- Anatomy of a neuron
- Overview of neuron structure and function
- The membrane potential
- Electrotonic and action potentials
- Saltatory conduction in neurons
- Neuronal synapses (chemical)
- The synapse
- Neurotransmitters and receptors
- Q & A: Neuron depolarization, hyperpolarization, and action potentials
- Overview of the functions of the cerebral cortex
Neurotransmitters and receptors
Different classes of neurotransmitters, and different types of receptors they bind to.
Did you know there are billions of neurons—and trillions of synapses—in your amazing brain? (No wonder you can learn anything, including neurobiology!) Most of your synapses are chemical synapses, meaning that information is carried by chemical messengers from one neuron to the next.
In the article on synapses, we discussed how synaptic transmission works. Here, we’ll focus on neurotransmitters, the chemical messengers released from neurons at synapses so that they can “talk” to neighboring cells. We’ll also look at the receptor proteins that let the target cell “hear” the message.
Neurotransmitters: Conventional and unconventional
There are many different kinds of neurotransmitters, and new ones are still being discovered! Over the years, the very idea of what makes something a neurotransmitter has changed and broadened. Because the definition has expanded, some recently discovered neurotransmitters may be viewed as "nontraditional” or “unconventional” (relative to older definitions).
We’ll discuss these unconventional neurotransmitters at the end of article. For now, let's start out by discussing the conventional ones.
The chemical messengers that act as conventional neurotransmitters share certain basic features. They are stored in synaptic vesicles, get released when enters the axon terminal in response to an action potential, and act by binding to receptors on the membrane of the postsynaptic cell.
Diagram of a synapse, showing neurotransmitters stored in synaptic vesicles inside the axon terminal. In response to an action potential, the vesicles fuse with the presynaptic membrane and release neurotransmitter into the synaptic cleft.
The conventional neurotransmitters can be divided into two main groups: small molecule neurotransmitters and neuropeptides.
Small molecule neurotransmitters
The small molecule neurotransmitters are (not too surprisingly!) various types of small organic molecules. They include:
- The amino acid neurotransmitters glutamate, GABA (γ-aminobutyric acid), and glycine. All of these are amino acids, though GABA is not an amino acid that's found in proteins.Glycine, glutamic acid, and GABA structures. All are amino acids.
- The biogenic amines dopamine, norepinephrine, epinephrine, serotonin, and histamine, which are made from amino acid precursors.
- The purinergic neurotransmitters ATP and adenosine, which are nucleotides and nucleosides.Adenosine structure.
- Acetylcholine, which does not fit into any of the other structural categories, but is a key neurotransmitter at neuromuscular junctions (where nerves connect to muscles), as well as certain other synapses.Acetylcholine structures.
The neuropeptides are each made up of three or more amino acids and are larger than the small molecule transmitters. There are a great many different neuropeptides. Some of them include the endorphins and enkephalins, which inhibit pain; Substance P, which carries pain signals; and Neuropeptide Y, which stimulates eating and may act to prevent seizures.
Amino acid sequence of enkephalin: N-Tyr-Gly-Gly-Phe-Met-C.
A neurotransmitter’s effects depend on its receptor
Some neurotransmitters are generally viewed as “excitatory," making a target neuron more likely to fire an action potential. Others are generally seen as “inhibitory," making a target neuron less likely to fire an action potential. For instance:
- Glutamate is the main excitatory transmitter in the central nervous system.
- GABA is the main inhibitory neurotransmitter in the adult vertebrate brain.
- Glycine is the main inhibitory neurotransmitter in the spinal cord.
However, "excitatory" and "inhibitory" aren't really clear-cut bins into which we can sort neurotransmitters. Instead, a neurotransmitter can sometimes have either an excitatory or an inhibitory effect, depending on the context.
How can that be the case? As it turns out, there isn’t just one type of receptor for each neurotransmitter. Instead, a given neurotransmitter can usually bind to and activate multiple different receptor proteins. Whether the effect of a certain neurotransmitter is excitatory or inhibitory at a given synapse depends on which of its receptor(s) are present on the postsynaptic (target) cell.
