The cell always sits here. Now suppose you have a positive battery
here. This positive battery may be the battery
due to the opening of sodium channels. It would be a positive battery inside
because as we said before, Sodium tends to go from the outside to the inside
because of this drop, or this change in concentration, very big outside small
inside. So in this case if I open a conductance G
for the sodium, my voltage will start to grow this direction, because the battery
of the sodium will be more and more if g of the conductance for the battery, for
the sodium will be strong, the voltage will go there.
So, your synaptic potential will be depolarizing.
Suppose you have another conductance, synaptic conductance.
This will be your battery, let's say for the case of potassium.
In this case whenever you open new conducts for the potassium your membrane
voltage will go below, the resting potential.
More negative, or more positive depending on the battery, this is exactly what is
written there. So this is what we will call depolarising
Synoptic potential. This is what we will call hyperpolarizing
Synoptic potential. This will be, again,
depolarization[SOUND] less, depolarized in the resting potential and this will be
hyperpolarization more polarized. Hyperpolarization.
And this is what a synapse can do. Some of the synapse will be depolarizing
synapses, depending on the battery. Some synapses will be hyperpolarizing
synapses depending on their battery. But that's what they can do, and they are
limited by the battery. You cannot get more voltage than the
battery. You are limited by this ceiling.
There is a ceiling to the synaptic voltage.
The ceiling is the battery. Maximum you will get the most positive
battery, or maximum you will get the most negative battery, depending on what
conductance is being opened. And that's what the synapse can do.
And just to summarize, what we just said is, we said that in your brain, there are
these batteries, which depends on the ion concentration of sodium, potassium,
chloride, calcium, not many ions. But the concentration difference of these
ions from inside to the outside determine the batteries.
And these are the ceiling or the floor, where the voltage of the Synapse can go.
It can go either so much positive and it can go so much negative, no more negative
and no more positive. That's your brain limitation, that's the
signal that the brain can generate and just to tell you that the maximum one.
Could be something let's say, like 200 micrometer 200 millivolts, or something
like that, more positive than the resting potential, something like that.
Most negative could go maybe to minus 90 or minus 100 millivolts.
More negative than the resting potential, so minus 24 mil-, millivolt more
negative, so you really cannot go much, you can either go this direction by minus
20 millivolts. You can maybe go this direction by plus
200 or 250 millivolts. And that's the limitation of your signal
in the brain. The brain cannot generate 110 volts.
Or in Israel 220 volts. It can only generate few 100 or so
milli-volts. Maybe 200th positive, maybe minus 90
negative. That's all.
That's all. So that's the regime of signals in the
brain, and these are the signals you have to deal with, in order to represent the
whole world. The whole world, is represented by
signals, that are moving between these two extremes, because, just because.
You are dependent on opening of r channels the signal's in the brain
mostly. This is true also for the action
potential, the spike, which we discuss next lesson.
It all depends on opening of ion channels or closing of ion channels.
And these iron channels are for particular, channel for particular ion.
For example, sodium or potassium or chloride or calcium, and this is
signified by a battery. And this battery is not too big.
That means you can not get more than this battery.
Maybe plus 200 for calcium. Maybe minus 90 for potassium.
But that's your limitiation. So these are the signlas in the brain.
This is the steady state case. The transient case depends also on this
part which you can see that develops the synaptic potential, takes time to
develop. So there is a time constant for the
synaptic potential. We don't need it to discuss it now,
exactly the equation, but in principle you understand whenever you open,
whenever you open a conductance like this for the synapse[SOUND] .
This is your synoptic conductance and you measure the voltage in the circuit, you
will get the synoptic voltage. It could be when you open this
conductance, you will get a synoptic voltage that looks like this because you
open the conductance for a given period of time and then it is closed.
The transmitter interacts with the receptor for a short period of time.
[SOUND] Then you'll get the synaptic voltage here, V of the synapse.
It will look like this if it's a positive, if it's a depolarizing signal.
It will go more positive than the resting potential, and it will attenuate/g.
I will correlate excitatory post synaptic potential, typically.
This is one case. In the other case, when the battery is
reversed, if the battery is reversed. And it's more negative inside, then when
I open these channels, when I change the synoptic conductant g, I will get, a
negative voltage I will call it inhibitory post synoptic potential.
It will go from the resting potential more negatively and then will attenuate
back to the resting potential. This will be my post synoptic potential
which is hyperpolarizing/g. I will collate inhibitory post synaptic
potential and then this one will be a positive post synaptic potential
excitatory. And these are the two synaptic potentials
in the brain. You have two types of synapsis, we've
already discussed it last lesson. Some of the synapsis tend to do negative
voltage, some synapses tend to do positive voltage relative to the resting
potential typically, we will call those that are doing positive voltage,
inhibitory. They tend to inhibit the activity of the
cell, we'll discuss what does it mean. And then, you have some of them, many of
them that are excitatory that tend to excite the cell to make it more
excitable, more firing, the cell, the cell will be more active when you have a
lot of them, and less active when you have a lot of them.