1 00:00:00,090 --> 00:00:02,490 The following content is provided under a Creative 2 00:00:02,490 --> 00:00:04,059 Commons license. 3 00:00:04,059 --> 00:00:06,330 Your support will help MIT OpenCourseWare 4 00:00:06,330 --> 00:00:10,720 continue to offer high quality educational resources for free. 5 00:00:10,720 --> 00:00:13,320 To make a donation or view additional materials 6 00:00:13,320 --> 00:00:17,280 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:17,280 --> 00:00:18,450 at ocw.mit.edu. 8 00:00:26,517 --> 00:00:27,850 PROFESSOR: So let's get started. 9 00:00:27,850 --> 00:00:31,231 [CHATTER] 10 00:00:31,231 --> 00:00:34,060 Oh, it's interesting that some of your questions 11 00:00:34,060 --> 00:00:37,810 had to do with things that I didn't quite cover in lecture, 12 00:00:37,810 --> 00:00:38,620 but that's fine. 13 00:00:41,160 --> 00:00:43,660 I'm going to cover them at the beginning of today's lecture. 14 00:00:48,640 --> 00:00:50,480 This is a really important question. 15 00:00:50,480 --> 00:00:54,040 How do you know what channel is at-- 16 00:00:54,040 --> 00:00:56,260 how do you know what ion a particular channel is 17 00:00:56,260 --> 00:00:58,150 conducting? 18 00:00:58,150 --> 00:00:59,650 It's actually hard to determine. 19 00:00:59,650 --> 00:01:05,650 There are various ways to do it, where you can specifically 20 00:01:05,650 --> 00:01:08,380 label the ion, or follow a particular ion, 21 00:01:08,380 --> 00:01:11,470 and address whether or not it's getting into a cell 22 00:01:11,470 --> 00:01:14,200 or across a membrane, when there's a particular ion 23 00:01:14,200 --> 00:01:15,550 channel present. 24 00:01:15,550 --> 00:01:17,620 But it's not completely trivial. 25 00:01:17,620 --> 00:01:20,140 One of the things I didn't have time to go through with you, 26 00:01:20,140 --> 00:01:22,670 but which is one of the PowerPoints though, 27 00:01:22,670 --> 00:01:27,730 is the fact that ion channels are really selective. 28 00:01:27,730 --> 00:01:32,560 And they don't conduct ions on the basis of size, OK? 29 00:01:32,560 --> 00:01:37,540 So you're not going to get a large ion channel that 30 00:01:37,540 --> 00:01:41,620 could accommodate a large ion, also accommodating a small ion. 31 00:01:41,620 --> 00:01:45,340 It's not a matter about of just opening up a space. 32 00:01:45,340 --> 00:01:47,620 There are also charge considerations 33 00:01:47,620 --> 00:01:52,060 where the ions actually interact with the molecules 34 00:01:52,060 --> 00:01:55,840 in the channel, in the actual pore through the membrane. 35 00:01:55,840 --> 00:01:59,860 And it's that interaction which selects for a particular ion. 36 00:01:59,860 --> 00:02:03,970 But this notion of exactly what ions channels are conducting 37 00:02:03,970 --> 00:02:08,590 has been many, many decades of work. 38 00:02:08,590 --> 00:02:10,609 And what I've written up here is correct. 39 00:02:10,609 --> 00:02:12,400 But if you want to explore it more with me, 40 00:02:12,400 --> 00:02:14,290 come and talk during office hours. 41 00:02:14,290 --> 00:02:16,540 And then a number of you started asking me 42 00:02:16,540 --> 00:02:19,000 about modulation of neurotransmitters, which I'll 43 00:02:19,000 --> 00:02:21,094 talk about in today's lecture. 44 00:02:21,094 --> 00:02:22,510 And these are clearly of interest, 45 00:02:22,510 --> 00:02:27,220 because many recreational drugs and medications modulate 46 00:02:27,220 --> 00:02:29,410 neurotransmitter amounts. 47 00:02:29,410 --> 00:02:30,770 And that is how they work. 48 00:02:30,770 --> 00:02:32,320 So for example, some of you might 49 00:02:32,320 --> 00:02:37,000 be taking things called SSRIs, specific serotonin re-uptake 50 00:02:37,000 --> 00:02:40,030 inhibitors like Prozac, which make 51 00:02:40,030 --> 00:02:42,760 you feel less anxious and better about things. 52 00:02:42,760 --> 00:02:47,380 And these act by prolonging serotonin activity, 53 00:02:47,380 --> 00:02:51,040 by preventing it from being retaken up into cells. 54 00:02:51,040 --> 00:02:54,100 And I'll touch on this at the beginning of the lecture. 55 00:02:54,100 --> 00:02:55,390 And it takes a while-- 56 00:02:55,390 --> 00:02:56,830 is what I've written here-- 57 00:02:56,830 --> 00:03:00,220 for these medications to start working, because you're really 58 00:03:00,220 --> 00:03:04,960 asking for a rearrangement of the whole synaptic process 59 00:03:04,960 --> 00:03:08,131 and the synaptic structure, in order that they can work. 60 00:03:08,131 --> 00:03:09,130 And here is another one. 61 00:03:09,130 --> 00:03:10,540 What about amphetamines? 62 00:03:10,540 --> 00:03:11,890 Are they neurotransmitters? 63 00:03:11,890 --> 00:03:12,730 No. 64 00:03:12,730 --> 00:03:15,440 They increase the release of dopamine, 65 00:03:15,440 --> 00:03:18,580 which is a neurotransmitter, and they also 66 00:03:18,580 --> 00:03:22,380 seem to inhibit re-uptake of dopamine and serotonin is all. 67 00:03:22,380 --> 00:03:26,110 I'll talk about re-uptake very briefly in a moment. 68 00:03:26,110 --> 00:03:28,390 For many medications, both-- 69 00:03:28,390 --> 00:03:30,970 actually, for many medications, no matter whether or not 70 00:03:30,970 --> 00:03:33,400 they affect your brain or other parts of your body, 71 00:03:33,400 --> 00:03:38,990 the precise mechanism of action is really not known. 72 00:03:38,990 --> 00:03:40,030 There are guesses. 73 00:03:40,030 --> 00:03:40,870 There's data. 74 00:03:40,870 --> 00:03:44,080 But the precise mechanism is often not known. 75 00:03:44,080 --> 00:03:48,290 So let's use that as a segue into our lecture. 76 00:03:48,290 --> 00:03:51,080 I won't have office hours today, due to my schedule. 77 00:03:51,080 --> 00:03:55,220 I will have them next Wednesday due to the vacation schedule. 78 00:03:55,220 --> 00:03:57,770 And you're welcome to email me in the meantime. 79 00:03:57,770 --> 00:03:58,280 All right. 80 00:04:07,610 --> 00:04:10,130 We've been walking through the cells involved 81 00:04:10,130 --> 00:04:12,410 in nervous system formation-- the connections 82 00:04:12,410 --> 00:04:13,900 between the cells. 83 00:04:13,900 --> 00:04:15,680 And now today we're going to finish 84 00:04:15,680 --> 00:04:18,290 talking about the connections between the cells 85 00:04:18,290 --> 00:04:22,730 and segue into the incredibly complex topic of circuits 86 00:04:22,730 --> 00:04:25,080 in the nervous system. 87 00:04:25,080 --> 00:04:27,020 So today, the first thing I want to talk about 88 00:04:27,020 --> 00:04:32,360 is regulating synapses. 89 00:04:32,360 --> 00:04:35,840 And you remember that we're talking 90 00:04:35,840 --> 00:04:36,910 about chemical synapses. 91 00:04:42,260 --> 00:04:47,469 And the second thing I want to talk about are circuits. 92 00:04:47,469 --> 00:04:48,260 Is there a problem? 93 00:04:48,260 --> 00:04:50,150 Can you hear me OK at the back? 94 00:04:50,150 --> 00:04:50,660 Thumbs up. 95 00:04:50,660 --> 00:04:51,450 Great. 96 00:04:51,450 --> 00:04:51,950 Good. 97 00:04:56,000 --> 00:04:59,180 When I introduced circuits-- when I introduced synapses 98 00:04:59,180 --> 00:05:03,940 to you, I told you that one of the reasons that there 99 00:05:03,940 --> 00:05:07,650 was this chemical synapse in the midst with a slow chemicals 100 00:05:07,650 --> 00:05:11,890 synapse, in the midst of this rapid electrical transmission, 101 00:05:11,890 --> 00:05:14,500 was because you could regulate synapses. 102 00:05:14,500 --> 00:05:17,420 And that is really what fine tunes us. 103 00:05:17,420 --> 00:05:20,860 It allows us to respond in a graded way to stimuli 104 00:05:20,860 --> 00:05:23,500 both from within the body and outside. 