Membrane Potential

by Jason Amores Sumpter
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in this video, we're going to introduce some terms. That will be very helpful for you as we move forward and talk about neuronal signaling or the signaling between neurons. And so the first term we're going to introduce here is electric current. And so electric current can be defined as the flow of electric charge. Now, when you think of an electric current, you may think of an image that looks something like this, where you have some kind of battery and we know that electrons, which can be symbolized with the symbol right here, will flow through a wire here and it will be used to power some kind of device such as this light bulb here. However, electric currents do not necessarily require the flow of electrons and in fact the electric current that's utilized by neuronal signaling is not going to be electrons. Instead it's going to be ions and the flow of ions will generate this electric current. Now the term electric potential refers to the electric potential energy per unit of charge and it is measured in units of volts, which can be abbreviated with the V. And the term voltage refers to the difference in electric potential between two points and results from differences in charge. And so if we look at our image on the left hand side over here, notice towards the top, we have these positive charges and towards the bottom we have these negative charges. And so because there is a difference, uh an electric potential between these two points, we have a positive charge here towards the top and again a negative charge here towards the bottom. What that means is that there exists a voltage here in this image. And if we were to take, say, a positively charged particle and place it here, what we would see is that this positively charged particle would repel all of the positive charges towards the top, and it would be attracted towards the negative charges towards the bottle. And so this positively charged particle would make its way and move towards the negative charges. Now when it comes to neuronal signaling, the electric potential can be created using an electrochemical gradient. And so an electrochemical gradient is really just the combination of a chemical concentration gradient and an electric potential gradient across the membrane. And so if we take a look at this image down below, which will notice is that it represents a biological membrane here in the middle. And which will notice is we have a bunch of ions on the left hand side which represents our extra cellular space. And we also have a bunch of ions on the right hand side which represents the intracellular space inside the cell. And what you'll notice is that when we look at the ion concentration gradients of these three ions, sodium ion, potassium ion, and chloride ion uh they have gradients that um exist as this image shows. So uh in the extra cellular space there tends to be a large concentration of sodium ion and as you cross the membrane towards the inside of the cell, there's a low concentration of sodium ions. So we can put low here and high here. And what you'll notice is that the potassium ion concentration is opposite of the sodium ion concentration. So there's a high concentration of potassium inside in the intracellular space and there's a low concentration of potassium on the outside of the cell. And then the chloride ion concentration resembles that of the sodium ion, where there's high concentration of chloride and ion on the outside of the cell and low concentration of chloride and i on on the inside of the cell. And so the cell is able to create these concentration gradients through utilizing these ion channels or this membrane protein here that can transport ions. And so we'll get to talk more about this transport ion in a different course, but ultimately what you'll see is that uh there are going to be uh sodium ions pumped towards the outside of the cell. So that's what creates the high sodium on the outside of the cell. And then there are going to be potassium ions pumped towards the inside of the cell. Uh and there will be more uh sodium pumped out than there is potassium pumped in. And so over time, what ends up happening is there is a build up of positive charge on the outside of the cell. And so that's something that's important to keep in mind that on the outside of the cell uh there is going to be an a net positive charge. And on the inside of the cell there's going to be a net negative charge across the membrane. And so what that means is that there exists a voltage across the membrane. And we refer to this electric potential. Uh we refer to this as the membrane potential. And so the membrane potential of course, is going to involve the biological membrane. Uh and so it's the difference and electric potential between the interior and exterior of a cell, which will be separated by the membrane and which will notice, is that when the cell is in a resting state, basically when the cell is in a normal resting state and it's not really doing anything, it will have a resting membrane potential. And that is really just the baseline membrane potential of the cell. And that resting membrane potential is going to be negative, meaning that the inside of the cell will be more negative with respect to the outside of the cell, which will be more positive. However, the membrane potential can change. And this will be critical when we start to look at these neuronal signaling. Uh And so hyper polarization is a term that refers to the membrane potential becoming more negative. And deep polarization is a term that refers to the membrane potential becoming more positive. And so we'll get to utilize these terms uh more as we move forward in our course and discussed, uh, neuronal signaling. And so that here concludes this video, I'll see you all in our next one.