Now here is an example of an electrical synapse between two neurons,
neuron A and neuron B.
Now electrical synapses function in a manner very similar to the way that
the connections between components in your cell phone or in your computer function.
They allow the activity from one side, in this case neuron A,
to be directly propagated to the other side, in this case neuron B.
So they changed the voltage on one side,
given some activity and changes on the other side.
And the way that neuron a communicates with neuron B, or vice versa,
is through what is known as a gap junction here.
And a gap junction is depicted here in terms of these.
So these are essentially ionic channels.
So here's one, here's another and so on.
And you notice that these ion channels span the membrane
of both neuron A and the membrane of neuron B.
And the result of this particular arrangement is that if you have
excitation on one side, due to for example, an action potential.
And maybe you have sodium ions on this side that are in
higher concentration than on the other side.
Then these channels allow these ions to migrate to the other side.
And the result of this movement of
ions from one side to the other is that you're going to have a change
in the membrane potential of neuron B as a result of some activity in neuron A.
So these types of electrical synopsis
are really useful when you want to have fast connections between two neurons.
And typically they're found in the case where you need to synchronize,
which is you want to make neurons fire simultaneously together.
It turns out to be a useful mechanism when you want to synchronize sets of neurons.
The other case where you would like to have these types of
fast electrical synapses or fast connection between neurones is when
you have to implement something like an escape reflex.
And that's something that's found for example in the crayfish.
So that's an example of an electrical synapses and what about chemical synapses?
So here's a depiction of a chemical synapse.
So suppose we have a neuron A and
a neuron B and here's the action potential, or spike coming in.
Now what we have on one side, on the side of neuron A are these bags which are known
as vesicles, so these are bags of neurotransmitter molecules.
So these are chemicals that are stored in these bags.
And so when an action potential spike comes in along the axon of neuron A it
causes these bags, these bags called reticules to
fuse with the membrane and in doing so this bags
release the neurotransmitter molecules into the gap between the two neurons.
Now this gap is called the synaptic clef.
And so when these neurotransmitter molecules stand
fuse with the receptors on the other side.
So these receptors are nothing but the chemically gated
ionic channels that we talked about in the previous lecture,
then these chemically gated channels start to open.
And so you know what happens when these channels are to open?
Well, they're going to allow some ions to either come inside or
go outside according to the concentration of the ions on the inside or the outside.
So suppose that these are channels that allow sodium ions to come in,
then you're going to have sodium ions coming in, into this neuron.
And as a result, you're going to have an increase
in the membrane potential of neuron B.
So you can see how a spike from neuron A, causes this
cascade of chemical events, and then that in turn causes these channels to open.
And that finally causes some changes in the membrane potential.
So you're going from electrical activity to chemical and
back to an electrical change again, on the side of neuron B.
Now you might ask yourself why evolution go to the trouble of constructing
such a complex electrical chemical an electrical connection when it could have
just use the electrical connections with gap junctions to begin with.
Any thoughts on that?
So why would something like this, like in a chemical synapse,
be useful compared to just a simple electrical synapse?