03,530 --> 00:18:05,750 so that we can actually discuss this 362 00:18:05,750 --> 00:18:07,180 with the diagram in front of you. 363 00:18:07,180 --> 00:18:09,270 This is number one on your handout. 364 00:18:09,270 --> 00:18:09,770 OK? 365 00:18:09,770 --> 00:18:14,540 So let's look at this and indicate it more clearly. 366 00:18:14,540 --> 00:18:18,660 These N normally in setting up binocular vision, 367 00:18:18,660 --> 00:18:22,330 this circle here is meant to be one eye, 368 00:18:22,330 --> 00:18:28,440 and this back here is meant to be one side of the tectum. 369 00:18:28,440 --> 00:18:41,840 Let me write on the board that in retinal tactile mapping 370 00:18:41,840 --> 00:18:47,300 the tectum can also be called the thalamus. 371 00:18:47,300 --> 00:18:48,030 All right. 372 00:18:48,030 --> 00:18:55,930 So what's important here is that these N neurons go back 373 00:18:55,930 --> 00:19:00,330 to this region of the tectum called the C. 374 00:19:00,330 --> 00:19:00,830 OK? 375 00:19:00,830 --> 00:19:02,610 And I see I have not wrote, I have not 376 00:19:02,610 --> 00:19:04,900 indicated tectum here or retina. 377 00:19:04,900 --> 00:19:05,930 I'll go back and do it. 378 00:19:05,930 --> 00:19:09,380 But you should write here, the circle here is the retina 379 00:19:09,380 --> 00:19:12,145 and this oval here is the tectum. 380 00:19:15,070 --> 00:19:17,450 And then we will be together. 381 00:19:17,450 --> 00:19:21,270 So these N neurons from the retina 382 00:19:21,270 --> 00:19:27,830 send out axons that get to this C or caudal region 383 00:19:27,830 --> 00:19:30,420 of the tectum. 384 00:19:30,420 --> 00:19:35,180 And these T neurons or temporal neurons from the retina 385 00:19:35,180 --> 00:19:39,000 send out axons that get to this R or rostral 386 00:19:39,000 --> 00:19:41,560 region of the tectum. 387 00:19:41,560 --> 00:19:45,840 So this is a long preamble to tell you two things. 388 00:19:45,840 --> 00:19:48,970 Firstly, these connections are what you always 389 00:19:48,970 --> 00:19:54,530 see in every animal with either binocular or monocular vision. 390 00:19:54,530 --> 00:19:57,620 Now, the way this is set up is actually quite confusing. 391 00:19:57,620 --> 00:20:02,170 And I want to, let's not worry about the details too much, 392 00:20:02,170 --> 00:20:06,570 but let's look at, as I see it I realize it's quite confusing. 393 00:20:06,570 --> 00:20:09,130 I took it from a very old paper, which 394 00:20:09,130 --> 00:20:11,690 I thought would be of interest, but as I see it here 395 00:20:11,690 --> 00:20:13,570 there's one point of confusion. 396 00:20:13,570 --> 00:20:17,140 But what I want to point out is an experiment 397 00:20:17,140 --> 00:20:21,860 that was done a long time ago by a guy called Roger Sperry which 398 00:20:21,860 --> 00:20:25,220 addressed this question of whether axons were guided 399 00:20:25,220 --> 00:20:27,460 in a particular way or whether or not 400 00:20:27,460 --> 00:20:29,500 they randomly found their way. 401 00:20:29,500 --> 00:20:32,130 And then by some kind of survival cue 402 00:20:32,130 --> 00:20:34,670 the connections in the nervous system were set up. 403 00:20:34,670 --> 00:20:38,610 So what Sperry did was to take this tectum 404 00:20:38,610 --> 00:20:42,310 and he rotated it 180 degrees. 405 00:20:42,310 --> 00:20:47,300 So the C is now 180 degrees reversed. 406 00:20:47,300 --> 00:20:52,140 And if he did that he had to break the axons that connected 407 00:20:52,140 --> 00:20:55,640 the normal retina and the tectum. 408 00:20:55,640 --> 00:20:56,700 OK? 409 00:20:56,700 --> 00:21:00,250 But what happened was that those axons grew back. 410 00:21:00,250 --> 00:21:02,750 This was done in frogs and they grew back. 411 00:21:06,090 --> 00:21:09,260 And there are two possible outcomes to this experiment. 412 00:21:09,260 --> 00:21:14,230 The first is that these nasal, don't you love it? 413 00:21:17,520 --> 00:21:19,411 Thank you so much. 414 00:21:19,411 --> 00:21:19,910 OK. 415 00:21:19,910 --> 00:21:24,280 The first is that these nasal neurons will grow back 416 00:21:24,280 --> 00:21:28,070 to this R region, which they would not normally do, 417 00:21:28,070 --> 00:21:31,220 but it's far away from them so maybe that's how they work. 418 00:21:31,220 --> 00:21:36,490 The second is that these nasal neurons will grow back 419 00:21:36,490 --> 00:21:40,140 to the C region, N to C in the same way 420 00:21:40,140 --> 00:21:42,130 that they would in the normal case, 421 00:21:42,130 --> 00:21:44,610 even though they're closer together. 