1 00:00:00,060 --> 00:00:02,400 The following content is provided under a Creative 2 00:00:02,400 --> 00:00:03,790 Commons license. 3 00:00:03,790 --> 00:00:06,000 Your support will help MIT OpenCourseWare 4 00:00:06,000 --> 00:00:10,120 continue to offer high quality educational resources for free. 5 00:00:10,120 --> 00:00:12,660 To make a donation, or to view additional materials 6 00:00:12,660 --> 00:00:16,590 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,590 --> 00:00:17,660 at ocw.mit.edu. 8 00:00:20,400 --> 00:00:27,185 ABBY NOYCE: So, talking about the nervous system. 9 00:00:27,185 --> 00:00:29,820 The nervous system is divided into 10 00:00:29,820 --> 00:00:31,490 the central nervous system, which 11 00:00:31,490 --> 00:00:34,850 is the brain and spinal cord. 12 00:00:34,850 --> 00:00:36,810 And then the peripheral nervous system, 13 00:00:36,810 --> 00:00:38,810 which is pretty much everything else. 14 00:00:38,810 --> 00:00:42,290 Peripheral nervous system is sensory receptors 15 00:00:42,290 --> 00:00:44,780 in the neurons that bring sensory information back 16 00:00:44,780 --> 00:00:46,640 to the central nervous system. 17 00:00:46,640 --> 00:00:50,300 It's motor neurons that send the movement commands out 18 00:00:50,300 --> 00:00:52,100 to your muscles. 19 00:00:52,100 --> 00:00:55,630 It's things like neurons that control digestion, 20 00:00:55,630 --> 00:00:59,869 and neurons that control how different glands behave. 21 00:00:59,869 --> 00:01:01,410 That's the peripheral nervous system. 22 00:01:01,410 --> 00:01:03,892 And that's not what we're going to focus on in this class, 23 00:01:03,892 --> 00:01:06,350 because we're interested in the processing that's happening 24 00:01:06,350 --> 00:01:08,200 in the central nervous system-- 25 00:01:08,200 --> 00:01:14,480 in the spinal cord, and in particular, in the brain. 26 00:01:14,480 --> 00:01:19,100 And the nervous system is made up of two main types of cells. 27 00:01:19,100 --> 00:01:23,840 Neurons are nerve cells, they're kind of the stars 28 00:01:23,840 --> 00:01:27,011 of the nervous system. 29 00:01:27,011 --> 00:01:29,510 They're the ones who do this kind of information processing, 30 00:01:29,510 --> 00:01:30,989 they have inputs and outputs-- 31 00:01:30,989 --> 00:01:32,780 and we'll look in a minute at the structure 32 00:01:32,780 --> 00:01:36,150 of a neuron, how that works. 33 00:01:36,150 --> 00:01:39,479 And then there's a second set of cells called glia. 34 00:01:39,479 --> 00:01:41,270 And we're going to run over the glia first, 35 00:01:41,270 --> 00:01:43,260 because they're simpler. 36 00:01:43,260 --> 00:01:45,530 There's a couple of different kinds of glia. 37 00:01:45,530 --> 00:01:50,180 The word glia comes from glue in Greek or Latin-- 38 00:01:50,180 --> 00:01:53,120 I'm guessing Latin, but I don't know. 39 00:01:53,120 --> 00:01:55,250 One of those kind of classic languages. 40 00:01:55,250 --> 00:01:56,700 AUDIENCE: It looks like Latin. 41 00:01:56,700 --> 00:01:59,810 ABBY NOYCE: Yeah, OK. 42 00:01:59,810 --> 00:02:03,380 And they got this name because the traditional view of this 43 00:02:03,380 --> 00:02:05,600 is that glia don't really do anything. 44 00:02:05,600 --> 00:02:08,820 They hang out, they're around neurons, they cushion them, 45 00:02:08,820 --> 00:02:11,090 they protect them, they're the support system. 46 00:02:11,090 --> 00:02:13,940 But the neurons are really the stars of the show. 47 00:02:13,940 --> 00:02:16,820 And more recently it's been shown that glia actually 48 00:02:16,820 --> 00:02:19,200 do a lot of really important things. 49 00:02:19,200 --> 00:02:22,570 So there's four types of glia. 50 00:02:22,570 --> 00:02:26,480 There's astrocytes, which do a ton of important stuff 51 00:02:26,480 --> 00:02:27,850 in the brain. 52 00:02:27,850 --> 00:02:32,690 Astrocytes are star shaped cells, thus the name. 53 00:02:37,430 --> 00:02:41,240 So astrocytes actually form syntaptic connections 54 00:02:41,240 --> 00:02:41,840 with neurons. 55 00:02:41,840 --> 00:02:44,360 So they actually can release a neurotransmitter, 56 00:02:44,360 --> 00:02:46,850 they can pick up neurotransmitters. 57 00:02:46,850 --> 00:02:49,670 They modulate-- have you guys heard 58 00:02:49,670 --> 00:02:51,870 of the blood-brain barrier? 59 00:02:51,870 --> 00:02:56,180 So for the most part your capillaries, your finest blood 60 00:02:56,180 --> 00:02:58,550 vessels, have kind of loose walls 61 00:02:58,550 --> 00:03:01,130 that nutrients and other stuff can go from your bloodstream 62 00:03:01,130 --> 00:03:03,850 into the cellular tissue. 63 00:03:03,850 --> 00:03:06,440 But in the brain, that's not true. 64 00:03:06,440 --> 00:03:09,950 The capillaries in the brain are joined by tight junctions. 65 00:03:09,950 --> 00:03:12,682 There's no space between them for things to creep through. 66 00:03:12,682 --> 00:03:14,390 There's actually very few things that you 67 00:03:14,390 --> 00:03:16,780 can ingest that will go from your bloodstream 68 00:03:16,780 --> 00:03:19,280 into your brain where all these delicate neurons are kicking 69 00:03:19,280 --> 00:03:21,890 around. 70 00:03:21,890 --> 00:03:24,422 And it's because of these astrocytes that they 71 00:03:24,422 --> 00:03:25,880 have little feet-- think if they're 72 00:03:25,880 --> 00:03:28,850 like a star-shaped cell, so all these little feet stick out. 73 00:03:28,850 --> 00:03:31,340 And line up right along the edge of the capillaries 74 00:03:31,340 --> 00:03:32,750 in your brain. 75 00:03:32,750 --> 00:03:35,150 And make them form these tight connections, 76 00:03:35,150 --> 00:03:36,560 these tight junctions. 77 00:03:36,560 --> 00:03:41,060 So that you don't get all of the random stuff 78 00:03:41,060 --> 00:03:44,750 you're eating on any given day messing up your nervous system. 79 00:03:44,750 --> 00:03:49,440 So astrocytes, they manage the blood-brain barrier. 80 00:03:49,440 --> 00:03:51,539 They change the amount of blood flow 81 00:03:51,539 --> 00:03:52,830 that's happening in your brain. 82 00:03:52,830 --> 00:03:56,330 So you guys probably know that one way that neuroscientists 83 00:03:56,330 --> 00:04:00,020 measure brain activity is by looking at how much blood 84 00:04:00,020 --> 00:04:01,550 flow is in the brain. 85 00:04:01,550 --> 00:04:03,770 And it's turning out that actually astrocytes, 86 00:04:03,770 --> 00:04:06,800 these support system cells, are what's 87 00:04:06,800 --> 00:04:08,540 controlling how much blood flow goes 88 00:04:08,540 --> 00:04:11,700 to different parts of your brain. 89 00:04:11,700 --> 00:04:14,240 So we've got astrocytes, these guys are pretty cool. 90 00:04:14,240 --> 00:04:16,573 There's lots of people doing cool research on these guys 91 00:04:16,573 --> 00:04:17,820 right now. 92 00:04:17,820 --> 00:04:22,070 And then there's two types of cells. 93 00:04:22,070 --> 00:04:28,370 One of them is Schwann cells. 94 00:04:28,370 --> 00:04:33,320 And one of them is oligodendroglia. 95 00:04:39,740 --> 00:04:45,350 And these both are cells that wrap around neurons 96 00:04:45,350 --> 00:04:48,500 with that tissue called myelin. 97 00:04:48,500 --> 00:04:50,840 And we'll talk about a myelin more in a little bit. 98 00:04:50,840 --> 00:04:53,240 But basically, a neuron that is myelinated 99 00:04:53,240 --> 00:04:56,960 is faster than a neuron that's unmyelinated. 100 00:04:56,960 --> 00:05:02,030 So these guys, Schwann cells in the peripheral nervous system 101 00:05:02,030 --> 00:05:06,240 and oligodendroglia in the central nervous system, 102 00:05:06,240 --> 00:05:09,200 are helping to speed up axonal transmission. 103 00:05:11,780 --> 00:05:15,590 And finally there's a set of glia called microglia 104 00:05:15,590 --> 00:05:18,797 that do immune reactions in the brain 105 00:05:18,797 --> 00:05:19,880 and in the nervous system. 106 00:05:23,720 --> 00:05:26,160 So glia, there's a lot of glia. 107 00:05:26,160 --> 00:05:28,410 There's more glia than neurons by mass 108 00:05:28,410 --> 00:05:30,100 in the central nervous system. 109 00:05:30,100 --> 00:05:32,830 But they're, for the most part, the support team. 110 00:05:32,830 --> 00:05:34,080 AUDIENCE: What's the last one? 111 00:05:34,080 --> 00:05:35,571 ABBY NOYCE: Microglia. 112 00:05:35,571 --> 00:05:38,560 AUDIENCE: What do they do? 113 00:05:38,560 --> 00:05:43,980 ABBY NOYCE: Microglia, they do inflammation. 114 00:05:43,980 --> 00:05:46,560 So if you get an injury of some kind and it's inflamed, 115 00:05:46,560 --> 00:05:48,300 there's more-- 116 00:05:48,300 --> 00:05:52,140 there's a signal that's sent out that brings your body's healing 117 00:05:52,140 --> 00:05:54,540 response to the area. 118 00:05:54,540 --> 00:05:56,590 Microglia do that in the central nervous system. 119 00:05:56,590 --> 00:06:00,927 They're also a big part of the immune response 120 00:06:00,927 --> 00:06:02,010 within the nervous system. 121 00:06:04,950 --> 00:06:07,880 So we've got four types of glia. 122 00:06:07,880 --> 00:06:14,210 And again, so the traditional view 123 00:06:14,210 --> 00:06:16,212 is that these guys are the supporting role. 124 00:06:16,212 --> 00:06:17,670 There's more information coming out 125 00:06:17,670 --> 00:06:20,010 showing that they're much more heavily involved 126 00:06:20,010 --> 00:06:22,109 in the sorts of elaborate processing 127 00:06:22,109 --> 00:06:24,150 that people think of as the nervous systems, what 128 00:06:24,150 --> 00:06:28,870 the nervous system does, than was originally thought. 129 00:06:28,870 --> 00:06:31,980 So besides glia-- 130 00:06:31,980 --> 00:06:34,618 I love sliding boards-- 131 00:06:34,618 --> 00:06:39,040 we'll put our glia up top. 132 00:06:39,040 --> 00:06:41,250 Then we have the rock stars of the nervous system. 133 00:06:41,250 --> 00:06:42,720 We get our neurons. 134 00:06:42,720 --> 00:06:45,010 So sketchy neuron diagram. 135 00:07:10,104 --> 00:07:11,050 OK. 136 00:07:11,050 --> 00:07:14,911 So we have a neuron. 137 00:07:14,911 --> 00:07:15,910 Neurons are fun to draw. 138 00:07:19,090 --> 00:07:24,220 So the main parts of a neuron that you should know about 139 00:07:24,220 --> 00:07:30,250 are, neurons have a kind of central cell body area. 140 00:07:30,250 --> 00:07:33,070 Here's our nucleus, it's how you know it's the cell body. 141 00:07:38,050 --> 00:07:39,640 And cell body has all of your kind 142 00:07:39,640 --> 00:07:40,942 of classic cellular structures. 143 00:07:40,942 --> 00:07:42,400 You've got your nucleus, you've got 144 00:07:42,400 --> 00:07:50,050 your Golgi apparatus, your ribosomes, lysosomes, 145 00:07:50,050 --> 00:07:52,757 all of this stuff, mitochondria. 146 00:07:52,757 --> 00:07:55,090 Mitochondria are kind of all over the place in a neuron, 147 00:07:55,090 --> 00:07:57,310 but they are there in the cell body. 148 00:07:57,310 --> 00:07:59,680 Another word for cell body is the soma, 149 00:07:59,680 --> 00:08:03,210 and you'll see that as well. 150 00:08:03,210 --> 00:08:06,430 So it's this kind of central-- it does all the stuff that 151 00:08:06,430 --> 00:08:07,630 keeps the cell alive. 152 00:08:12,350 --> 00:08:16,280 And then, so on this particular neuron, this is a neuron-- 153 00:08:16,280 --> 00:08:18,310 information on this neuron-- 154 00:08:18,310 --> 00:08:21,070 that's not mine. 155 00:08:21,070 --> 00:08:27,240 Information on this neuron is going this way. 156 00:08:27,240 --> 00:08:29,400 So around, on this side of the neuron, 157 00:08:29,400 --> 00:08:34,150 we've got our input, information input zone, 158 00:08:34,150 --> 00:08:35,860 with all of these little spines. 159 00:08:35,860 --> 00:08:37,808 These are the dendrites. 160 00:08:44,600 --> 00:08:45,760 They're all over the place. 161 00:08:45,760 --> 00:08:50,470 And the dendrites take in information from other neurons. 