105 00:05:23,500 --> 00:05:29,110 It allows the body to adapt in ways that are not all or none, 106 00:05:29,110 --> 00:05:31,240 where action potentials are all or none. 107 00:05:31,240 --> 00:05:34,690 The overall response of the body clearly isn't. 108 00:05:34,690 --> 00:05:36,250 It's very nuanced. 109 00:05:36,250 --> 00:05:40,810 And all of this has to do with regulating chemical synapses. 110 00:05:40,810 --> 00:05:43,660 And that's what we'll talk about for a few moments. 111 00:05:43,660 --> 00:05:46,540 You can regulate chemical synapses 112 00:05:46,540 --> 00:05:48,980 by changing the amount of neurotransmitter. 113 00:06:02,280 --> 00:06:04,900 And if you think about this for a moment, 114 00:06:04,900 --> 00:06:07,620 if you think about the synapse, there's 115 00:06:07,620 --> 00:06:11,340 the neurotransmitter released into the space between the two 116 00:06:11,340 --> 00:06:15,090 cells, diffuses across the synaptic cleft, 117 00:06:15,090 --> 00:06:18,900 and then does something to the post synaptic cell, potentially 118 00:06:18,900 --> 00:06:21,250 to lead to an action potential. 119 00:06:21,250 --> 00:06:24,060 Now if that neurotransmitter stuck around 120 00:06:24,060 --> 00:06:26,550 in the space between the cells, it 121 00:06:26,550 --> 00:06:29,190 would keep stimulating the postsynaptic cell 122 00:06:29,190 --> 00:06:31,010 over and over. 123 00:06:31,010 --> 00:06:33,240 And as more neurotransmitter was released, 124 00:06:33,240 --> 00:06:36,390 so the postsynaptic cell would be further stimulated. 125 00:06:36,390 --> 00:06:39,750 And you would get to a point where the post synaptic neuron 126 00:06:39,750 --> 00:06:42,720 was completely over stimulated. 127 00:06:42,720 --> 00:06:45,630 And that's clearly not a way to regulate responsiveness 128 00:06:45,630 --> 00:06:46,990 to any stimuli. 129 00:06:46,990 --> 00:06:51,420 So neurotransmitter does not stay in the synaptic cleft 130 00:06:51,420 --> 00:06:52,320 for very long. 131 00:06:57,720 --> 00:07:00,980 So it's changing the amount of neurotransmitter 132 00:07:00,980 --> 00:07:03,860 via degradation. 133 00:07:07,580 --> 00:07:11,450 Once neurotransmitter is released, in some cases, 134 00:07:11,450 --> 00:07:14,060 it's degraded by specific enzymes. 135 00:07:14,060 --> 00:07:19,530 For example, in the case of acetylcholine, 136 00:07:19,530 --> 00:07:23,490 there's a particular enzyme that breaks down acetylcholine. 137 00:07:23,490 --> 00:07:27,960 And if that enzyme is inhibited, you go into respiratory shock. 138 00:07:27,960 --> 00:07:31,180 You cannot breathe anymore, because you have to activate 139 00:07:31,180 --> 00:07:36,460 and inactivate the muscles via the nerves, as you breathe. 140 00:07:36,460 --> 00:07:40,360 You can also regulate the amount of neurotransmitter 141 00:07:40,360 --> 00:07:49,090 by something called re-uptake, sometimes called reabsorption, 142 00:07:49,090 --> 00:07:51,265 where the neurotransmitter-- 143 00:07:51,265 --> 00:07:56,140 absorb-- tion-- where the neurotransmitter is released. 144 00:07:56,140 --> 00:08:00,490 And then it's taken up by the presynaptic cell, which is 145 00:08:00,490 --> 00:08:02,440 a frugal way of doing things. 146 00:08:02,440 --> 00:08:03,940 It doesn't have to keep synthesizing 147 00:08:03,940 --> 00:08:05,950 the neurotransmitter, and that-- 148 00:08:05,950 --> 00:08:09,520 so reabsorb-- re-uptake by the presynaptic cell. 149 00:08:14,460 --> 00:08:18,910 And that is the case for serotonin and dopamine. 150 00:08:25,330 --> 00:08:28,960 And then in some cases, you can regulate 151 00:08:28,960 --> 00:08:31,630 the synthesis, the amount of neurotransmitter 152 00:08:31,630 --> 00:08:33,370 that's being made. 153 00:08:33,370 --> 00:08:36,640 And the big class of neurotransmitters regulated 154 00:08:36,640 --> 00:08:40,299 in this way are the endorphins, which 155 00:08:40,299 --> 00:08:43,270 are a group of peptide neurotransmitters 156 00:08:43,270 --> 00:08:46,870 that are the natural opiates of the body, the natural pain 157 00:08:46,870 --> 00:08:48,700 regulators of the body. 158 00:08:48,700 --> 00:08:53,740 And all of these processes are regulatable 159 00:08:53,740 --> 00:08:57,100 both for modulating normal synapses, 160 00:08:57,100 --> 00:09:02,320 and also in all cases, for medication targets, 161 00:09:02,320 --> 00:09:03,700 for drug targets-- 162 00:09:03,700 --> 00:09:15,045 so normal modulation and drug targets. 163 00:09:19,410 --> 00:09:20,840 Let's look at a couple of slides. 164 00:09:27,420 --> 00:09:31,650 Acetylcholine is a neurotransmitter 165 00:09:31,650 --> 00:09:35,280 that binds its receptor on the postsynaptic membrane. 166 00:09:35,280 --> 00:09:38,460 And after it's done so, acetylcholinesterase, 167 00:09:38,460 --> 00:09:44,040 the E here, comes and breaks it down and stops it restimulating 168 00:09:44,040 --> 00:09:47,280 the postsynaptic cell. 169 00:09:47,280 --> 00:09:52,680 Many nerve gases-- sarin was one of the famous ones that was 170 00:09:52,680 --> 00:09:55,680 used in the Japanese underground some years ago-- 171 00:09:55,680 --> 00:09:58,380 inhibit acetylcholinesterase. 172 00:09:58,380 --> 00:10:00,120 And in that case, you get a buildup 173 00:10:00,120 --> 00:10:01,860 of acetylcholine in the post-- 174 00:10:01,860 --> 00:10:03,600 in the synaptic cleft. 175 00:10:03,600 --> 00:10:06,840 You get repeated stimulation of the postsynaptic cell. 176 00:10:06,840 --> 00:10:10,020 And that leads to, as I said, respiratory 177 00:10:10,020 --> 00:10:13,240 paralysis and death. 178 00:10:13,240 --> 00:10:19,250 Our troops overseas have with them vials of atropine. 179 00:10:19,250 --> 00:10:23,780 Atropine is a competitive inhibitor of acetylcholine-- 180 00:10:23,780 --> 00:10:26,690 binds to the receptor and prevents acetylcholine 181 00:10:26,690 --> 00:10:27,830 from binding. 182 00:10:27,830 --> 00:10:30,230 And in the case of a nerve gas attack, 183 00:10:30,230 --> 00:10:32,510 if you inject yourself with atropine, 184 00:10:32,510 --> 00:10:35,960 you'll stop the acetylcholine from working. 185 00:10:35,960 --> 00:10:37,340 And you'll be OK. 186 00:10:37,340 --> 00:10:39,080 You get a little kind of floppy, but you 187 00:10:39,080 --> 00:10:41,600 can survive, because they're ultimate mechanisms 188 00:10:41,600 --> 00:10:44,440 of stimulating those nerves. 189 00:10:44,440 --> 00:10:44,940 OK. 190 00:10:47,520 --> 00:10:50,550 Here's serotonin, the re-uptake pathway. 191 00:10:50,550 --> 00:10:53,910 Serotonin is released as all neurotransmitters, 192 00:10:53,910 --> 00:10:56,850 and then it's reabsorbed by the presynaptic cell. 193 00:10:56,850 --> 00:10:59,820 And SSRIs, whose mechanism of action 194 00:10:59,820 --> 00:11:02,520 is really not understood, do something 195 00:11:02,520 --> 00:11:06,290 to block the re-uptake of serotonin. 196 00:11:06,290 --> 00:11:06,830 All right. 197 00:11:09,650 --> 00:11:14,420 The other way, clearly, that one could modulate 198 00:11:14,420 --> 00:11:18,500 how often synapses are active, or how often 199 00:11:18,500 --> 00:11:21,430 the postsynaptic cell is stimulated, 200 00:11:21,430 --> 00:11:24,320 is by modulating the receptors. 