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.