How Neurons Communicate

Welcome to Lesson Six

Deep inside your brain is something neuroscientists call a neural network. Right now, there are billions of neurons firing, signaling, and communicating nonstop. Think of it as a city that never sleeps.

Welcome back to The Psychology Notebook. This is lesson six in our Psych 101 series.

In this lesson, we’re going inside the brain to watch how one message travels across billions of neurons in real time.

By the end of this lesson, you’ll understand how neurons communicate, what neurotransmitters actually do, and why a tiny gap between brain cells controls everything from your mood to your memory.

Let’s get started.

An Incoming Message

Today, something urgent is happening. A message has arrived, and it absolutely must be delivered. The question is: Will it make it?

Your brain doesn’t just automatically send every signal it receives. It has to decide. And that decision process—it’s the reason you can focus right now or completely zone out.

In a moment, we will explain why this is so important. But now there’s an incoming message inside one neuron. Let’s call it neuron 24B.

The cell body, also known as the soma, is receiving incoming data.

“All right, everyone. Signals coming in from the dendrites. Let’s see what we’re working with.”

Excitatory and Inhibitory Signals

Now, the dendrites are the neuron’s receivers. They pick up chemical signals from neighboring neurons.

And here’s where it gets interesting. Not all signals say the same thing.

Some are excitatory. They’re saying, “Fire. Send the message. Go.”

Others are inhibitory. They’re saying, “Slow down. Don’t send it. Stop.”

The cell body has to add all of these up. This is called summation, and it’s one of the most important concepts in neural communication.

“We’ve got three excitatory signals pushing us forward and two inhibitory signals telling us to hold. Let’s total it up.”

Threshold and the All-or-None Principle

If the excitatory input becomes strong enough, the neuron reaches something called threshold, which is the minimum level of stimulation needed to trigger a response.

Think of it like a tipping point.

“Threshold reached. Send the signal.”

Here’s the part that surprises a lot of psychology students. Once the threshold is reached, the neuron fires at full strength.

No half signals. No partial firing.

This is called the all-or-none principle. The neuron either fires completely or not at all.

And when it fires, something powerful is released. That’s where Spark comes in.

Meet Spark: The Action Potential

The electrical signal that fires is called the action potential. And this tiny electrical event is the foundation of how your brain communicates.

Every thought you’ve ever had, every movement you’ve ever made, and every emotion you’ve ever felt started with an action potential, just like this one.

“I’ve been activated. What’s the mission?”

“Deliver this message to the next neuron. Move.”

Spark launches down the neuron. But there’s a problem ahead, because every neuron has something at the end of it that no electrical signal can cross on its own.

We’ll get there in a moment. First, Spark has to travel.

The Axon and Myelin

Spark shoots down the axon, the neuron’s long communication highway.

This is the part of the neuron that carries the electrical signal away from the cell body toward the next neuron.

But speed matters. Your brain can’t afford slow signals, especially in emergencies. So, how does it speed things up?

That’s where myelin comes in.

Myelin is a fatty insulating layer wrapped around the axon by specialized cells called glial cells.

Think of it like the rubber insulation around an electrical wire. It prevents signal loss and dramatically increases speed.

Nodes of Ranvier and Saltatory Conduction

But here’s the clever part. Myelin doesn’t cover the axon continuously. Instead, there are tiny gaps along the axon called nodes of Ranvier.

Rather than traveling smoothly down the entire axon, the action potential actually jumps from node to node.

This process is called saltatory conduction. The word comes from Latin and literally means “to leap.” And this jumping dramatically increases speed.

Neural signals can travel up to 100 meters per second. That’s why you can pull your hand away from a hot stove before you even consciously feel the pain.

But Spark is about to reach the end of the road. And that’s where things get interesting.

Reaching the Synapse

Spark arrives at the axon terminal, and suddenly the road ends.

“Wait, where’s the rest of the path?”

What Spark is looking at is called the synapse, and it’s one of the most important structures in neuroscience.

A synapse is a microscopic gap between two neurons.

And here’s a critical detail. Neurons never actually touch each other. There is always a space between them.

Spark is an electrical signal, but electricity cannot jump across this gap. So the brain switches strategies.

At this point, communication changes from electrical to chemical.

And the molecules responsible for carrying the message across? They’re about to enter the scene.

Neurotransmitters

When the action potential reaches the axon terminal, it triggers the release of neurotransmitters: chemical messengers stored in tiny sacs called vesicles.

These chemicals are the only thing capable of carrying the signal across the synapse.

And each neurotransmitter has a very specific role in the brain.

Let me introduce you to some of the most important ones you’ll encounter in psychology.

Dopamine

“Important message. I’m in. I handle reward, motivation, and pleasure.”

Dopamine plays a major role in the brain’s reward system. It helps drive motivation, reinforces learning, and is involved in pleasure experiences.

But dopamine must stay balanced.

Too little dopamine is associated with Parkinson’s disease, while excessive dopamine activity in certain pathways has been linked to schizophrenia.

Serotonin

“Everyone relax. Mood, sleep, and appetite are my department.”

Serotonin helps regulate mood, sleep, and digestion.