201 00:11:24,320 --> 00:11:26,750 Intuitively, if you have more receptors, 202 00:11:26,750 --> 00:11:29,630 the neurotransmitter has more places to bind. 203 00:11:29,630 --> 00:11:34,620 It can send a greater signal by changing membrane potential, 204 00:11:34,620 --> 00:11:37,400 if you decrease the number of receptors, and so on. 205 00:11:37,400 --> 00:11:39,990 And it turns out, if you modify the receptors, 206 00:11:39,990 --> 00:11:41,900 if you put phosphate groups on them, 207 00:11:41,900 --> 00:11:45,590 sometimes you can also change how well they act. 208 00:11:45,590 --> 00:11:50,330 So the other thing to do to modulate synaptic activity 209 00:11:50,330 --> 00:11:54,080 is by changing the receptors. 210 00:11:59,270 --> 00:12:03,380 And there are really three ways to do this-- 211 00:12:03,380 --> 00:12:17,190 change in number or change in the type-- 212 00:12:17,190 --> 00:12:23,750 the subtype of receptor-- where there might be 213 00:12:23,750 --> 00:12:25,850 a subtle change in amino acid. 214 00:12:25,850 --> 00:12:27,680 Because you are using now a different gene 215 00:12:27,680 --> 00:12:29,660 to make the receptor. 216 00:12:29,660 --> 00:12:37,930 You can increase or change the affinity for neurotransmitter. 217 00:12:45,870 --> 00:12:48,720 And you can change receptor responsiveness. 218 00:12:55,270 --> 00:12:58,360 All of these three things have got something 219 00:12:58,360 --> 00:13:04,285 to do with learning and memory. 220 00:13:07,220 --> 00:13:08,700 They have to do with addiction. 221 00:13:13,160 --> 00:13:16,950 And all of these changes are slow. 222 00:13:16,950 --> 00:13:23,310 They occur over minutes, days, weeks, even. 223 00:13:27,400 --> 00:13:30,140 And for some, we understand how these changes occur, 224 00:13:30,140 --> 00:13:31,820 but for many, we don't. 225 00:13:31,820 --> 00:13:35,800 The outcome of changing the receptors-- 226 00:13:35,800 --> 00:13:41,250 and I see we have a board issue here. 227 00:13:41,250 --> 00:13:42,960 So I'm going to put this on-- 228 00:13:42,960 --> 00:13:44,960 actually, let me put this board down-- 229 00:13:49,410 --> 00:13:51,950 and this board up. 230 00:13:51,950 --> 00:13:54,050 One of the outcomes of changing all 231 00:13:54,050 --> 00:13:56,660 of these parameters about the receptors 232 00:13:56,660 --> 00:13:59,090 is that over a long period of time, 233 00:13:59,090 --> 00:14:02,270 you really change how a synapse works. 234 00:14:02,270 --> 00:14:05,834 And you can change how a synapse works through these parameters, 235 00:14:05,834 --> 00:14:09,830 by repeatedly stimulating that synapse, OK? 236 00:14:09,830 --> 00:14:11,780 This is what practice does. 237 00:14:11,780 --> 00:14:13,750 When you practice your musical instrument, 238 00:14:13,750 --> 00:14:17,450 or you practice your biochemistry problems, 239 00:14:17,450 --> 00:14:19,490 and you do it over and over, you're 240 00:14:19,490 --> 00:14:24,140 changing the synapses that allow you to engage these problems. 241 00:14:24,140 --> 00:14:28,640 And these processes have got names. 242 00:14:28,640 --> 00:14:31,070 So let me just complete this. 243 00:14:31,070 --> 00:14:37,850 So by changing receptors so that repeated synaptic stimulation-- 244 00:14:47,180 --> 00:14:49,916 changes the responsiveness of the synapse-- 245 00:14:57,130 --> 00:15:03,290 generally, as I say, through the receptors in this case. 246 00:15:03,290 --> 00:15:04,650 OK. 247 00:15:04,650 --> 00:15:07,760 And there are two outputs. 248 00:15:07,760 --> 00:15:12,320 One, you can increase the synaptic response. 249 00:15:12,320 --> 00:15:14,270 You can make it more likely that there'll 250 00:15:14,270 --> 00:15:17,700 be an action potential. 251 00:15:17,700 --> 00:15:20,230 And that would take place at excitatory synapses. 252 00:15:27,230 --> 00:15:30,590 And this process is known as a long term potentiation. 253 00:15:35,900 --> 00:15:38,540 It's the stuff you want when you're 254 00:15:38,540 --> 00:15:40,250 trying to learn something. 255 00:15:40,250 --> 00:15:43,350 It's believed to be the way memory works. 256 00:15:43,350 --> 00:15:46,340 You can also decrease the response 257 00:15:46,340 --> 00:15:48,620 of a particular synapse, and that 258 00:15:48,620 --> 00:15:50,800 would work if it was an inhibitory synapse. 259 00:15:55,130 --> 00:15:57,935 And in that case, the process is called long term-- 260 00:16:00,730 --> 00:16:01,810 let's just write it out-- 261 00:16:01,810 --> 00:16:04,450 long term depression. 262 00:16:08,180 --> 00:16:12,920 Both of these processes have been shown to work in the lab, 263 00:16:12,920 --> 00:16:14,480 in culture-- 264 00:16:14,480 --> 00:16:16,850 hard to do those experiments on real animals. 265 00:16:16,850 --> 00:16:19,610 But it's believed that that's how memory works. 266 00:16:19,610 --> 00:16:23,460 So in your first handout, I've drawn out for you the idea. 267 00:16:23,460 --> 00:16:25,400 Here is a normal axon. 268 00:16:25,400 --> 00:16:28,370 The presynaptic and the postsynaptic cell-- 269 00:16:28,370 --> 00:16:29,930 neurotransmitters released. 270 00:16:29,930 --> 00:16:31,790 It engages receptors. 271 00:16:31,790 --> 00:16:36,480 And at some frequency, there's an action potential elicited. 272 00:16:36,480 --> 00:16:40,420 If you change-- if you repeatedly stimulate that axon 273 00:16:40,420 --> 00:16:41,220 over time-- 274 00:16:41,220 --> 00:16:45,810 days, minutes-- you change the receptor spectrum 275 00:16:45,810 --> 00:16:47,640 on the postsynaptic cell. 276 00:16:47,640 --> 00:16:51,120 And in long term, potentiation responsiveness increases. 277 00:16:51,120 --> 00:16:53,280 Whereas in long term depression, you 278 00:16:53,280 --> 00:16:56,400 decrease receptor number, or some other parameter, 279 00:16:56,400 --> 00:17:00,946 and you lead to decreased responsiveness. 280 00:17:00,946 --> 00:17:01,446 OK. 281 00:17:04,890 --> 00:17:14,020 Let us move on to our second topic, which 282 00:17:14,020 --> 00:17:21,510 is that of circuits, which refers 283 00:17:21,510 --> 00:17:24,240 to multiple connectivities-- 284 00:17:24,240 --> 00:17:29,130 multiple synapses forming in a way that is stable, 285 00:17:29,130 --> 00:17:33,285 and that can lead to particular outcomes of particular stimuli. 286 00:17:38,380 --> 00:17:40,920 Let's look at your next handout to try 287 00:17:40,920 --> 00:17:44,020 to get a sense of where we are. 288 00:17:44,020 --> 00:17:46,660 So I've diagrammed this out for you. 289 00:17:46,660 --> 00:17:48,510 And I've started here with some kind 290 00:17:48,510 --> 00:17:52,500 of sensory neuron, an interneuron, and a motor 291 00:17:52,500 --> 00:17:53,580 neuron-- 292 00:17:53,580 --> 00:17:57,081 three connected neurons. 293 00:17:57,081 --> 00:17:59,080 You could put a lot of these other things called 294 00:17:59,080 --> 00:18:00,850 interneurons in the way. 295 00:18:00,850 --> 00:18:03,460 Sensory neurons receive signals. 296 00:18:03,460 --> 00:18:07,660 Motor neurons tell things to happen, like muscles to work. 297 00:18:07,660 --> 00:18:09,790 And interneurons, of which there can be many, 298 00:18:09,790 --> 00:18:12,610 are the connectors between sensory and motor neurons. 299 00:18:15,290 --> 00:18:18,030 There's an input, and there's an output. 300 00:18:18,030 --> 00:18:20,360 And what happens in between-- 301 00:18:20,360 --> 00:18:22,130 this is a synthesis of what we've 302 00:18:22,130 --> 00:18:25,810 talked about over the last couple of lectures. 303 00:18:25,810 --> 00:18:29,710 The sensory neuron makes multiple synapses 304 00:18:29,710 --> 00:18:31,000 onto the interneuron. 305 00:18:31,000 --> 00:18:33,670 They can be excitatory or inhibitory. 306 00:18:33,670 --> 00:18:35,005 They're summated. 