POTENCIAL DE MEMBRANA 1: Definición y factores que lo establecen — Transcript

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1. 0:09How’s it going? Welcome to Medical
2. 0:11Led. Today we will talk about a very
3. 0:14important topic to continue with this
4. 0:16video program that we have practically
5. 0:18dedicated to electrophysiology, and the
6. 0:21topic today is the membrane potential.
7. 0:24This concept is super important to know
8. 0:26before getting into all the mechanisms
9. 0:28and processes. related to the action
10. 0:31potential. First, what is the membrane
11. 0:33potential? What does this concept refer
12. 0:36to? Here is the definition for you. The
13. 0:39membrane potential is the difference
14. 0:41between the intracellular charges
15. 0:43represented by the Greek letter psi.
16. 0:47Psi i means psi of inside, that is,
17. 0:49inside the cell. And psi obviously has
18. 0:53to do with the difference in relation
19. 0:56to the extracellular charges, which is
20. 0:59noted as psi of outside. Right, the
21. 1:03membrane potential will depend on
22. 1:05several factors. The most important are
23. 1:08these three here, which I have put in
24. 1:10bullet points and that we will discuss
25. 1:13little by little. The first one,
26. 1:16intracellular electronegativity. For
27. 1:20this, I left you a drawing that will
28. 1:23help us understand why
29. 1:24electronegativity is involved here in
30. 1:27the membrane potential. Well, the
31. 1:30drawing basically represents a neuron.
32. 1:33The neuron has a body, which would be
33. 1:36this part here, right? It also has
34. 1:39something we call dendrites, which are
35. 1:41these extensions that allow it to
36. 1:43receive signals from the outside
37. 1:45environment or from other neurons as
38. 1:47well, okay? From the soma comes what is
39. 1:51called an axon, which is practically
40. 1:53this extension of the neuron that ends
41. 1:55in what would be a synaptic button,
42. 1:57right? If we were to zoom in on the
43. 2:01axon membrane here, we would find
44. 2:04something like this. Right. We have
45. 2:08here, if you notice, a blue line that
46. 2:10delimits two spaces. An extracellular
47. 2:15space, which would be this one here,
48. 2:18and an intracellular space. Okay? Both
49. 2:22inside and outside the cell we have
50. 2:24different charges, and the charges are
51. 2:26provided precisely by the positive and
52. 2:28negative ions present. For example,
53. 2:31here on the intracellular side we have
54. 2:33a positive charge that is representing
55. 2:35a potassium ion, right? This other one
56. 2:38that is negative could be representing
57. 2:39a phosphate. This positive one here
58. 2:42could be another potassium and so on,
59. 2:43right? Same thing out here, look, out
60. 2:45here there is a positive charge that
61. 2:47could be sodium, right? We know that
62. 2:49sodium is the most abundant cation in
63. 2:51this space. We can also have negative
64. 2:53charges, such as what? Like chlorine.
65. 2:55Okay? And if you notice, in most of
66. 2:58these regions, both inside and outside,
67. 3:00the charges are practically balanced.
68. 3:03There is a positive, a negative, a
69. 3:05positive, and a negative. However, as
70. 3:08we get right up to the lipid membrane,
71. 3:11we will notice that this changes. The
72. 3:14fact is that negative charges
73. 3:16predominate in the intracellular space.
74. 3:19If you look here, that is, right up
75. 3:21against the membrane, we have many
76. 3:22negative charges. Negative, negative,
77. 3:24negative. An occasional positive one.
78. 3:26Maybe a potassium there, right? That
79. 3:28moved in closer. Negative, negative,
80. 3:30positive, negative, negative, negative.