422 00:21:44,610 --> 00:21:47,420 And the outcome of the experiment was unequivocal 423 00:21:47,420 --> 00:21:49,070 and it was fantastic and it was one 424 00:21:49,070 --> 00:21:52,030 of the reasons he got a Nobel Prize, which 425 00:21:52,030 --> 00:21:56,240 was that you always get these nasal neurons growing back 426 00:21:56,240 --> 00:21:59,390 to the C region and these temporal neurons 427 00:21:59,390 --> 00:22:00,990 growing back to the R region. 428 00:22:04,011 --> 00:22:04,510 All right? 429 00:22:04,510 --> 00:22:08,630 So this is a very long preamble for a very complex axon writing 430 00:22:08,630 --> 00:22:12,400 system, but the bottom line here that I want you to have 431 00:22:12,400 --> 00:22:16,710 is that these retinal neurons knew which part of the brain 432 00:22:16,710 --> 00:22:18,950 they had to project back to. 433 00:22:18,950 --> 00:22:20,710 And that was very strong evidence 434 00:22:20,710 --> 00:22:23,160 that there was guidance of these neurons 435 00:22:23,160 --> 00:22:26,031 to the correct region of the brain. 436 00:22:26,031 --> 00:22:26,530 All right. 437 00:22:26,530 --> 00:22:30,469 So let's move on to how this guidance takes place. 438 00:22:30,469 --> 00:22:32,010 And I'm going to come back at the end 439 00:22:32,010 --> 00:22:34,070 to talk about survival because that's also 440 00:22:34,070 --> 00:22:35,500 an important part of this. 441 00:22:35,500 --> 00:22:38,730 And the part of the cell that you need to know 442 00:22:38,730 --> 00:22:40,625 is the growth cone. 443 00:22:46,670 --> 00:22:48,560 Now, some lectures ago I gave you 444 00:22:48,560 --> 00:22:51,110 a lecture about 3-dimensional structure. 445 00:22:51,110 --> 00:22:54,600 And we talked in some detail about cell migration 446 00:22:54,600 --> 00:22:58,920 and how cells move in a mechanistic sense. 447 00:22:58,920 --> 00:23:02,820 What we're going to talk about now is very related to that, 448 00:23:02,820 --> 00:23:06,630 but instead of talking about a whole cell moving 449 00:23:06,630 --> 00:23:10,080 we're going to talk about just part of a cell moving. 450 00:23:10,080 --> 00:23:12,030 We're going to talk about this growth cone. 451 00:23:12,030 --> 00:23:14,320 So what is the growth cone? 452 00:23:14,320 --> 00:23:18,400 The growth cone is the axon tip. 453 00:23:18,400 --> 00:23:23,700 It has both sensory function because it's 454 00:23:23,700 --> 00:23:26,910 got various receptors of the type we've talked about, 455 00:23:26,910 --> 00:23:32,460 more in a moment, and it also has motor function 456 00:23:32,460 --> 00:23:40,750 because it has all those actin-based microfilaments that 457 00:23:40,750 --> 00:23:43,580 allow it to move in exactly the same way 458 00:23:43,580 --> 00:23:46,340 that we saw the whole cell moving. 459 00:23:46,340 --> 00:23:49,370 But you remember, of course, that axons are long 460 00:23:49,370 --> 00:23:51,330 and they're far away from the cell body, 461 00:23:51,330 --> 00:23:54,710 so we're just talking about the axon growing now. 462 00:23:54,710 --> 00:23:57,560 This is number two on your handout, 463 00:23:57,560 --> 00:24:01,260 and this is a diagram of the end of an axon in the growth cone. 464 00:24:01,260 --> 00:24:04,210 The axon's structure, the length of the axon 465 00:24:04,210 --> 00:24:06,650 is stabilized by microtubules. 466 00:24:06,650 --> 00:24:11,640 And subsequent to the ending of where the microtubules are, 467 00:24:11,640 --> 00:24:14,430 the cell sends out a series of processes 468 00:24:14,430 --> 00:24:17,550 that are called filopodia or lamellipodia. 469 00:24:17,550 --> 00:24:19,120 I've given you the singular here. 470 00:24:19,120 --> 00:24:20,240 Filopodia are thin. 471 00:24:20,240 --> 00:24:21,760 Lamellipodia are fatter. 472 00:24:21,760 --> 00:24:22,540 It doesn't matter. 473 00:24:22,540 --> 00:24:26,100 And it's in these filopodia and lamellipodia 474 00:24:26,100 --> 00:24:30,330 that there are microfilaments that, you should know, 475 00:24:30,330 --> 00:24:32,980 are comprised of actin polymers. 