How do neurones communicate with one another? How are impulses sent across the brain? Neurones are the basic units of the nervous system, integral to many functions, and they number in the billions. Neurones are essential in how information from the outside world is received, integrated, processed, and passed on to other neurones or non-neuronal structures (known as effectors). They communicate through nerve impulses, which are enabled by neurotransmitters. So, what are neurotransmitters?
- We are going to delve into the world of neurotransmitters in psychology.
- First, we will provide a neurotransmitter definition.
- Then, we will explore how neurotransmitters work, discussing neurotransmitter function and providing examples of neurotransmitters.
- Furthermore, we will highlight the different types of neurotransmitters.
- Finally, we will discuss the different disorders associated with neurotransmitter dysfunction.
Fig. 1 - Neurotransmitters are chemical messengers in the nervous system.
What are neurotransmitters?
Neurotransmitters aid the communication between neurones.
Neurotransmitters are chemical messengers that send signals from neurones to other neurones or receiving structures.
Receiving structures include glands, muscles, and organs, formally known as effector organs. They are tissues or organs that respond to the signal de neurones and the nervous system as a whole is sending them, like for example excreting hormones, contracting, etc.
There is a synaptic cleft between neurones, a little space where neurotransmitters pass from one neurone to another. The synaptic cleft is where many important processes occur and where neurotransmitters 'work'.
What is a Neurotransmitter's Function?
A neurotransmitter's function is to aid communication between neurones, or between neurones and effector organs. They transmit the information that allows animals to understand their environment and emit a response to (changes in) it.
The Release of Neurotransmitters at Synapses
We can summarise neurotransmitter release in the following way: neurotransmitters are the chemical messengers released by presynaptic neurones (the cell that sends the signals) into the synaptic cleft, to bind to specific receptors on postsynaptic neurones or non-neural cells (the cell that receives the signals).
Presynaptic neurones are the neurones that send the signal, i.e. that releases de neurotransmitters. Neurotransmitters are released at the end of the presynaptic neurone's axon (axon terminal).
Postsynaptic neurones are the neurones that receive the signal, i.e. that generate a response (an action potential) to the neurotransmitters binding to their receptors. Signal reception usually happens at the dendrites. A postsynaptic neuron can be presynaptic too if it releases neurotransmitters after receiving and processing the neurotransmitters from another neuron.
The synaptic cleft is the very small space between the pre- and postsynaptic neurones.
Fig. 2. Synapse diagram.
Let's have a look at the process of synaptic communication and neurotransmitter release:- A presynaptic neurone has received a stimulus and generates an action potential that travels all the way to its axon terminal. There, the change in membrane potential carried by the action potential initiates the process of neurotransmitter release: the vesicles carrying neurotransmitters fuse with the neurone's membrane, so that the neurotransmitters are released to the synaptic cleft.
An action potential is a change in the electrical charge in a cell. Only certain cells, like neurones, can produce an action potential. Usually, the internal side of the membrane has a negative charge compared to the outside of a cell's membrane. However, when an action potential is initiated, the inside of the cell turns more positive. Not all changes in membrane potential transform into action potentials, but when they do this change in charge initiates other processes in the cell. In neurones, positive charge change usually travels down the axon to the axon terminal.
Membrane potential refers to the difference in electrical charge across the membrane of a neurone between its internal environment and external environment.
- In the synaptic cleft, neurotransmitters diffuse and reach the postsynaptic cell. They will then bind to their specific receptors on the postsynaptic membrane, and issue a reaction in the cell.
- The neurotransmitters can then affect the membrane potential of the receiving neurone or cell, which may cause another action potential in the receiving neurone or cell (depolarisation) or stabilise it (hyperpolarisation).
Neurotransmitters can have an excitatory or inhibitory effect, which we will discuss soon! A neurotransmitter's function and effect on the receiving cell (postsynaptic neurone) varies depending on the neurotransmitter released and the receptor the receiving cell has to receive the neurotransmitter.