81. 3:32The opposite of what happens on the
82. 3:34other side. Outside the cell, we now
83. 3:37have a predominance of positive charges
84. 3:39. It could be sodium, sodium, sodium,
85. 3:41sodium, one negative. That could be
86. 3:43chlorine there, right? Sneaking in,
87. 3:44positive, positive. What does this tell
88. 3:49us? Well, basically, if we had a
89. 3:52voltmeter, right? that would allow us
90. 3:59to measure the millivolts inside and
91. 4:01outside using electrodes. I place one
92. 4:04here and another that goes into the
93. 4:06intracellular fluid. If I measure the
94. 4:11difference between one and the other,
95. 4:12what will happen? Well, the negative
96. 4:16charge right in this part will be
97. 4:19compared to the positive charge, and so
98. 4:23I will find that the intracellular part
99. 4:26is more negative. This difference is
100. 4:30exactly what we call membrane potential
101. 4:33, okay? And membrane potentials also
102. 4:35vary according to the cell type. For
103. 4:38example, here we have some resting
104. 4:40membrane potential values, okay? In the
105. 4:43case of our example, the neuron has a
106. 4:45membrane potential of -70 mV. What does
107. 4:48this mean? That the inside, that is,
108. 4:51the intracellular space, is 70 mV more
109. 4:54negative than the extracellular part.
110. 4:58Yes, it varies a little here in
111. 5:00skeletal muscle. If you notice here,
112. 5:02the difference is even greater, it is
113. 5:05-95 mV. Cardiac muscle -90. Smooth
114. 5:08muscle -60. Okay? So, this means that
115. 5:12the difference in smooth muscle is not
116. 5:14as great as the one we have, for
117. 5:15example, here in skeletal muscle. These
118. 5:20four cells that we saw here, cells and
119. 5:23muscle tissue itself, stand out because
120. 5:26they are excitable tissues, which we
121. 5:28saw in the previous video. I will leave
122. 5:32the information for that video here for
123. 5:34you to check out. What happens? Lastly,
124. 5:38the erythrocyte is not an excitable
125. 5:40cell, right? However, it will also have
126. 5:42a membrane potential. This one, if you
127. 5:45notice, is not that pronounced, okay?
128. 5:47The difference is only -1 mV. Okay?
129. 5:52With this, we would then be talking
130. 5:54about the first factor. I repeat, the
131. 5:56first factor that determines the
132. 5:58potential would be intracellular
133. 6:00electronegativity. What are the next
134. 6:03ones? We have the diffusion of
135. 6:05potassium and sodium. Okay? Let's use
136. 6:07this same neuron example. Now I'll
137. 6:09expand it a little bit more here. What
138. 6:11do we have? Well, we have here the
139. 6:16extracellular and here the
140. 6:17intracellular. All right. They are
141. 6:20flipped here with respect to the image,
142. 6:21but basically we will also distinguish
143. 6:23them by the type of ions inside and
144. 6:25outside. The extracellular space, what
145. 6:28does it have? Mostly it has sodium ions
146. 6:30, right? Since we had just seen in past
147. 6:34videos that sodium outside the cell had
148. 6:37what? An approximate concentration of
149. 6:43140, right? 140 milliequivalents. On
150. 6:47the other hand, inside the cell, sodium
151. 6:49, if you look, well, it's nothing like
152. 6:51that, right? It is around 14
153. 6:53milliequivalents. There is a much lower
154. 6:56concentration of sodium inside than
155. 6:58there is outside. Conversely, potassium
156. 7:02, potassium is what predominates in the
157. 7:03intracellular space, around 140
158. 7:05milliequivalents. Now, out here,
159. 7:09outside the cell, how many are there?
160. 7:12Well, there is a difference and it is
161. 7:14between 3.5 and 5 milliequivalents. Now
162. 7:18, what does this tell us? The fact that
163. 7:20there is a difference will mark a
164. 7:22tendency for the ions to leave
165. 7:24depending on the concentrations. First,
166. 7:27what should we keep in mind? Potassium
167. 7:30leaves the cell because of this
168. 7:32concentration difference, thanks to the
169. 7:34fact that there are specific channels
170. 7:35that allow it to leave, okay? They are
171. 7:38channels that are open so that
172. 7:40potassium can leave and go from one
173. 7:43side to the other. Okay? So, let's draw
174. 7:45the line here. All right, that will be
175. 7:50it, right? Let's imagine that inside
176. 7:53the neuron we have precisely that
177. 7:55membrane potential I mentioned, right?