476 00:24:32,980 --> 00:24:35,430 And I've shown you also receptors 477 00:24:35,430 --> 00:24:38,990 on this growth cone that can bind 478 00:24:38,990 --> 00:24:42,630 two molecules in the extracellular matrix 479 00:24:42,630 --> 00:24:46,480 or can bind two molecules on another cell. 480 00:24:51,861 --> 00:24:52,360 OK. 481 00:24:52,360 --> 00:24:54,890 This is what a growth cone really looks like. 482 00:24:54,890 --> 00:24:56,980 This is a time-lapse of minutes. 483 00:24:56,980 --> 00:24:59,750 And you can see that this is a very dynamic structure. 484 00:24:59,750 --> 00:25:02,690 The cell is sending out processes and retracting them, 485 00:25:02,690 --> 00:25:04,730 sending them out and retracting them. 486 00:25:04,730 --> 00:25:08,720 And the sense that one has when one watches axons sending out 487 00:25:08,720 --> 00:25:11,130 their processes is that they really 488 00:25:11,130 --> 00:25:12,970 are feeling around their environment 489 00:25:12,970 --> 00:25:17,140 and trying to decide where they should go. 490 00:25:17,140 --> 00:25:19,660 I want to remind you this is a diagram from one 491 00:25:19,660 --> 00:25:22,450 of your previous lectures from Formation 4. 492 00:25:22,450 --> 00:25:26,210 The processes by which the axon is moving and sending out 493 00:25:26,210 --> 00:25:28,470 its processes are exactly the same 494 00:25:28,470 --> 00:25:31,710 as the processes that change during mesenchymal cell 495 00:25:31,710 --> 00:25:34,670 migration, but we're just talking about sending out 496 00:25:34,670 --> 00:25:37,960 part of the membrane and not the whole cell moving. 497 00:25:37,960 --> 00:25:38,460 All right. 498 00:25:38,460 --> 00:25:41,310 So what is important in this process? 499 00:25:41,310 --> 00:25:47,401 Well, what seems to be important are several things. 500 00:25:47,401 --> 00:25:47,900 OK. 501 00:25:54,500 --> 00:25:57,800 And one set of things that are important 502 00:25:57,800 --> 00:26:02,040 are called short-range guidance cues. 503 00:26:06,770 --> 00:26:09,990 Short-range refers to how far they can work. 504 00:26:09,990 --> 00:26:12,450 Whether these are diffusible molecules or not. 505 00:26:12,450 --> 00:26:14,830 And I'll talk about long-range ones in a moment. 506 00:26:14,830 --> 00:26:17,470 The distinction between them is a little semantic. 507 00:26:17,470 --> 00:26:20,880 But short-range guidance cues include 508 00:26:20,880 --> 00:26:23,820 things in the extracellular matrix 509 00:26:23,820 --> 00:26:30,020 and include things that are irrevocably stuck onto cells so 510 00:26:30,020 --> 00:26:33,350 that you can really only get interactions directly 511 00:26:33,350 --> 00:26:37,740 between two adjacent cells or between a cell 512 00:26:37,740 --> 00:26:44,040 and something that it is directly just opposed to. 513 00:26:44,040 --> 00:26:47,770 And I want to talk about two classes of cues. 514 00:26:47,770 --> 00:26:50,390 Oh, and them I'm going to indicate later 515 00:26:50,390 --> 00:26:54,610 on long-range guidance cues. 516 00:26:57,660 --> 00:27:00,936 So let me first talk about cell adhesion molecules. 517 00:27:09,590 --> 00:27:14,090 We've talked about the extracellular matrix a bunch. 518 00:27:14,090 --> 00:27:18,260 Laminin, in particular, is a very important molecule 519 00:27:18,260 --> 00:27:23,140 for guiding axons through their paths to where they are going. 520 00:27:23,140 --> 00:27:25,810 This is a movie showing an axon. 521 00:27:25,810 --> 00:27:28,150 This ought to be a movie showing an axon trying 522 00:27:28,150 --> 00:27:31,880 to decide what substrate it wants to migrate on. 523 00:27:31,880 --> 00:27:33,080 Here is an axon. 524 00:27:33,080 --> 00:27:35,300 There it's sending out its growth cone. 525 00:27:35,300 --> 00:27:39,120 And this axon has been put onto a dish that's 526 00:27:39,120 --> 00:27:41,450 got two different kinds of coatings. 527 00:27:41,450 --> 00:27:46,490 On the top half it's got laminin and on the bottom half 528 00:27:46,490 --> 00:27:49,340 it's got polylysine. 529 00:27:49,340 --> 00:27:52,890 And if you look at this you can see 530 00:27:52,890 --> 00:27:54,940 that it's sending out processes that 531 00:27:54,940 --> 00:27:57,910 are touching the polylysine, but as it 532 00:27:57,910 --> 00:28:01,500 goes on it decides it's really not interested in interacting 533 00:28:01,500 --> 00:28:05,000 with the polylysine and it's interested in interacting 534 00:28:05,000 --> 00:28:05,960 with the laminin. 