Fig. 3: (a) The synaptic cleft is the space between the axon terminal or terminal button of one neurone and the dendrite of another neurone. (b) In this pseudo-coloured image from a scanning electron microscope, a terminal button (green) has been opened to reveal the synaptic vesicles (orange and blue) inside.
Neurotransmitters trigger or inhibit an impulse in the next neurone. After release, neurotransmitters are removed from the synaptic cleft through several processes, such as reuptake and the breakdown of the neurotransmitter.
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 diffuses from the PREsynaptic neurone, across the synaptic cleft, to the POSTsynaptic neurone.
What happens to neurotransmitters after they have completed their function?
Neurotransmitters need to be eliminated after a short time in the synaptic cleft because if not the neuronal signal would not stop and neurones or effector cells would not be able to go back to their resting state. In other words, it is essential for terminating synaptic transmission.
Neurotransmitters can be removed from the synaptic cleft through three methods:
- Diffusion: diffusion is the most basic system for eliminating neurotransmitters from the synaptic cleft. In this method, neurotransmitters will just move away from the cleft and away from the pre- and postsynaptic cells.
- Reuptake: either the presynaptic cell or glial cells surrounding the neurons can reabsorb the neurotransmitters. Sometimes, the receptors bound to the neurotransmitters in the postsynaptic cells can be absorbed via endocytosis too.
- Enzymatic degradation: enzymes that break down certain types of neurotransmitters can be released to the synaptic cleft, and the resulting products of the digestion be absorbed by the surrounding cells.
After each signal, the synaptic cleft is cleared of neurotransmitters to enable the processing of upcoming signals independently. However, if a new neurotransmitter release occurs before the previous one is eliminated, the two signals work together.
What are the Different Types of Neurotransmitters?
Depending on the reaction of the postsynaptic cell, neurotransmitters can be classified as excitatory or inhibitory. They can also be divided depending on their chemical structure:
- Amino acid neurotransmitters (glutamate and GABA)
- Monoamine neurotransmitters (epinephrine, dopamine, serotonin)
- Peptide neurotransmitters (oxytocin, somatostatin)
Monoamine neurotransmitters are molecules formed by an amino group (-NH2) bound to an aromatic ring by a two-carbon chain (-CH2-CH2). All monoamines are derived from the aromatic amino acids phenylalanine, tyrosine, or tryptophan.
As we said before, the receptor has a role in how a neurotransmitter affects the postsynaptic neurone. In the same way, each type of neurotransmitter will impact the postsynaptic neurone differently.
Research into the topic of neurotransmitters highlights how scientists are still discovering more and more neurotransmitter types every day, so we cannot say we have found all the different types of neurotransmitters.
Excitatory and Inhibitory Neurotransmitters
This table summarises the differences between excitatory and inhibitory neurotransmitters.
| Neurotransmitter type | Definition | Examples |
| Excitatory neurotransmitters | These neurotransmitters increase the chances of a resulting action potential occurring in the postsynaptic neurone or receiving cell. | Glutamate, epinephrine, norepinephrine |
| Inhibitory neurotransmitters | These neurotransmitters decrease the chances of a resulting action potential occurring in the postsynaptic neurone or receiving cell. | GABA, glycine and serotonin |
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.
Neurotransmitter Examples
There's a vast array of neurotransmitters, and they can have different effects on their target cell. Let's explore the various examples of neurotransmitters.
- Noradrenaline mobilises the brain and body for action, involved in the fight-or-flight response. It is also known as norepinephrine. Despite their similarities, it works more specifically on receptors than epinephrine, mainly affecting muscle contraction.
- Glutamate is one of the primary excitatory neurotransmitters involved in learning, memory and cognition and a precursor of GABA. An excess of glutamate can result in issues such as epilepsy and cases of excitotoxicity.
- Epinephrine also plays a role in the fight-or-flight response, increasing heart rate and glucose levels in the blood, and is released by the adrenal glands.
- Acetylcholine affects the central and peripheral nervous system and is responsible for activities such as smooth muscle contraction in the autonomic nervous system.