307 00:18:35,005 --> 00:18:37,720 The interneuron then decides whether to make 308 00:18:37,720 --> 00:18:40,000 an action potential or not-- 309 00:18:40,000 --> 00:18:43,360 action potential-- yes or no in each case. 310 00:18:43,360 --> 00:18:45,460 If it makes an action potential, it 311 00:18:45,460 --> 00:18:49,330 sends its signal to the next neuron, the motor neuron. 312 00:18:49,330 --> 00:18:52,390 That motor neuron is getting a bunch of inputs-- 313 00:18:52,390 --> 00:18:54,640 excitatory and inhibitory. 314 00:18:54,640 --> 00:18:57,160 And it, again, has to decide whether or not there's 315 00:18:57,160 --> 00:18:59,080 an action potential, OK? 316 00:18:59,080 --> 00:19:02,260 So that's the context that puts circuits, 317 00:19:02,260 --> 00:19:06,040 where we have been discussing the process of setting up 318 00:19:06,040 --> 00:19:07,840 the connections in the nervous system. 319 00:19:11,040 --> 00:19:15,170 The thing about circuits-- and here's a term you should know-- 320 00:19:15,170 --> 00:19:20,515 is that they have to do with axon pathfinding. 321 00:19:26,650 --> 00:19:29,470 And so let's, for a moment, forget this diagram, which 322 00:19:29,470 --> 00:19:33,580 is a diagram of what there is in the adult. 323 00:19:33,580 --> 00:19:37,670 And let's think about how these circuits are set up. 324 00:19:37,670 --> 00:19:41,080 The circuits that we have in our brains and then our bodies 325 00:19:41,080 --> 00:19:45,640 are of the close to uncountable. 326 00:19:45,640 --> 00:19:49,120 I'm not sure we ever will count them in an animal as complex 327 00:19:49,120 --> 00:19:50,140 as ourselves. 328 00:19:50,140 --> 00:19:53,050 It's been done in caenorhabditis, 329 00:19:53,050 --> 00:19:54,910 which has got 1,000 cells-- the little 330 00:19:54,910 --> 00:19:57,700 worm we talked about-- it's got 1,000 cells. 331 00:19:57,700 --> 00:20:00,300 And all of the connections in caenorhabditis 332 00:20:00,300 --> 00:20:01,600 have been mapped. 333 00:20:01,600 --> 00:20:04,480 And they are extremely complex. 334 00:20:04,480 --> 00:20:06,340 But that's just a handful of neurons. 335 00:20:06,340 --> 00:20:08,590 When you're talking about 10 to the 10th, 10 336 00:20:08,590 --> 00:20:13,380 to the 11th neurons, those connections are enormous. 337 00:20:13,380 --> 00:20:16,350 But we can ask some basic questions, OK? 338 00:20:16,350 --> 00:20:18,190 And let's just state it. 339 00:20:18,190 --> 00:20:21,460 Neurons are connected to form circuits. 340 00:20:29,750 --> 00:20:31,845 We can say that they're functional circuits, 341 00:20:31,845 --> 00:20:32,345 if you like. 342 00:20:35,090 --> 00:20:36,860 That is implicit. 343 00:20:36,860 --> 00:20:39,620 And the big question is, how do these circuits 344 00:20:39,620 --> 00:20:40,515 know where to form? 345 00:20:48,670 --> 00:20:51,430 Or if you like the restatement, how 346 00:20:51,430 --> 00:20:53,590 do the neurons know where to go-- 347 00:21:04,030 --> 00:21:06,380 know to go and where to connect? 348 00:21:14,800 --> 00:21:17,290 And there are two kind of intuitive answers, 349 00:21:17,290 --> 00:21:19,810 both of which turn out to be correct. 350 00:21:19,810 --> 00:21:22,960 Either the neurons just go every which way. 351 00:21:22,960 --> 00:21:25,420 And if it works, that's retained, 352 00:21:25,420 --> 00:21:29,710 and the rest of the connections just are dissolved and go away. 353 00:21:29,710 --> 00:21:31,210 And that's true. 354 00:21:31,210 --> 00:21:36,040 But the bigger truth is that the neurons are told where to go. 355 00:21:36,040 --> 00:21:40,530 So you can phrase these that the neurons undergo 356 00:21:40,530 --> 00:21:47,800 some kind of random process where neurons survive 357 00:21:47,800 --> 00:21:49,555 if connections are made. 358 00:21:57,470 --> 00:22:01,910 Or there's some kind of guided process 359 00:22:01,910 --> 00:22:04,390 where the neurons are told where to go. 360 00:22:11,730 --> 00:22:17,192 And both of these turn out, after many, many years of work, 361 00:22:17,192 --> 00:22:17,775 to be correct. 362 00:22:21,020 --> 00:22:23,620 Let's look at some slides here. 363 00:22:23,620 --> 00:22:26,560 This is work from Professor Fee, over in Brain and Cognitive 364 00:22:26,560 --> 00:22:27,070 Science. 365 00:22:27,070 --> 00:22:30,730 He studies these little birds and their song circuit, 366 00:22:30,730 --> 00:22:34,390 which forms and reforms during the life of the bird. 367 00:22:34,390 --> 00:22:36,920 The circuit is complicated. 368 00:22:36,920 --> 00:22:38,350 These are not single neurons. 369 00:22:38,350 --> 00:22:40,180 They are bundles of neurons. 370 00:22:40,180 --> 00:22:42,430 But it's kind of mappable. 371 00:22:42,430 --> 00:22:43,920 And this is one of the circuits. 372 00:22:43,920 --> 00:22:47,560 It's got something to do with song within the bird brain. 373 00:22:47,560 --> 00:22:49,630 It's not the entire circuit. 374 00:22:49,630 --> 00:22:53,330 That's one of the things that's going on here on campus. 375 00:22:53,330 --> 00:22:57,050 But when you start thinking about things like language-- 376 00:22:57,050 --> 00:22:59,090 I really like this slide. 377 00:22:59,090 --> 00:23:02,090 Because these are activity plots of the brain that 378 00:23:02,090 --> 00:23:06,050 can be done in a number of ways by monitoring oxygen uptake, 379 00:23:06,050 --> 00:23:07,850 or glucose use. 380 00:23:07,850 --> 00:23:10,310 But you can look and see different parts 381 00:23:10,310 --> 00:23:11,970 of the brain are active. 382 00:23:11,970 --> 00:23:13,950 And when you think about language-- 383 00:23:13,950 --> 00:23:15,860 here are these four parts of the brain. 384 00:23:15,860 --> 00:23:18,140 Actually, here is a huge part of the brain 385 00:23:18,140 --> 00:23:20,480 used to generate words. 386 00:23:20,480 --> 00:23:24,320 Each of these parts of the brain are millions and millions 387 00:23:24,320 --> 00:23:27,150 of neurons, which are connected to one another 388 00:23:27,150 --> 00:23:29,240 within these regions. 389 00:23:29,240 --> 00:23:33,110 And then each of these regions is connected to one another. 390 00:23:33,110 --> 00:23:35,750 And each of those regions is connected 391 00:23:35,750 --> 00:23:40,280 to the output, which would be all of your vocal apparatus. 392 00:23:40,280 --> 00:23:43,490 And all of this is connected to your auditory apparatus, 393 00:23:43,490 --> 00:23:47,000 to your visual system, so that you can read words. 394 00:23:47,000 --> 00:23:49,250 They can be processed, and so on. 395 00:23:49,250 --> 00:23:51,830 The connections that give language 396 00:23:51,830 --> 00:23:54,920 probably take a very large part of your brain. 397 00:23:54,920 --> 00:23:58,940 I would throw out 15% to 20% of your brain 398 00:23:58,940 --> 00:24:02,030 is involved in some aspect of language-- 399 00:24:02,030 --> 00:24:05,270 receiving, generating, or output. 400 00:24:05,270 --> 00:24:08,570 And the circuits there, as I said, are enormous. 401 00:24:08,570 --> 00:24:09,200 OK. 402 00:24:09,200 --> 00:24:11,540 So do neurons know where to go? 403 00:24:11,540 --> 00:24:12,380 Let me go with you. 404 00:24:12,380 --> 00:24:13,610 This isn't on your handout. 405 00:24:13,610 --> 00:24:15,360 This is to look on the screen. 406 00:24:15,360 --> 00:24:17,690 Do neurons know where to connect? 407 00:24:17,690 --> 00:24:21,630 There was a very famous experiment done by Sperry, 408 00:24:21,630 --> 00:24:23,750 some many years ago-- 409 00:24:23,750 --> 00:24:27,560 Roger Sperry-- who got a Nobel Prize for his work-- 410 00:24:27,560 --> 00:24:29,540 that involved a frog. 411 00:24:29,540 --> 00:24:34,160 And it involved figuring out where the neurons in the retina 412 00:24:34,160 --> 00:24:35,030 went. 413 00:24:35,030 --> 00:24:38,620 So it turns out, if you look at your eyes, 414 00:24:38,620 --> 00:24:42,650 OK, there are neurons that go from your eye 415 00:24:42,650 --> 00:24:45,590 back into your brain. 