162 00:08:50,470 --> 00:08:53,256 Or if you're out by a sensory receptor, 163 00:08:53,256 --> 00:08:55,630 then they might take information from a sensory receptor, 164 00:08:55,630 --> 00:08:57,250 from the photo receptors in your eyes, 165 00:08:57,250 --> 00:08:59,260 or the touch receptors in your skin. 166 00:08:59,260 --> 00:09:05,399 Dendrites take information in, information goes this way. 167 00:09:05,399 --> 00:09:06,940 All of the information that comes in, 168 00:09:06,940 --> 00:09:09,990 you might have a neuron coming in here, 169 00:09:09,990 --> 00:09:16,090 and a neuron coming in here, all sending in different signals. 170 00:09:16,090 --> 00:09:20,450 And that information is integrated 171 00:09:20,450 --> 00:09:21,980 at the base of the axon here. 172 00:09:21,980 --> 00:09:25,250 This long tail is the axon. 173 00:09:25,250 --> 00:09:26,720 Information is integrated here. 174 00:09:34,980 --> 00:09:40,380 And so there's either an excitatory input 175 00:09:40,380 --> 00:09:42,750 or an inhibitory input coming in. 176 00:09:42,750 --> 00:09:44,800 The neuron adds all of these things up. 177 00:09:44,800 --> 00:09:47,640 And if it reaches a certain threshold amount, 178 00:09:47,640 --> 00:09:50,310 then it sends a nerve impulse-- 179 00:09:50,310 --> 00:09:52,890 shazam-- down the axon. 180 00:09:56,280 --> 00:09:57,300 And it fires. 181 00:09:57,300 --> 00:10:00,547 It releases neurotransmitter, a chemical-- 182 00:10:00,547 --> 00:10:02,130 there's a bunch of different chemicals 183 00:10:02,130 --> 00:10:04,020 that can be neurotransmitter-- 184 00:10:04,020 --> 00:10:05,880 out through these axon terminals. 185 00:10:08,430 --> 00:10:12,000 Where, hopefully, it's received by another neuron. 186 00:10:12,000 --> 00:10:14,400 It's the next one in the chain. 187 00:10:14,400 --> 00:10:17,430 So vo-ahm. 188 00:10:17,430 --> 00:10:22,510 Neurons send information. 189 00:10:22,510 --> 00:10:25,090 So the thing about neurons is that they 190 00:10:25,090 --> 00:10:28,690 use two different types of communication. 191 00:10:28,690 --> 00:10:33,647 They use an electrical signal within the cell. 192 00:10:33,647 --> 00:10:35,230 So when there's information coming in, 193 00:10:35,230 --> 00:10:37,690 and it's doing this integration and transmitting it 194 00:10:37,690 --> 00:10:41,320 along the axon, that's an electrical change. 195 00:10:41,320 --> 00:10:43,600 Then when it's communicating with other neurons 196 00:10:43,600 --> 00:10:46,800 at a synapse, at a gap between two neurons like this, 197 00:10:46,800 --> 00:10:49,400 it's using a chemical signaler. 198 00:10:53,700 --> 00:10:54,200 Questions? 199 00:10:54,200 --> 00:10:56,600 AUDIENCE: Can you say that again? 200 00:10:56,600 --> 00:10:57,410 ABBY NOYCE: Yes. 201 00:10:57,410 --> 00:11:01,280 So the nervous system uses two different types 202 00:11:01,280 --> 00:11:04,380 of signaling within a cell. 203 00:11:04,380 --> 00:11:10,280 So when it's taking input from other neurons and kind 204 00:11:10,280 --> 00:11:12,740 of trying to add it all up, at this integration zone, 205 00:11:12,740 --> 00:11:17,030 at the axon hillock here, it's using electrical signaling. 206 00:11:17,030 --> 00:11:20,270 Within the cell, it passes an electrical signal 207 00:11:20,270 --> 00:11:24,080 called the action potential down the axon. 208 00:11:24,080 --> 00:11:27,420 And once that electrical signal reaches the axon terminals, 209 00:11:27,420 --> 00:11:30,032 the very tips, we change modes. 210 00:11:30,032 --> 00:11:31,490 And it causes the neuron to release 211 00:11:31,490 --> 00:11:36,451 a chemical, a neurotransmitter, into the synapse. 212 00:11:36,451 --> 00:11:36,950 Synapse. 213 00:11:41,230 --> 00:11:43,610 So the synapse is that gap between one certain nerve 214 00:11:43,610 --> 00:11:44,990 cell and the next one. 215 00:11:44,990 --> 00:11:47,150 So the axon terminals of this nerve cell 216 00:11:47,150 --> 00:11:51,210 will scooch right onto the dendrites, usually, 217 00:11:51,210 --> 00:11:52,691 of the next one. 218 00:11:52,691 --> 00:11:54,440 And send more information along the chain. 219 00:11:59,687 --> 00:12:08,490 So let's talk about what actually 220 00:12:08,490 --> 00:12:10,790 happens at a synapse in a little bit more detail. 221 00:12:10,790 --> 00:12:11,790 We'll do that one first. 222 00:12:11,790 --> 00:12:14,164 And then we'll go back and we'll do the electrical stuff. 223 00:12:31,190 --> 00:12:35,670 So here's our axon. 224 00:12:35,670 --> 00:12:39,510 Here's our axon coming in to an axon terminal. 225 00:12:39,510 --> 00:12:41,420 I'll just draw one, although reality, 226 00:12:41,420 --> 00:12:44,310 an axon will have tons and tons and tons of axon terminals. 227 00:12:44,310 --> 00:12:47,190 An axon can have hundreds of these. 228 00:12:47,190 --> 00:12:51,320 And here is-- scroll up again. 229 00:12:51,320 --> 00:13:00,830 Let's say a dendrite of the next cell. 230 00:13:00,830 --> 00:13:03,350 So here's our synapse. 231 00:13:03,350 --> 00:13:07,280 So what's going to happen is the nerve 232 00:13:07,280 --> 00:13:09,500 impulse, the action potential, is going to come down 233 00:13:09,500 --> 00:13:12,720 the axon of the previous cell. 234 00:13:12,720 --> 00:13:14,450 And that's which kind of signaling, 235 00:13:14,450 --> 00:13:15,409 electrical or chemical? 236 00:13:15,409 --> 00:13:16,283 AUDIENCE: Electrical. 237 00:13:16,283 --> 00:13:18,470 ABBY NOYCE: Electrical, it's coming within the cell. 238 00:13:18,470 --> 00:13:24,210 When it gets here, it's going to trigger the cell-- 239 00:13:24,210 --> 00:13:25,190 back up a second. 240 00:13:25,190 --> 00:13:27,810 So cells have a membrane, right? 241 00:13:27,810 --> 00:13:30,060 High school biology, we're all good with this, a lipid 242 00:13:30,060 --> 00:13:31,384 bilayer. 243 00:13:31,384 --> 00:13:33,050 And so one of the things about membranes 244 00:13:33,050 --> 00:13:35,840 is that different kinds of ions can only go through a membrane 245 00:13:35,840 --> 00:13:37,790 if there's a channel for them there. 246 00:13:37,790 --> 00:13:39,290 So we're going to talk a bunch today 247 00:13:39,290 --> 00:13:41,780 about different kinds of channels in membranes 248 00:13:41,780 --> 00:13:43,710 and what makes them open and close. 249 00:13:43,710 --> 00:13:46,490 So this electrical potential comes along 250 00:13:46,490 --> 00:13:52,670 and it opens up a bunch of calcium channels, right here 251 00:13:52,670 --> 00:13:55,830 at the axon terminal. 252 00:13:55,830 --> 00:13:57,382 So we get our calcium. 253 00:13:57,382 --> 00:13:58,340 AUDIENCE: Calcium ions? 254 00:13:58,340 --> 00:13:59,630 ABBY NOYCE: Calcium ions, yes. 255 00:13:59,630 --> 00:14:01,880 Ca2+. 256 00:14:01,880 --> 00:14:06,620 And these calcium ions are going to go, fa-shoom, 257 00:14:06,620 --> 00:14:10,340 into the axon terminals. 258 00:14:10,340 --> 00:14:14,150 The electrical signal coming down the axon 259 00:14:14,150 --> 00:14:16,250 triggers this influx of calcium. 260 00:14:16,250 --> 00:14:19,190 Calcium is used all over the place in the nervous system 261 00:14:19,190 --> 00:14:22,100 as a signal of these different kinds. 262 00:14:22,100 --> 00:14:24,650 In this case, calcium is going to come in 263 00:14:24,650 --> 00:14:30,260 and it's going to cause the release of neurotransmitters. 264 00:14:30,260 --> 00:14:34,430 So neurotransmitter is actually usually produced 265 00:14:34,430 --> 00:14:40,010 in the cell body, where all the complicated cellular mechanisms 266 00:14:40,010 --> 00:14:42,120 for building things are. 267 00:14:42,120 --> 00:14:45,260 And it's going to be packaged up into little vesicles, 268 00:14:45,260 --> 00:14:47,700 into little packages. 269 00:14:47,700 --> 00:14:50,210 And so at any given point in time at the axon terminal, 270 00:14:50,210 --> 00:14:52,790 there's going to be a bunch of these little vesicles 271 00:14:52,790 --> 00:14:56,732 full of neurotransmitter hanging out. 272 00:14:56,732 --> 00:14:58,190 And when the calcium comes in, it's 273 00:14:58,190 --> 00:15:01,850 going to cause one of these to actually scooch over 274 00:15:01,850 --> 00:15:05,000 to the membrane and bind with it. 275 00:15:05,000 --> 00:15:08,275 So that the neurotransmitter is released into the synapse. 276 00:15:13,982 --> 00:15:16,442 AUDIENCE: So the calcium binds to membrane of the vesicle 277 00:15:16,442 --> 00:15:17,440 or the actual neuron? 278 00:15:17,440 --> 00:15:20,920 ABBY NOYCE: The calcium binds to a receptor 279 00:15:20,920 --> 00:15:25,000 that's on the vesicle membrane, which then tells the vesicle 280 00:15:25,000 --> 00:15:26,620 to scooch over next to the membrane 281 00:15:26,620 --> 00:15:29,805 and kind of merge with it and release the neurotransmitter. 282 00:15:32,449 --> 00:15:34,740 And that's about all the detail I can give you on that. 283 00:15:34,740 --> 00:15:37,280 My bio-chem is not where I would like it to be. 284 00:15:37,280 --> 00:15:38,821 It's one of the things I want to take 285 00:15:38,821 --> 00:15:40,340 this year, a little bit better. 286 00:15:40,340 --> 00:15:45,080 OK, so you get neurotransmitter released. 287 00:15:45,080 --> 00:15:47,510 And then what? 288 00:15:47,510 --> 00:15:51,140 Well, it's got to be noticed by the second cell. 289 00:15:51,140 --> 00:15:54,170 That this cell over here has to have 290 00:15:54,170 --> 00:15:56,870 a way of knowing that there is neurotransmitter 291 00:15:56,870 --> 00:15:58,520 in the synapse. 292 00:15:58,520 --> 00:16:02,190 So it's going to have receptor proteins. 293 00:16:02,190 --> 00:16:05,060 These are our neurotransmitter receptors. 294 00:16:05,060 --> 00:16:06,660 They're hanging out. 295 00:16:06,660 --> 00:16:08,420 AUDIENCE: Can we zoom in again? 296 00:16:08,420 --> 00:16:11,000 ABBY NOYCE: Can we zoom in again? 297 00:16:11,000 --> 00:16:12,890 Let's see. 298 00:16:12,890 --> 00:16:14,750 Here's our synapse. 299 00:16:14,750 --> 00:16:19,144 And we've had a vesicle do its little merging trick. 300 00:16:19,144 --> 00:16:20,810 So the neurotransmitter's been released. 301 00:16:25,571 --> 00:16:27,110 And what happens is-- 302 00:16:29,650 --> 00:16:32,150 and I can't draw this in a whole lot more detail. 303 00:16:32,150 --> 00:16:34,850 The way this works is think of receptor protein 304 00:16:34,850 --> 00:16:36,950 as being like a lock and a key, is the analogy 305 00:16:36,950 --> 00:16:38,280 that everybody uses. 306 00:16:38,280 --> 00:16:42,770 So when a particular molecule comes along, and in this case, 307 00:16:42,770 --> 00:16:45,150 it's this particular neurotransmitter, 308 00:16:45,150 --> 00:16:48,740 it's scooches right into a slot in the receptor protein that's 309 00:16:48,740 --> 00:16:51,560 just the right shape for it. 310 00:16:51,560 --> 00:16:54,080 And it causes something to happen. 311 00:16:54,080 --> 00:16:56,420 And depending on what kind of neurotransmitter 312 00:16:56,420 --> 00:16:58,940 and what kind of receptor we're looking at, 313 00:16:58,940 --> 00:17:01,040 you'll see different changes. 314 00:17:01,040 --> 00:17:04,490 So the neurotransmitter is released into the synapse. 315 00:17:04,490 --> 00:17:09,369 It binds to the receptor. 316 00:17:09,369 --> 00:17:12,069 Maybe it'll open a channel. 317 00:17:12,069 --> 00:17:14,440 That's a pretty typical response. 318 00:17:14,440 --> 00:17:18,819 Maybe, sometimes, it causes something else 319 00:17:18,819 --> 00:17:23,109 to be released from the receptor and into the cell 320 00:17:23,109 --> 00:17:25,630 and cause another string of chemical reactions. 321 00:17:31,961 --> 00:17:36,120 But the thing that's important is that in some way or another, 322 00:17:36,120 --> 00:17:38,670 this neurotransmitter that's released 323 00:17:38,670 --> 00:17:41,400 changes the probability of the next cell firing. 324 00:17:41,400 --> 00:17:44,850 It makes it more likely to fire or less likely to fire. 325 00:18:00,290 --> 00:18:04,940 So once it's so-- backing up from the beginning, 326 00:18:04,940 --> 00:18:06,920 electrical signal and action potential 327 00:18:06,920 --> 00:18:11,600 comes down the axon, triggers calcium, 328 00:18:11,600 --> 00:18:16,880 calcium ions to flow in to the axon terminal. 