- Dopamine is more commonly associated with reward pathways in the brain, involved in motivation and feelings of pleasure. Dysfunction of dopamine is linked closely 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. 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.)
- GABA (gamma-aminobutyric acid), formed from glutamate, inhibits or reduces the excitatory neurotransmitters that affect the central nervous system. It is also associated with moods and emotions. As glutamate is one of the primary excitatory neurotransmitters, GABA is one of the most inhibitory neurotransmitters.
- 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.
Serotonin and dopamine are also examples of neuromodulators. Neuromodulators are molecules that affect neuronal signalling.
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 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 reward and motivation. As a result, it is closely linked to drug abuse and addiction, as drugs impact the release and reuptake of dopamine and artificially extend these feelings.
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 essentially prolongs 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.
Fig. 3: Dopamine is a neurotransmitter involved in the reward pathways of the brain.Neurotransmitter Disorders in Psychology
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, which is 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 unnecessary action (this is why people who suffer from anxiety have increased heart rates and sweating levels and experience panic.)
- Schizophrenia: 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 take antipsychotic medications that influence their dopamine levels. Typical antipsychotics block dopamine receptors in the brain, preventing dopamine uptake. Atypical antipsychotics affect dopamine and other neurotransmitters, such as serotonin.
Those with depression often use drugs that increase serotonin levels by preventing serotonin reuptake. Thus, serotonin remains in the synapses for longer.
We often refer to drugs affecting neurotransmitters as agonists or antagonists.
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. 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 excitatory and inhibitory neurotransmitters. It applies to antagonists, too, as antagonist drugs will reduce the excitatory or inhibitory effect.
Differences between Neurotransmitters and Hormones
Neurotransmitters and hormones are both molecules generated in the body that serve for communication between tissues, organs and organ systems. However, even if the function is similar, hormones and neurotransmitters are very different. They vary in their:
- chemical properties,
- location,
- speed of action,
- duration of effect,
- target cells
Let's summarise the differences in the following table:
| Differences | Neurotransmitters | Hormones |
| Chemical properties | Small molecules | Large molecules |
| Effect location | Synapses | Endocrine glands, target organs |
| Effect speed | Rapid, milliseconds to seconds | Slower, seconds to minutes |
| Duration of effect | Brief, seconds to minutes | Longer, minutes to hours |
| Target cells | Neurons, muscle cells | Cells throughout the body |
| Function | Regulate synaptic transmission, communication from neurons | Regulate various physiological processes, such as growth and development, metabolism, and reproduction |
Neurotransmitters - Key takeaways
- Neurotransmitters are chemical messengers in the brain that allow neurones to communicate with one another over the synaptic cleft. Neurotransmitters are released from the presynaptic neurone through synaptic vesicles, into the synaptic cleft, and received by the postsynaptic neurone or non-neural cell (effector organs).
- The neurotransmitters will then affect the membrane potential of the receiving neurone or cell, which may cause another action potential in the receiving neurone or cell (depolarisation), or stabilise it (hyperpolarisation).
- We can categorise neurotransmitters into two main classifications: excitatory or inhibitory neurotransmitters. We can also classify neurotransmitters into different types, examples including monoamine neurotransmitters and amino acid neurotransmitters, to name a few.
- Excitatory neurotransmitters increase the chances of an action potential occurring in the postsynaptic neurone. Inhibitory neurotransmitters decrease the chances of an action potential occurring in the postsynaptic neurone.
- Dopamine is an example of a neurotransmitter with excitatory and inhibitory effects depending on what receptor it binds to in the postsynaptic neurone.
References
- Fig. 2: Diagram of synaptic transmission by Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020). 3.2 Cells of the Nervous System. In Psychology 2e. OpenStax. https://openstax.org/books/psychology-2e/pages/3-2-cells-of-the-nervous-system (credit b: modification of work by Tina Carvalho, NIH-NIGMS; scale-bar data from Matt Russell)
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