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Neurotransmitters

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Neurotransmitters

Neurotransmitters are chemical messengers that send signals from neurones to receiving cells.

These cells could be another neurone, a gland, or muscle. There is a synaptic cleft between each neurone, a little space where neurotransmitters pass from one neurone to another. The synaptic cleft is where many important processes occur and where neurotransmitters ‘work’.

How do neurotransmitters work?

Neurotransmitters are the chemical messengers sent by presynaptic neurones (the cell that sends the signals) to postsynaptic neurones (the cell that receives the signals) through the small space between them (the synaptic cleft). This process occurs when an action potential is generated and transmitted to the synapse. This, then, triggers the release of neurotransmitters from the axon terminal (the ends of the axons).

For a neurotransmitter to reach the synaptic cleft, it travels in small synaptic vesicles (tiny bubble-like structures containing the neurotransmitters) from the presynaptic neurone. These synaptic vesicles then release the neurotransmitters into the synaptic cleft, where they will find binding sites (receptors) on the receiving cell.

A neurotransmitter’s function and effect on the receiving cell (postsynaptic neurone) varies depending on the receptor they have to receive the neurotransmitter.

This process is what happens when one neurone communicates with another. As neurones don’t connect, an electrical signal from one neurone is transmitted to another. What happens essentially is that each neurone triggers an electrical impulse in the next neurone.

There are a lot of keywords here that can be confusing. An excellent way to remember them is to prioritise learning the main ones. The prefixes (pre- and post-) indicate the direction of where the neurotransmitter is going.

For instance, a neurotransmitter travels from the PREsynaptic neurone, through the synaptic cleft, to the POSTsynaptic neurone. The synaptic cleft is the little space between the presynaptic and the postsynaptic neurone.

Overall, synaptic transmission is where an action potential (a term we covered in our Process of Synaptic Transmission article) starts the presynaptic neurone by preparing the synaptic vesicles with the neurotransmitter inside.

What are the different classifications of neurotransmitters?

As we said before, the receptor has a role in how a neurotransmitter affects the postsynaptic neurone. In the same way, different types of neurotransmitters also impact the postsynaptic neurone.

When a neurotransmitter is released into the synaptic cleft, it creates a reaction.

We can divide neurotransmitters into two main classifications. It essentially boils down to whether or not the neurotransmitter will cause an action potential in the postsynaptic neurone.

Excitatory

Inhibitory

These neurotransmitters increase the likelihood of a resulting action potential occurring in the postsynaptic neurone or receiving cell.

These neurotransmitters decrease the likelihood of a resulting action potential occurring in the postsynaptic neurone or receiving cell.

The neurotransmitter will cause an action potential by affecting and influencing the ion flow across cell membranes of the neurones to cause an excitatory or inhibitory effect. Neurotransmitters can also act as modulators: they can significantly impact many neurons and other neurotransmitters simultaneously. They often work with other neurotransmitters to enhance the impact of excitation or inhibition.

Excitatory neurotransmitters

Here are some examples of excitatory neurotransmitters:

  • Noradrenaline mobilises the brain and body for action. It is also known as norepinephrine (the names can be used interchangeably). It works more specifically on receptors than epinephrine despite their similarities.
  • Glutamate is one of the primary excitatory neurotransmitters. An excess of it can result in issues such as epilepsy and cases of excitotoxicity.
  • Epinephrine plays a role in the fight-or-flight response and is released by the adrenal glands. It is also known as adrenaline.
  • Acetylcholine is responsible for activating skeletal muscles and organs in the autonomic nervous system.
  • Dopamine is more commonly associated with reward pathways in the brain. Dysfunctions of dopamine are closely linked to schizophrenia and Parkinson’s disease. It can also have inhibitory effects. That is why the type of receptor it binds to is essential in distinguishing its effects.

Inhibitory neurotransmitters

Here are some examples of inhibitory neurotransmitters:

  • GABA (gamma-aminobutyric acid) inhibits or reduces the excitatory neurotransmitters that affect the nervous system. It is also associated with moods and emotions. As glutamate is one of the main excitatory neurotransmitters, GABA is one of the most inhibitory neurotransmitters. GABA acts as a balancing effect to glutamate in some instances.
  • Serotonin (5-HT) is closely associated with the modulation of moods and emotions and sleep regulation. Emotional disorders such as depression are often related to the dysfunction of serotonin.
  • Dopamine is a neurotransmitter that also inhibits movements and aids in coordination. This is why when dopamine levels are low, such as in cases of Parkinson’s disease, a person may struggle with movement disorders (a key symptom of Parkinson’s disease.)