Let's make this more concrete by looking at an example. The neurotransmitter acetylcholine is excitatory at the neuromuscular junction in skeletal muscle, causing the muscle to contract. In contrast, it is inhibitory in the heart, where it slows heart rate. These opposite effects are possible because two different types of acetylcholine receptor proteins are found in the two locations.
Cell type specificity in response to acetylcholine.
Left panel: skeletal muscle cell. The acetylcholine molecule binds to a ligand-gated ion channel, causing it to open and allowing positively charged ions to enter the cell. This event promotes muscle contraction.
Right panel: cardiac muscle cell. The acetylcholine molecule binds to a G protein-coupled receptor, triggering a downstream response that leads to inhibition of muscle contraction.
- The acetylcholine receptors in skeletal muscle cells are called nicotinic acetylcholine receptors. They are ion channels that open in response to acetylcholine binding, causing depolarization of the target cell.
- The acetylcholine receptors in heart muscle cells are called muscarinic acetylcholine receptors. They are not ion channels, but trigger signaling pathways in the target cell that inhibit firing of an action potential.
Types of neurotransmitter receptors
As the example above suggests, we can divide the receptor proteins that are activated by neurotransmitters into two broad classes:
- Ligand-activated ion channels: These receptors are membrane-spanning ion channel proteins that open directly in response to ligand binding.
- Metabotropic receptors: These receptors are not themselves ion channels. Neurotransmitter binding triggers a signaling pathway, which may indirectly open or close channels (or have some other effect entirely).
Ligand-activated ion channels
The first class of neurotransmitter receptors are ligand-activated ion channels, also known as ionotropic receptors. They undergo a change in shape when neurotransmitter binds, causing the channel to open. This may have either an excitatory or an inhibitory effect, depending on the ions that can pass through the channel and their concentrations inside and outside the cell.
Ligand-activated ion channels are large protein complexes. They have certain regions that are binding sites for the neurotransmitter, as well as membrane-spanning segments that make up the channel.
Diagram of ligand-activated channel. When neurotransmitter binds to the channel, it opens and cations flow down their concentration gradient and into the cell, causing a depolarization.
Ligand-activated ion channels typically produce very quick physiological responses. Current starts to flow (ions start to cross the membrane) within tens of microseconds of neurotransmitter binding, and the current stops as soon as the neurotransmitter is no longer bound to its receptors. In most cases, the neurotransmitter is removed from the synapse very rapidly, thanks to enzymes that break it down or neighboring cells that take it up.
Activation of the second class of neurotransmitter receptors only affects ion channel opening and closing indirectly. In this case, the protein to which the neurotransmitter binds—the neurotransmitter receptor—is not an ion channel. Signaling through these metabotropic receptors depends on the activation of several molecules inside the cell and often involves a second messenger pathway. Because it involves more steps, signaling through metabotropic receptors is much slower than signaling through ligand-activated ion channels.
Diagram of one way that a metabotropic receptor can act. The ligand binds to the receptor, which triggers a signaling cascade inside the cell. The signaling cascade causes the ion channel to open, allowing cations to flow down their concentration gradient and into the cell, resulting in a depolarization.
Some metabotropic receptors have excitatory effects when they're activated (make the cell more likely to fire an action potential), while others have inhibitory effects. Often, these effects occur because the metabotropic receptor triggers a signaling pathway that opens or closes an ion channel. Alternatively, a neurotransmitter that binds to a metabotropic receptor may change how the cell responds to a second neurotransmitter that acts through a ligand-activated channel. Signaling through metabotropic receptors can also have effects on the postsynaptic cell that don’t involve ion channels at all.
Conventional neurotransmitters and their receptor types
|Neurotransmitter||Ligand-activated ion channel receptor(s)?||Metabotropic receptor(s)?|
|Acetylcholine||Yes (excitatory )||Yes|
This table isn't a comprehensive listing, but it does cover some of the most well-known conventional neurotransmitters.
