Voiceover: In this video I want to talk
about the types of neurotransmitter receptors.
Neurons are often referred to
as excitatory or inhibitory,
but more accurately it's the synapse
that's excitatory or inhibitory,
and even more specifically,
it's the combination of the neurotransmitter
that's released at the synapse
and the receptor that it binds to
on the post synaptic membrane.
Because many neurotransmitters
can bind to multiple types of receptors
so that the neurotransmitter can sometimes
be excitatory and it can sometimes
be inhibitory to the target cell
so that this axon terminal might be releasing
this neurotransmitter here at the synapse
and perhaps when it binds to this purple receptor,
that causes an excitatory potenetial
in the target cell,
but if it binds to this orange receptor
on the post synaptic membrane,
that would cause inhibition of the target cell,
would cause inhibitory potential.
When the target cell is another neuron,
excitatory or inhibitory synapses
can be scattered around all over
the surface of the neuron.
Or there are many neurons
where the dendrites are receiving
predominately excitatory synapses
so that there can be a large number
of neurons synapsing on the dendrites
and releasing neurotransmitters
that will cause depolarizations in the dendrites.
So let me just draw little purple pluses in here
to represent that these are excitatory synapses.
And then many neurons like this will have
more inhibitory synapses on the soma.
So I'll just draw some little orange minus signs here
to represent that these could be
inhibitory synapses on the soma.
And then often neurons have synapses
on their axon terminals so that other axon terminals
are synapsing on the axon terminal
of the target neuron and these will often be a mix
of excitatory and inhibitory synapses.
And there's a big variety in how the synapses
are set up on neurons.
And there would be a mix of some
excitatory and inhibitory synapses
at all these locations.
But for neurons that are set up like this,
kind of a general way of thinking about how
information would flow to the neuron is that
they can receive lots of excitatory
input through the dendrites,
causing depolarizations to spread down
the dendrites into the soma.
But then if these neurons are inhibiting the soma,
that can block those depolarizations from
reaching the trigger zone
to trigger an action potential.
And then when action potentials are conducted
from the trigger zone down the axon
to the axon terminal,
the excitatory and inhibitory synapses
on the axon terminal can increase or decrease
the amount of neurotransmitter that is released
when an action potential reaches the axon terminal.
So that there can be fine tuning of the
output of the neuron at multiple levels
all the way from the dendrites through the soma
and to the actual individual axon terminals,
can have their output turned up or turned down
in response to the information flowing in
from all these synapses.
So that one way you can categorize synapses
is if they're predominately excitatory
or predominately inhibitory.
But there are some other big differences
in neurotransmitter receptors and
how they pass information from
neurotransmitters in the chemical synapse
to the target cell.
So let me just draw a big axon terminal here
that's releasing neurotransmitter
into the synaptic cleft.
And this will be the post synaptic membrane
of the target cell.
And I'm gonna draw the two big types
of neurotransmitter receptors here.
Let me draw this one in grey
with its little receptor to bind
to the neurotransmitter.
And I'll draw this other one in yellow over here
with its little pocket to
bind to its neurotransmitter.
So this first type of neurotransmitter receptor
is called ionotropic.
And ionotropic neurotransmitter receptors
are ligand gated ion channels.
So they have ion right in the name.
And when their ligand binds to their receptor,
which in this case is their neurotransmitter,
they open and allow certain ions to pass.
And when the neurotransmitter leaves the receptor,
then they close and they don't let ions
pass through anymore.
The ionotropic neurotransmitter receptors
cause graded potentials when they're activated,
which have a rapid effect on the potential
of the near by membrane that is brief and local.
The target cell will usually be excited
if the activated ionotropic neurotransmitter
receptor allows sodium or calcium ions to pass
because they will usually flow into the neuron,
bringing their positive charges in
and causing depolarization.
Or the ionotropic neurotransmitter receptor
will usually cause inhibition of the target cell
if it allows chloride or potassium ions to pass.
Chloride ions will usually flow into the neuron,
bringing negative charges in,
making it more negative inside
and potassium ions will usually flow out of
the neuron, carrying their positive charges
outside and also making it more negative inside.
Now the other big class of neurotransmitter
receptors are called metabotropic.
And these are not ion channels.
Instead, when their neurotransmitter
binds to the receptor,
they activate second messengers inside the neuron.
And there are a number of different
types of second messenger systems
that can be activated by
metabotopic neurotransmitter receptors.
And these second messengers
can do a lot of different things.
They can go and effect the
behavior of ion channels,
causing them to be more or less active.
Or they can change the activity of
proteins inside the neuron.
And some can even effect the activity of genes
and change the pattern of gene
expression inside of neurons.
When these metabotropic neurotransmtter
receptors are activated,
the response of the target cell occurs
more slowly than it does with
activation of ionotropic
neurotransmitter receptors, but the effects
may be larger and they may be more widespread
because there can be amplification
through these second messenger systems.
The effects of activation of these
second messenger systems by metabotropic
receptors may involve just brief excitation
or inhibition of the target cell,
or it may cause longer lasting changes
to how the target cell behaves,
either when it's at rest or
when it's responding to input.