535 00:28:05,960 --> 00:28:07,620 And that's where it lands up. 536 00:28:07,620 --> 00:28:14,020 Other things that you should be aware of that 537 00:28:14,020 --> 00:28:17,250 are important in axon guidance in terms of cell adhesion 538 00:28:17,250 --> 00:28:21,150 are the molecules cadherins that we talked about previously. 539 00:28:21,150 --> 00:28:25,116 Now, some of these are attractive. 540 00:28:33,800 --> 00:28:36,790 In other words, axons will like to grow 541 00:28:36,790 --> 00:28:42,820 on a particular substrate with particular cell adhesive, 542 00:28:42,820 --> 00:28:46,450 with particular cell adhesive or cell interactions. 543 00:28:46,450 --> 00:28:54,550 So attractive cues will promote axon outgrowth. 544 00:28:58,620 --> 00:29:02,970 And one can counter this with repulsive cues 545 00:29:02,970 --> 00:29:08,070 or repulsive signals which will inhibit axon outgrowth. 546 00:29:10,920 --> 00:29:13,490 And this is going to be true both for the cell adhesion 547 00:29:13,490 --> 00:29:16,880 molecules I'll talk to you about and for other molecules 548 00:29:16,880 --> 00:29:17,650 later on. 549 00:29:24,520 --> 00:29:29,810 So let's move onto diffusible molecules or the longer range 550 00:29:29,810 --> 00:29:31,650 molecules. 551 00:29:31,650 --> 00:29:32,150 OK. 552 00:29:50,220 --> 00:29:53,620 And I want to give you, to illustrate 553 00:29:53,620 --> 00:29:57,790 this, an example of a very beautiful axon guidance 554 00:29:57,790 --> 00:29:58,820 phenomenon. 555 00:29:58,820 --> 00:30:02,490 And so we're on number three on your handout. 556 00:30:02,490 --> 00:30:05,280 And this is the example I want to discuss with you. 557 00:30:05,280 --> 00:30:09,860 It has to do with two sets of neurons in the spinal cord 558 00:30:09,860 --> 00:30:16,150 and the brain that are called commissural neurons 559 00:30:16,150 --> 00:30:18,120 and trochlear neurons. 560 00:30:18,120 --> 00:30:24,400 This diagram is a section, it's a cartoon 561 00:30:24,400 --> 00:30:27,770 of a section through a spinal cord. 562 00:30:27,770 --> 00:30:30,030 Spinal cord transverse section. 563 00:30:30,030 --> 00:30:32,780 If you lay on the ground and someone cuts you in half, 564 00:30:32,780 --> 00:30:35,450 that way that is a transverse section. 565 00:30:35,450 --> 00:30:36,120 OK? 566 00:30:36,120 --> 00:30:38,880 So here's the hole in the middle of the spinal cord, 567 00:30:38,880 --> 00:30:42,320 this is the roof plate of the spinal cord nearest the skin, 568 00:30:42,320 --> 00:30:44,260 and something called the floor plate 569 00:30:44,260 --> 00:30:46,690 which is deepest in your body. 570 00:30:46,690 --> 00:30:52,580 The commissural axons grow from this dorsal region 571 00:30:52,580 --> 00:30:55,720 down towards the floor plate. 572 00:30:55,720 --> 00:30:58,530 In contrast, these trochlear axons 573 00:30:58,530 --> 00:31:00,830 that I've shown in blue here grow 574 00:31:00,830 --> 00:31:05,060 from the floor plate away towards the roof plate. 575 00:31:05,060 --> 00:31:10,920 And this data is shown very beautifully 576 00:31:10,920 --> 00:31:16,360 by staining these cells with appropriate markers here. 577 00:31:16,360 --> 00:31:18,400 And I apologize if you're red-green color blind. 578 00:31:18,400 --> 00:31:19,860 I know this is an issue, but this 579 00:31:19,860 --> 00:31:22,180 is how the stains are actually done. 580 00:31:22,180 --> 00:31:23,990 These are the real colors of the stains. 581 00:31:23,990 --> 00:31:26,590 So up here in the top of the spinal cord, 582 00:31:26,590 --> 00:31:30,430 these red dots are the cell bodies of neurons. 583 00:31:30,430 --> 00:31:33,920 And the green are the axons that are sending out 584 00:31:33,920 --> 00:31:34,870 their processes. 585 00:31:34,870 --> 00:31:36,930 And these are the commissural axons 586 00:31:36,930 --> 00:31:40,060 and they are growing down, down, down towards the floor plate. 587 00:31:40,060 --> 00:31:41,880 And actually eventually will cross 588 00:31:41,880 --> 00:31:44,270 to the other side of the embryo. 589 00:31:44,270 --> 00:31:48,900 So how do these axons know where to go? 590 00:31:48,900 --> 00:31:54,990 Well, one model, one model suggests 591 00:31:54,990 --> 00:31:57,950 that these axons are being told where 592 00:31:57,950 --> 00:32:01,930 to go by this little region at the bottom called the floor 593 00:32:01,930 --> 00:32:02,630 plate. 