The Synaptic Battery

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Из курса от партнера Hebrew University of Jerusalem

Synapses, Neurons and Brains

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Hebrew University of Jerusalem

Synapses, Neurons and Brains

701 оценки

These are very unique times for brain research. The aperitif for the course will thus highlight the present “brain-excitements” worldwide. You will then become intimately acquainted with the operational principles of neuronal “life-ware” (synapses, neurons and the networks that they form) and consequently, on how neurons behave as computational microchips and how they plastically and constantly change - a process that underlies learning and memory. Recent heroic attempts to realistically simulate large cortical networks in the computer will be highlighted (e.g., “the Blue Brain Project”) and processes related to perception, cognition and emotions in the brain will be discussed. For dessert we will deliberate on the future of brain research, including the questions of “brain and art”, consciousness and free will. For more information see the course promo below and read “About the course.”

Из урока

Electrifying Brains –Passive Electrical Signals

In this module we will discuss the "Electrifying brain – passive electrical signals". We will show that neurons are electrical device and learn what enables neurons to become “electrifying”. Here we will describe only the passive (vs. active) electrical properties of neurons. We will show that, at the quiescent state, the difference in electric potential across the cell’s membrane is always negative inside the cell (“the “resting potential”); we next show that the membrane behaves like an electrical (resistance-capacitance) RC circuit and highlight the notion of “membrane time constant” and, consequently, the ability of neurons to summate (in time) successive (synaptic) inputs (“electrical memory”) – a fundamental mechanism utilized by the brain. We will also show that when the synapse is activated, it generates an analog electrical signal (“the post-synaptic potential”, PSP) in the receiving (“post-synaptic”) cell. Most interestingly, there are two types of synapses in the brain – “excitatory” and “inhibitory” – we will discuss how these two opposing signals interact in the receiving ``neuron. This module is more technical than the more descriptive first two lessons; we encourage those of you who are not familiar with basic electricity (resistance, capacitance, Ohms law and Kirchoff’s law) to read about these in the sources links for this week’s lecture.

The Cell as a RC Circuit11:01

The Voltage Equation for the Passive Cell9:20

The Membrane Time Constant14:03

Temporal Summation9:31

The Resting Potential8:54

The Synaptic Potential Part 19:38

The Synaptic Conductance6:19

The Synaptic Battery10:34

The Synaptic Potential Part 28:02

The Voltage Equation for the Synapse and EPSP and IPSP13:33

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Idan Segev
Professor
Computational Neuroscience

So I want to emphasize, elaborate a little bit more about this new channels

that we just discussed. The red channels which we call the

synaptic channels signifies by the conductance GS, G synapse.

Unlike the resting channels, the resting conductance GR.

So what are these channels? What can they do?

Let me tell you something that these rather generally in, in, in, in, in

living cells in general, but in particular, nerve cells, is that there

are difference in iron concentrations. Between the inside of the cell and the

outside of the cell. For example, we know that the nerve cells

there is a lot of sodium channels, Na, sodium channels outside of the cell and

much little inside the cell. And for example in case of potassium, the

opposite is true. You have a lot of potassium channels

inside and less on the outside. So there is a gradient of particular

ions, specific ions, a lot of Sodium channels outside versus inside.

A lot of Potassium channels inside versus outside, and this is in the steady state

in the resting case. Now, what happens let's look at the case

for when, this, the synaptic channels are very specific channels, and they are

only, uniquely enable the flow of sodium. Let's say that you now open a particular

sodium channels. Let's say that the receptor that is

attached to a transmitter opens a specific channel for sodium, and only for

sodium. In this case because of the gradient,

because of the difference in the concentration of sodium outside versus

inside. Whenever you have a chance, a new chance

of a new channel to be opened, and this channel particularly enables only this

ion to flow, sodium will flow inside. And, if this sodium channel will be

opened. Sodium will flow, it will flow inside,

from the outside to the inside. Just because there is this difference in

concentration and whenever you have a chance in this case, the conductance

enables, the new conductors enable the flow of ions of sodium from outside to

inside. What will happen then?

Then the sodium that goes inside the cell will make the cell more positive.

The cell will become more positive. This means that, because of the flow of

sodium inside, you will get depolarization inside the cell.

So this is, why you, we can think of what of, of a battery for the synapse.

The battery signifies in some sense, the direction of flow, of the, of the ion,

and of course the charge of this ion. For example, for sodium, I will say that

if the synaptic conductance is conductance for sodium, I can draw here,

the conductance is a sodium conductance, gNa, for example.

And the battery, that will signify the tendency of the direction of flow from

the outside to the inside, the tendency to make the inside more positive, I will

put it as a battery that is more positive, inside the cell.

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.

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