178. 7:57-90. Right, this will be the resting
179. 8:01membrane potential by pure logic. What
180. 8:08do we assume when potassium leaves the
181. 8:10cell? Meaning, the positive charge that
182. 8:13potassium has goes from inside to
183. 8:14outside. What do we assume could happen
184. 8:17to my membrane potential? It will
185. 8:19change, of course, but which way will
186. 8:21it change? If I am losing positive
187. 8:24charges from the inside, that tells me
188. 8:26that the potential will be even more
189. 8:28negative, okay? So the potential would
190. 8:30no longer be -90, it would be what? -94
191. 8:33, or even less. If it kept leaving,
192. 8:37well, even more so. However, what
193. 8:39happens? Sodium, for its part, will
194. 8:42also have a passive movement, but it
195. 8:44will do so in the other direction,
196. 8:46because sodium predominates in the
197. 8:48extracellular space and it will enter
198. 8:50through specific sodium channels. So,
199. 8:54it will go this way. What happens now
200. 8:59if I am putting positive charges inside
201. 9:01the cell? Right? I am gaining positive
202. 9:05charges. So, the potential difference
203. 9:07that will happen now will not go down,
204. 9:09but will go up. Okay? So, basically,
205. 9:12what is happening? Potassium leaves,
206. 9:14sodium enters. Now, you are surely
207. 9:17wondering something. Okay, I am letting
208. 9:19potassium out, right? And the potassium
209. 9:21is slowly, but surely, leaving and the
210. 9:23sodium is slowly entering, it is slowly
211. 9:26entering. I mean, a moment will arrive,
212. 9:28if we follow that logic, where almost
213. 9:29all the potassium is outside and almost
214. 9:31all the sodium is inside. How do I
215. 9:33maintain these concentrations? And
216. 9:35above all, how do I maintain this
217. 9:37membrane potential? Well, now the third
218. 9:39factor comes into play, which is
219. 9:41precisely the sodium-potassium ATPase
220. 9:43pump. What function will this pump have
221. 9:46? As we saw in the membrane transport
222. 9:50video, which I’ll leave up here for
223. 9:53you to check out, this pump is in
224. 9:56charge of translocating two potassium
225. 9:58ions inside, that is, it brings in two
226. 10:01potassiums and at the same time it will
227. 10:04take three sodiums out of the cell,
228. 10:07okay? So, three sodiums will go this
229. 10:14way. That is, outward, and two
230. 10:18potassiums inward. Got it. That is how
231. 10:25the sodium-potassium ATPase pump works,
232. 10:27right? And I repeat, why is "ATPase"
233. 10:30added? Well, because for this pump to
234. 10:33work, it precisely needs ATP molecules,
235. 10:36right? Which we basically obtain from
236. 10:39aerobic metabolism and metabolic
237. 10:40pathways that, surely thanks to the
238. 10:42videos we have uploaded in recent
239. 10:44months, you are already familiar with,
240. 10:46right? They are metabolic pathways that
241. 10:49properly allow for generating cellular
242. 10:51respiration and, therefore, ATP. Well,
243. 10:54now, finishing up this video, all that
244. 10:56remains is for me to thank you for
245. 10:58continuing to watch all the Medical
246. 11:00content. The second part of the video
247. 11:04will be precisely about a couple of
248. 11:06equations that are necessary to
249. 11:07integrate today's knowledge, which
250. 11:09would be the Nernst potential and the
251. 11:11Goldman equation. We will leave those
252. 11:14for the second part, and I hope you
253. 11:16enjoyed this video.