Types of neurotransmitter receptors

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.