594 00:32:02,630 --> 00:32:07,440 And this was tested in the following way. 595 00:32:07,440 --> 00:32:10,650 One can go into a developing spinal cord, 596 00:32:10,650 --> 00:32:13,770 and one can cut out different pieces of it. 597 00:32:13,770 --> 00:32:17,090 So you can cut out a piece of the dorsal spinal cord 598 00:32:17,090 --> 00:32:21,320 and you can stick it close to a piece of the floor plate 599 00:32:21,320 --> 00:32:25,300 or you can stick it close to a piece of the roof plate. 600 00:32:25,300 --> 00:32:28,820 And then you can look and see what happens. 601 00:32:28,820 --> 00:32:32,500 And what happens with time is that neurons 602 00:32:32,500 --> 00:32:36,520 will develop in these pieces of dorsal spinal cord. 603 00:32:36,520 --> 00:32:41,720 In the case of a set of tissues that's 604 00:32:41,720 --> 00:32:44,400 got the dorsal spinal cord and the floor plate, 605 00:32:44,400 --> 00:32:48,260 you can see very clearly that axons will grow out 606 00:32:48,260 --> 00:32:50,370 towards the floor plate. 607 00:32:50,370 --> 00:32:52,740 However, when you have the roof plate there, 608 00:32:52,740 --> 00:32:55,470 the axons do not grow out from the spinal cord. 609 00:32:55,470 --> 00:32:56,710 There's no outgrowth. 610 00:32:56,710 --> 00:33:00,750 And this type of experiment indicated that the floor plate 611 00:33:00,750 --> 00:33:04,930 was attracting these commissural axons to it, 612 00:33:04,930 --> 00:33:08,110 and they were growing out because of this attraction. 613 00:33:08,110 --> 00:33:10,960 And here's what a real experiment looks like. 614 00:33:10,960 --> 00:33:13,280 Here's a piece of dorsal spinal cord, 615 00:33:13,280 --> 00:33:16,410 here's a piece of floor plate, and here in the control 616 00:33:16,410 --> 00:33:18,500 experiment there was dorsal spinal cord. 617 00:33:18,500 --> 00:33:20,300 And this is just a chunk of tissue culture 618 00:33:20,300 --> 00:33:22,690 cells instead of the roof plate. 619 00:33:22,690 --> 00:33:25,430 And what you see here are these streaks coming out 620 00:33:25,430 --> 00:33:28,760 from the dorsal spinal cord explants towards the floor 621 00:33:28,760 --> 00:33:29,260 plate. 622 00:33:29,260 --> 00:33:31,570 And these streaks are the axons growing out. 623 00:33:31,570 --> 00:33:34,380 So this is actually a very robust assay. 624 00:33:34,380 --> 00:33:37,420 And so using this assay, some years 625 00:33:37,420 --> 00:33:39,930 ago Marc Tessier-Lavigne's lab tried 626 00:33:39,930 --> 00:33:43,160 to find what the floor plate was secreting 627 00:33:43,160 --> 00:33:46,040 to attract these commissural axons. 628 00:33:46,040 --> 00:33:50,040 And it was a long, hard road that involved dissecting 35, 629 00:33:50,040 --> 00:33:53,450 000 chick brains in order to do biochemistry to get 630 00:33:53,450 --> 00:33:56,730 the proteins that were doing this attraction of these 631 00:33:56,730 --> 00:33:58,320 commissural axons. 632 00:33:58,320 --> 00:34:00,440 But they got something in the end. 633 00:34:00,440 --> 00:34:05,020 And what they got is a protein called netrin-1. 634 00:34:08,500 --> 00:34:11,610 Panel B is a section through the spinal cord, 635 00:34:11,610 --> 00:34:14,380 and you cannot see it very well, except for this black glob 636 00:34:14,380 --> 00:34:15,300 at the bottom. 637 00:34:15,300 --> 00:34:17,659 But what this is the floor plate. 638 00:34:17,659 --> 00:34:20,570 And it's been stained for netrin-1 RNA. 639 00:34:20,570 --> 00:34:23,090 And you can see this netrin-1 RNA is only 640 00:34:23,090 --> 00:34:24,659 in the floor plate region. 641 00:34:24,659 --> 00:34:26,770 And if you look at the protein, it's 642 00:34:26,770 --> 00:34:28,330 also in the floor plate region. 643 00:34:28,330 --> 00:34:33,600 Although, it diffuses a little bit out of that region. 644 00:34:33,600 --> 00:34:36,239 Is netrin-1 really important for making 645 00:34:36,239 --> 00:34:38,130 these axons grow in the right place? 646 00:34:38,130 --> 00:34:39,070 Well, yes. 647 00:34:39,070 --> 00:34:42,670 Because if you take a mouse and knock out, 648 00:34:42,670 --> 00:34:45,550 you genetically mutate the netrin-1 gene, 649 00:34:45,550 --> 00:34:48,400 the commissural axons don't grow properly. 