Neurotransmitters

What are neurotransmitters?

Neurotransmitters aid the communication between neurones.

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).

Fig. 2. Synapse diagram.

• 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.

• 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).

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.

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)

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.

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.

When we consider the effects of neurotransmitters such as serotonin, GABA, and epinephrine on the body, we can say that neurotransmitters significantly impact behaviours.

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

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.

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.

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: