Okay, now let's discuss exactly
what is going on in the brain that gives rise to the
signals that, that, that, what was observed by Berger on that EEG.
So what I, what I've shown, what I'm
showing you here, is a, a cartoon, a schematic.
It's actually derived from Kandel's principles of neural science.
And what we're depicting here are some,
some neurons that are sitting in the neocortex.
And,
as you can see, here's the surface of the cortex.
And, I've drawn some two, two electrodes
that are actually right on the cortical surface.
In the EEG, they'd be much farther away on the scalp.
And in this, in this cartoon of a cortex, I've
shown two ensembles of large projection neurons.
These are the kind of neurons that sit in layer five of the neocortex.
And as you can see, there are three neurons
here, and they have axons that are coming from the
thalamus and those axons make synapses in the relatively
proximal part of the dendrites of these large projection neurons.
On the right you see another set of three neurons.
And here the axons are coming, say from the
other side of the brain, through the corpus callosum.
And those synapses are being made on the more distal part of the dendrite.
Now, looking at, looking at this cartoon, one thing, the first thing
you can appreciate is that these
large projection neurons have a similar orientation.
In other words, look at the way the dendrites are all oriented
upward, in this case, towards the surface of, of, of the cortex.
Secondly, let's consider the way the inputs
that come into these neurons, might arise.
So,
we'll look at an action, let's take an action potential.
So, here's your action potential, right here.
It's coming in from the thalamus, and it's going to move
down along the axon, and eventually reach the synapse of this neuron.
And when it does, it'll give rise to an excitatory postsynaptic potential.
Now, associated with that, is a flow of ions,
in this case, I'm showing sodium going into the dendrite.
That flow of ions
creates, in this case, a sink, because you have a
positive charge going from the
extracellular space, into the intercellular space.
So that region is going to be relatively negatively charged.
Of course wherever you have a sink you have
a source and that's creating flow of current, current.
And if we look at that flow.
In this case we have positivity more in the distal region of the dendrite.
And negatively close, closer to the cell body.
And, if you can imagine, this electrode what it, what it is reading in
terms of voltage changes, it's, it's sensing
that there's more positivity closer to the electrode.
So, the actual readout of that electrode will show a positive charge.
In this case the deflection of the EEG is downward denoting positivity.
Okay, so the, the summation of the electrical activity
in the groups of neurons is what is exactly detected by the scalp electrode.
Now let's take a look at a different orientation.
In this case the EPS P's, and now I'm showing all
three at once because they're arriving close at the same time.
These EPS P's are all going to lead to a sink in the more distal part
of dendrite, right, and the source will be more proximal.
And in this case, as you can see, the EEG is detecting
a more negative charge and shows the upward deflection as depicted here.