[SOUND]. Okay. So, up until now I really spoke only about the case where everything was, so to speak, passive. Passive resistor, passive capacitor, and a battery, passive battery, the battery that these constant. So, this was minus 70 millivolts. And this had some value depending on the cell type and, and size. And this has the capacitance depending on the capacitance of the cell. I didn't tell you much about the values of R and of C but I want to tell you now, just to give you a ballpark, just to tell you that this R multiplied by C. This time constant that I just mentioned, R multiplied by C, R multiplied by C, which I call the membrane time constant, is on the order in central neurons in our brain, on the order of 20 milliseconds or so. Milli is a thousandth of, 10 to the power of 3. So, this is 20 millisecond, much smaller than a second. And that's about the correct time of a typical RC multiplication, RC properties of a bunch of membrane in the brain. About twenty milliseconds, it could go down to 10 milliseconds, maybe 5 milliseconds, depending on how leaky is the membrane of this particular cell. But you may remember 20 milliseconds time constant. Okay. So, this was the passive situation here. [SOUND]. And now I want to go into the synaptic potential. Because I told you before, nobody from the outside inject currents into cells. So who, where, is the source of current to cells? And these are the synapses. So, let me draw symbolically a synapse onto this cell. Let's draw it like this. This will symbolize an axon terminating on the sol, on, on the cell. So, remember that we spoke early on, when we spoke on the anatomy of cells. I hope you remember that there were, typically there were dendrites with spines. So, this was a piece of dendrite[SOUND] and there was an axon going here,[SOUND] and there was this varicosity in the axon. Something like that. So, this was the axon. [SOUND]. And I, you remember that there were these vesicles inside the axon full of transmitter. [SOUND]. And in the dendrite, in the dendritic spine, there were these receptors. [SOUND]. So, this was the receptors,[SOUND] and these were the transmitter, the vesicles with the transmitter. [SOUND]. And there was interaction between them. So, sometimes, when there was an action potential, when there was a spike here, and we'll talk about spike later, there was a communication through the synapse and this recep-, this receptors absorbed, so to speak, interacted with a transmitter,[SOUND] that was now distributed in this space between, in the very small space. Not as big as I drew here but a very small space between the pre-synaptic axon and the post-synaptic dendritic spine through these receptors and transmitter interaction. So, now, I'm now trying to explain to you what happens in the membrane. When you have, in this case, rather than a, an extra cell, an external source of injection, which was an electrode, a real source of injection or communication, which is the synapse. So, this would be my synapse now. So, I want to tell you how do we think electrically as in a synapse as an electrical device. You saw that there is a chemical interaction because there is the vesicles that released the transmitter. There are the receptors that picks up this transmitter. What happens then? We know today that something very interesting is happening when the transmitter[SOUND] interacts with the receptor. We know, that there are new ion channels open in this membrane. Just because of this interacter, interaction, between the transmitter and the receptor, there are these new[SOUND] channels, which enables new current flow into the cells . So I, I now schematically mark the interaction between the transmitter and the receptor. This interaction is expressed itself, by the opening of ion channels, channels that enable flow of current. And depending on the type of channels with, whether the current would flow from the inside, outside or from the outside, inside. We'll talk about these channels later on. I just want to, to explain to you, why do I look now at the synapse as an electrical device? There is of course the chemical interaction but eventually at the post-synaptic side at the axon, sorry, the dendrite side the dendritic spine side there. Eventually, there is current flow inside or outside through the opening of new ion channels, new path for current. So, if I look at the electrical circuit, the passive one. And I want to add these particular new channels that are being opened in the membrane of the cell. So, it was the same cell before but now I added new channels. Let's call it direct channels. This will be our synaptic channel. And these channels in the membrane are opened only when there was a transmitter release. An interaction with a tra-, with a receptor. I will now draw electrically what I just described by words. I'm saying that in the membrane, in the post-synaptic membrane, there are these channels which behave like a resistor or a conductance. We shall call it g-conductance of the synaptic channels, g-synapse. These are the new channels that are being opened, that enables new current to flow from the outside of the cell to the inside or vice versa. And the direction of current will depend on the value[SOUND] of this synaptic battery, E[SOUND] synapse. Okay. If this battery of the synaptic ion channels, we shall talk about the battery in a second, is more positive inside. The opening of these ion channels will make the membrane more positive inside than the resting potential. Or otherwise, if this is more negative than the resting potential, then the cell will become more negative due to the opening of the synaptic channels. So, what I told you is something not easy to grasp. The fact that the transmitter release opens in the membrane of the post-synaptic cell, new ion channels, the red ion channels. And these red ion channels enable particular specific ions to flow either from the outside to the inside or from the inside to the outside, depending on which channel is being opened. We'll talk about type of channels in a second. But this is now a full circuit that, that represent both the passive part that you already studied beforehand. And also the synaptic part, the synaptic conductance part. So, this is a full post-synaptic membrane consisting of passive properties and synaptic conductance and capacitor.
