So I want to emphasize, elaborate a little bit more about this new channels that we just discussed. The red channels which we call the synaptic channels signifies by the conductance GS, G synapse. Unlike the resting channels, the resting conductance GR. So what are these channels? What can they do? Let me tell you something that these rather generally in, in, in, in, in living cells in general, but in particular, nerve cells, is that there are difference in iron concentrations. Between the inside of the cell and the outside of the cell. For example, we know that the nerve cells there is a lot of sodium channels, Na, sodium channels outside of the cell and much little inside the cell. And for example in case of potassium, the opposite is true. You have a lot of potassium channels inside and less on the outside. So there is a gradient of particular ions, specific ions, a lot of Sodium channels outside versus inside. A lot of Potassium channels inside versus outside, and this is in the steady state in the resting case. Now, what happens let's look at the case for when, this, the synaptic channels are very specific channels, and they are only, uniquely enable the flow of sodium. Let's say that you now open a particular sodium channels. Let's say that the receptor that is attached to a transmitter opens a specific channel for sodium, and only for sodium. In this case because of the gradient, because of the difference in the concentration of sodium outside versus inside. Whenever you have a chance, a new chance of a new channel to be opened, and this channel particularly enables only this ion to flow, sodium will flow inside. And, if this sodium channel will be opened. Sodium will flow, it will flow inside, from the outside to the inside. Just because there is this difference in concentration and whenever you have a chance in this case, the conductance enables, the new conductors enable the flow of ions of sodium from outside to inside. What will happen then? Then the sodium that goes inside the cell will make the cell more positive. The cell will become more positive. This means that, because of the flow of sodium inside, you will get depolarization inside the cell. So this is, why you, we can think of what of, of a battery for the synapse. The battery signifies in some sense, the direction of flow, of the, of the ion, and of course the charge of this ion. For example, for sodium, I will say that if the synaptic conductance is conductance for sodium, I can draw here, the conductance is a sodium conductance, gNa, for example. And the battery, that will signify the tendency of the direction of flow from the outside to the inside, the tendency to make the inside more positive, I will put it as a battery that is more positive, inside the cell. So this is very important. I just showed you, that this synaptic battery, E of the synapse, actually represents the direction of current flow, the tendency in this case to make the inside of the cell more positive. Because, just because it was Sodium flowing from outside to inside, just because the channels are very specific to Sodium. So this will be the synaptic potential, which depends on this case of this Sodium. Suppose you have another case. [SOUND]. And this is the case,[SOUND], where your channels are very specific to potassium. And I just told you that you have a lot of potassium inside. [SOUND]. And less potassium outside. This means that when I open a specific channel for the potassium, the potassium will escape to the outside. Just because there is a lot of potassium inside, little potassium outside, the potassium will tend to flow from inside to outside if you enable it by opening a channel due to this transmitter interaction. So if you have a particular receptors, that upon interaction with the transmitter, opens potassium channels, you will lose positive charge from the inside and the inside will become more negative. In this case In this case, my battery will look like this. My synaptic battery will look like this. It will have a conductance, synaptic conductance, which is potassium based, gk or g sine. But now the battery related to this synapse will be such that it is more negative inside. Its more negative inside. Just because when you enter open the ion channels for potassium you will make the inside more positive because potassium tends to flow outside of the cell. Leaving the cell more negative inside. So you see that these particular branch, the synaptic branch always consists of the conductance, which depends on which ion is being opened, which ion conductance is specifically being opened. Sometimes it is a combination of more than one. I just gave you two examples of a very specific ion channels. Once with a particular conductance only enabling sodium to flow in. And in this case, the battery representing this particular conductance is a battery depending only on the relationship, on the condu, on the concentration of the Pota, on the Sodium outside versus inside. And in this case, because Sodium flows inside you will make the inside more positive, this will be a positive battery representing the Sodium so to speak battery. And in the case of potassium because in the case of potassium, potassium leaves the cell because the concentration is high inside low outside it tends to leave the cell when you open a conductance. So you have a synaptic conductance, gs, which depends on potassium. And, in this case, this synaptic battery, is synapse, is negative inside, because the tendency of the potassium is to leave the cell making the inside more negative than before, so this will be a battery that makes the inside more negative than before, and this is signifies, signified by this, by this negativity. So, this is basically the principle of why the synapse generates current source at the post-synaptic membrane. You can see there are no electrodes here. There are only ion channels, that could be open, could be open. They are there, the channels are waiting, or the receptors are waiting. If there is a transmitter interacting with this receptor, the receptor changes the configuration in the membrane, enabling new current to flow from the outside to the inside, or, depending on the channel, from the inside to the outside. That's why I can think of a synapse postsynaptically as the current source, but not a linear current source, not a simple current source. Because in this case the current source depends on coductance change. First there is a conductance change and this conductance change is those channels that are being opened. New channels being opened by the transmitter and depending on which ion channel or channel or channels being opened you have a battery that signifies what we call the driving force. Where is the driving force for this aisle to flow? Is the driving force from the outside to the inside? That means that the inside would become more positive if it's a sodium channel or whether the driving force is in the opposite direction. Is it from the inside to the outside in case of potassium, then the inside would become negative. So that, this is this branch, the synaptic branch, the electrical representation of the post-synaptic membrane where the voltage we will see soon is developed due to the activity of the synapse.