416 00:24:45,590 --> 00:24:47,750 The neurons on the side of your nose 417 00:24:47,750 --> 00:24:49,100 are called the nasal neurons. 418 00:24:49,100 --> 00:24:50,810 And the neurons on the side of you, 419 00:24:50,810 --> 00:24:54,110 or the outside of your face, are called temporal neurons. 420 00:24:54,110 --> 00:24:57,290 And what Sperry did was to take an adult frog 421 00:24:57,290 --> 00:25:00,470 and rotate the eye 180 degrees so 422 00:25:00,470 --> 00:25:03,740 that the nasal neurons would now be on the outside of the head. 423 00:25:03,740 --> 00:25:07,460 The temporal neurons would be on the inside of the head, OK? 424 00:25:07,460 --> 00:25:10,370 And for a while after the operation, 425 00:25:10,370 --> 00:25:13,170 the frog was really confused. 426 00:25:13,170 --> 00:25:17,020 But after a while, it actually recovered. 427 00:25:17,020 --> 00:25:19,460 Now let's lay out this-- 428 00:25:19,460 --> 00:25:21,990 and the recovery, as I'm going to tell you, 429 00:25:21,990 --> 00:25:27,000 told Sperry and the world that neurons were told where to go. 430 00:25:27,000 --> 00:25:30,300 So here is the retina depicted as a circle. 431 00:25:30,300 --> 00:25:33,060 And here's a part of the brain called the optic tectum, 432 00:25:33,060 --> 00:25:34,740 that the neurons connect to. 433 00:25:34,740 --> 00:25:36,860 It's the first part of their circuit. 434 00:25:36,860 --> 00:25:38,850 Here are the temporal neurons connecting 435 00:25:38,850 --> 00:25:41,670 to the C region of the brain, and the nasal 436 00:25:41,670 --> 00:25:45,560 neurons connecting to the R region. 437 00:25:45,560 --> 00:25:49,870 So after this rotation, there were two possibilities. 438 00:25:49,870 --> 00:25:53,870 One was that the axons growing out. 439 00:25:53,870 --> 00:25:55,030 So let's just be clear. 440 00:25:55,030 --> 00:25:58,330 In this rotation, these axons were severed. 441 00:25:58,330 --> 00:25:59,500 They were cut. 442 00:25:59,500 --> 00:26:02,890 They had to regrow and find their targets, if they could. 443 00:26:02,890 --> 00:26:06,610 So one possibility would be that the axons would grow out 444 00:26:06,610 --> 00:26:09,440 of this retina, and they'd grow everywhere. 445 00:26:09,440 --> 00:26:11,440 And they wouldn't make really wrong connections. 446 00:26:11,440 --> 00:26:13,090 And things would be a mess. 447 00:26:13,090 --> 00:26:15,340 In actual fact, what really happened 448 00:26:15,340 --> 00:26:17,800 was that the nasal neurons, even though they 449 00:26:17,800 --> 00:26:19,780 were on the wrong side of the face, 450 00:26:19,780 --> 00:26:21,820 found their way to the R region. 451 00:26:21,820 --> 00:26:24,670 And the tactile neurons found their way to the C region, 452 00:26:24,670 --> 00:26:27,370 just like they had connected to before. 453 00:26:27,370 --> 00:26:29,740 And this was a really profound experiment 454 00:26:29,740 --> 00:26:33,040 that told investigators that axons knew where to go. 455 00:26:33,040 --> 00:26:35,110 And we know the molecular basis of this now, 456 00:26:35,110 --> 00:26:37,240 but I'm not going to tell you about it. 457 00:26:41,360 --> 00:26:47,360 In order to understand more about this guidance process, 458 00:26:47,360 --> 00:26:50,390 you need to understand a part of the axon 459 00:26:50,390 --> 00:26:53,990 that we haven't discussed, and a time in the neurons life 460 00:26:53,990 --> 00:26:56,060 that we haven't discussed either. 461 00:26:56,060 --> 00:27:00,230 And that is a time when neurons are growing. 462 00:27:00,230 --> 00:27:04,340 Most neuronal growth takes place during embryonic development, 463 00:27:04,340 --> 00:27:06,570 but it continues throughout life. 464 00:27:06,570 --> 00:27:09,590 And so this is a developmental process, but also 465 00:27:09,590 --> 00:27:12,640 in that experiment, that was an adult frog. 466 00:27:12,640 --> 00:27:13,310 OK. 467 00:27:13,310 --> 00:27:20,240 So guidance of neurons-- 468 00:27:20,240 --> 00:27:26,630 and as we'll discuss, particularly axons-- 469 00:27:26,630 --> 00:27:33,740 occurs during development and repair-- 470 00:27:33,740 --> 00:27:34,970 and learning as well. 471 00:27:34,970 --> 00:27:42,140 We can put that up as well-- during development, repair, 472 00:27:42,140 --> 00:27:44,529 and learning. 473 00:27:44,529 --> 00:27:46,070 Although I'd say for learning there's 474 00:27:46,070 --> 00:27:48,330 a bit of a question mark whether that's true. 475 00:27:48,330 --> 00:27:49,990 But I think it's probably true. 476 00:27:49,990 --> 00:27:51,900 OK. 477 00:27:51,900 --> 00:27:54,390 And the cell that we need to consider 478 00:27:54,390 --> 00:27:57,885 is a committed neuron, which is called a neuroblast. 479 00:27:57,885 --> 00:27:59,460 It doesn't matter. 480 00:27:59,460 --> 00:28:02,580 But here's your committed neuron. 481 00:28:02,580 --> 00:28:06,120 You have to think back to past lectures-- 482 00:28:06,120 --> 00:28:08,220 a neuron that knows that it's going to be a neuron 483 00:28:08,220 --> 00:28:11,220 but hasn't decided to-- hasn't become one yet-- 484 00:28:11,220 --> 00:28:13,140 hasn't differentiated. 485 00:28:13,140 --> 00:28:15,660 It's called a neuroblast. 486 00:28:15,660 --> 00:28:17,400 And here it is. 487 00:28:17,400 --> 00:28:20,160 It's just a cell body with the nucleus. 488 00:28:24,480 --> 00:28:28,980 And over time, this neuroblast sends out 489 00:28:28,980 --> 00:28:31,740 processes that are called neurites. 490 00:28:31,740 --> 00:28:33,600 They initially look the same. 491 00:28:33,600 --> 00:28:35,760 Soon some of them become dendrites. 492 00:28:35,760 --> 00:28:38,760 And one of them becomes an axon-- 493 00:28:38,760 --> 00:28:52,680 some neurite, axon, plus dendrite outgrowth. 494 00:28:56,580 --> 00:29:02,980 So you have a cell now with some processes. 495 00:29:02,980 --> 00:29:06,730 And one of these is going to be the axon. 496 00:29:06,730 --> 00:29:09,250 And that axon grows. 497 00:29:09,250 --> 00:29:11,560 And it's during that growth process 498 00:29:11,560 --> 00:29:13,380 that it figures out where to go. 499 00:29:13,380 --> 00:29:14,800 The cell body doesn't move. 500 00:29:14,800 --> 00:29:16,580 The dendrites don't move. 501 00:29:16,580 --> 00:29:20,110 It's the axon that is doing the pathfinding. 502 00:29:20,110 --> 00:29:40,230 So the axon extends and eventually finds its target. 503 00:29:45,880 --> 00:29:52,120 And this extension is a growth process that we'll talk about. 504 00:29:52,120 --> 00:29:53,440 OK. 505 00:29:53,440 --> 00:29:55,570 For every-- a nerve-- 506 00:29:55,570 --> 00:29:57,400 when we talk about nerves-- 507 00:29:57,400 --> 00:29:58,660 there are actually many-- 508 00:29:58,660 --> 00:30:00,840 many neurons, many axons. 509 00:30:00,840 --> 00:30:03,070 There are bundles of axons. 510 00:30:03,070 --> 00:30:07,030 And there is something about the first axon 511 00:30:07,030 --> 00:30:09,760 to find a target, that's very important-- 512 00:30:09,760 --> 00:30:12,600 the pioneer axon. 513 00:30:12,600 --> 00:30:15,510 The first one finds the target. 514 00:30:18,895 --> 00:30:19,895 And then others follow-- 515 00:30:25,000 --> 00:30:28,920 the same path that lays down a path. 516 00:30:28,920 --> 00:30:36,350 And they form bundles, also called fascicles. 517 00:30:36,350 --> 00:30:41,735 And these fascicles together make the nerve. 