Neurotransmitters

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.

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.

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:

Inhibitory neurotransmitters

Here are some examples of inhibitory neurotransmitters:

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.

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.

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:

Neurotransmitter 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.