The Nervous System, Part 2 - Action! Potential!: Crash Course Anatomy & Physiology #9 — Transcript

Full transcript

1. 0:00What if everything you did, and thought, and felt could be communicated by pushing a button?
2. 0:04It’d be like using the world’s simplest app -- one that just sends out a little ping,
3. 0:07always at the same volume and length -- to communicate everything from, “It sure is
4. 0:11cold in here,” to, “I love churros,” to, “Boy, I sure would like to breathe sometime soon.”
5. 0:16Well, that is actually exactly how your neurons send ALL the impulses responsible for every
6. 0:22one of your actions, thoughts, and emotions.
7. 0:24When a neuron is stimulated enough, it fires an electrical impulse that zips down its axon
8. 0:29to its neighboring neurons.
9. 0:31But they’ve only got one signal that they can send, and it only transmits at one uniform
10. 0:36strength and speed.
11. 0:37What they can vary is the frequency or number of pulses -- like this [buzz buzz buzz] is
12. 0:42distinct from this [buzz buzz buzz buzz buzz buzz buzz].
13. 0:45And your brain can translate these signals, reading them like binary code, organizing
14. 0:49them by location, sensation, magnitude, and importance, so that you know the difference
15. 0:52between “turn up the thermostat” and “Oh my gosh I’m on fire.”
16. 0:55That buzz, that nerve impulse, is called the action potential.
17. 0:59It’s one of the most fundamental aspects of anatomy and physiology, and really life in general.
18. 1:03It’s happening inside of you right now. And we want to make sure that you understand
19. 1:06what all that buzz is about.
20. 1:18Before we delve into how neurons communicate, we’ve first got to understand a little bit
21. 1:21of our old friend electricity.
22. 1:23Basically, think of your body as a sack of batteries.
23. 1:25NO, I mean, you don’t look like a sack of batteries, I’m just saying that, your body
24. 1:29as a whole is electrically neutral, with equal amounts of positive and negative charges floating
25. 1:33around. But certain areas are more positively or negatively charged than others.
26. 1:37And because opposite charges attract, we need barriers, or membranes, to keep positive and
27. 1:41negative charges separate until we’re ready to use the energy that their attraction creates.
28. 1:46In other words, we keep ‘em separated to build potential.
29. 1:49A battery just sitting on its own has both a positive and negative end, and the potential
30. 1:53to release energy. But it doesn’t do anything until it’s hooked up to a flashlight or
31. 1:58a phone or a kids’ toy that lets those charges move toward each other, on the way converting
32. 2:03electricity into light, or sound, or children’s laughter.
33. 2:06In much the same way, each neuron in your body is like its own little battery with its
34. 2:09own separated charges.
35. 2:11It just needs an event to trigger the action that brings those charges together.
36. 2:15If you’re thinking that this sounds more like engineering than anatomy, that might
37. 2:18not be a bad thing. It might even help to think of your neurons in the same terms an
38. 2:22electrician might use.
39. 2:23Voltage, for example, is the measure of potential energy generated by separated charges. It’s
40. 2:28measured in volts, but in the case of your body, we use millivolts because it’s a pretty small amount.
41. 2:33In a cell, we refer to this difference in charge as the membrane potential. The bigger
42. 2:37the difference between the positive and negative areas, the higher the voltage, and the larger the potential.
43. 2:42And just like there’s voltage in your body, there’s also current -- the flow of electricity
44. 2:46from one point to another. The amount of charge in a current is related both to its voltage and its resistance.
45. 2:51Resistance is just whatever’s getting in the way of the current. Something with a high
46. 2:55resistance is an insulator, like plastic, and something with a low resistance is a conductor, like metal.
47. 3:00Now, when we talk about these concepts in terms of you, we’re typically talking about
48. 3:04how currents indicate the flow of positively or negatively charged ions across the resistance
49. 3:09of your cells’ membranes.
50. 3:11And again, these membranes separate the charges, so they’re what provide the potential to
51. 3:16convert the electricity into something useful.
52. 3:18K, now that we’ve got Electricity 101 down, let's see how it works inside your nervous system.
53. 3:22A resting neuron is like a battery just sitting in that sack that is you. When it’s just
54. 3:28sitting there, it’s more negative on the inside of the cell, relative to the extracellular
55. 3:31space around it.