329 00:18:16,880 --> 00:18:19,490 The calcium causes these vesicles 330 00:18:19,490 --> 00:18:23,750 to bind with the membrane and release the neurotransmitter. 331 00:18:23,750 --> 00:18:27,560 The neurotransmitters is now floating around in the synapse. 332 00:18:27,560 --> 00:18:32,700 And it binds to a receptor on the postsynaptic cell. 333 00:18:32,700 --> 00:18:34,540 So people talk about at a synapse, 334 00:18:34,540 --> 00:18:36,290 you've got a presynaptic cell, the one who 335 00:18:36,290 --> 00:18:38,330 releases the neurotransmitter. 336 00:18:38,330 --> 00:18:42,140 And a postsynaptic cell, the one who has the receptors. 337 00:18:42,140 --> 00:18:44,390 And they cause some change to happen. 338 00:18:44,390 --> 00:18:47,840 Some change in the probability that the postsynaptic cell 339 00:18:47,840 --> 00:18:48,800 will in turn fire. 340 00:18:52,890 --> 00:18:57,440 AUDIENCE: It always has to do with the probability? 341 00:18:57,440 --> 00:18:59,982 ABBY NOYCE: It's not going to be 100% sure, because-- 342 00:18:59,982 --> 00:19:02,440 AUDIENCE: No, I mean, like it always has to do with whether 343 00:19:02,440 --> 00:19:03,790 or not the next one's? 344 00:19:03,790 --> 00:19:05,360 ABBY NOYCE: Yeah, it's going to change whether the next one 345 00:19:05,360 --> 00:19:05,780 fires. 346 00:19:05,780 --> 00:19:07,363 I mean, the exception to this is going 347 00:19:07,363 --> 00:19:09,830 to be like the sort of synapse that you 348 00:19:09,830 --> 00:19:12,830 see where our motor nerve comes down and hits a muscle, 349 00:19:12,830 --> 00:19:13,950 it's going to change. 350 00:19:13,950 --> 00:19:16,350 It's going to make the muscle contract, for example. 351 00:19:16,350 --> 00:19:18,969 So that's not quite a synapse, but very much like one. 352 00:19:18,969 --> 00:19:20,510 Again, it's a neurotransmitter that's 353 00:19:20,510 --> 00:19:22,970 released that affects the next thing. 354 00:19:22,970 --> 00:19:26,960 So yeah, whether or not they fire 355 00:19:26,960 --> 00:19:30,230 is basically how neurons process information. 356 00:19:30,230 --> 00:19:32,780 So each one gets all of this input 357 00:19:32,780 --> 00:19:35,600 and then decides whether or not it's 358 00:19:35,600 --> 00:19:37,790 going to send an action potential on 359 00:19:37,790 --> 00:19:39,510 down to the next neurons in its chain. 360 00:19:42,360 --> 00:19:45,680 So once it binds with the receptor, 361 00:19:45,680 --> 00:19:49,490 it's important that it not stay there forever. 362 00:19:49,490 --> 00:19:52,060 Neurons fire pretty quickly. 363 00:19:52,060 --> 00:19:55,940 The maximum rate of firing for a neuron 364 00:19:55,940 --> 00:19:58,280 is about 1,200 impulses a second. 365 00:20:01,100 --> 00:20:05,750 So it's important that this receptor does its thing. 366 00:20:05,750 --> 00:20:10,060 And then, fa-shoom, gets the transmitter out of there. 367 00:20:10,060 --> 00:20:13,530 To clear it out, let it kind of reset a little bit. 368 00:20:13,530 --> 00:20:17,150 And there's two main ways that that neurotransmitter 369 00:20:17,150 --> 00:20:19,520 is cleared out of the synapse. 370 00:20:19,520 --> 00:20:23,616 There are enzymes that break down neurotransmitters. 371 00:20:28,576 --> 00:20:30,550 AUDIENCE: On the outside? 372 00:20:30,550 --> 00:20:31,883 ABBY NOYCE: In the synapse, yep. 373 00:20:34,900 --> 00:20:39,536 So you might have an enzyme, this is a hungry enzyme. 374 00:20:39,536 --> 00:20:42,120 [ROARS SOFTLY] It's going to break down 375 00:20:42,120 --> 00:20:43,410 the neurotransmitter. 376 00:20:45,912 --> 00:20:47,370 The other thing that can happen is, 377 00:20:47,370 --> 00:20:49,770 you can actually have the original cell 378 00:20:49,770 --> 00:20:57,600 can have its own receptors for the neurotransmitter 379 00:20:57,600 --> 00:21:00,030 that actually [SUCKS] act like vacuum cleaners 380 00:21:00,030 --> 00:21:02,280 and slurp it back up, out of the synapses. 381 00:21:02,280 --> 00:21:04,170 This is called reuptake. 382 00:21:04,170 --> 00:21:07,170 So the neurotransmitter is reuptaken 383 00:21:07,170 --> 00:21:10,890 by the presynaptic cell, who then can just repackage it back 384 00:21:10,890 --> 00:21:14,280 into a vesicle and be ready to use it again 385 00:21:14,280 --> 00:21:16,290 the next time a neurotransmitter comes along. 386 00:21:16,290 --> 00:21:19,095 So there's a lot of recycling of neurotransmitter as well. 387 00:21:19,095 --> 00:21:21,420 AUDIENCE: Wait, so what's it called? 388 00:21:21,420 --> 00:21:24,360 ABBY NOYCE: That process is called the reuptake. 389 00:21:24,360 --> 00:21:26,408 AUDIENCE: Do all the neurotransmitters always 390 00:21:26,408 --> 00:21:29,630 get recycled? 391 00:21:29,630 --> 00:21:31,380 ABBY NOYCE: More or less, not all of them. 392 00:21:31,380 --> 00:21:32,940 So for any given neurotransmitter, 393 00:21:32,940 --> 00:21:35,400 you're going to usually see either an enzyme that breaks it 394 00:21:35,400 --> 00:21:36,350 down. 395 00:21:36,350 --> 00:21:38,350 And usually that will break it down all the way, 396 00:21:38,350 --> 00:21:40,320 it'll break it down to some precursor that is then 397 00:21:40,320 --> 00:21:41,500 brought back into the cell. 398 00:21:41,500 --> 00:21:45,380 And it's remanufactured from that precursor. 399 00:21:45,380 --> 00:21:46,800 But if it's reuptaken, then yeah, 400 00:21:46,800 --> 00:21:48,660 it will just get repackaged and used again. 401 00:22:01,440 --> 00:22:03,820 Oh hey, terminology, I forgot that. 402 00:22:03,820 --> 00:22:06,630 So there's two main types of receptors, like I said. 403 00:22:06,630 --> 00:22:09,740 One of the most common types of receptors 404 00:22:09,740 --> 00:22:14,580 are ionotropic receptors. 405 00:22:14,580 --> 00:22:17,627 And an ionotropic receptor opens an ion channel 406 00:22:17,627 --> 00:22:19,210 when the neurotransmitter binds to it. 407 00:22:26,090 --> 00:22:29,335 So one of the most common of these is-- 408 00:22:29,335 --> 00:22:33,010 let me see if I can get this right-- 409 00:22:33,010 --> 00:22:35,797 glutamate, which is a really common excitatory 410 00:22:35,797 --> 00:22:36,505 neurotransmitter. 411 00:22:36,505 --> 00:22:43,150 It has an ionotropic receptor that lets in calcium ions. 412 00:22:43,150 --> 00:22:45,940 So it lets calcium ions in. 413 00:22:45,940 --> 00:22:47,540 Again, you're getting an ion inflow 414 00:22:47,540 --> 00:22:49,900 through an ionotropic receptor. 415 00:22:49,900 --> 00:22:53,470 And then the receptors that caused these longer-term slower 416 00:22:53,470 --> 00:22:58,060 changes by releasing, usually, a protein that does-- 417 00:22:58,060 --> 00:23:00,760 then causes other cellular changes-- 418 00:23:00,760 --> 00:23:02,530 are called metabotropic receptors. 419 00:23:02,530 --> 00:23:04,390 And they're slower, but they tend 420 00:23:04,390 --> 00:23:07,336 to cause longer lasting changes in the cell. 421 00:23:07,336 --> 00:23:08,710 They might make it more sensitive 422 00:23:08,710 --> 00:23:10,750 to this neurotransmitter, or less sensitive, 423 00:23:10,750 --> 00:23:14,800 cause it to put out more receptors so it is more-- 424 00:23:14,800 --> 00:23:16,797 again, increasing the sensitivity. 425 00:23:47,670 --> 00:23:50,864 AUDIENCE: And then some would do both? 426 00:23:50,864 --> 00:23:52,280 ABBY NOYCE: Some neurotransmitters 427 00:23:52,280 --> 00:23:55,010 will have both types of receptors, 428 00:23:55,010 --> 00:23:56,870 but at any given synapse, you're only 429 00:23:56,870 --> 00:23:59,670 going to see one or the other. 430 00:23:59,670 --> 00:24:06,260 So like acetylcholine has both an ionotropic receptor type 431 00:24:06,260 --> 00:24:07,670 and a metabotropic receptor type, 432 00:24:07,670 --> 00:24:09,080 but you're not going to see that at the same synapse. 433 00:24:09,080 --> 00:24:11,690 You'll see some over here and some over there, 434 00:24:11,690 --> 00:24:13,865 usually, because they do different kinds of things. 435 00:24:25,100 --> 00:24:28,060 People feeling pretty OK about this? 436 00:24:28,060 --> 00:24:29,468 Cool. 437 00:24:29,468 --> 00:24:30,426 All right. 438 00:24:34,260 --> 00:24:37,890 So that makes something a neurotransmitter? 439 00:24:37,890 --> 00:24:40,500 If you are a brain scientist, or if you were a brain scientist 440 00:24:40,500 --> 00:24:42,720 50 years ago, more like, and you were 441 00:24:42,720 --> 00:24:47,100 looking at a substance that was existing in the brain. 442 00:24:47,100 --> 00:24:50,120 And you wanted to prove that it was a neurotransmitter. 443 00:24:50,120 --> 00:24:52,300 What sort of criteria might you look for? 444 00:25:02,657 --> 00:25:04,740 How would you know it's a neurotransmitter and not 445 00:25:04,740 --> 00:25:08,040 just something that is kicking around loose in the brain? 446 00:25:08,040 --> 00:25:09,300 Doing something else, maybe. 447 00:25:09,300 --> 00:25:10,440 Modulating something else. 448 00:25:16,128 --> 00:25:18,510 AUDIENCE: Because it came out of the cell? 449 00:25:18,510 --> 00:25:19,330 ABBY NOYCE: Good. 450 00:25:19,330 --> 00:25:21,460 So it's released by the presynaptic cell. 451 00:25:26,784 --> 00:25:28,620 AUDIENCE: It does something when it gets up 452 00:25:28,620 --> 00:25:32,180 to-- when it binds to the receptor on the next neuron. 453 00:25:32,180 --> 00:25:33,010 ABBY NOYCE: Yeah. 454 00:25:33,010 --> 00:25:34,590 So exactly. 455 00:25:34,590 --> 00:25:36,190 So when you stick it-- 456 00:25:36,190 --> 00:25:41,170 so if you take the substance and you put it on this neuron, 457 00:25:41,170 --> 00:25:44,125 then it changes how that neuron behaves. 458 00:25:59,050 --> 00:26:00,960 AUDIENCE: Can I just read the other two here? 459 00:26:00,960 --> 00:26:02,950 ABBY NOYCE: Sure. 460 00:26:02,950 --> 00:26:04,250 You've got [INAUDIBLE] 461 00:26:04,250 --> 00:26:07,050 AUDIENCE: Number three, after it has transmitted its signal 462 00:26:07,050 --> 00:26:10,240 to this neuron, it must be deactivated rapidly. 463 00:26:10,240 --> 00:26:11,924 ABBY NOYCE: Yeah, OK. 464 00:26:11,924 --> 00:26:14,340 AUDIENCE: And number four, it must have the same effect 465 00:26:14,340 --> 00:26:16,640 on the postsynaptic neuron. 466 00:26:16,640 --> 00:26:18,860 When applied experimentally, the signal 467 00:26:18,860 --> 00:26:21,822 is once secreted by presynaptic neuron. 468 00:26:21,822 --> 00:26:22,530 ABBY NOYCE: Good. 469 00:26:22,530 --> 00:26:23,946 OK, so another of thing to look at 470 00:26:23,946 --> 00:26:26,820 is whether the presynaptic neuron has the mechanism 471 00:26:26,820 --> 00:26:29,250 for synthesizing this molecule. 472 00:26:29,250 --> 00:26:32,900 Does it have the enzymes that it needs to do that? 473 00:26:57,170 --> 00:27:04,150 OK, so we've talked about what's happening at the synapse. 474 00:27:04,150 --> 00:27:09,220 Let's talk about how these chemicals, 475 00:27:09,220 --> 00:27:12,430 these neurotransmitters, actually are causing changes 476 00:27:12,430 --> 00:27:14,080 on these postsynaptic neurons. 477 00:27:23,040 --> 00:27:27,000 So getting back to this whole lipid bilayer membrane idea. 478 00:27:30,300 --> 00:27:33,157 Neurons have a bilayer. 479 00:27:33,157 --> 00:27:34,740 Remember, these are the cell membranes 480 00:27:34,740 --> 00:27:37,110 that you all probably saw in bio at some point. 481 00:27:37,110 --> 00:27:42,210 They look like head, two tails, head two tails, head two tails, 482 00:27:42,210 --> 00:27:43,226 two tails, head. 483 00:27:46,149 --> 00:27:47,940 All the lipids lined up against each other. 484 00:27:51,580 --> 00:27:55,170 Yes, hydrophilic heads and hydrophobic tails. 485 00:27:55,170 --> 00:27:57,180 So and it's those hydrophobic tails-- 486 00:27:57,180 --> 00:27:59,040 AUDIENCE: [INAUDIBLE] 487 00:27:59,040 --> 00:28:01,635 ABBY NOYCE: Ah, we were wondering. 488 00:28:01,635 --> 00:28:02,843 Come on in, Ms. [? Nagitt. ?] 489 00:28:02,843 --> 00:28:03,590 AUDIENCE: Sorry. 