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.