Low serotonin activity is commonly associated with depression and anxiety. That’s why many antidepressant medications known as SSRIs work by increasing the availability of serotonin in the brain.

Acetylcholine

“Ready, set, move.”

Acetylcholine is essential for muscle movement, attention, and memory.

It’s the neurotransmitter that signals your muscles to contract, allowing your body to move.

Declining acetylcholine activity is also linked to Alzheimer’s disease, which affects memory and cognitive function.

GABA

“All right, everyone, let’s calm things down.”

GABA, short for gamma-aminobutyric acid, is the brain’s primary inhibitory neurotransmitter.

It slows neural activity and helps keep the brain from becoming overstimulated.

Without enough GABA, neural activity can become excessive, which can contribute to seizures and anxiety disorders.

Glutamate

“Time to wake the brain up.”

Glutamate is the brain’s main excitatory neurotransmitter. It plays a crucial role in learning and memory formation.

But like dopamine, balance matters.

Too much glutamate can damage neurons through a process known as excitotoxicity.

Endorphins

“Pain, stress—I’ve got backup.”

Endorphins are the body’s natural painkillers.

They’re released during exercise, stress, and injury, and they bind to many of the same receptors as opioid drugs.

That famous runner’s high people talk about—that’s endorphins at work.

Norepinephrine

“Threat detected. Everyone, stay sharp.”

Norepinephrine is your brain’s alarm system.

It controls alertness, attention, and your fight-or-flight response.

When danger hits, or even when you’re just stressed about a deadline, norepinephrine floods your system to keep you focused and ready to act.

Too little is linked to fatigue and difficulty concentrating. Too much keeps you in a constant state of anxiety and hypervigilance.

It’s the reason your heart races before a big exam, even when you know you studied.

Crossing the Synaptic Cleft

Now, all of these neurotransmitters are packaged into vesicles waiting at the axon terminal.

They’re ready to cross the synapse, but they can’t just attach anywhere. They need to find the right target.

And this is where one of the most elegant systems in biology comes into play.

The vesicles fuse with the membrane of the axon terminal and release neurotransmitters into the synaptic cleft, the tiny, fluid-filled space between neurons.

Once released, these chemical messengers drift across the gap toward the next neuron.

But they can’t attach just anywhere. Each neurotransmitter must find a receptor that matches its exact shape.

This is known as the lock-and-key model.

“Delivering to the movement receptor.”

“Perfect fit.”

“Reward pathway confirmed. Message delivered.”

Each receptor is designed to accept only one specific neurotransmitter.

Dopamine fits dopamine receptors. Serotonin fits serotonin receptors.

If the shapes don’t match, the message can’t be delivered.

When the correct neurotransmitter binds to its receptor, it either excites or inhibits the next neuron, influencing whether that neuron will fire its own action potential.

And just like that, the signal continues its journey.

Clearing the Signal

But the synapse can’t stay flooded with neurotransmitters forever.

If these chemicals stayed in the synapse too long, neurons would keep firing over and over again.

So, the brain has a cleanup system. And what happens next is just as important as the signal itself.

After neurotransmitters deliver their message, they can’t stay in the synapse forever. If they did, neurons would continue firing again and again, and the signal would never stop.

So the brain has a cleanup system. This process is called reuptake.

Reuptake and Recycling

During reuptake, the presynaptic neuron reabsorbs the neurotransmitters it released, pulling them back into the axon terminal for recycling and reuse.

“Message delivered.”

“All right, everyone, back to the terminal. Let’s reset the system.”

Some neurotransmitters are recycled through reuptake. Others are broken down by specialized enzymes in the synapse.

Either way, the goal is the same: clear the synapse so the next signal can be sent.

This constant cycle of release, signaling, and cleanup happens billions of times every second inside your brain.

SSRIs and Neural Communication

Now, here’s where this becomes incredibly important for psychology and medicine.

Remember when we talked about antidepressants called SSRIs?

SSRI stands for selective serotonin reuptake inhibitor.

These medications work by blocking the reuptake of serotonin, which leaves more serotonin available in the synapse.

That increased availability can help improve mood in people experiencing depression.

So when scientists design treatments for mental health conditions, they’re not guessing. They’re targeting the exact communication system you’re learning about right now.

The Message Continues

The next neuron receives the message.

Its dendrites detect the incoming neurotransmitters. The cell body adds up the signals. Threshold is reached, and a brand-new action potential fires.

“Message received. Spark 2, begin transmission.”

And just like that, the signal continues its journey.

Inside each neuron, communication is electrical. But between neurons, communication is chemical.

That constant shift—electricity inside, chemistry between—is what allows your brain to send information across billions of cells.

This process happens billions of times every second in the human brain.

Every thought you have, every memory you form, every movement you make depends on this exact system.

Coming Up Next

If you’re enjoying this series and want to keep learning psychology in a way that actually makes sense, make sure you subscribe to The Psychology Notebook so you don’t miss the next lesson.

In the next lesson, lesson seven, we zoom out and explore the nervous system and explain why stress, safety, and survival can make your brain feel like the world is dangerous even when it isn’t.

Thanks for learning with me, and I’ll see you in the next lesson.