490 00:28:03,590 --> 00:28:04,548 ABBY NOYCE: No worries. 491 00:28:08,955 --> 00:28:10,072 AUDIENCE: [INAUDIBLE] 492 00:28:10,072 --> 00:28:11,030 ABBY NOYCE: No worries. 493 00:28:14,860 --> 00:28:17,500 So it's those hydrophobic tails that 494 00:28:17,500 --> 00:28:20,110 are the reason that ions, that charged particles, 495 00:28:20,110 --> 00:28:21,440 can't get through the membrane. 496 00:28:21,440 --> 00:28:22,856 Those hydrophobic tails don't want 497 00:28:22,856 --> 00:28:27,800 to interact with anything that's got a charge on it like that. 498 00:28:27,800 --> 00:28:36,790 So some ions in a neuron-- 499 00:28:36,790 --> 00:28:39,820 in a neuron there's some channels-- remember, 500 00:28:39,820 --> 00:28:43,540 we talked about ions can go through a membrane 501 00:28:43,540 --> 00:28:45,890 if there is a channel for them. 502 00:28:45,890 --> 00:28:52,200 So, for example, most of the time potassium, good old K+, 503 00:28:52,200 --> 00:28:55,450 potassium ions, there are potassium channels that are 504 00:28:55,450 --> 00:28:56,250 always open. 505 00:28:56,250 --> 00:29:00,040 Potassium can kind of go in and out of the neurons 506 00:29:00,040 --> 00:29:01,750 whenever it wants to. 507 00:29:01,750 --> 00:29:03,880 And potassium is a big player here. 508 00:29:06,742 --> 00:29:08,630 So potassium floats free. 509 00:29:08,630 --> 00:29:12,950 And then on the inside of the neuron, 510 00:29:12,950 --> 00:29:16,460 are a bunch of proteins, different kinds 511 00:29:16,460 --> 00:29:20,540 of proteins doing different things, that are negatively 512 00:29:20,540 --> 00:29:21,680 charged. 513 00:29:21,680 --> 00:29:24,830 Protein and ions. 514 00:29:24,830 --> 00:29:31,890 And outside, there tends to be a lot of sodium. 515 00:29:31,890 --> 00:29:36,140 And sodium is positively charged. 516 00:29:36,140 --> 00:29:39,960 And then potassium, free range, potassium 517 00:29:39,960 --> 00:29:41,630 can go wherever it wants. 518 00:29:41,630 --> 00:29:44,060 So there end up being two competing 519 00:29:44,060 --> 00:29:46,640 forces on the potassium. 520 00:29:49,810 --> 00:29:52,710 The potassium is positively charged, the inside of the cell 521 00:29:52,710 --> 00:30:00,360 is negatively charged, potassium wants to scooch itself inside. 522 00:30:00,360 --> 00:30:03,000 So if we're looking just at a cell membrane here-- 523 00:30:03,000 --> 00:30:10,680 here's our membrane-- in, out. 524 00:30:10,680 --> 00:30:13,880 Here's our sodiums, sodiums, lots of sodiums. 525 00:30:17,910 --> 00:30:20,808 Proteins inside. 526 00:30:27,490 --> 00:30:33,090 So there is an electrostatic force pushing the potassium 527 00:30:33,090 --> 00:30:36,914 into the cell, because the inside of the cell is negative 528 00:30:36,914 --> 00:30:38,580 and the outside of the cell is positive. 529 00:30:38,580 --> 00:30:41,430 So potassium, because it's positively charged, 530 00:30:41,430 --> 00:30:45,310 is getting pulled in by that negative charge 531 00:30:45,310 --> 00:30:47,650 on the inside of the cell. 532 00:30:47,650 --> 00:30:59,390 So we have an electrostatic gradient pulling potassium in. 533 00:30:59,390 --> 00:31:01,640 Because, again, there's open potassium channels. 534 00:31:01,640 --> 00:31:05,612 Potassium can pretty much drift freely across the membrane 535 00:31:05,612 --> 00:31:06,445 as much as it wants. 536 00:31:09,100 --> 00:31:14,170 But as potassium starts to come into the cell, 537 00:31:14,170 --> 00:31:16,180 there starts to be-- 538 00:31:16,180 --> 00:31:18,190 eventually, there's going to be more potassium 539 00:31:18,190 --> 00:31:20,620 inside the cell than outside. 540 00:31:20,620 --> 00:31:26,530 And at that point, you start to see osmotic pressure. 541 00:31:26,530 --> 00:31:28,120 There's also a force on the potassium 542 00:31:28,120 --> 00:31:30,460 to diffuse evenly throughout the intracellular 543 00:31:30,460 --> 00:31:32,830 and the extracellular spaces. 544 00:31:32,830 --> 00:31:34,220 So potassium starts coming in. 545 00:31:34,220 --> 00:31:36,410 But eventually, there's a diffusion gradient. 546 00:31:42,060 --> 00:31:44,880 So not all of the potassium ends up inside. 547 00:31:44,880 --> 00:31:47,130 Some amount of it has to stay outside. 548 00:31:47,130 --> 00:31:50,340 And what the neuron ends up with is this point 549 00:31:50,340 --> 00:31:55,620 where the electrostatic pull pulling the potassium in, 550 00:31:55,620 --> 00:31:59,850 and the diffusion pole pushing the potassium out 551 00:31:59,850 --> 00:32:05,296 are balanced at a certain ratio of charge and potassiums 552 00:32:05,296 --> 00:32:05,795 inside. 553 00:32:09,690 --> 00:32:11,715 AUDIENCE: Polarity? 554 00:32:11,715 --> 00:32:13,590 ABBY NOYCE: Yeah, at a certain concentration, 555 00:32:13,590 --> 00:32:14,381 you could say that. 556 00:32:16,992 --> 00:32:19,290 AUDIENCE: But this is just over the certain section 557 00:32:19,290 --> 00:32:20,082 of the axon? 558 00:32:20,082 --> 00:32:21,540 ABBY NOYCE: This is when the neuron 559 00:32:21,540 --> 00:32:24,710 is at rest, when it's not actively sending off an action 560 00:32:24,710 --> 00:32:25,210 potential. 561 00:32:25,210 --> 00:32:29,790 This is going to be true across the entire cell. 562 00:32:29,790 --> 00:32:33,090 So at rest, the potential, the difference 563 00:32:33,090 --> 00:32:37,335 in charge across the membrane, is about-- 564 00:32:37,335 --> 00:32:38,700 it depends on who you ask. 565 00:32:38,700 --> 00:32:41,740 We'll say minus 70 millivolts. 566 00:32:41,740 --> 00:32:44,760 So it's more negative inside than outside. 567 00:32:44,760 --> 00:32:48,150 And that's the point at which the potassium 568 00:32:48,150 --> 00:32:53,382 inside and outside are balanced across both these forces. 569 00:32:53,382 --> 00:32:56,100 It ranges usually between about minus 50 millivolts 570 00:32:56,100 --> 00:32:58,570 and minus 80 millivolts. 571 00:32:58,570 --> 00:33:01,010 There's differences for different kinds of neurons. 572 00:33:01,010 --> 00:33:03,030 There's differences for different species. 573 00:33:03,030 --> 00:33:05,370 Minus 70 is a good middle-of-the-road number. 574 00:33:08,310 --> 00:33:10,890 So this potential, this is called the resting potential. 575 00:33:17,960 --> 00:33:20,410 And again that potential word just 576 00:33:20,410 --> 00:33:22,060 refers to the difference in charge 577 00:33:22,060 --> 00:33:24,730 between the inside of the cell and the outside of the cell. 578 00:33:36,682 --> 00:33:37,678 We good? 579 00:33:47,650 --> 00:33:52,440 So remember, we talked about these ionotropic tropic 580 00:33:52,440 --> 00:33:54,240 receptor types. 581 00:33:54,240 --> 00:33:56,830 They open up ion channels. 582 00:33:56,830 --> 00:33:59,940 They're going to let other ions in or out of the cell. 583 00:33:59,940 --> 00:34:01,970 Usually, not a potassium channel. 584 00:34:01,970 --> 00:34:03,390 Letting more or less potassium in 585 00:34:03,390 --> 00:34:05,260 isn't going to make a big difference. 586 00:34:05,260 --> 00:34:09,570 But you might see ionotropic receptors 587 00:34:09,570 --> 00:34:13,320 that open sodium channels, that open chlorine, which 588 00:34:13,320 --> 00:34:18,030 is a negatively charged ion, channels that open calcium 589 00:34:18,030 --> 00:34:19,230 channels. 590 00:34:19,230 --> 00:34:22,260 And any of these ions coming in, being 591 00:34:22,260 --> 00:34:28,820 able to cross this membrane is going to change the potential. 592 00:34:28,820 --> 00:34:33,310 So if, for example, this neurotransmitter 593 00:34:33,310 --> 00:34:41,139 was a channel that let in sodium, then 594 00:34:41,139 --> 00:34:43,989 we're going to get some positive ions coming in. 595 00:34:43,989 --> 00:34:47,199 It might go from being minus 70 millivolts to, 596 00:34:47,199 --> 00:34:50,053 say, minus 60 millivolts. 597 00:34:50,053 --> 00:34:51,219 And that would be localized. 598 00:34:51,219 --> 00:34:55,610 That would be like right here, where the receptor is. 599 00:34:55,610 --> 00:34:58,540 And so one of the big ways in which receptors change 600 00:34:58,540 --> 00:35:00,930 the probability of the postsynaptic cell, 601 00:35:00,930 --> 00:35:03,880 the next cell firing, is by letting 602 00:35:03,880 --> 00:35:07,210 in these different ions that are changing 603 00:35:07,210 --> 00:35:11,000 the potential across the cell. 604 00:35:11,000 --> 00:35:15,430 And if this cell keeps firing lots and lots of times, 605 00:35:15,430 --> 00:35:17,009 more sodium is going to come in. 606 00:35:17,009 --> 00:35:18,550 More sodium's going to come in, maybe 607 00:35:18,550 --> 00:35:21,814 we're going to have sodium come in at this receptor. 608 00:35:21,814 --> 00:35:22,980 And it might change it more. 609 00:35:22,980 --> 00:35:28,090 It might make it minus 50 millivolts. 610 00:35:28,090 --> 00:35:29,280 And again, that's localized. 611 00:35:29,280 --> 00:35:32,560 So the difference is going to get weaker as it goes away 612 00:35:32,560 --> 00:35:33,509 from the synapse. 613 00:35:33,509 --> 00:35:36,050 So it might get all the way down to minus 50 millivolts right 614 00:35:36,050 --> 00:35:38,710 at the synapse, a little bit further it might still 615 00:35:38,710 --> 00:35:40,960 be minus 60 millivolts. 616 00:35:40,960 --> 00:35:43,630 Further away it might still be minus 70 millivolts. 617 00:35:48,330 --> 00:35:53,730 But if we've got lots of synapses 618 00:35:53,730 --> 00:35:56,954 all doing this at once-- 619 00:35:56,954 --> 00:35:58,800 we'll have another one come in-- 620 00:35:58,800 --> 00:36:03,810 and they're all sending in more sodium into this cell, 621 00:36:03,810 --> 00:36:06,774 then this cell is going to depolarize 622 00:36:06,774 --> 00:36:07,690 all over the membrane. 623 00:36:07,690 --> 00:36:09,090 It's going to become-- 624 00:36:09,090 --> 00:36:12,810 that potential difference is going to get smaller. 625 00:36:12,810 --> 00:36:17,470 And at this all-important integration zone, the axon 626 00:36:17,470 --> 00:36:21,160 hillock, there's a threshold. 627 00:36:21,160 --> 00:36:24,270 If the axon hillock gets to this threshold 628 00:36:24,270 --> 00:36:27,670 value of about minus 50 millivolts, 629 00:36:27,670 --> 00:36:32,280 then, shazam, the neuron sends off 630 00:36:32,280 --> 00:36:35,670 an action potential that goes flying down the axon 631 00:36:35,670 --> 00:36:39,720 and causes it to release neurotransmitter down 632 00:36:39,720 --> 00:36:41,284 at the axon terminals. 633 00:36:53,384 --> 00:36:56,095 AUDIENCE: And would these synapses all 634 00:36:56,095 --> 00:36:59,880 be from the same cell, or can they be from other? 635 00:36:59,880 --> 00:37:01,770 ABBY NOYCE: They could be from a whole bunch 636 00:37:01,770 --> 00:37:02,645 of different neurons. 637 00:37:02,645 --> 00:37:05,580 So these are clearly pretty simplified sketches. 638 00:37:05,580 --> 00:37:08,320 Like, a neuron like this, like if it's 639 00:37:08,320 --> 00:37:10,140 a pyramidal neuron in the cortex, 640 00:37:10,140 --> 00:37:13,150 could have inputs from hundreds different cells or more. 641 00:37:13,150 --> 00:37:16,660 The number of synapses in your brain is staggeringly huge. 642 00:37:16,660 --> 00:37:18,796 And I don't have it on hand today. 643 00:37:18,796 --> 00:37:20,780 AUDIENCE: Would they mix together? 644 00:37:20,780 --> 00:37:24,210 ABBY NOYCE: Yeah, so you'd get input from different cells. 645 00:37:24,210 --> 00:37:27,240 Usually, you'll see either cells that are all in the same region 646 00:37:27,240 --> 00:37:30,690 talking directly to each other, or cells 647 00:37:30,690 --> 00:37:34,072 bringing in input from far away and kind of integrating it. 648 00:37:34,072 --> 00:37:36,030 So you'll see cells doing both of those things, 649 00:37:36,030 --> 00:37:38,310 but usually not quite mixed together. 650 00:37:42,010 --> 00:37:44,632 AUDIENCE: Does the neurotransmitter have to come 651 00:37:44,632 --> 00:37:46,590 from all different-- from all of the dendrites, 652 00:37:46,590 --> 00:37:48,591 or could an action-- 653 00:37:48,591 --> 00:37:50,340 ABBY NOYCE: So it depends on a couple of-- 654 00:37:50,340 --> 00:37:51,790 AUDIENCE: --terminal in the right place? 655 00:37:51,790 --> 00:37:53,830 ABBY NOYCE: It depends on a couple of things. 656 00:37:53,830 --> 00:37:57,852 It depends on-- if just this one cell was firing, 657 00:37:57,852 --> 00:38:00,060 but it kept firing, and kept firing, and kept firing, 658 00:38:00,060 --> 00:38:01,560 kept firing, and so that sodium gets 659 00:38:01,560 --> 00:38:04,210 to keep coming in, and coming in, and coming in, 660 00:38:04,210 --> 00:38:08,700 then you'll get summation over time, sort of. 661 00:38:08,700 --> 00:38:12,600 And that could be enough to get this area to that threshold 662 00:38:12,600 --> 00:38:13,800 value. 663 00:38:13,800 --> 00:38:16,485 Or you could also see this one fires a little bit, 664 00:38:16,485 --> 00:38:18,360 and that one fires a little bit, and that one 665 00:38:18,360 --> 00:38:20,040 fires a little bit. 666 00:38:20,040 --> 00:38:22,950 And you get spatial summation, where you're adding up inputs 667 00:38:22,950 --> 00:38:24,960 from different parts of the cell, that 668 00:38:24,960 --> 00:38:27,750 bring it to the same thing. 669 00:38:27,750 --> 00:38:32,310 But, for example, what happens if maybe at this synapse, 670 00:38:32,310 --> 00:38:36,120 these receptors let in chlorine ions. 671 00:38:36,120 --> 00:38:37,312 What's going to happen? 672 00:38:37,312 --> 00:38:40,270 AUDIENCE: [INAUDIBLE] 673 00:38:40,270 --> 00:38:42,250 ABBY NOYCE: It gets more polarized, right. 674 00:38:42,250 --> 00:38:43,330 It gets hyperpolarized. 675 00:38:43,330 --> 00:38:46,490 It might go to minus 80 millivolts. 676 00:38:46,490 --> 00:38:49,660 So if you've got an inhibitory synapse, like this, 677 00:38:49,660 --> 00:38:51,710 it's making this cell less likely to fire. 678 00:38:51,710 --> 00:38:54,070 By making it hyperpolarized, by making 679 00:38:54,070 --> 00:38:58,750 it harder for an excitatory input 680 00:38:58,750 --> 00:39:01,810 to bring it all the way down to that threshold value. 681 00:39:01,810 --> 00:39:05,440 So this is how neurons integrate lots of information. 682 00:39:05,440 --> 00:39:07,360 They have inhibitory synapses that 683 00:39:07,360 --> 00:39:09,460 make them less likely to fire. 684 00:39:09,460 --> 00:39:12,950 And excitatory synapses that make them more likely to fire. 685 00:39:12,950 --> 00:39:15,460 Two different kinds of input coming in. 686 00:39:15,460 --> 00:39:18,640 And they add them up, and if, at any given point in time, 687 00:39:18,640 --> 00:39:23,186 the membrane potential at the axon hillock here-- 688 00:39:23,186 --> 00:39:26,070 this is the axon hillock. 689 00:39:26,070 --> 00:39:27,730 And it's this just kind of little bump, 690 00:39:27,730 --> 00:39:32,530 right where the axon starts going off from the cell. 691 00:39:32,530 --> 00:39:34,930 If it gets to minus 50 millivolts right there, 692 00:39:34,930 --> 00:39:37,120 then it triggers a chain of stuff 693 00:39:37,120 --> 00:39:41,275 that is the action potential. 694 00:39:41,275 --> 00:39:44,390 AUDIENCE: Is it always [INAUDIBLE] 50? 695 00:39:44,390 --> 00:39:45,640 ABBY NOYCE: It's pretty close. 696 00:39:45,640 --> 00:39:48,340 Again, there's a little bit of variation among the cells, 697 00:39:48,340 --> 00:39:50,380 and among species. 698 00:39:50,380 --> 00:39:53,710 It's usually about 20 to 25 millivolts less 699 00:39:53,710 --> 00:39:56,230 than the resting potential that's usually-- 700 00:39:56,230 --> 00:39:58,400 so you'll see a range in the resting potential 701 00:39:58,400 --> 00:40:01,712 and you'll see a correlating range in the threshold. 702 00:40:01,712 --> 00:40:04,308 AUDIENCE: So what [INAUDIBLE] the different types 703 00:40:04,308 --> 00:40:05,960 of synapses [INAUDIBLE]. 704 00:40:05,960 --> 00:40:08,370 ABBY NOYCE: So there's excitatory synapses 705 00:40:08,370 --> 00:40:13,310 that makes the cell more likely to fire by depolarizing it. 706 00:40:13,310 --> 00:40:16,290 So they're going to usually let in a positive ion that 707 00:40:16,290 --> 00:40:18,540 brings the potential of the cells 708 00:40:18,540 --> 00:40:20,640 closer to zero, closer to being even. 709 00:40:24,600 --> 00:40:27,500 And there's inhibitory synapses that make 710 00:40:27,500 --> 00:40:30,225 the cell less likely to fire. 711 00:40:30,225 --> 00:40:31,850 And usually they'll do that by bringing 712 00:40:31,850 --> 00:40:37,970 in a negatively charged ion that hyperpolarizes the membrane, 713 00:40:37,970 --> 00:40:41,930 makes it get even more extreme difference between the inside 714 00:40:41,930 --> 00:40:42,650 and the outside. 715 00:40:42,650 --> 00:40:45,770 So it makes it harder for it to reach that threshold 716 00:40:45,770 --> 00:40:48,020 value at the axon hillock. 717 00:40:48,020 --> 00:40:49,499 AUDIENCE: Couldn't they [INAUDIBLE] 718 00:40:49,499 --> 00:40:51,748 tell the cell to release positive [INAUDIBLE] already? 719 00:40:54,504 --> 00:40:55,170 ABBY NOYCE: Hmm? 720 00:40:55,170 --> 00:40:57,120 AUDIENCE: Tell the cell to release positives, 721 00:40:57,120 --> 00:40:59,862 and that would also bring it down. 722 00:40:59,862 --> 00:41:01,195 ABBY NOYCE: I'm sorry, I don't-- 723 00:41:01,195 --> 00:41:03,320 AUDIENCE: If the cell body releases positive ions 724 00:41:03,320 --> 00:41:06,920 instead of taking them in, that would also? 725 00:41:06,920 --> 00:41:09,250 ABBY NOYCE: Yeah, so you might-- 726 00:41:09,250 --> 00:41:12,160 AUDIENCE: That would be similar to if it took in a negative. 727 00:41:12,160 --> 00:41:14,620 ABBY NOYCE: Yeah, I don't know if that-- 728 00:41:14,620 --> 00:41:16,240 I do not know that that happens, which 729 00:41:16,240 --> 00:41:18,640 is not to say it doesn't, but it's not something 730 00:41:18,640 --> 00:41:19,980 that I know about. 731 00:41:19,980 --> 00:41:21,640 Because the two positive ions that 732 00:41:21,640 --> 00:41:24,500 are kind of at the center of how neurons work are sodium, 733 00:41:24,500 --> 00:41:25,600 which is-- 734 00:41:25,600 --> 00:41:27,820 the force is going to be pushing sodium in. 735 00:41:27,820 --> 00:41:29,437 And potassium, which doesn't really-- 736 00:41:29,437 --> 00:41:30,520 which is already balanced. 737 00:41:30,520 --> 00:41:33,549 So these aren't like active transport channels. 738 00:41:33,549 --> 00:41:35,590 They aren't using energy to pull these things in. 739 00:41:35,590 --> 00:41:38,920 They're just taking advantage of the gradient forces that 740 00:41:38,920 --> 00:41:42,070 want to push the ions into the cell already. 741 00:41:42,070 --> 00:41:45,910 So all the sodium's outside, the sodium wants to go in. 742 00:41:45,910 --> 00:41:48,119 The chlorine is all like-- the chlorine's all outside 743 00:41:48,119 --> 00:41:48,910 and wants to go in. 744 00:41:48,910 --> 00:41:51,130 The sodium has got both an electrical and a diffusion 745 00:41:51,130 --> 00:41:53,710 gradient pushing in the same direction. 746 00:41:53,710 --> 00:41:58,480 And when you have an action potential, 747 00:41:58,480 --> 00:42:00,170 then it takes advantage of that. 748 00:42:00,170 --> 00:42:02,260 But before we talk about that, let's 749 00:42:02,260 --> 00:42:06,170 take five minutes to get up, walk around, stretch. 750 00:42:06,170 --> 00:42:08,650 So we have an action potential, an electrical signal 751 00:42:08,650 --> 00:42:11,500 coming down the axon terminal. 752 00:42:11,500 --> 00:42:12,525 And then what happens? 753 00:42:16,216 --> 00:42:18,600 AUDIENCE: A calcium ion [INAUDIBLE].. 754 00:42:18,600 --> 00:42:19,350 ABBY NOYCE: Right. 755 00:42:19,350 --> 00:42:23,430 Voltage-gated calcium channels open up. 756 00:42:23,430 --> 00:42:26,526 Calcium goes where? 757 00:42:26,526 --> 00:42:28,260 [INTERPOSING VOICES] 758 00:42:28,260 --> 00:42:30,780 ABBY NOYCE: In, binds to the vesicles, good. 759 00:42:30,780 --> 00:42:33,450 What do the vesicles do, someone else? 760 00:42:33,450 --> 00:42:34,220 Someone else? 761 00:42:34,220 --> 00:42:36,510 AUDIENCE: Binds to the presynaptic membrane. 762 00:42:36,510 --> 00:42:39,250 ABBY NOYCE: Yeah , they merge with the membrane. 763 00:42:39,250 --> 00:42:44,835 And what do they do? 764 00:42:44,835 --> 00:42:46,710 AUDIENCE: The let go of the neurotransmitter. 765 00:42:46,710 --> 00:42:48,210 ABBY NOYCE: Yeah, they open up, they 766 00:42:48,210 --> 00:42:51,210 release the neurotransmitter into the synapse. 767 00:42:51,210 --> 00:42:53,986 And then what happens? 768 00:42:53,986 --> 00:42:54,944 AUDIENCE: [INAUDIBLE] 769 00:42:54,944 --> 00:42:55,652 ABBY NOYCE: Sure. 770 00:42:55,652 --> 00:42:58,776 AUDIENCE: The neurotransmitter binds to the receptor proteins 771 00:42:58,776 --> 00:43:00,700 of that postsynaptic neuron? 772 00:43:00,700 --> 00:43:01,854 ABBY NOYCE: Right. 773 00:43:01,854 --> 00:43:04,020 Usually on the dendrites of the postsynaptic neuron. 774 00:43:04,020 --> 00:43:07,700 But you'll also see synapses right on the cell body. 775 00:43:07,700 --> 00:43:11,150 You'll even see synapses onto like the axon terminals 776 00:43:11,150 --> 00:43:12,650 of other cells, that actually affect 777 00:43:12,650 --> 00:43:14,150 just how much neurotransmitter gets 778 00:43:14,150 --> 00:43:16,040 released and things like that. 779 00:43:16,040 --> 00:43:19,850 But the prototypical synapse is from the axon of one neuron 780 00:43:19,850 --> 00:43:21,830 to the dendrites of the next. 781 00:43:21,830 --> 00:43:24,844 So they'll bind to the receptors here. 782 00:43:24,844 --> 00:43:26,510 And what happens when a neurotransmitter 783 00:43:26,510 --> 00:43:27,343 binds to a receptor? 784 00:43:32,649 --> 00:43:35,194 AUDIENCE: It either causes ion channels to open, or-- 785 00:43:35,194 --> 00:43:35,860 [RINGTONE MUSIC] 786 00:43:35,860 --> 00:43:36,330 AUDIENCE: Oh my god. 787 00:43:36,330 --> 00:43:37,238 AUDIENCE: I'm sorry. 788 00:43:37,238 --> 00:43:40,013 AUDIENCE: --it causes more or less receptor proteins 789 00:43:40,013 --> 00:43:44,260 to appear on the membrane. 790 00:43:44,260 --> 00:43:46,950 ABBY NOYCE: Yeah, so it either causes ion channels to open 791 00:43:46,950 --> 00:43:50,760 or it triggers a cascade of biochemical effects 792 00:43:50,760 --> 00:43:53,070 that cause long-term changes, one of which 793 00:43:53,070 --> 00:43:56,640 can be creating more membrane proteins. 794 00:43:56,640 --> 00:43:58,920 So that depends on if it's an ionotropic 795 00:43:58,920 --> 00:44:03,480 or a metabotropic receptor. 796 00:44:03,480 --> 00:44:07,740 And if it lets in ions, what happens? 797 00:44:07,740 --> 00:44:12,630 What happens if it lets in positive ions, like sodium? 798 00:44:12,630 --> 00:44:14,520 What happens to the postsynaptic cell 799 00:44:14,520 --> 00:44:17,493 when those positive ions come in? 800 00:44:17,493 --> 00:44:20,860 AUDIENCE: The action potential depolarizes. 801 00:44:20,860 --> 00:44:23,220 ABBY NOYCE: The membrane potential, yeah. 802 00:44:23,220 --> 00:44:25,110 So it goes from its resting potential, 803 00:44:25,110 --> 00:44:27,660 it gets less polarized. 804 00:44:27,660 --> 00:44:30,000 And so is that excitatory or inhibitory? 805 00:44:30,000 --> 00:44:31,200 AUDIENCE: Excitatory? 806 00:44:31,200 --> 00:44:32,730 ABBY NOYCE: Yes, good. 807 00:44:32,730 --> 00:44:35,130 And if it's a negative ion, like chlorine, coming in? 808 00:44:38,164 --> 00:44:39,330 Then it's inhibitory, right. 809 00:44:39,330 --> 00:44:41,340 It's going to hyperpolarize the membrane, 810 00:44:41,340 --> 00:44:45,450 make the difference between outside and inside bigger. 811 00:44:45,450 --> 00:44:49,800 And make the neuron less likely to fire, because, of course, 812 00:44:49,800 --> 00:44:51,240 the neuron wants-- 813 00:44:51,240 --> 00:44:55,080 in order to fire, the membrane potential, right in here, 814 00:44:55,080 --> 00:44:58,140 has got to reach that threshold level of minus 50 millivolts. 815 00:45:00,860 --> 00:45:02,290 Cool. 816 00:45:02,290 --> 00:45:02,790 OK. 817 00:45:05,830 --> 00:45:11,770 So what happens-- if it reaches this minus 50 millivolt 818 00:45:11,770 --> 00:45:12,930 channel-- 819 00:45:12,930 --> 00:45:17,440 we are going to do a zoomed in on one of these boards. 820 00:45:17,440 --> 00:45:19,600 When it reaches this minus 50-- 821 00:45:19,600 --> 00:45:22,480 need more layers-- all right. 822 00:45:22,480 --> 00:45:25,390 When it reaches this minus 50 millivolt level-- 823 00:45:25,390 --> 00:45:28,525 so here's our axon. 824 00:45:28,525 --> 00:45:37,170 [EXHALES] All along the length of our axon 825 00:45:37,170 --> 00:45:42,350 are voltage-gated sodium channels. 826 00:45:42,350 --> 00:45:45,520 So they are channel proteins, like we've been talking about. 827 00:45:45,520 --> 00:45:48,910 The ion that they let through is sodium. 828 00:45:48,910 --> 00:45:51,550 And these guys open or close depending 829 00:45:51,550 --> 00:45:54,910 on what the potential, the voltage, across the membrane 830 00:45:54,910 --> 00:45:57,010 is. 831 00:45:57,010 --> 00:46:01,090 And this is why that minus 50 millivolt 832 00:46:01,090 --> 00:46:02,740 threshold is important. 833 00:46:02,740 --> 00:46:07,340 Because once the potential reaches minus 50 millivolts, 834 00:46:07,340 --> 00:46:12,100 this sodium channel-- 835 00:46:12,100 --> 00:46:14,200 and, of course, there's like a bajillion of these, 836 00:46:14,200 --> 00:46:15,880 right, there's tons of them-- 837 00:46:15,880 --> 00:46:20,634 opens, which way is sodium going to go, in or out? 838 00:46:20,634 --> 00:46:21,518 AUDIENCE: In. 839 00:46:21,518 --> 00:46:22,450 ABBY NOYCE: In, right. 840 00:46:22,450 --> 00:46:26,220 Sodium comes in. 841 00:46:26,220 --> 00:46:32,140 And because there's tons of these all along, 842 00:46:32,140 --> 00:46:37,370 lots and lots and lots of sodium comes in. 843 00:46:37,370 --> 00:46:39,130 So if you were measuring the potential 844 00:46:39,130 --> 00:46:41,890 across the membrane right here, with like an electrode 845 00:46:41,890 --> 00:46:44,090 on the inside, and an electrode on the outside, 846 00:46:44,090 --> 00:46:47,680 and measuring the difference between them, what you'd see 847 00:46:47,680 --> 00:46:48,770 is-- 848 00:46:48,770 --> 00:46:50,590 here's zero. 849 00:46:50,590 --> 00:46:54,360 Here's our resting potential, minus 70. 850 00:46:54,360 --> 00:46:56,047 You see the neuron just hanging out. 851 00:46:56,047 --> 00:46:57,880 And as input start coming, in it might go up 852 00:46:57,880 --> 00:47:03,430 to that all-important minus 50 threshold level. 853 00:47:03,430 --> 00:47:06,640 And then as the sodium comes flooding in, 854 00:47:06,640 --> 00:47:09,700 the potential is going to depolarize. 855 00:47:09,700 --> 00:47:12,880 It's going to go way past the zero line. 856 00:47:12,880 --> 00:47:15,310 It's actually going to go all the way up to about-- 857 00:47:20,030 --> 00:47:20,530 way up. 858 00:47:20,530 --> 00:47:25,090 It's going to go all the way up to about positive 50 859 00:47:25,090 --> 00:47:27,490 or 60 millivolts, in the other direction from it, 860 00:47:27,490 --> 00:47:33,650 because so much sodium just comes pouring in right at once. 861 00:47:33,650 --> 00:47:35,770 So it's this extreme depolarization. 862 00:47:35,770 --> 00:47:38,770 It actually flips the potential across the membrane. 863 00:47:53,090 --> 00:47:59,480 All right, and then these sodium channels are on a timer. 864 00:47:59,480 --> 00:48:04,250 They open very briefly, a few milliseconds, they close. 865 00:48:04,250 --> 00:48:09,830 But in the meantime, the cell is now full of sodium. 866 00:48:09,830 --> 00:48:14,914 And in the meantime, when we get this extreme depolarization, 867 00:48:14,914 --> 00:48:17,330 when it goes all the way to the interior of the cell being 868 00:48:17,330 --> 00:48:21,831 positive relative to the exterior, that triggers-- 869 00:48:21,831 --> 00:48:23,330 and they'd be right in here, but I'm 870 00:48:23,330 --> 00:48:25,330 going to draw them on the other side for space-- 871 00:48:25,330 --> 00:48:28,370 we're going to have another set of voltage triggered channels. 872 00:48:28,370 --> 00:48:31,550 These ones are for potassium. 873 00:48:31,550 --> 00:48:35,190 Good, old potassium. 874 00:48:35,190 --> 00:48:38,930 And so remember that potassium was at its equilibrium point 875 00:48:38,930 --> 00:48:40,670 when the neuron's at rest. 876 00:48:40,670 --> 00:48:44,330 It's balancing the electrostatic gradient pulling it in 877 00:48:44,330 --> 00:48:46,149 and the diffusion gradient pushing it out. 878 00:48:46,149 --> 00:48:47,690 But now with the interior of the cell 879 00:48:47,690 --> 00:48:50,910 is more positive than the exterior, 880 00:48:50,910 --> 00:48:54,620 the electrostatic gradient is pushing potassium out. 881 00:48:54,620 --> 00:48:57,920 The diffusion gradient is pushing potassium out. 882 00:48:57,920 --> 00:49:00,350 There's already more of them inside than outside. 883 00:49:00,350 --> 00:49:03,140 And so this flood of potassium-- 884 00:49:03,140 --> 00:49:10,050 gates open, potassium goes pouring out of the cell. 885 00:49:10,050 --> 00:49:14,750 And so this actually ends up bringing the spike back down. 886 00:49:14,750 --> 00:49:17,510 It dips a little bit down below the resting potential, 887 00:49:17,510 --> 00:49:18,980 because potassium is just going out 888 00:49:18,980 --> 00:49:22,266 so fast, with such extremeness. 889 00:49:22,266 --> 00:49:24,250 AUDIENCE: So it's like-- 890 00:49:24,250 --> 00:49:26,770 could it compare to simple harmonic motion, 891 00:49:26,770 --> 00:49:29,570 where if you displace a pendulum, 892 00:49:29,570 --> 00:49:31,500 it's not just going to stop at the bounds. 893 00:49:31,500 --> 00:49:33,333 It's going to move back the other direction. 894 00:49:33,333 --> 00:49:34,580 ABBY NOYCE: Yeah, sure. 895 00:49:34,580 --> 00:49:36,700 I don't know if that's a good par-- 896 00:49:36,700 --> 00:49:39,646 I don't know if the underlying principles are similar enough 897 00:49:39,646 --> 00:49:41,270 for that to be a good parallel, but you 898 00:49:41,270 --> 00:49:42,465 can think of it that way. 899 00:49:42,465 --> 00:49:44,772 AUDIENCE: What I mean is like, you can't expect 900 00:49:44,772 --> 00:49:46,230 it to stop at the bounds, it's just 901 00:49:46,230 --> 00:49:48,021 going to go a bit further than it needs to. 902 00:49:48,021 --> 00:49:49,420 ABBY NOYCE: Just a bit. 903 00:49:49,420 --> 00:49:49,920 Yeah. 904 00:49:49,920 --> 00:49:53,270 So it actually undershoots here. 905 00:49:53,270 --> 00:49:57,410 And again, just like these guys are on a timer, 906 00:49:57,410 --> 00:49:58,990 potassium channels are on a timer. 907 00:49:58,990 --> 00:50:01,100 So they're going to close. 908 00:50:01,100 --> 00:50:05,657 And then we're back to just our resting set. 909 00:50:05,657 --> 00:50:07,490 So remember, there's some potassium channels 910 00:50:07,490 --> 00:50:08,614 that are open all the time. 911 00:50:08,614 --> 00:50:11,640 Potassium can do some amount of moving back and forth. 912 00:50:11,640 --> 00:50:14,540 But this is like complete potassium permeability 913 00:50:14,540 --> 00:50:16,010 when this happens, so the potassium 914 00:50:16,010 --> 00:50:18,860 can move much faster than it can when the neuron's 915 00:50:18,860 --> 00:50:20,240 at its resting state. 916 00:50:20,240 --> 00:50:22,540 Yes? 917 00:50:22,540 --> 00:50:26,380 AUDIENCE: Won't it eventually mess up the gradient 918 00:50:26,380 --> 00:50:30,272 in terms of confusion, because there's more sodium inside? 919 00:50:30,272 --> 00:50:31,105 ABBY NOYCE: So, yes. 920 00:50:31,105 --> 00:50:33,460 So the final thing that happens is that 921 00:50:33,460 --> 00:50:39,090 there's a sodium-potassium pump along the membrane that takes-- 922 00:50:39,090 --> 00:50:41,130 sodium-potassium pump is going to take-- 923 00:50:41,130 --> 00:50:42,330 let's see. 924 00:50:42,330 --> 00:50:53,950 Three sodiums and two potassiums and swap them. 925 00:50:53,950 --> 00:50:56,690 And this is not taking advantage of the diffusion gradient. 926 00:50:56,690 --> 00:50:58,830 This actually takes energy. 927 00:50:58,830 --> 00:51:01,890 This is one of the ways that your neurons maintain 928 00:51:01,890 --> 00:51:04,740 that resting potential in its correct balance, 929 00:51:04,740 --> 00:51:07,200 is this sodium-potassium pump. 930 00:51:07,200 --> 00:51:10,410 Some people say that like more than half of the energy 931 00:51:10,410 --> 00:51:13,880 your brain uses is just that constant sodium 932 00:51:13,880 --> 00:51:17,570 out, potassium in, sodium out, potassium in, pump 933 00:51:17,570 --> 00:51:19,480 that's running all the time. 934 00:51:19,480 --> 00:51:30,820 So these guys reset the neuron back to its resting potential. 935 00:51:30,820 --> 00:51:33,580 Now the thing that happens is-- 936 00:51:33,580 --> 00:51:35,580 so we've talked about this happening right here, 937 00:51:35,580 --> 00:51:37,440 at the axon hillock. 938 00:51:37,440 --> 00:51:41,340 But this axon goes and goes and goes and goes and goes. 939 00:51:41,340 --> 00:51:45,000 And all along it-- we'll run it behind our graph there-- 940 00:51:45,000 --> 00:51:48,880 all along it are these voltage-gated sodium channels. 941 00:51:48,880 --> 00:51:50,980 So these sodium channels needed it 942 00:51:50,980 --> 00:51:55,530 to be depolarized past that minus 50 millivolts state. 943 00:51:55,530 --> 00:51:57,600 And then they bring in sodium. 944 00:51:57,600 --> 00:52:00,450 And the sodium is going to diffuse a little bit. 945 00:52:00,450 --> 00:52:03,090 And that means that the next receptor 946 00:52:03,090 --> 00:52:05,400 in the chain, this next piece, is 947 00:52:05,400 --> 00:52:08,290 going to be depolarized past that minus 50 millivolt 948 00:52:08,290 --> 00:52:08,790 threshold. 949 00:52:08,790 --> 00:52:11,210 So this one is going to open. 950 00:52:11,210 --> 00:52:14,340 And sodium goes in and it's going to diffuse a little bit. 951 00:52:14,340 --> 00:52:17,250 And then this one's going to open. 952 00:52:17,250 --> 00:52:19,380 And sodium is going to go in. 953 00:52:19,380 --> 00:52:21,750 And so this is how the signal gets 954 00:52:21,750 --> 00:52:25,020 propagated all the way down the length of the axon. 955 00:52:25,020 --> 00:52:29,870 Each set of voltage-gated sodium channels 956 00:52:29,870 --> 00:52:31,830 lets in sodium, which depolarizes 957 00:52:31,830 --> 00:52:34,320 the next chunk of the axon. 958 00:52:37,188 --> 00:52:41,030 AUDIENCE: So it's the same threshold for all-- 959 00:52:41,030 --> 00:52:42,334 ABBY NOYCE: All of these, yep. 960 00:52:42,334 --> 00:52:44,250 So the first place where you start seeing them 961 00:52:44,250 --> 00:52:45,990 is at the axon hillock there. 962 00:52:45,990 --> 00:52:48,450 That's where you first see these voltage-gated channels. 963 00:52:48,450 --> 00:52:53,370 And then all down the axon, they just keep staying there. 964 00:52:53,370 --> 00:52:56,070 All the way out to the axon terminals, where, remember, 965 00:52:56,070 --> 00:52:59,590 we have a voltage-gated calcium channel. 966 00:52:59,590 --> 00:53:04,430 So when this change in potential hits the terminals, 967 00:53:04,430 --> 00:53:08,190 it's that voltage across the membrane change that 968 00:53:08,190 --> 00:53:10,800 triggers calcium to come in to do 969 00:53:10,800 --> 00:53:12,885 the release of neurotransmitter at the synapse. 970 00:53:19,820 --> 00:53:22,685 Questions? 971 00:53:22,685 --> 00:53:26,180 Want to walk through it one more time? 972 00:53:26,180 --> 00:53:28,910 Yeah, OK. 973 00:53:28,910 --> 00:53:37,200 So at the axon hillock, we start to see these voltage-gated-- 974 00:53:37,200 --> 00:53:41,430 so they're triggered by the potential across the membrane-- 975 00:53:41,430 --> 00:53:42,400 sodium channels. 