All of the neurotransmitters we have discussed so far can be considered “conventional” neurotransmitters. More recently, several classes of neurotransmitters have been identified that don’t follow all of the usual rules. These are considered “unconventional” or “nontraditional” neurotransmitters.
Two classes of unconventional transmitters are the endocannabinoids and the gasotransmitters (soluble gases such as nitric oxide, , and carbon monoxide, ). These molecules are unconventional in that they are not stored in synaptic vesicles and may carry messages from the postsynaptic neuron to the presynaptic neuron. Also, rather than interacting with receptors on the plasma membrane of their target cells, the gasotransmitters can cross the cell membrane and act directly on molecules inside the cell.
Other unconventional messengers will probably be discovered as we learn more and more about how neurons work. As these new chemical messengers are discovered, we may have to further change our idea of what it means to be a neurotransmitter.
Want to join the conversation?
- If I understand correctly, the point in having different types of neurotransmitters is that they do different things. But if a neuron has only two states, firing and not firing, how can different neurotransmitters do different things?(9 votes)
- The membrane potential has to reach a certain threshold for firing; this is known as summation (for which there are spatial and temporal components) and occurs at the axon hillock. Certain populations of neurons only express receptors for certain neurotransmitters. Excitatory and inhibitory NTs work with or against one another to bring the membrane potential closer to or farther from that firing threshold. Look on the wikipedia pages for summation, EPSPs, and IPSPs, for more information.(3 votes)
- What happens if receptor sites for the NT were blocked(3 votes)
- If the receptor sites for the neurotransmitter are blocked, the neurotransmitter is not able to act on that receptor. Most of the time, the neurotransmitter will then be taken back up by the neuron that released it, in a process known as "reuptake". However, in the case of Acetylcholine, there will be multiple copies of the enzyme known as acetylcholinesterase within the synapse that will break it down.(4 votes)
- I want to know about brain structure and transactions in centres.(4 votes)
- I do not know what :transactions_ you are speaking of, but I found this.
If you are interested in more brain related things check that out. And if you have specific question come back:
- how many receptors on a garden variety human brain neuron?(2 votes)
- Neuropeptide Y stimulates eating, according to this article. What does that mean?(2 votes)
- It means that Neuropeptide Y stimulates processes related to increased food intake, such as greater production of saliva from salivary glands, gut motility and subjective feeling of empty stomach and hunger.
This paper explains the experimental procedure:
- what are membrane spanning segments?(2 votes)
- intrinsic channel proteins. Basically channel proteins that span the cell membrane(1 vote)
- What would happen if neurotransmitters stayed attached to the receptors at the synapse?(1 vote)
- If a neurotransmitter were to stay attached to the receptors it would essentially block that receptor from other neurotransmitters. When neurotransmitters bind to receptors, those receptors become activated. Activated receptors would open or close ion channels, which would affect the membrane potential of the postsynaptic cell. However, the opening or closing of those channels are brief. Thus, if neurotransmitters stayed attached to the receptors they would effectively act as a receptor blocker. For example, naloxazone irreversibly binds to mu-opioid receptors, which prevent them from being activated from opioids.(2 votes)
- what determines if a neurotransmitter is excitatory or inhibitory?(1 vote)
- Receptors for that neurotransmitter determines whether it'll have an excitatory or inhibitory effect. If the receptor for that neurotransmitter is ionotropic, the activation of that receptor will open or close certain ion channels, thereby altering the membrane potential of the postsynaptic cell.(2 votes)
- Hi, can I know what's the difference between muscarinic and nicotinic receptors? Does both of it produce a sympathetic and parasympathetic response? What about the excitatory and inhibitory response? Do both muscarinic and nicotinic receptors exhibit these responses?(1 vote)
- What happens with the unmyelinated axons?(1 vote)