Neurotransmitter Systems

Pamela Brown
Nov 29, 2025
5 min read

Your brain is not one big electrical signal. It’s actually hundreds of chemical conversations happening at once. These chemicals are neurotransmitters, and they control everything from motivation to movement. You’ve probably heard of dopamine and serotonin. Dopamine gets called the “pleasure” molecule and serotonin the “happiness” molecule. While those labels have tiny grains of truth, they’re mostly oversimplified marketing slogans. The real science is far more interesting, much more complex, and honestly, way more useful.

What are Neurotransmitters

Instead of thinking of neurotransmitters as “moods in molecule form,” it’s better to think of them as chemical messengers. I covered the details of synaptic transmission in my last post, but here’s a quick reminder:

An action potential reaches the axon terminal and triggers a chain of events that releases neurotransmitters into the synaptic cleft. Once in the cleft, some of those molecules bind to receptors on the postsynaptic neuron. Depending on the type of receptor, they can make the neuron’s membrane potential more positive (excitatory) or more negative (inhibitory). But a neuron isn’t necessarily controlled by one signal. It's the combination of all the excitatory and inhibitory inputs arriving at the same time that determines whether it fires its own action potential. It’s a constant balancing act.

When a Neuron Needs to Fire

There are some places in the body where a synapse is designed to always cause an action potential. A perfect example is the neuromuscular junction (NMJ), where motor neurons communicate with skeletal muscle using acetylcholine (ACh). At the NMJ:

The presynaptic neuron releases a huge amount of ACh.
The postsynaptic membrane (the muscle fiber) has tons of ACh receptors.
The synapse is structured so that the voltage always depolarizes enough to cross threshold.

This design is important because when you need to move—especially in an emergency—you cannot afford for the signal to fail. If you step on a nail or touch a hot stove, your peripheral nervous system makes sure the “MOVE NOW” command fires automatically and reliably. This is a good reminder that neurotransmitters don’t always operate in subtle, modulatory ways. Some systems are built for speed and certainty.

How We Define a Neurotransmitter

A chemical is categorized as a neurotransmitter based on function, not chemistry. A molecule is defined as a true neurotransmitter if it meets the following conditions.

It must be:

Produced and stored in a neuron.
Released in response to stimulation.
Capable of producing a response in a postsynaptic cell.

That means very different chemicals, from amino acids to peptides, can count as neurotransmitters as long as they meet these criteria.

The Three Major Types of Neurotransmitters

Classic neurotransmitters fall into three different categories outlined below:

Type	Description	Examples
Amino Acid	Fast, direct, and responsible for major excitation / inhibition	Glutamate, GABA
Amines	Modulatory, slower, influence circuits and mood	Dopamine, serotonin, norepinephrine
Peptides	Slower, emotional, and long-term signaling	Substance P, endorphins, oxytocin

Key Neurotransmitters (NT) You Should Know

The following are some important neurotransmitter that you should be aware of as well as their main effects on a postsynaptic neuron:

Glutamate	Main excitatory NT
GABA	Main inhibitory NT
Dopamine	Motivation predictor; expectation of reward
Serotonin	Mood balance, appetite, sleep
Acetylcholine	Muscle activation, attention
Norepinephrine	Arousal, stress response (AKA adrenaline)
Endorphins	Pain modulation and reward (famous for the “runner’s high”)

A Closer Look at Dopamine

Dopamine surges not when a reward is received, but when it's anticipated—making it a powerful engine of motivation. This neurochemical drives us toward goals, novelty, and progress, which is why the pursuit often feels more rewarding than the achievement itself. It’s the reason the journey—striving, improving, advancing—can be more satisfying than the destination. This is also why Friday tends to be a favorite day of the week. Studies show that more people prefer Fridays over Sundays despite the fact that Sunday is part of the weekend. Anticipation is powerful. If you want a great deep dive, The Molecule of More by Daniel Z. Lieberman MD, and Michael E. Long. It is an amazing introduction to dopamine’s enormous influence on human behavior.

Serotonin: Why It’s Not the “Happiness Molecule”

SSRIs, or selective serotonin reuptake inhibitors, increase serotonin levels in the synaptic cleft by blocking reuptake, which gives serotonin more time to bind to receptors. But this doesn’t automatically equal “more happiness.”

Here’s the more accurate story:

Serotonin influences mood stability, emotional regulation, sleep, and appetite.
SSRIs are slow because they change receptor sensitivity, not just serotonin levels.
Serotonin affects how stable your emotional baseline is, not whether you feel joy.

So yes, calling serotonin the “happiness molecule” is overly simplistic. It’s more like the “emotional thermostat.”

Too Much Neurotransmitter ≠ Better

More does not mean better. In fact, too much of a neurochemical can be just as detrimental as too little. Neurotransmitters need to stay in balance. A good example is epilepsy. Many seizures involve runaway excitation, often driven by an excess of glutamate or not enough of the inhibitory GABA. This shows that mental health and brain function aren’t about “add more chemical.” They’re about regulating signaling dynamics.

Most psychiatric and neurological conditions involve:

Mistimed release
Disrupted receptor sensitivity
Altered feedback loops
Faulty clearance or reuptake

…not simple chemical deficiencies.

Researchers now know that most psychiatric disorders stem from disrupted neurotransmitter signaling dynamics rather than simply “too much” or “too little”. Even tiny shifts in how neurotransmitters are released, cleared, or detected by receptors can reshape the activity of entire neural circuits. Because neurons operate in interconnected networks, not in isolation, a small change in one part of a circuit can ripple outward into attention, mood, memory, and behavior.

New Discoveries

For years, people believed there were only a handful of classical neurotransmitters. Now we know:

Neuropeptides are everywhere. Gasotransmitters like nitric oxide and carbon monoxide also act as NTs.

Endocannabinoids (lipid-based messengers) challenge our classic definition of what a NT is, and some cells even co-release multiple neurotransmitters at once (called cotransmission). The list keeps growing as our technology improves. Our knowledge of neuroscience is far from complete.

Practical Neuroscience Moment

Now that you know how powerful neurotransmitters are, here are three science-backed ways to naturally support healthy NT balance - with no supplements or gimmicks required:

Exercise - increases serotonin and dopamine receptor sensitivity.

Learning new things - increases dopamine release.

Sleep - restores neurotransmitter levels and receptor function

Final Thoughts

There is so much more to say about neurotransmitters. Entire books have been written about single molecules, and we’re still discovering new ones. Even tiny changes in their timing, release, or receptor activity can completely shift how we think, feel, and function. The more we understand these chemical conversations, the better we understand ourselves, and the better we can shape a healthier, more resilient brain.

*Neuromuscular Junction image provided by Servier Medical Art (https://smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).