976 00:53:42,400 --> 00:53:45,900 They let positively charged sodium ions in. 977 00:53:45,900 --> 00:53:50,250 And they're triggered when you get to that minus 50 millivolts 978 00:53:50,250 --> 00:53:52,080 threshold potential. 979 00:53:52,080 --> 00:53:54,990 These guys start to open. 980 00:53:54,990 --> 00:53:59,910 And as they open, sodium comes pouring in. 981 00:53:59,910 --> 00:54:01,680 Tons and tons and tons of sodium gets 982 00:54:01,680 --> 00:54:05,690 shoved in to the inside of the axon. 983 00:54:05,690 --> 00:54:09,440 There's a diffusion gradient pushing the sodium in. 984 00:54:09,440 --> 00:54:11,980 The sodium is all outside, it wants to spread out evenly. 985 00:54:11,980 --> 00:54:14,730 So it's being pushed into the axon by that. 986 00:54:14,730 --> 00:54:17,940 There's also an electrostatic gradient pushing it in. 987 00:54:17,940 --> 00:54:21,720 Because at minus 50 millivolts, the inside 988 00:54:21,720 --> 00:54:23,230 is more negative than the outside. 989 00:54:23,230 --> 00:54:28,440 The positively charged ions are drawn to that negative charge. 990 00:54:28,440 --> 00:54:31,740 So sodium floods in. 991 00:54:31,740 --> 00:54:33,240 Then these guys close. 992 00:54:33,240 --> 00:54:37,650 They're only open for a little while and they close up again. 993 00:54:37,650 --> 00:54:42,630 But all of that sodium coming in depolarizes the neuron-- 994 00:54:42,630 --> 00:54:44,430 not just that mine is 50 millivolts 995 00:54:44,430 --> 00:54:46,000 that we were talking about before-- 996 00:54:46,000 --> 00:54:49,110 way up, so that the inside is at like plus 50. 997 00:54:49,110 --> 00:54:52,260 The inside is more positive than the outside because of all 998 00:54:52,260 --> 00:54:55,260 these positively charged sodium ions that came, 999 00:54:55,260 --> 00:54:57,350 shoom, coming in. 1000 00:55:01,470 --> 00:55:05,820 Then once it gets way up so that the inside is 1001 00:55:05,820 --> 00:55:08,340 more positive than the outside, we 1002 00:55:08,340 --> 00:55:10,596 have another set of voltage-gated channels. 1003 00:55:10,596 --> 00:55:11,970 And like I said, you're these are 1004 00:55:11,970 --> 00:55:13,678 going to be interspersed with these guys. 1005 00:55:13,678 --> 00:55:15,870 I just drew them down here to give us some talking 1006 00:55:15,870 --> 00:55:18,010 space, thinking space. 1007 00:55:18,010 --> 00:55:21,120 And these are potassium channels. 1008 00:55:21,120 --> 00:55:25,766 So even at rest, the membrane is kind of permeable to potassium. 1009 00:55:25,766 --> 00:55:27,390 There are some potassium channels open. 1010 00:55:27,390 --> 00:55:30,420 But it's going to diffuse slowly across the membrane. 1011 00:55:30,420 --> 00:55:33,570 But when these guys open up, when these voltage-gated 1012 00:55:33,570 --> 00:55:36,330 channels open up-- again, there's tons of them-- 1013 00:55:36,330 --> 00:55:39,710 potassium now has both a diffusion gradient, 1014 00:55:39,710 --> 00:55:42,510 there's more potassium inside than outside. 1015 00:55:42,510 --> 00:55:44,400 And an electrostatic gradient because now 1016 00:55:44,400 --> 00:55:47,910 the inside's positive, pushing these positive potassium 1017 00:55:47,910 --> 00:55:49,620 ions out. 1018 00:55:49,620 --> 00:55:56,616 So the potassium pours out through these channels. 1019 00:55:56,616 --> 00:56:01,210 And the potassium, so much potassium comes out of the cell 1020 00:56:01,210 --> 00:56:03,910 that it not only goes back down to minus 70 millivolts, 1021 00:56:03,910 --> 00:56:06,760 it actually goes a little bit below that. 1022 00:56:06,760 --> 00:56:09,970 It undershoots before coming back up 1023 00:56:09,970 --> 00:56:11,550 to the resting potential. 1024 00:56:16,250 --> 00:56:18,610 So after the sodium and potassium 1025 00:56:18,610 --> 00:56:22,450 have done their thing, we have a sodium-potassium pump 1026 00:56:22,450 --> 00:56:26,950 that uses energy, that actually pushes sodium out of the cell 1027 00:56:26,950 --> 00:56:27,850 and potassium in. 1028 00:56:27,850 --> 00:56:32,650 It's kind of cleaning up after this extreme membrane potential 1029 00:56:32,650 --> 00:56:34,720 thing that just happened. 1030 00:56:34,720 --> 00:56:38,770 So these guys are using ATP, using energy, 1031 00:56:38,770 --> 00:56:43,810 to push this stuff back into its proper resting alignment. 1032 00:56:43,810 --> 00:56:45,850 And then this has propagated all the way 1033 00:56:45,850 --> 00:56:48,410 down the length of the neuron because all 1034 00:56:48,410 --> 00:56:51,810 of these sodium channels that are making it go 1035 00:56:51,810 --> 00:56:53,305 are voltage-gated. 1036 00:56:53,305 --> 00:56:58,030 So each time the sodium comes in, it diffuses a little bit, 1037 00:56:58,030 --> 00:57:01,960 it depolarizes the next section of the membrane, 1038 00:57:01,960 --> 00:57:04,780 and that opens the next set of sodium gates, which 1039 00:57:04,780 --> 00:57:06,430 lets in more sodium, which depolarizes 1040 00:57:06,430 --> 00:57:08,620 the next section of membrane. 1041 00:57:08,620 --> 00:57:12,090 And it goes, shazam. 1042 00:57:21,630 --> 00:57:24,300 So when people talk about a neuron firing, usually 1043 00:57:24,300 --> 00:57:25,800 what they're really talking about is 1044 00:57:25,800 --> 00:57:27,765 a transmission of that-- 1045 00:57:27,765 --> 00:57:30,210 or a nerve impulse-- 1046 00:57:30,210 --> 00:57:32,160 they're talking about this action potential 1047 00:57:32,160 --> 00:57:37,220 that travels down the neuron's axon to the axon terminals. 1048 00:57:45,630 --> 00:57:46,902 Who thinks they get it? 1049 00:57:46,902 --> 00:57:48,610 Raise your hand, if you think you get it. 1050 00:57:48,610 --> 00:57:50,734 Raise your hand if you think you get it well enough 1051 00:57:50,734 --> 00:57:53,290 to explain it to somebody else. 1052 00:57:53,290 --> 00:57:56,260 All right, cool. 1053 00:57:56,260 --> 00:57:57,020 Reasonably good. 1054 00:58:01,920 --> 00:58:03,080 Anyone want to look over-- 1055 00:58:03,080 --> 00:58:05,279 AUDIENCE: How does it restore at the end? 1056 00:58:05,279 --> 00:58:08,501 When the potential [INAUDIBLE] 1057 00:58:08,501 --> 00:58:10,000 AUDIENCE: Is it because of the pump? 1058 00:58:10,000 --> 00:58:13,657 ABBY NOYCE: Yeah, the sodium-potassium pump 1059 00:58:13,657 --> 00:58:15,240 throws guys out, and these guys in, it 1060 00:58:15,240 --> 00:58:18,224 goes back to it's resting. 1061 00:58:18,224 --> 00:58:20,640 And so the sodium is all getting pushed out, and remember, 1062 00:58:20,640 --> 00:58:23,907 the potassium still has some amount of freedom of movement. 1063 00:58:23,907 --> 00:58:26,490 So this clears all the potassium out and then clears all the-- 1064 00:58:26,490 --> 00:58:27,810 AUDIENCE: Oh, is it the sodium [INAUDIBLE] 1065 00:58:27,810 --> 00:58:29,040 and the potassium coming in. 1066 00:58:29,040 --> 00:58:31,734 ABBY NOYCE: Sodium goes out, potassium comes in. 1067 00:58:31,734 --> 00:58:33,150 So as the sodium gets cleared out, 1068 00:58:33,150 --> 00:58:35,520 the potassium can float around and get itself back 1069 00:58:35,520 --> 00:58:38,430 to its original resting potential. 1070 00:58:38,430 --> 00:58:40,380 Really, the important thing this pump is doing 1071 00:58:40,380 --> 00:58:42,463 is getting all that sodium back out of your cells. 1072 00:58:51,502 --> 00:58:53,960 So one of the important things-- back to glia for a minute. 1073 00:58:53,960 --> 00:58:57,340 One of the really important thing that astrocytes do 1074 00:58:57,340 --> 00:59:02,560 is they monitor the levels of potassium floating around 1075 00:59:02,560 --> 00:59:05,980 in the extracellular fluid, outside between the cells. 1076 00:59:05,980 --> 00:59:09,940 Because if you think about it, when a neuron fires, 1077 00:59:09,940 --> 00:59:12,390 this potassium all comes pouring out of the neuron 1078 00:59:12,390 --> 00:59:14,200 and into the extracellular fluid, 1079 00:59:14,200 --> 00:59:15,880 into the space between them. 1080 00:59:15,880 --> 00:59:19,270 And that could mess up the other neurons around it, 1081 00:59:19,270 --> 00:59:20,920 because the resting potential is based 1082 00:59:20,920 --> 00:59:23,325 on the balance of potassium. 1083 00:59:23,325 --> 00:59:25,450 So if all of a sudden, we dump a lot more potassium 1084 00:59:25,450 --> 00:59:28,200 into the extracellular fluid, then 1085 00:59:28,200 --> 00:59:30,340 the balance between the diffusion 1086 00:59:30,340 --> 00:59:32,800 gradient and the electrostatic gradient is going to change. 1087 00:59:32,800 --> 00:59:34,909 The potential of those neurons is going to change. 1088 00:59:34,909 --> 00:59:36,200 And it could all get messed up. 1089 00:59:36,200 --> 00:59:39,250 So astrocytes actually soak up some of that potassium 1090 00:59:39,250 --> 00:59:40,564 and release it back. 1091 00:59:40,564 --> 00:59:42,784 AUDIENCE: Wouldn't that pump actually depolarize 1092 00:59:42,784 --> 00:59:51,720 the cell, because you're letting more positive ions out than in? 1093 00:59:51,720 --> 00:59:57,050 ABBY NOYCE: Yes, but because potassium can diffuse slowly, 1094 00:59:57,050 --> 01:00:02,780 but freely, then its electrostatic gradient 1095 01:00:02,780 --> 01:00:04,736 pulls in some more potassium to balance it. 1096 01:00:08,046 --> 01:00:08,920 Like we talked about. 1097 01:00:08,920 --> 01:00:10,794 So there's a diffusion gradient on potassium, 1098 01:00:10,794 --> 01:00:13,600 pushing it out of the cell at rest. 1099 01:00:13,600 --> 01:00:15,909 And an electrostatic gradient that pulls it in. 1100 01:00:15,909 --> 01:00:17,950 So this would have those off-balance for a while, 1101 01:00:17,950 --> 01:00:20,158 so that you'd have the electrostatic gradient pulling 1102 01:00:20,158 --> 01:00:23,851 in more potassium than is being pushed out by the diffusion. 1103 01:00:23,851 --> 01:00:24,350 Yeah? 1104 01:00:24,350 --> 01:00:28,580 AUDIENCE: Would people being under the influence of drugs, 1105 01:00:28,580 --> 01:00:30,940 would that affect the way that's being done? 1106 01:00:30,940 --> 01:00:31,690 ABBY NOYCE: Yeah. 1107 01:00:31,690 --> 01:00:37,600 So most drugs that affect the way your brain works actually 1108 01:00:37,600 --> 01:00:40,990 work at the synapse. 1109 01:00:40,990 --> 01:00:44,230 They'll either act like a neurotransmitter at a synapse 1110 01:00:44,230 --> 01:00:47,615 or they'll block a synapse so that its neurotransmitter 1111 01:00:47,615 --> 01:00:48,115 can't-- 1112 01:00:51,362 --> 01:00:52,070 train of thought. 1113 01:00:52,070 --> 01:00:53,050 AUDIENCE: Bind to the receptor? 1114 01:00:53,050 --> 01:00:53,560 ABBY NOYCE: Yeah, so its neurotransmitter 1115 01:00:53,560 --> 01:00:54,640 can't bind properly. 1116 01:00:54,640 --> 01:00:56,320 They'll get in that spot, they'll 1117 01:00:56,320 --> 01:01:00,010 stick well enough to fit in that spot in the receptor. 1118 01:01:00,010 --> 01:01:01,570 But they won't have some key property 1119 01:01:01,570 --> 01:01:03,202 necessary to actually activate it. 1120 01:01:03,202 --> 01:01:04,868 AUDIENCE: Would affects would that have? 1121 01:01:04,868 --> 01:01:06,700 Would it just wait until the drug wore off? 1122 01:01:06,700 --> 01:01:07,780 ABBY NOYCE: Depends. 1123 01:01:07,780 --> 01:01:10,810 So there's actually-- ever gotten like Novocaine 1124 01:01:10,810 --> 01:01:12,520 at the dentist or something? 1125 01:01:12,520 --> 01:01:14,110 Novocaine works. 1126 01:01:14,110 --> 01:01:16,660 It's only localized, but it works 1127 01:01:16,660 --> 01:01:23,270 by actually blocking those voltage-gated sodium channels. 1128 01:01:23,270 --> 01:01:27,130 So the pain nerves in that area can't send signals 1129 01:01:27,130 --> 01:01:29,590 because they have no way of propagating an action 1130 01:01:29,590 --> 01:01:30,830 potential. 1131 01:01:30,830 --> 01:01:33,190 So your pain receptors are jumping up and down going, 1132 01:01:33,190 --> 01:01:35,890 oh my god, somebody is cutting holes in your mouth. 1133 01:01:35,890 --> 01:01:43,150 But this signal cannot get to your brain because these gates 1134 01:01:43,150 --> 01:01:46,140 are blocked by the Novocaine. 