518 00:30:46,300 --> 00:30:48,160 The part of the axon that's really 519 00:30:48,160 --> 00:30:51,880 important for this process is the very tip of the axon, 520 00:30:51,880 --> 00:30:54,670 and it has a special name called the growth cone. 521 00:31:00,830 --> 00:31:08,080 So the axon tip, or the growth cone, is crucial. 522 00:31:08,080 --> 00:31:10,820 And we can draw it on the board. 523 00:31:10,820 --> 00:31:13,760 If we now draw an axon-- 524 00:31:18,010 --> 00:31:19,540 and we've blown it up now. 525 00:31:19,540 --> 00:31:22,810 So usually we blow up and we lose bits, 526 00:31:22,810 --> 00:31:24,790 but now we're not going to lose any bit of it. 527 00:31:24,790 --> 00:31:26,410 Here's the axon. 528 00:31:26,410 --> 00:31:35,230 And down the length of the axon are parallel microtubules-- 529 00:31:35,230 --> 00:31:36,950 many of them. 530 00:31:36,950 --> 00:31:43,180 And these microtubules both stabilize the axon 531 00:31:43,180 --> 00:31:47,650 and also transport substances to and from the cell body. 532 00:31:50,530 --> 00:31:56,980 So they stabilize the shape of very long axons, 533 00:31:56,980 --> 00:32:04,870 and they also transport, like little railway tracks, 534 00:32:04,870 --> 00:32:09,290 substances to and from the cell body. 535 00:32:09,290 --> 00:32:12,070 That's not the tip of the axon. 536 00:32:12,070 --> 00:32:16,150 At the very tip, the microtubules end, 537 00:32:16,150 --> 00:32:21,940 and they interdigitate with these finger-like protrusions, 538 00:32:21,940 --> 00:32:23,560 which are very dynamic. 539 00:32:23,560 --> 00:32:26,060 That means they change all the time. 540 00:32:26,060 --> 00:32:31,330 And these protrusions protrude because of polymerized actin. 541 00:32:31,330 --> 00:32:33,539 And you might be saying, we heard about that already. 542 00:32:33,539 --> 00:32:34,038 Yes. 543 00:32:34,038 --> 00:32:35,920 You've heard about that in morphogenesis. 544 00:32:35,920 --> 00:32:37,480 We've talked about this. 545 00:32:37,480 --> 00:32:42,250 Cell biology is cell biology, whether it's about neurons, 546 00:32:42,250 --> 00:32:45,760 or cells in your stomach, or cells in your-- 547 00:32:45,760 --> 00:32:48,130 that give rise to the hairs on your head. 548 00:32:48,130 --> 00:32:50,330 All of these cells are just cells. 549 00:32:50,330 --> 00:32:53,800 So there are these protrusions at the end. 550 00:32:56,690 --> 00:32:58,950 So this whole thing is the axon. 551 00:32:58,950 --> 00:33:03,810 But the very end is the growth cone. 552 00:33:03,810 --> 00:33:06,100 This very end is the growth cone. 553 00:33:09,890 --> 00:33:21,590 And these protrusions go by the name of filopodia, 554 00:33:21,590 --> 00:33:27,350 filamentous, and lamellipodia, more flattened feet. 555 00:33:31,550 --> 00:33:35,060 And they protrude because of the polymerization of actin 556 00:33:35,060 --> 00:33:36,165 within them. 557 00:33:36,165 --> 00:33:48,800 So there's F-actin leads to protrusions. 558 00:33:52,410 --> 00:33:54,070 Well, that's one important thing. 559 00:33:54,070 --> 00:33:56,050 And I'll show you a movie in a moment 560 00:33:56,050 --> 00:33:58,330 to show you that as an axon grows, 561 00:33:58,330 --> 00:34:01,330 it's sending out zillions of these things all the time 562 00:34:01,330 --> 00:34:04,370 that are feeling their way around the place. 563 00:34:04,370 --> 00:34:05,292 Actually, I'll-- no. 564 00:34:05,292 --> 00:34:07,750 I won't show you now, because there's an intervening slide. 565 00:34:07,750 --> 00:34:11,980 But the other thing you should know is that like all cells, 566 00:34:11,980 --> 00:34:15,760 there are receptors on the surface of the growth cone 567 00:34:15,760 --> 00:34:20,170 that are also sampling what ligands are in the environment. 568 00:34:20,170 --> 00:34:26,790 And if the ligands are favorable, 569 00:34:26,790 --> 00:34:30,540 they will either stabilize these protrusions 570 00:34:30,540 --> 00:34:33,000 and make more of them to take place. 571 00:34:33,000 --> 00:34:36,015 So here are receptors all over the place. 572 00:34:39,192 --> 00:34:40,425 This is a receptor. 573 00:34:45,080 --> 00:34:55,570 And if receptors contact ligands, 574 00:34:55,570 --> 00:35:00,730 then again, you will get F-actin formed. 575 00:35:00,730 --> 00:35:03,100 And protrusions will be formed. 576 00:35:03,100 --> 00:35:05,260 And they will be stabilized. 577 00:35:05,260 --> 00:35:06,910 OK. 578 00:35:06,910 --> 00:35:11,230 The growth cone is paramount to axon outgrowth. 579 00:35:11,230 --> 00:35:15,430 There are signals which both attract axons towards them, 580 00:35:15,430 --> 00:35:19,110 and signals which repel axons. 581 00:35:19,110 --> 00:35:22,080 We can call those, not surprisingly, 582 00:35:22,080 --> 00:35:23,270 attractive signals. 583 00:35:26,440 --> 00:35:29,910 We'll talk about some in a moment. 584 00:35:29,910 --> 00:35:33,390 And in that case, the growth cone extends. 585 00:35:36,770 --> 00:35:40,350 And it extends-- not to beat a dead horse 586 00:35:40,350 --> 00:35:46,230 here-- but because F-actin increases or is stabilized. 587 00:35:46,230 --> 00:35:48,570 And the flip is that the growth cone 588 00:35:48,570 --> 00:35:54,400 can be destabilized and literally collapse 589 00:35:54,400 --> 00:35:56,170 under repulsive signals. 590 00:36:03,690 --> 00:36:10,140 The growth cone collapses. 591 00:36:10,140 --> 00:36:16,380 And it collapses because F-actin now becomes 592 00:36:16,380 --> 00:36:20,580 G globula, or unpolymerized actin, 593 00:36:20,580 --> 00:36:24,150 using exactly the same processes that we talked about 594 00:36:24,150 --> 00:36:26,530 during the morphogenesis lecture. 595 00:36:26,530 --> 00:36:28,050 Let's look at a couple of slides. 596 00:36:28,050 --> 00:36:29,830 You don't have these. 597 00:36:29,830 --> 00:36:31,870 So just look on the screen. 598 00:36:31,870 --> 00:36:36,390 This is a cell drawn in Professor Lodish's book. 599 00:36:36,390 --> 00:36:38,640 Here's the leading edge of the cell. 600 00:36:38,640 --> 00:36:40,950 The direction of the cell is where there 601 00:36:40,950 --> 00:36:44,875 is lots of polymerized actin. 602 00:36:44,875 --> 00:36:46,640 This I've just drawn on the board. 603 00:36:46,640 --> 00:36:48,410 And here is an axonal growth code. 604 00:36:48,410 --> 00:36:50,860 And I really like this movie. 605 00:36:50,860 --> 00:36:53,990 It's a time lapse taken over 10 minutes. 606 00:36:53,990 --> 00:36:58,280 And you can see all these things at the end and on the sides-- 607 00:36:58,280 --> 00:37:00,700 these protrusions from the cell-- very active. 608 00:37:00,700 --> 00:37:01,450 They're forming. 609 00:37:01,450 --> 00:37:05,410 They're disaggregating, forming again and disaggregating 610 00:37:05,410 --> 00:37:07,300 as the cell-- those of the lamellipodia 611 00:37:07,300 --> 00:37:08,620 and the filopodia-- 612 00:37:08,620 --> 00:37:12,160 as the cell is feeling its way through the environment-- 613 00:37:12,160 --> 00:37:14,920 trying to find somewhere to go, OK? 614 00:37:14,920 --> 00:37:18,380 So this is a real exploratory process by the cell. 615 00:37:20,981 --> 00:37:21,480 Good. 616 00:37:24,180 --> 00:37:27,770 What are these guidance signals? 617 00:37:27,770 --> 00:37:30,230 Well, this is not a mystery either. 618 00:37:30,230 --> 00:37:32,540 Ligands-- we know about receptors. 619 00:37:32,540 --> 00:37:35,110 You know about the guidance signals are ligands. 620 00:37:54,550 --> 00:38:00,170 And these ligands, when they're bound to a receptor, 621 00:38:00,170 --> 00:38:03,830 lead to signal transduction and axon outgrowth. 622 00:38:03,830 --> 00:38:11,690 So ligands-- and we'll put in parentheses, plus receptors-- 623 00:38:11,690 --> 00:38:23,380 to signal transduction-- and a change in the growth cone. 624 00:38:30,670 --> 00:38:32,990 There are two kinds of guidance signals 625 00:38:32,990 --> 00:38:36,840 that are reasonably separate from one another. 626 00:38:36,840 --> 00:38:41,260 They're called short range and long range guidance signals. 627 00:38:41,260 --> 00:38:47,590 Short range signals require contact 628 00:38:47,590 --> 00:38:52,060 between the axon and the extracellular matrix, 629 00:38:52,060 --> 00:38:54,500 or the axon and another cell. 630 00:38:54,500 --> 00:38:58,150 So there's some localized accumulation of a signal, 631 00:38:58,150 --> 00:39:04,360 and that has to be directly contacted by the growth cone-- 632 00:39:04,360 --> 00:39:13,255 so require axon ECM, or cell contact. 633 00:39:17,260 --> 00:39:20,470 These signals are not diffusible. 634 00:39:23,770 --> 00:39:27,010 Therefore-- and they include things 635 00:39:27,010 --> 00:39:32,740 like laminin, which is a part of the extracellular matrix, 636 00:39:32,740 --> 00:39:35,830 but also an axon guidance signal. 637 00:39:35,830 --> 00:39:41,740 And then there are long range signals, 638 00:39:41,740 --> 00:39:44,470 which would be the flip of the short range. 639 00:39:44,470 --> 00:39:45,640 These are diffusible. 640 00:39:48,970 --> 00:39:52,320 And they can be concentration dependent in their effect. 641 00:39:56,690 --> 00:39:58,370 And we talked about things like this 642 00:39:58,370 --> 00:40:03,220 previously when we talked about morphogens-- 643 00:40:03,220 --> 00:40:06,910 other ligands that can act at different concentrations 644 00:40:06,910 --> 00:40:08,650 in different ways. 645 00:40:08,650 --> 00:40:09,190 OK. 646 00:40:09,190 --> 00:40:12,010 And an example that I'll explore more with you 647 00:40:12,010 --> 00:40:13,225 is the netrin protein. 648 00:40:22,910 --> 00:40:23,410 All right. 649 00:40:23,410 --> 00:40:25,960 Let me see what I have here. 650 00:40:25,960 --> 00:40:27,140 OK, this is nice. 651 00:40:27,140 --> 00:40:29,540 This is on your-- this is your next handout. 652 00:40:29,540 --> 00:40:32,950 And this is an assay for short range guidance signals. 653 00:40:32,950 --> 00:40:34,960 It's called a stripe assay. 654 00:40:34,960 --> 00:40:37,000 And this was how it was found. 655 00:40:37,000 --> 00:40:42,520 What those nasal and temporal retinal neurons grew on-- 656 00:40:42,520 --> 00:40:44,890 the idea is you take a plastic dish, 657 00:40:44,890 --> 00:40:48,010 and you put stripes of different molecules on the dish. 658 00:40:48,010 --> 00:40:51,726 And then you put neurons all the way along one side of the dish. 659 00:40:51,726 --> 00:40:52,600 And you look at them. 660 00:40:52,600 --> 00:40:54,496 You ask where they grow. 661 00:40:54,496 --> 00:40:55,870 And they'll choose where to grow. 662 00:40:55,870 --> 00:40:57,550 And if they like one of the molecules, 663 00:40:57,550 --> 00:41:00,370 if they can interact with one of the molecules on the dish, 664 00:41:00,370 --> 00:41:02,440 they will grow in particular stripes, 665 00:41:02,440 --> 00:41:03,570 and not in other stripes. 666 00:41:03,570 --> 00:41:05,570 And that gives you your experiment and control 667 00:41:05,570 --> 00:41:07,670 in one dish. 668 00:41:07,670 --> 00:41:09,560 This is what it looks like. 669 00:41:09,560 --> 00:41:13,990 So here are the neurons from the nasal side of the retina. 670 00:41:13,990 --> 00:41:16,330 And those nasal neurons, which project 671 00:41:16,330 --> 00:41:21,910 to the R side of the tectum, grow on our membranes, 672 00:41:21,910 --> 00:41:25,240 but not on C membranes, which is where they don't go. 673 00:41:25,240 --> 00:41:27,940 You may not have this, but you can go back and look at this 674 00:41:27,940 --> 00:41:29,430 later on. 675 00:41:29,430 --> 00:41:31,450 All right. 676 00:41:31,450 --> 00:41:34,660 But let's move on now to an example 677 00:41:34,660 --> 00:41:37,230 that I want to spend a bit of time on. 678 00:41:37,230 --> 00:41:44,390 And the example of a long range signal is that the netrin 679 00:41:44,390 --> 00:41:45,550 in the spinal cord. 680 00:41:51,170 --> 00:41:53,240 And what we're going to talk about 681 00:41:53,240 --> 00:41:56,060 are two types of neurons, one of which 682 00:41:56,060 --> 00:41:59,120 are growing down the spinal cord from the back, more 683 00:41:59,120 --> 00:42:02,030 towards the belly, and another type of neuron that's 684 00:42:02,030 --> 00:42:05,540 growing more from the belly side of the spinal cord, back 685 00:42:05,540 --> 00:42:07,790 towards the back, OK? 686 00:42:07,790 --> 00:42:10,520 And those neurons always know where to go. 687 00:42:10,520 --> 00:42:14,660 And it turns out they are told where to go by the same signal. 688 00:42:14,660 --> 00:42:17,270 So there are two types of neurons. 689 00:42:17,270 --> 00:42:23,970 There are these things called commissural neurons. 690 00:42:29,820 --> 00:42:35,890 And they grow ventrally, or down, 691 00:42:35,890 --> 00:42:38,170 towards something called the floor plate. 692 00:42:38,170 --> 00:42:40,990 I'll show you in the diagram. 693 00:42:40,990 --> 00:42:43,090 And then there are these other ones 694 00:42:43,090 --> 00:42:52,910 called trochlear neurons that grow dorsally, or up. 695 00:42:52,910 --> 00:42:57,230 And they grow away from this thing called the floor plate. 696 00:42:57,230 --> 00:43:02,450 And it turns out, using an explant assay, 697 00:43:02,450 --> 00:43:04,440 that I'll go through with you in your slides-- 698 00:43:04,440 --> 00:43:06,980 but you should understand-- 699 00:43:06,980 --> 00:43:14,180 it was found that a single molecule called netrin, which 700 00:43:14,180 --> 00:43:22,440 is a secreted ligand, a secreted protein, that's 701 00:43:22,440 --> 00:43:24,135 expressed in the floor plate-- 702 00:43:31,190 --> 00:43:34,840 which we'll talk about in a moment-- 703 00:43:34,840 --> 00:43:48,130 is attractive for the commissural neurons 704 00:43:48,130 --> 00:43:51,625 and repulsive for the trochlear neurons. 705 00:43:57,850 --> 00:44:01,990 And it also turns out, as we'll go through in a moment, 706 00:44:01,990 --> 00:44:04,660 that this has to do with different receptors 707 00:44:04,660 --> 00:44:09,190 and different receptor dimers, which bind the same ligand. 708 00:44:09,190 --> 00:44:11,860 And so here there is something called, 709 00:44:11,860 --> 00:44:13,950 for the commissural neurons, there 710 00:44:13,950 --> 00:44:16,690 is something called a DCC-- 711 00:44:16,690 --> 00:44:19,355 DCC receptor dimer. 712 00:44:23,790 --> 00:44:26,220 And for the trochlear neurons, there 713 00:44:26,220 --> 00:44:33,472 is a DCC UNC5 receptor pair. 714 00:44:33,472 --> 00:44:34,930 And this will not mean much to you, 715 00:44:34,930 --> 00:44:36,590 but now you can write it down. 716 00:44:36,590 --> 00:44:39,130 And then you can go through your slides. 717 00:44:39,130 --> 00:44:44,530 And you'll have a reference point on right with you. 718 00:44:44,530 --> 00:44:45,830 OK. 719 00:44:45,830 --> 00:44:48,570 So here is the diagram. 720 00:44:48,570 --> 00:44:51,450 The cell bodies of the commissural neurons 721 00:44:51,450 --> 00:44:54,270 are up in this region of the spinal cord 722 00:44:54,270 --> 00:44:55,280 called the roof plate. 723 00:44:55,280 --> 00:44:57,630 It's the top of the spinal cord. 724 00:44:57,630 --> 00:45:00,510 And on the bottom of the spinal cord, near your belly, 725 00:45:00,510 --> 00:45:03,900 there is a cone-shaped group of cells 726 00:45:03,900 --> 00:45:06,630 that forms this thing called the floor plate. 727 00:45:06,630 --> 00:45:08,910 The floor plate doesn't actually make neurons. 728 00:45:08,910 --> 00:45:11,820 It turns out to be a really important source of signals. 729 00:45:11,820 --> 00:45:13,860 It's an organizer, if you like. 730 00:45:13,860 --> 00:45:17,880 And it not only organizes these axons in the spinal cord, 731 00:45:17,880 --> 00:45:20,647 it actually also organizes your midline. 732 00:45:20,647 --> 00:45:22,230 And it's one of the reasons, if you're 733 00:45:22,230 --> 00:45:25,750 missing the midline of the body, things go wrong. 734 00:45:25,750 --> 00:45:27,850 It's because the floor plates are not there. 735 00:45:27,850 --> 00:45:30,030 So the floor plate is an organizer. 736 00:45:30,030 --> 00:45:32,610 They are the commissural neurons growing towards it. 737 00:45:32,610 --> 00:45:35,880 And here are the trochlear neurons growing away 738 00:45:35,880 --> 00:45:38,950 from the floor plate. 739 00:45:38,950 --> 00:45:41,440 This is what it looks like if you do an immunostain. 740 00:45:41,440 --> 00:45:43,180 The cell bodies are in red. 741 00:45:43,180 --> 00:45:45,190 And here are the commissural neurons 742 00:45:45,190 --> 00:45:47,090 coming down in the spinal cord. 743 00:45:47,090 --> 00:45:49,300 And this is a section through the spinal cord, OK? 744 00:45:49,300 --> 00:45:52,600 You've cut through-- cut through at the waist 745 00:45:52,600 --> 00:45:55,330 and then turned the section on its side. 746 00:45:55,330 --> 00:45:58,280 So you're looking into the spinal cord, 747 00:45:58,280 --> 00:46:01,270 which would be coming out in its length from the board-- 748 00:46:01,270 --> 00:46:03,920 from the screen. 749 00:46:03,920 --> 00:46:04,460 All right. 750 00:46:04,460 --> 00:46:08,840 Here's an explant assay to figure out whether or not 751 00:46:08,840 --> 00:46:10,670 that floor plate has got something 752 00:46:10,670 --> 00:46:14,180 to do with the direction that those commissural neurons grow. 753 00:46:14,180 --> 00:46:17,550 So on your handout, the idea was to take 754 00:46:17,550 --> 00:46:19,830 a piece of dorsal spinal cord that 755 00:46:19,830 --> 00:46:22,620 hadn't started to send neurons out yet, 756 00:46:22,620 --> 00:46:26,190 and to culture it together in the laboratory, 757 00:46:26,190 --> 00:46:29,460 in a plastic dish, with some fluid nutrients, 758 00:46:29,460 --> 00:46:31,305 and so on, and ask what happened. 759 00:46:31,305 --> 00:46:34,680 And if you did that, that dorsal spinal cord 760 00:46:34,680 --> 00:46:39,480 sent out neurons towards the floor plate. 761 00:46:39,480 --> 00:46:41,940 On the other hand, you can see that that experiment was 762 00:46:41,940 --> 00:46:44,940 specific, because if you put the dorsal spinal cord together 763 00:46:44,940 --> 00:46:47,940 with some roof plate, nothing happened. 764 00:46:47,940 --> 00:46:49,830 There was no outgrowth. 765 00:46:49,830 --> 00:46:52,800 So there was something special about this floor plate 766 00:46:52,800 --> 00:46:56,490 that elicited that commissural acts on outgrowth. 767 00:46:56,490 --> 00:46:58,890 And the idea is that the floor plate was attracting 768 00:46:58,890 --> 00:47:01,530 the commissural axons. 769 00:47:01,530 --> 00:47:03,040 This is what it really looks like. 770 00:47:03,040 --> 00:47:05,550 Here's a chunk of dorsal spinal cord and floor plate. 771 00:47:05,550 --> 00:47:07,410 And here are the growing axons. 772 00:47:07,410 --> 00:47:10,310 And here's the control experiment. 773 00:47:10,310 --> 00:47:10,810 All right. 774 00:47:10,810 --> 00:47:14,060 So what is the protein involved? 775 00:47:14,060 --> 00:47:17,270 Well, the idea was that it was something in the floor plate. 776 00:47:17,270 --> 00:47:19,130 And it was really hard to find this, 777 00:47:19,130 --> 00:47:21,110 because there's not much of it. 778 00:47:21,110 --> 00:47:23,870 Professor Tessier-Lavigne, who is now 779 00:47:23,870 --> 00:47:27,210 president of Rockefeller University, but at the time 780 00:47:27,210 --> 00:47:30,140 he was running a research laboratory, and he 781 00:47:30,140 --> 00:47:32,870 and many undergraduates, and graduate students, 782 00:47:32,870 --> 00:47:36,840 and post-docs, and so on, went ahead and dissected many, 783 00:47:36,840 --> 00:47:38,480 many little floor plates. 784 00:47:38,480 --> 00:47:40,760 And they also found that the brain of the chicken 785 00:47:40,760 --> 00:47:42,950 contained the same kind of activity. 786 00:47:42,950 --> 00:47:45,920 So they dissected out many thousands-- 787 00:47:45,920 --> 00:47:48,190 I believe it was 35,000-- 788 00:47:48,190 --> 00:47:51,480 chick brains and spinal cords. 789 00:47:51,480 --> 00:47:54,050 And they did biochemistry on this material. 790 00:47:54,050 --> 00:47:57,246 And they used the explant assay that I just showed you, 791 00:47:57,246 --> 00:47:58,620 where instead of the floor plate, 792 00:47:58,620 --> 00:48:01,820 you'd have a little pellet of material in which you 793 00:48:01,820 --> 00:48:03,770 had soaked the material that you've 794 00:48:03,770 --> 00:48:07,550 purified by biochemistry, from your smushed up, 795 00:48:07,550 --> 00:48:09,200 dissected brains. 796 00:48:09,200 --> 00:48:12,200 And from this-- it was really successful-- from this, 797 00:48:12,200 --> 00:48:14,700 they got a single protein. 798 00:48:14,700 --> 00:48:16,700 Here's the RNA for the protein. 799 00:48:16,700 --> 00:48:19,040 That's right expressed in the floor plate. 800 00:48:19,040 --> 00:48:20,090 That's what the white is. 801 00:48:20,090 --> 00:48:20,890 That's the RNA. 802 00:48:20,890 --> 00:48:22,760 This is an in situ hybridisation. 803 00:48:22,760 --> 00:48:24,320 And they called it netrin. 804 00:48:24,320 --> 00:48:27,560 Netrin protein diffuses away from the floor plate, 805 00:48:27,560 --> 00:48:30,380 but it's still mostly on this ventral side 806 00:48:30,380 --> 00:48:31,880 of the spinal cord. 807 00:48:31,880 --> 00:48:35,360 And you could show that netrin was important, 808 00:48:35,360 --> 00:48:39,150 because if you meet a mouse that lacked netrin-- 809 00:48:39,150 --> 00:48:41,060 here's the mouse that lacks netrin-- 810 00:48:41,060 --> 00:48:43,280 the commissural neurons go all over the place. 811 00:48:43,280 --> 00:48:45,920 They really don't know where to go. 812 00:48:45,920 --> 00:48:47,120 All right. 813 00:48:47,120 --> 00:48:51,870 The netrin receptors, as I drew on the board-- 814 00:48:51,870 --> 00:48:53,979 you can look on your last handout-- 815 00:48:53,979 --> 00:48:54,520 are two-fold. 816 00:48:57,780 --> 00:49:02,460 One of them are called DCC, the tyrosine kinases. 817 00:49:02,460 --> 00:49:06,600 They also activate GTPases and do some other signal 818 00:49:06,600 --> 00:49:07,950 transduction. 819 00:49:07,950 --> 00:49:12,090 And when netrin binds to a dimer of DCC, 820 00:49:12,090 --> 00:49:14,580 you get F-actin that's made. 821 00:49:14,580 --> 00:49:16,230 Microtubules grow. 822 00:49:16,230 --> 00:49:18,420 And the growth cone extends. 823 00:49:18,420 --> 00:49:24,540 On the other hand, when you get this hetero dimer DCC and UNC5, 824 00:49:24,540 --> 00:49:29,530 or a different homodimer, you get cytoskeletal remodeling, 825 00:49:29,530 --> 00:49:34,210 G-actin made, and the growth cone collapses. 826 00:49:34,210 --> 00:49:35,880 OK. 827 00:49:35,880 --> 00:49:36,710 Close enough. 828 00:49:36,710 --> 00:49:38,840 We'll stop there.