[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.

The Synaptic Potential Part 1

Synapses, Neurons and Brains

Meet the Instructors