Serotonin and dopamine are also examples of neuromodulators.

When we consider the effects of neurotransmitters such as serotonin, GABA, and epinephrine on the body, we can say that neurotransmitters significantly impact behaviours. Serotonin may make a person more calm and relaxed, whilst epinephrine (adrenaline), an essential factor in the fight-or-flight response, can have the complete opposite effect on behaviour. A person will feel more alert and anxious and experience feelings of fear with epinephrine.

Similarly, dopamine significantly affects behaviours, primarily when these behaviours result in dopamine release.

The ventral tegmental area (VTA) is one of the major areas associated with dopamine within the brain, as it has a lot of dopaminergic neurones. It is connected to the substantial nigra, another hotspot of the dopaminergic areas in the brain.

The VTA is very heavily associated with feelings of rewards and motivation. As a result, it is closely linked to drug abuse and addiction.

The drug cocaine has an inhibitory effect on dopamine transporters within these two dopaminergic areas: it inhibits dopamine reuptake in the VTA. Thus, dopamine in these dopaminergic areas remains in the synapses for longer, extending the rewarding feelings. This is what causes that infamous feeling of euphoria when people take cocaine. Cocaine is essentially just prolonging the effects of dopamine in the brain’s reward pathways.

Addiction becomes a problem when it takes more quantities of cocaine to produce the same effect.

What are the different types of neurotransmitters?

Neurotransmitters differ in categorising, but we can typically separate them into three major types: amino acids, monoamines, and peptides.

There are further categories to organise neurotransmitters. There are up to six categories overall. They are known as gasotransmitters, purines, and trace amines. Acetylcholine has its class!

Amino acids

Neurotransmitters that are classified within the amino acid type are:

  • Glutamate.

  • GABA.

Monoamines

Neurotransmitters that are classified within the monoamine type are:

  • Epinephrine.

  • Dopamine.

  • Serotonin.

Peptides

Neurotransmitters that are classified within the peptide type are:

  • Oxytocin.
  • Somatostatin.

Here is a chart indicating the biosynthetic precursor(s) of neurotransmitters. It shows you how some of the neurotransmitters are made up:

Neurotransmitters Biosynthetic Precursors StudySmarterNeurotransmitter biosynthetic precursor chart

What are some disorders associated with neurotransmitter dysfunction?

When things are going right with neurotransmitters, you can navigate your daily life without much hindrance. Your brain can operate and react accordingly to different situations.

However, certain disorders can occur when there is an imbalance in neurotransmitters:

  • Depression is a mood disorder associated with serotonin and dopamine, amongst other key neurotransmitters. Low serotonin levels are commonly associated with depression.
  • Anxiety is linked to noradrenaline/norepinephrine, associated with the fight-or-flight response. Dysfunction here can induce feelings of fear, and the need to run as the body starts to prepare for this (this is why people who suffer from anxiety have increased heart rates, levels of sweating, and feelings of general panic.)
  • Schizophrenia: this disorder is associated with excess dopamine levels in the brain, which can result in symptoms such as hallucinations and movement disorders (rocking back and forth, speech poverty, and avolition.)

Medications for these disorders often affect the neurotransmitter associated with them. For instance, schizophrenic patients often have antipsychotic medications that influence their dopamine levels. They work by blocking dopamine receptors in the brain, preventing dopamine uptake.

Those with depression often use drugs that increase serotonin levels by preventing the reuptake of serotonin. Thus, serotonin remains in the synapses for longer.

We often refer to drugs affecting neurotransmitters as agonists or antagonists:

  1. Agonists work by aiding the uptake of neurotransmitters at the receptors on the postsynaptic neurone/receiving cell. They bind to the synaptic receptors and facilitate the effects of the neurotransmitter.