56. 3:32This difference is known as the neuron’s resting membrane potential, and it sits at
57. 3:36around -70 millivolts.
58. 3:37Where do those charges come from?
59. 3:39Outside of a resting neuron, there’s a bunch of positive sodium ions floating around, just
60. 3:44lingering outside the membrane.
61. 3:45Inside, the neuron holds potassium ions that are positive as well, but they’re mingled
62. 3:50with bigger, negatively-charged proteins. And since there are more sodium ions outside
63. 3:55than there are potassium ions inside, the cell’s interior has an overall negative charge.
64. 4:00When a neuron has a negative membrane potential like this, it is said to be polarized.
65. 4:04Now, these ions didn’t just show up in this arrangement on their own. This is all orchestrated
66. 4:08by one of the most important bits of machinery in your nervous system, the sodium-potassium pump.
67. 4:14This little protein straddles the membrane of the neuron, and there are tons of them
68. 4:18all along the axon. For every two potassium ions it pumps into the cell, it pumps out
69. 4:22three sodium ions.
70. 4:24This creates a difference in the concentration of sodium and potassium, and a difference
71. 4:27in charges -- making it more positive outside the neuron.
72. 4:31This difference is an electrochemical gradient, and you probably know enough about biology
73. 4:35by now to know that NATURE HATES GRADIENTS! It wants to even out all of those inequalities,
74. 4:41in concentration and in charge, to restore balance.
75. 4:44But the only way to even out that gradient, is for the ions to pass across the membrane.
76. 4:48Thankfully, the sodium-potassium pump isn’t the only way in or out of the cell -- the
77. 4:52membrane is also riddled with ion channels, large proteins that can provide safe passage
78. 4:57across the membrane, when their respective gates are open.
79. 5:00And these gates open and close for different reasons, depending on their structure and purpose.
80. 5:05Most are voltage-gated channels, which open at certain membrane potentials, and close
81. 5:09at others. For example, sodium channels in your neurons like to open around -55 mV.
82. 5:14But some others are ligand gated channels -- they only open up when a specific neurotransmitter,
83. 5:19like serotonin, or a hormone latches on to it.
84. 5:21And then we also have mechanically gated channels, which open in response to
85. 5:25physically stretching the membrane.
86. 5:27In any case, when the gates do open, ions quickly diffuse across that membrane down
87. 5:31their electrochemical gradient, evening out the concentrations, and running away from
88. 5:35other positively charged ions.
89. 5:37This movement of ions is the key to all electrical events in neurons, and thus is the force behind
90. 5:42every. single. thing. we think, do, and feel.
91. 5:45Of course, not all of your body’s electrical responses are the same. And neither are the
92. 5:48flows of ions going in and out of your neurons.
93. 5:50If only a few channels open, and only a bit of sodium enters the cell, that causes just
94. 5:55a little change in the membrane potential in a localized part of the cell. This is called
95. 5:59a graded potential.
96. 6:00But in order to send long-distance signals all the way along an axon, you need a bigger
97. 6:04change -- one big enough to trigger those voltage-gated channels.
98. 6:07That is an action potential!
99. 6:09And your best bet for making that happen is to depolarize that resting neuron -- I mean,
100. 6:14cause a big enough change in its membrane potential that it’ll trigger the voltage-gated
101. 6:18channels to open.
102. 6:19It all starts with your neuron sitting there at resting state. All of the ion channels
103. 6:23are closed, and the inner voltage is resting at -70 mV.
104. 6:26And then something happens! Some environmental stimulus occurs -- say like a spider brushes
105. 6:31up against a tiny hair on your knee -- triggering those sodium channels to open, increasing
106. 6:35the charge inside the membrane.
107. 6:37Now, the stimulus -- and the resulting change -- have to be strong enough to cross a threshold
108. 6:41for the true action potential to kick in and that threshold is about -55 mV.
109. 6:46Remember that number. Because this is an all-or- nothing phenomenon. If the stimulus is too weak, and
110. 6:51the change doesn’t hit that level, it’s like a false alarm -- the neuron just returns
111. 6:55to its resting state.
112. 6:56But kind of like Doc Brown hitting 1.21 gigawatts in the Delorean, once it hits that threshold
113. 7:01-- you’re not going to travel in time, but you are going to see some serious action potential.
