And then you stop completely injecting the current.

There will be a attenuation until you drop back to the initial condition which

we will later call it resting potential. So, what we saw here is something very

important. Really, really important for cells.

The fact that when you have a consequence of several inputs, several currents, one

after the other with the appropriate time difference.

If it's not too long the time difference, then the reminiscence the remainders of

the previous voltage here will be the initial condition for the next buildup.

And you can see that the voltage is buildup, so this amplitude will be

smaller than the next amplitude, which will be smaller than the third amplitude,

because they build one on top of the other.

This phenomena is called Temporal Summation, because I summate in time.

One response, and then the second one on top of the first response, and then the

third one on top of the second response, and they summate one on top of the other.

And this is because you have a memory to the system, electrical memory.

There is some memory to the first input. Here is the first input.

There is some memory to it because it takes time to get rid of it.

And then you get the second input on top of the memory of the first one, and you

build up. This is called Temporal Summation.

And it's all due to the fact that you have time constant.

Let me just say one thing that is very important, and that of course, if you

would inject, if you would inject continuously the current.

Unlike what we did, if I would have injected the current continuously, then

of course, I will get a buildup

[SOUND].

That is larger than what you would get if you had these intermissions.

Though the maximal you can get, of course, is when you continuously inject

the current. Whenever you stop the current, you, of

course, lose something of the voltage because of this attenuation.

So this IR that we mentioned before, is the maximum that you can get.

And if you inject in some intermissions, you will get the peak will be always less

than the voltage if you would have injected continously.

That's one important aspect. But still, the fact that there, there is

not too long a difference in time between the first, and the second and the third.

And meaning that the time difference here is on the order of tao, so this is on the

order of the time constant of the membrane.

It could be two time constant but not too long because then the voltage will

attenuate back to rest, to, to the initial state.

If this, if the intermission here between the currents is on the order of the time

constant, then there will be this Temporal Summation.

There will be buildup of one voltage on top of the shoulder of the previous

voltage, and this is Temporal Summation. And this means that when you have

consequence input like synaptic inputs. One after the other after the other, that

will tend, the voltage response, will tend to summate one on top of the other.

What would have happened for example if I would inject here a negative current,

like this? What would have happened if my current

number four would be not positive inside, but rather I inject negative current

inside the cell. This would mean that at that point, here,

I will push the voltage down. Because I now push the voltage in the

negative direction. Indeed, if this is strong enough, I may

go very, very much below, even below the resting potential depending how this

strong this voltage this current is. This is hyperpolarizing current, this is

current that makes the cell more negative inside.

This is carrying the voltage toward negativity.

And when I complete, when I cease injecting the current, [SOUND] then the

voltage will attenuate back, [SOUND] into the resting potential here, into the

reference potential. So this will be also a summation of a

positive response. Depolarization.

And a negative response, hyper-polarization due to

hyper-polarization current. So this interplay between positive

currents trying to depolarize the cell, negative currents trying to carry the

voltage back or even more negative relative to the initial state.

This interplay between positive and negative is exactly what synapses are

doing. Because as we'll hear soon, some synapses

inject positive current, some synapses inject negative current in the cell.

And this will be a Temporal Summation of consequence of several currents.

One positive, second positive, third positive, fourth negative and so on in

time. This is Temporal Summation.

It's a very, very critical important, machinery.

The way that neuron work when they summate inputs is governed by this

Temporal Summation principle. Which is eventually all results from the

fact that you have a membrane time constant memory that takes time, because

of this memory, takes time to get rid of the previous input.

And that's why you can summate the next one on top of it.

If the time difference is correct. So you have to remember the notion of

Temporal Summation.