650 00:34:48,400 --> 00:34:51,270 So here's a wild type mouse spinal cord, here 651 00:34:51,270 --> 00:34:54,139 are these axons growing down to the floor plate, 652 00:34:54,139 --> 00:34:58,770 and here is a mutant mouse that's lacking netrin-1. 653 00:34:58,770 --> 00:35:02,210 And these commissural axons grow but they never 654 00:35:02,210 --> 00:35:03,520 get to the floor plate. 655 00:35:03,520 --> 00:35:08,670 They don't really know where to go. 656 00:35:08,670 --> 00:35:15,790 Now, if you look back at handout three -- 657 00:35:15,790 --> 00:35:22,030 -- there was something else about this floor plate region 658 00:35:22,030 --> 00:35:25,480 in that these trochlear axons were growing away from 659 00:35:25,480 --> 00:35:27,040 the floor plate. 660 00:35:27,040 --> 00:35:30,280 And so one could construct the ultimate hypothesis 661 00:35:30,280 --> 00:35:32,020 that the floor plate was actually 662 00:35:32,020 --> 00:35:36,320 repelling these trochlear axons from growing towards it. 663 00:35:36,320 --> 00:35:39,410 And it turns out that is exactly what's happened, 664 00:35:39,410 --> 00:35:44,190 and netrin-1 can act both as a chemo attractant 665 00:35:44,190 --> 00:35:45,600 and a chemo repellant. 666 00:35:49,000 --> 00:35:53,880 And it does so, and this is number five on your handouts. 667 00:35:53,880 --> 00:35:57,430 It does so by using two different receptors, 668 00:35:57,430 --> 00:36:01,120 one of which makes it attractive to neurons and the other which 669 00:36:01,120 --> 00:36:02,470 makes it repulsive. 670 00:36:02,470 --> 00:36:06,740 So here's netrin indicated as a diffusible molecule. 671 00:36:06,740 --> 00:36:11,020 It binds to a receptor called DCC which was initially 672 00:36:11,020 --> 00:36:14,460 implicated in colon cancer called deleted in colon cancer. 673 00:36:14,460 --> 00:36:16,840 It's not clear if it really is still involved in cancer 674 00:36:16,840 --> 00:36:17,580 or not. 675 00:36:17,580 --> 00:36:20,340 But here's netrin binding to its receptor. 676 00:36:20,340 --> 00:36:23,480 And subsequent there are a bunch of signal transduction 677 00:36:23,480 --> 00:36:26,820 that happens that changes whether or not 678 00:36:26,820 --> 00:36:29,120 these axons move to the floor plate 679 00:36:29,120 --> 00:36:32,210 and whether or not they, in fact, grow out at all. 680 00:36:32,210 --> 00:36:35,110 In the case of repulsion, it turns out 681 00:36:35,110 --> 00:36:38,190 that there's another receptor called unc-5. 682 00:36:38,190 --> 00:36:44,290 And that makes a connection with the DCC receptor. 683 00:36:44,290 --> 00:36:48,480 There's an interaction between the unc-5 and the DCC receptor. 684 00:36:48,480 --> 00:36:51,410 Netrin still binds to the DCC receptor, 685 00:36:51,410 --> 00:36:55,520 but the net result of this is a different signal transduction 686 00:36:55,520 --> 00:36:58,260 pathway activated such that you now 687 00:36:58,260 --> 00:37:02,180 get repulsion of these neurons by activation 688 00:37:02,180 --> 00:37:04,040 of this duet of receptors. 689 00:37:04,040 --> 00:37:06,330 So for this diffusible process, I've 690 00:37:06,330 --> 00:37:10,010 given you the example of netrin that 691 00:37:10,010 --> 00:37:14,375 can act both as an attractant and a repellant. 692 00:37:18,640 --> 00:37:22,720 And many, I have no idea how to spell repellant. 693 00:37:22,720 --> 00:37:24,560 Repellant, I think it's got an E. 694 00:37:24,560 --> 00:37:25,226 OK. 695 00:37:25,226 --> 00:37:25,725 Repellant. 696 00:37:28,750 --> 00:37:31,170 And there are now several receptors known 697 00:37:31,170 --> 00:37:35,000 that are able to be, there are several guidance cues known 698 00:37:35,000 --> 00:37:37,380 that can either be attractants or repellants. 699 00:37:37,380 --> 00:37:41,200 What happens when a growth cone sees a repulsive signal? 700 00:37:41,200 --> 00:37:43,500 This movie will show you. 701 00:37:43,500 --> 00:37:45,240 That is the growth cone collapsing. 702 00:37:45,240 --> 00:37:47,560 Let me show it to you again. 703 00:37:47,560 --> 00:37:50,700 So you start off with a pretty robust growth cone, 704 00:37:50,700 --> 00:37:58,340 and it collapses when you put a little drop of some guidance 705 00:37:58,340 --> 00:38:01,550 cue that it doesn't like up in this corner of the slide. 