114. 7:04At that threshold, the voltage-gated sodium channels open, and there are tons of these,
115. 7:09so all of the positive sodium ions rush in, making the cell massively depolarized -- so
116. 7:14much so that it actually goes positive, up to about positive 40 mV.
117. 7:18This is action potential in … action.
118. 7:21It’s really just a temporary reversal of a membrane potential -- a brief depolarization
119. 7:26caused by changes in currents.
120. 7:27And unlike graded potentials, which are small and localized, an action potential kicks off
121. 7:32a biological chain reaction, which sends that electrical signal down the axon.
122. 7:36Because each of your neurons has lots of voltage-gated sodium channels. So when a few in one area
123. 7:40open, that local current is strong enough to change the voltage around them. And that
124. 7:45triggers their neighbors, which triggers the voltage around them, and so on down the line.
125. 7:49As soon as all that’s underway, the process of repolarization kicks in. This time the
126. 7:53voltage-gated potassium ion channels open up, letting those potassium ions flow out,
127. 7:58in an attempt to rebalance the charges.
128. 8:00If anything, it goes too far at first, and the membrane briefly goes through hyperpolarization:
129. 8:05Its voltage drops to -75 or so mV, before all of the gates close and the sodium-potassium
130. 8:11pumps take over and bring things back to their resting level.
131. 8:14Now when part of an axon is in the middle of all this, and its ion channels are open,
132. 8:18it can’t respond to any other stimulus, no matter how strong. This is called the refractory
133. 8:23period, and it’s there to help prevent signals from traveling in both directions down the
134. 8:27axon at once.
135. 8:28So that is the surprisingly simple app that your nervous system uses to let you experience the world.
136. 8:34And because the voltages in this process are always pretty much the same -- the initial threshold
137. 8:37around -55 mV, and the peak at depolarization at +40 mV -- your neurons only communicate
138. 8:43in a single, monotone buzz.
139. 8:45It doesn’t matter if it’s a spider on your knee or an elephant, a paper cut or stab
140. 8:50wound, the strength of that action potential is always the same.
141. 8:54What does change is the frequency of the buzz.
142. 8:56A weak stimulus tends to trigger less frequent action potentials. And that includes if the
143. 9:01stimulus is coming from you, like your brain telling your muscles to perform some task.
144. 9:04If I need to do something delicate, like pick up an egg, the signal is low-frequency: [buzz...buzz...buzz...]
145. 9:10But a more intense signal -- like trying to crush a can -- increases the frequency of
146. 9:14those action potentials to tell your muscles to contract harder, and the message turns
147. 9:17into something that you can’t ignore -- [buzzbuzzbuzzbuzz]
148. 9:20Action potentials also vary by speed, or conduction velocity.
149. 9:24They’re fastest in pathways that govern things like reflexes, for example, but they’re
150. 9:27slower in places like your glands, guts, and blood vessels.
151. 9:30And the factor that affects a neuron’s transmission speed the most, is whether there’s a myelin
152. 9:34sheath on its axon.
153. 9:35Axons coated in insulating myelin conduct impulses faster than non-myelinated ones,
154. 9:40partly because, instead of just triggering one channel at a time in a chain reaction,
155. 9:44a current can effectively leap from one gap in the myelin to the next.
156. 9:48These little gaps are the delightfully named Nodes of Ranvier, and this kind of propagation
157. 9:53is known as saltatory conduction, from the Latin word for “leaping.”
158. 9:57But what happens when an action potential hits the end of its axon and is ready to do
159. 10:01more than leap … and jump all the way to another neuron?
160. 10:04That you will find out next time!
161. 10:06Today you learned how your body is kinda like a big bag o’ batteries, and how ion channels
162. 10:10in your neurons regulate this electrochemistry to create an action potential, from resting
163. 10:15state to depolarization to repolarization and a brief bout of hyperpolarization.
164. 10:20Thanks for watching, especially to all of our Subbable subscribers, who make Crash Course
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167. 10:28This episode was written by Kathleen Yale. The script was edited by Blake de Pastino,
168. 10:32and our consultant is Dr. Brandon Jackson. It was directed by Nicholas Jenkins and Michael
169. 10:36Aranda, and our graphics team is Thought Café.
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