  2. Antagonists work by binding to synaptic receptors as well. However, they reduce the effects of a neurotransmitter.

Do not confuse this with excitatory and inhibitory effects. Agonists will aid the effect of the neurotransmitter, both if it’s excitatory or inhibitory. It applies to antagonists, too, as antagonist drugs will reduce the excitatory or inhibitory effect.


Neurotransmitters - Key takeaways

  • Neurotransmitters are chemical messengers in the brain that allow neurones to communicate with one another over the synaptic gap. Neurotransmitters are released from the presynaptic neurone through synaptic vesicles, into the synaptic cleft, and received by the postsynaptic neurone.
  • There are two main types we can categorise neurotransmitters into, and it essentially boils down to whether or not the neurotransmitter will cause an action potential in the postsynaptic neurone. They can be excitatory or inhibitory.
  • Excitatory neurotransmitters increase the likelihood of an action potential occurring in the postsynaptic neurone. Inhibitory neurotransmitters decrease the likelihood of an action potential occurring in the postsynaptic neurone.
  • Dopamine is an example of a neurotransmitter that can have excitatory and inhibitory effects depending on what receptor it binds to in the postsynaptic neurone.
  • We can classify neurotransmitters into amino acids, peptides, and monamines.

Frequently Asked Questions about Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals between neurones to receiving cells. These could be another neurone, a gland, or muscle.

They do so in different ways, depending on the neurotransmitter. Some will make a person behave calmly and more relaxed, whilst others cause feelings of anxiety to behave more erratically. For instance, dopamine gives you a sense of motivation and is a significant part of the reward pathways in the brain. If you do something that activates this, it will provide you with a 'good feeling', making you more inclined to repeat the behaviour. 

Key neurotransmitters of interest in psychology include serotonin, dopamine, glutamate, and GABA. 

Neurotransmitters work by allowing presynaptic neurones to communicate with receiving cells (the postsynaptic neurones) through the small space between them (the synaptic cleft).


Typically, this occurs when an action potential is generated and transmitted to the synapse, which triggers the release of neurotransmitters from the axon terminal (the ends of the axons). Neurotransmitters then bind to the receptors and either inhibit or excite the postsynaptic neurone, stopping or continuing the action potential. 

Final Neurotransmitters Quiz

Question

What is a neurotransmitter?

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Answer

It is a chemical messenger that allows for communication between one neurone and another.

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Why do neurones need neurotransmitters to communicate?

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Answer

The synaptic cleft means that neurones do not connect, so neurotransmitters bridge this gap.

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What is the synaptic cleft?

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It is the small gap between neurones.

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What are synaptic vesicles?


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They are the small, bubble-like structures that hold neurotransmitters in the presynaptic neurone, and release them into the synaptic cleft.

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What is the presynaptic neuron?


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The neuron proceeding the synaptic cleft from which the action potential originates.

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What is the postsynaptic neuron?


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The neuron that receives the neurotransmitter.

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What does the function of a neurotransmitter depend on?


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The type of neurotransmitter, as well as the receptor it binds to.

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What is an excitatory neurotransmitter?


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Answer

These neurotransmitters increase the likelihood of a resulting action potential occurring in the postsynaptic neurone or receiving cell.

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What is an inhibitory neurotransmitter?


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These neurotransmitters decrease the likelihood of a resulting action potential occurring in the postsynaptic neurone or receiving cell.

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Question

What are some examples of excitatory neurotransmitters?


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Answer

Glutamate, norepinephrine/noradrenalin, epinephrine, dopamine.

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What are some examples of inhibitory neurotransmitters?


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Answer

GABA, serotonin, dopamine.

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How does a neurotransmitter affect behaviours? 

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Answer

They affect behaviours by inducing certain feelings. Serotonin may make a person calmer and more relaxed, whilst epinephrine, an essential factor in the fight-or-flight response, can have the complete opposite effect on behaviour. A person will feel more alert and anxious and experience feelings of fear with epinephrine.

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What can we classify neurotransmitter types into?


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Answer

Amino acids, peptides, monoamines, gasotransmitters, purines, and trace amines. Acetylcholine has its own class.

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What neurotransmitters are amino acids?


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Answer

Glutamate, GABA.

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Question

Name a monoamine neurotransmitter.


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Answer

Epinephrine, dopamine, or serotonin.

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