706 00:38:01,550 --> 00:38:05,110 And what happens is that that actin cytoskeleton that 707 00:38:05,110 --> 00:38:09,340 is there just collapses, depolymerizes, and that axon 708 00:38:09,340 --> 00:38:14,120 tries to get as far away from the repulsive cue as it can. 709 00:38:14,120 --> 00:38:19,370 So we've talked about short-range and long-range 710 00:38:19,370 --> 00:38:20,170 guidance cues. 711 00:38:20,170 --> 00:38:23,610 We've talked about attractive and repulsive guidance cues. 712 00:38:23,610 --> 00:38:28,380 We've talked about how one single guidance 713 00:38:28,380 --> 00:38:31,760 cue can be both an attractant and a repellant. 714 00:38:31,760 --> 00:38:36,110 And now I want to get back to this very old hypothesis 715 00:38:36,110 --> 00:38:38,190 that the way that you actually got 716 00:38:38,190 --> 00:38:41,420 synaptic connections and the way you've got circuitry set up 717 00:38:41,420 --> 00:38:45,810 was by some kind of random way that involved survival 718 00:38:45,810 --> 00:38:48,370 of neurons that made the appropriate connections. 719 00:38:48,370 --> 00:38:51,890 In fact, that is true once you get neurons 720 00:38:51,890 --> 00:38:53,880 to where they're finally going. 721 00:38:53,880 --> 00:38:55,900 So you're setting up neurons, they're 722 00:38:55,900 --> 00:38:59,680 going along their path set up by some guidance cues. 723 00:38:59,680 --> 00:39:02,430 And of course you should be asking the question, well, 724 00:39:02,430 --> 00:39:05,890 how do the guidance cues get there in the first place, 725 00:39:05,890 --> 00:39:07,060 right? 726 00:39:07,060 --> 00:39:07,560 OK? 727 00:39:07,560 --> 00:39:11,400 So that's a question you can go and sit and think about. 728 00:39:11,400 --> 00:39:17,450 But in the end what happens is that you stabilize connections. 729 00:39:17,450 --> 00:39:19,180 And you do it in two ways. 730 00:39:30,570 --> 00:39:37,840 Both of which, or one of which seems to be activity dependent, 731 00:39:37,840 --> 00:39:42,410 or another of which is activity independent. 732 00:39:42,410 --> 00:39:46,360 By activity dependent, I mean that the nerve has to fire. 733 00:39:46,360 --> 00:39:48,560 There has to be synaptic activity. 734 00:39:48,560 --> 00:39:50,580 And that will maintain the connection. 735 00:39:50,580 --> 00:39:53,970 If there is not synaptic activity you will not. 736 00:39:53,970 --> 00:39:56,960 And there are some cases where that is not the case. 737 00:39:56,960 --> 00:40:00,280 This is number six on your handout. 738 00:40:00,280 --> 00:40:04,320 When neurons grow out to the neuromuscular junction 739 00:40:04,320 --> 00:40:08,030 they form a synapse, or when the neurons grow out to the muscle 740 00:40:08,030 --> 00:40:10,030 they form a synapse with the muscle. 741 00:40:10,030 --> 00:40:12,860 The synapse is initially a rather weak one, 742 00:40:12,860 --> 00:40:18,590 and over time there are proteins that are activated. 743 00:40:18,590 --> 00:40:20,730 Acetylcholine receptors, for example, 744 00:40:20,730 --> 00:40:23,890 that are activated that set up the synapse 745 00:40:23,890 --> 00:40:26,920 and stabilize it and maintain it. 746 00:40:26,920 --> 00:40:32,110 So that is one example of stabilizing connections. 747 00:40:32,110 --> 00:40:36,300 Another very famous example is growth factors 748 00:40:36,300 --> 00:40:39,560 that are survival signals. 749 00:40:39,560 --> 00:40:42,140 One of the most famous is NGF which 750 00:40:42,140 --> 00:40:44,240 stands for nerve growth factor. 751 00:40:44,240 --> 00:40:47,720 And this is a protein that is involved 752 00:40:47,720 --> 00:40:50,770 in making certain sets of neurons 753 00:40:50,770 --> 00:40:53,190 survive when they get to their targets. 754 00:40:53,190 --> 00:40:56,680 There seems to be a limiting amount of nerve growth factor 755 00:40:56,680 --> 00:40:59,290 around in specific regions of the animal, 756 00:40:59,290 --> 00:41:03,470 and only those neurons that can capture enough of that nerve 757 00:41:03,470 --> 00:41:06,850 growth factor on their receptors will survive. 758 00:41:06,850 --> 00:41:10,920 So there are neuronal survival signals that may or may not 759 00:41:10,920 --> 00:41:15,260 require a neuronal activity for their effect. 760 00:41:15,260 --> 00:41:17,210 OK. 761 00:41:17,210 --> 00:41:19,990 So two final things. 762 00:41:19,990 --> 00:41:22,720 One is this retinotectal map. 763 00:41:22,720 --> 00:41:27,410 We talked about the outcome of the Sperry experiments 764 00:41:27,410 --> 00:41:31,140 and how you got the neurons going back to where they are. 765 00:41:31,140 --> 00:41:33,650 What's the molecular basis for that? 766 00:41:33,650 --> 00:41:36,390 Well, you can show very beautifully 767 00:41:36,390 --> 00:41:41,700 that the neurons prefer to grow on a particular part 768 00:41:41,700 --> 00:41:43,780 of membrane that you give them. 769 00:41:43,780 --> 00:41:47,140 So, for example, the retinal neurons 770 00:41:47,140 --> 00:41:50,750 will choose to grow on the anterior rostral and not 771 00:41:50,750 --> 00:41:53,360 on the more posterior caudal neurons. 772 00:41:53,360 --> 00:41:57,810 And you can show that the reason for that 773 00:41:57,810 --> 00:42:02,230 is due to a class of molecules called the ephrins. 774 00:42:02,230 --> 00:42:06,020 Ephrins are secreted molecules, but they are short range. 775 00:42:06,020 --> 00:42:09,150 They work just on the cell next door. 776 00:42:09,150 --> 00:42:11,560 And you can show, I'm flashing this up, 777 00:42:11,560 --> 00:42:13,740 I know you can go and look at it later on, 778 00:42:13,740 --> 00:42:17,930 that wherever there is ephrin you 779 00:42:17,930 --> 00:42:20,540 do not see axonal outgrowth. 780 00:42:20,540 --> 00:42:24,070 So ephrins are inhibitory to axonal outgrowth. 781 00:42:24,070 --> 00:42:27,090 And I'm going to refer you to, I'm not going to go through it. 782 00:42:27,090 --> 00:42:29,795 I'm going to refer you to number eight on your handout. 783 00:42:29,795 --> 00:42:31,170 I'm not going to go through this. 784 00:42:31,170 --> 00:42:32,060 You can look at it. 785 00:42:32,060 --> 00:42:34,220 But I will just tell you that there 786 00:42:34,220 --> 00:42:38,840 are different combinations of ephrins and Eph receptors 787 00:42:38,840 --> 00:42:43,150 which guide retinal axons to the correct part of the tectum. 788 00:42:43,150 --> 00:42:45,990 The last thing that I want to spend the last minute on 789 00:42:45,990 --> 00:42:49,940 is a particularly fascinating point. 790 00:42:49,940 --> 00:42:54,200 And that is the question of nerve regeneration, 791 00:42:54,200 --> 00:42:58,180 and nerve regeneration in the central nervous system. 792 00:42:58,180 --> 00:43:01,900 So we believe right now that the same, in fact, we 793 00:43:01,900 --> 00:43:04,870 know that the cues that tell axons 794 00:43:04,870 --> 00:43:07,820 where to go in the first place in an embryo 795 00:43:07,820 --> 00:43:11,280 are maintained in the adult nervous system. 796 00:43:11,280 --> 00:43:14,010 And we also know that if you cut one of the nerves 797 00:43:14,010 --> 00:43:16,380 in your peripheral nervous system in your arm 798 00:43:16,380 --> 00:43:18,120 it will grow back fine. 799 00:43:18,120 --> 00:43:21,410 But we also know if you sever your spinal cord 800 00:43:21,410 --> 00:43:23,340 it will not grow back. 801 00:43:23,340 --> 00:43:25,380 And there has been intense research 802 00:43:25,380 --> 00:43:28,890 to try to figure out what it is that is making 803 00:43:28,890 --> 00:43:32,830 the axons in your spinal cord not grow back to where they 804 00:43:32,830 --> 00:43:35,670 were, although, in fact they should. 805 00:43:35,670 --> 00:43:39,160 Here's a diagram of an intact CNS axon. 806 00:43:39,160 --> 00:43:40,880 If it's severed of course you're going 807 00:43:40,880 --> 00:43:43,760 to get a fragment that's not connected to a cell body, 808 00:43:43,760 --> 00:43:45,510 but the other part of it is still 809 00:43:45,510 --> 00:43:47,260 connected to the cell body. 810 00:43:47,260 --> 00:43:50,780 And what happens is that that dies, that cell dies. 811 00:43:50,780 --> 00:43:53,020 It undergoes apoptosis and dies. 812 00:43:53,020 --> 00:43:56,500 And it's really not clear why it doesn't regenerate. 813 00:43:56,500 --> 00:43:59,630 One hypothesis is that there it is a poor growth environment 814 00:43:59,630 --> 00:44:03,120 in the extracellular matrix around the axon in the CNS. 815 00:44:03,120 --> 00:44:06,030 Another class of proteins of great interest 816 00:44:06,030 --> 00:44:09,620 are the Nogos and related ligands 817 00:44:09,620 --> 00:44:14,374 which are believed to be inhibitory to axon regrowth. 818 00:44:14,374 --> 00:44:16,540 And there's a lot of interest in this kind of thing. 819 00:44:16,540 --> 00:44:17,373 And I'll stop there. 820 00:44:17,373 --> 00:44:18,880 Thanks.