1135 01:01:46,140 --> 01:01:49,810 So the sodium can't come in, the action potential can't happen, 1136 01:01:49,810 --> 01:01:52,210 the information from your pain receptors 1137 01:01:52,210 --> 01:01:54,990 just doesn't get there. 1138 01:01:54,990 --> 01:01:58,350 AUDIENCE: That's cool. 1139 01:01:58,350 --> 01:01:59,670 ABBY NOYCE: Yeah. 1140 01:01:59,670 --> 01:02:00,460 It's cool. 1141 01:02:00,460 --> 01:02:03,770 Best class I took in college with psychopharmacology. 1142 01:02:03,770 --> 01:02:05,520 Talks about all different classes of drugs 1143 01:02:05,520 --> 01:02:07,570 and what they do, different neurons. 1144 01:02:10,090 --> 01:02:11,860 Not just illegal drugs, like psych drugs, 1145 01:02:11,860 --> 01:02:15,270 like antidepressants or anti-anxiety drugs. 1146 01:02:15,270 --> 01:02:16,420 All sorts of cool stuff. 1147 01:02:16,420 --> 01:02:20,590 Another one that interferes with these, with the sodium gates, 1148 01:02:20,590 --> 01:02:23,560 is the toxin that gets into shellfish 1149 01:02:23,560 --> 01:02:25,540 that are exposed to red tide. 1150 01:02:25,540 --> 01:02:30,100 So red tide is an algae that blooms periodically. 1151 01:02:30,100 --> 01:02:33,130 And then shellfish are filter feeders, so they eat the algae. 1152 01:02:33,130 --> 01:02:36,040 And there's a toxin in it and it's not just localized, 1153 01:02:36,040 --> 01:02:38,560 like Novocaine, it gets all your neurons. 1154 01:02:38,560 --> 01:02:40,690 So like the neurons that go to your muscles 1155 01:02:40,690 --> 01:02:42,820 that tell you to breathe. 1156 01:02:42,820 --> 01:02:43,667 Yeah, not good. 1157 01:02:43,667 --> 01:02:45,750 This one will kill you and that is how it does it. 1158 01:02:48,880 --> 01:02:52,810 OK so the other thing I wanted to talk about, really quickly, 1159 01:02:52,810 --> 01:02:57,850 is we talked about Schwann cells and oligodendroglia 1160 01:02:57,850 --> 01:03:01,630 or oligodendrocytes, some people will call them. 1161 01:03:01,630 --> 01:03:05,860 And what these guys do is they myelinate axons. 1162 01:03:05,860 --> 01:03:09,400 They produce this fatty substance called myelin 1163 01:03:09,400 --> 01:03:10,840 inside their cells. 1164 01:03:10,840 --> 01:03:15,050 And they wrap themselves around an axon. 1165 01:03:15,050 --> 01:03:17,470 So an axon that is myelinated-- 1166 01:03:17,470 --> 01:03:20,310 so this is going to be like the white matter in your brain. 1167 01:03:20,310 --> 01:03:22,600 Or the long-distance nerves that go out 1168 01:03:22,600 --> 01:03:25,047 through most of your peripheral nervous system, that 1169 01:03:25,047 --> 01:03:25,630 is myelinated. 1170 01:03:25,630 --> 01:03:30,850 And it's going to have this stuff coating it. 1171 01:03:30,850 --> 01:03:33,017 And it leaves these little gaps. 1172 01:03:33,017 --> 01:03:34,850 But it's actually going to be able to do one 1173 01:03:34,850 --> 01:03:35,280 all the way around. 1174 01:03:35,280 --> 01:03:36,910 It's actually going to be wrapped 1175 01:03:36,910 --> 01:03:38,190 all the way around the axon. 1176 01:03:43,370 --> 01:03:44,415 So this is myelin. 1177 01:03:47,390 --> 01:03:49,280 People usually refer to it as a myelin sheath 1178 01:03:49,280 --> 01:03:52,470 that coats the axon. 1179 01:03:52,470 --> 01:03:53,870 So again, Schwann cells are doing 1180 01:03:53,870 --> 01:03:56,120 this in the peripheral nervous system, like your motor 1181 01:03:56,120 --> 01:04:00,980 and sensory nerves, where some of them, the faster ones. 1182 01:04:00,980 --> 01:04:04,490 And oligodendroglia are doing it in the central nervous system 1183 01:04:04,490 --> 01:04:06,500 in your brain and spinal cord. 1184 01:04:06,500 --> 01:04:09,530 And what happens with this stuff is, 1185 01:04:09,530 --> 01:04:12,980 if we look at how we were just showing how transmission works, 1186 01:04:12,980 --> 01:04:17,780 with all of these channels along the ends, 1187 01:04:17,780 --> 01:04:21,890 on a myelinated neuron, conduction is faster. 1188 01:04:21,890 --> 01:04:27,210 Because what happens is you get a channel here, 1189 01:04:27,210 --> 01:04:30,050 or a bunch of channels here, letting in sodium. 1190 01:04:30,050 --> 01:04:33,830 And then instead of having to open channels all the way along 1191 01:04:33,830 --> 01:04:41,130 here, it's just diffused, and you get a change in potential. 1192 01:04:41,130 --> 01:04:49,351 So the conduction actually hops between the bits of sheath. 1193 01:04:49,351 --> 01:04:50,725 AUDIENCE: So it would take longer 1194 01:04:50,725 --> 01:04:53,850 if you had to wait for all the ions 1195 01:04:53,850 --> 01:04:56,610 to diffuse all the way down to the end. 1196 01:04:56,610 --> 01:04:58,864 ABBY NOYCE: It's faster. 1197 01:04:58,864 --> 01:05:01,530 And I think this is a hole in my physics understanding of what's 1198 01:05:01,530 --> 01:05:02,130 going on. 1199 01:05:02,130 --> 01:05:04,064 AUDIENCE: [INAUDIBLE] localized, so 1200 01:05:04,064 --> 01:05:07,630 that you can have [INAUDIBLE] more dramatic changes 1201 01:05:07,630 --> 01:05:08,649 in potential? 1202 01:05:08,649 --> 01:05:10,440 ABBY NOYCE: Yeah, maybe because you've only 1203 01:05:10,440 --> 01:05:12,773 got a few spots where you can see that change happening, 1204 01:05:12,773 --> 01:05:13,980 it's all concentrated. 1205 01:05:13,980 --> 01:05:14,700 I don't know. 1206 01:05:14,700 --> 01:05:16,860 I feel like it's faster. 1207 01:05:16,860 --> 01:05:19,260 It's a lot faster. 1208 01:05:19,260 --> 01:05:21,141 It's called saltatory conduction, 1209 01:05:21,141 --> 01:05:22,890 which is from the Latin word for to dance, 1210 01:05:22,890 --> 01:05:25,110 which is kind of cool. 1211 01:05:25,110 --> 01:05:28,920 Because it's like this little potential change skipping down 1212 01:05:28,920 --> 01:05:31,132 the axon, boink-boink-boink. 1213 01:05:38,700 --> 01:05:40,710 So yeah, it's faster because it hops 1214 01:05:40,710 --> 01:05:42,900 between each of these places. 1215 01:05:42,900 --> 01:05:45,060 And the physics of why that happens 1216 01:05:45,060 --> 01:05:46,770 have never quite made sense to me. 1217 01:05:46,770 --> 01:05:48,269 And I was looking at it again today, 1218 01:05:48,269 --> 01:05:51,960 and just being like, OK, I don't get it, but I believe you. 1219 01:05:51,960 --> 01:05:54,210 So if anyone manages to read it and figure it out, 1220 01:05:54,210 --> 01:05:55,800 please let me know. 1221 01:05:55,800 --> 01:06:01,215 AUDIENCE: I think it's similar with, say, we had a chain of 1222 01:06:01,215 --> 01:06:02,125 [INAUDIBLE]. 1223 01:06:02,125 --> 01:06:05,040 So it's going from this side to the other. 1224 01:06:05,040 --> 01:06:07,770 And then, say, had a race. 1225 01:06:07,770 --> 01:06:11,050 Have a chain of students versus just one kid. 1226 01:06:11,050 --> 01:06:13,489 The one kid has to run to touch this wall, 1227 01:06:13,489 --> 01:06:15,530 and then run across the room to get to that wall. 1228 01:06:15,530 --> 01:06:17,904 The other kids just to have to slap each others' hands as 1229 01:06:17,904 --> 01:06:21,260 soon as they get the previous. 1230 01:06:21,260 --> 01:06:23,240 ABBY NOYCE: Yeah. 1231 01:06:23,240 --> 01:06:25,100 AUDIENCE: That's a good analogy. 1232 01:06:25,100 --> 01:06:27,150 ABBY NOYCE: Sure. 1233 01:06:27,150 --> 01:06:28,950 So anyway, it's faster because it only 1234 01:06:28,950 --> 01:06:31,230 has to do it these individual sections, 1235 01:06:31,230 --> 01:06:35,162 it doesn't have to do it along the whole stretch of the axon. 1236 01:06:35,162 --> 01:06:39,210 AUDIENCE: Also, if you had the axon completely covered 1237 01:06:39,210 --> 01:06:40,744 with the-- 1238 01:06:40,744 --> 01:06:41,910 ABBY NOYCE: With the myelin. 1239 01:06:41,910 --> 01:06:43,775 AUDIENCE: No, with the channels. 1240 01:06:43,775 --> 01:06:44,760 ABBY NOYCE: Ah, OK. 1241 01:06:44,760 --> 01:06:45,940 AUDIENCE: That would like-- 1242 01:06:45,940 --> 01:06:47,315 that would mean that you wouldn't 1243 01:06:47,315 --> 01:06:50,670 have just sections, but as soon as there 1244 01:06:50,670 --> 01:06:54,900 was a certain potential, it'd start immediately 1245 01:06:54,900 --> 01:06:56,730 taking in the sodium. 1246 01:06:56,730 --> 01:06:59,140 And that would actually-- the effect 1247 01:06:59,140 --> 01:07:02,260 would diffuse down the axon, as it goes further and further. 1248 01:07:02,260 --> 01:07:05,069 It's like diminishing returns. 1249 01:07:05,069 --> 01:07:06,360 ABBY NOYCE: Yeah, I don't know. 1250 01:07:06,360 --> 01:07:08,750 Maybe I'll look at it again tonight. 1251 01:07:08,750 --> 01:07:11,330 So saltatory conduction in myelinated neurons, 1252 01:07:11,330 --> 01:07:14,750 myelinated neurons are faster than unmyelinated neurons. 1253 01:07:14,750 --> 01:07:17,494 Have you ever noticed that you stub your toe, 1254 01:07:17,494 --> 01:07:19,160 and you get two different kinds of pain. 1255 01:07:19,160 --> 01:07:21,070 You go, oh my gosh, ow, that hurts. 1256 01:07:21,070 --> 01:07:23,660 And it's really fast and then it goes away very quickly. 1257 01:07:23,660 --> 01:07:25,285 And then there's this second like, ooh, 1258 01:07:25,285 --> 01:07:26,180 ow, that really hurt. 1259 01:07:26,180 --> 01:07:28,388 Like there's these two very different characteristics 1260 01:07:28,388 --> 01:07:31,230 of pain that you get for any kind of sharp injury like that. 1261 01:07:31,230 --> 01:07:34,640 So the first kind of pain comes from one kind of pain receptor. 1262 01:07:34,640 --> 01:07:36,930 And it's on these very fast myelinated nerves. 1263 01:07:36,930 --> 01:07:40,040 So it comes to your brain first. 1264 01:07:40,040 --> 01:07:41,960 We don't need a demonstration. 1265 01:07:41,960 --> 01:07:47,440 And the second kind is on these slower nerves. 1266 01:07:47,440 --> 01:07:49,340 So it doesn't get to your brain as quickly, 1267 01:07:49,340 --> 01:07:51,320 but it doesn't adapt. 1268 01:07:51,320 --> 01:07:54,210 The first kind is like, it tends to adapt. 1269 01:07:54,210 --> 01:07:56,480 So your pain receptors-- it's like, OK, OK, 1270 01:07:56,480 --> 01:07:58,610 we know we know it hurt, and it goes away. 1271 01:07:58,610 --> 01:08:01,089 It just stops listening to the pain receptors. 1272 01:08:01,089 --> 01:08:02,630 The second kind says, nope, I'm going 1273 01:08:02,630 --> 01:08:03,920 to keep telling the brain about it, 1274 01:08:03,920 --> 01:08:05,253 keep telling the brain about it. 1275 01:08:05,253 --> 01:08:07,812 So it doesn't get there as fast, but it lingers a lot longer. 1276 01:08:07,812 --> 01:08:09,770 And so that's a difference between a myelinated 1277 01:08:09,770 --> 01:08:12,130 and an unmyelinated nerve carrying the same message. 1278 01:08:15,220 --> 01:08:17,703 All right, anyone have questions? 1279 01:08:24,021 --> 01:08:24,520 Cool. 1280 01:08:24,520 --> 01:08:26,420 AUDIENCE: So is it the myelinated 1281 01:08:26,420 --> 01:08:28,149 that lingers, or is it the? 1282 01:08:28,149 --> 01:08:30,609 ABBY NOYCE: No, the myelinated is fast. 1283 01:08:30,609 --> 01:08:33,060 And those are usually-- 1284 01:08:33,060 --> 01:08:36,172 and those are, in this case, in that pain transmitter case, 1285 01:08:36,172 --> 01:08:37,630 in particular, those are the nerves 1286 01:08:37,630 --> 01:08:43,090 that adapt to the continued input 1287 01:08:43,090 --> 01:08:46,060 and start ignoring it pretty quickly. 1288 01:08:46,060 --> 01:08:48,964 And then these other neurons are slower, 1289 01:08:48,964 --> 01:08:50,130 but they're more persistent. 1290 01:08:50,130 --> 01:08:51,970 They're just like nope, keep telling brain. 1291 01:08:51,970 --> 01:08:52,761 Keep telling brain. 1292 01:08:52,761 --> 01:08:54,040 Yup, it still hurts. 1293 01:08:54,040 --> 01:08:55,560 Yep, it still hurts. 1294 01:08:55,560 --> 01:08:58,547 Hi, your toe still hurts. 1295 01:08:58,547 --> 01:09:00,380 You guys know what I'm talking about, right? 1296 01:09:03,080 --> 01:09:04,630 OK.