Inside the Brain: Neurons and How They Communicate

1. From Whole Brain to Tiny Cells

In the previous module, you saw the big picture of the brain: major regions and what they do.

Now we zoom in to the microscopic level.

Inside your brain are about 86 billion neurons (nerve cells). These neurons:

• Receive information
• Process it
• Send signals to other cells

Together, they form networks that create everything you:

• Sense (sights, sounds, smells)
• Feel (emotions, pain, pleasure)
• Do (move, speak, plan, remember)

In this module you will:

1. Label the main parts of a neuron
2. Follow how an electrical signal (action potential) travels along a neuron
3. See how chemical signals (neurotransmitters) cross the gap between neurons
4. Connect this to real thoughts and behaviors

Keep in mind: neurons communicate using both electricity and chemistry. We will track a signal step by step, from one neuron to the next.

2. The Neuron: Main Parts and What They Do

Imagine a neuron as a tree with a long tail:

• Cell body (soma) – like the tree trunk
• Contains the nucleus (with DNA)
• Keeps the cell alive and decides whether to fire a signal

• Dendrites – like many small branches
• Receive incoming signals from other neurons
• Convert chemical messages into tiny electrical changes

• Axon – like a long tail or cable
• Carries the electrical signal (action potential) away from the cell body
• Can be very long (up to about 1 meter in humans for some neurons)

• Myelin sheath – like insulation on a wire
• Fatty coating around parts of the axon (made by glial cells)
• Speeds up electrical signal transmission

• Axon terminals (terminal buttons)
• Tiny branches at the end of the axon
• Make contact with other cells at synapses
• Release neurotransmitters (chemical messengers)

Visualize this:

• A roundish cell body in the center
• Bushy dendrites around it
• One long axon stretching out
• Axon covered in myelin "sausages" with small gaps
• Axon ending in many small terminals touching other cells

You will label these parts in the next activity.

3. Label the Neuron (Mental Sketch Exercise)

Without looking back, try this mental labeling exercise.

1. Close your eyes and imagine a neuron.
2. Picture each structure and say (or write) what it does.

Use this checklist:

• Dendrites – What do they receive? From where?
• Cell body (soma) – What is inside it that controls the cell?
• Axon – In which direction does it carry signals relative to the cell body?
• Myelin sheath – What does it help the signal do?
• Axon terminals – What do they release?

Now check yourself with this quick fill‑in (answer in your head or on paper):

1. Signals usually travel from (A) dendrites → (B) ____ → (C) axon → (D) ____ terminals.
2. The myelin sheath helps signals travel more f_____ and more r________.

Reveal the answers when you’re ready:

1. A: dendrites → B: cell body (soma) → C: axon → D: axon terminals
2. fast; reliably

If any part felt fuzzy, reread Step 2 and repeat this exercise once.

4. Resting Potential: A Neuron at Rest but Ready

Before a neuron sends a big signal, it sits in a resting state called the resting membrane potential.

Key idea: the inside of the neuron is slightly more negative than the outside.

• Typical value: about −70 millivolts (mV)
• This difference is created by:
• Ion pumps (especially the sodium–potassium pump)
• Ion channels that let some ions in or out

Important ions:

• Na⁺ (sodium) – more outside the neuron
• K⁺ (potassium) – more inside the neuron
• Cl⁻ (chloride) and negatively charged proteins also contribute

The resting potential means the neuron is polarized:

• Inside: more negative
• Outside: more positive

This is like a charged battery: no current flowing yet, but energy is stored and ready to be used.

You don’t need all the molecular details now. Just remember:

• A neuron at rest is not doing nothing; it is actively maintaining this charge difference
• This difference is what makes fast electrical signaling possible

5. Action Potential: The Electrical Signal in Motion

When a neuron receives enough input, it fires an action potential—a brief, rapid electrical signal that travels along the axon.

1. Trigger: Threshold

• Inputs on dendrites and the cell body slightly change the membrane potential
• If the membrane potential reaches a certain level (the threshold, often around −55 mV), the neuron fires
• This is often described as all‑or‑none: it either fires fully or not at all

2. Rising phase (Depolarization)

• Voltage‑gated Na⁺ channels open
• Na⁺ rushes into the cell
• Inside becomes less negative and then briefly positive (around +30 mV)

3. Falling phase (Repolarization)

• Na⁺ channels close
• Voltage‑gated K⁺ channels open
• K⁺ flows out, bringing the charge back down

4. Undershoot (Hyperpolarization)

• The membrane potential briefly becomes more negative than the resting level
• Then it returns to the resting potential as channels reset

5. Propagation along the axon

• This process repeats in segments along the axon
• In myelinated axons, the action potential jumps between gaps in the myelin called nodes of Ranvier (saltatory conduction)

Think of it like a wave in a stadium: each person stands up and sits down in order. The wave moves, but the people stay in place. Similarly, the action potential moves along the axon, but ions only move locally.

6. Real‑World Example: Touching a Hot Stove

Let’s connect this to something familiar: accidentally touching a hot stove.

1. Sensory receptors in your skin detect high temperature.
2. These receptors trigger action potentials in sensory neurons.
3. The action potentials travel along the axons up your arm toward your spinal cord.
4. In the spinal cord, those neurons make synapses with other neurons.
5. Some of those neurons send action potentials back down to muscles in your arm, telling them to contract.
6. You pull your hand away—often before you consciously feel pain.
7. Signals also travel up to your brain, where you become aware of the pain and heat.

Notice what’s happening:

• Action potentials carry information quickly along long distances (like wires)
• Synapses and neurotransmitters pass the message from one neuron to the next

The same basic pattern applies to seeing a ball and deciding to catch it, hearing your name, or remembering a song. Different parts of the brain are involved, but the communication rules are the same.

7. Synapses: Where Neurons Talk to Each Other

Neurons usually do not touch directly. They communicate across a tiny gap called a synapse.

Most synapses in the brain are chemical synapses (there are also electrical synapses, but they are less common in the human brain).

Components of a typical chemical synapse:

• Presynaptic neuron – the sending neuron
• Has axon terminals filled with tiny sacs called synaptic vesicles
• Vesicles contain neurotransmitters

• Synaptic cleft – the tiny gap (about 20–40 nanometers wide)

• Postsynaptic neuron – the receiving neuron
• Has receptors on its membrane (often on dendrites)
• These receptors bind specific neurotransmitters

What happens at a synapse:

1. An action potential reaches the axon terminal of the presynaptic neuron.
2. This causes calcium (Ca²⁺) channels to open; Ca²⁺ enters the terminal.
3. Ca²⁺ triggers vesicles to fuse with the membrane and release neurotransmitters into the synaptic cleft.
4. Neurotransmitters diffuse across the cleft.
5. They bind to receptors on the postsynaptic neuron.
6. This changes ion flow in the postsynaptic neuron, making it more or less likely to fire its own action potential.
7. Neurotransmitters are then removed (broken down, taken back up, or diffuse away) so the signal doesn’t last forever.

This chemical step is slower than the electrical step, but it allows for flexibility and modulation (signals can be strengthened, weakened, or combined in complex ways).

8. Quick Check: Action Potential vs Synapse

Test your understanding of where electrical and chemical signals happen.

9. Neurotransmitters: Different Chemicals, Different Effects

Neurotransmitters are chemical messengers. Different neurotransmitters have different typical roles, depending on where and how they act.

Common examples (based on current neuroscience understanding up to early 2026):

• Glutamate
• Main excitatory neurotransmitter in the brain
• Makes postsynaptic neurons more likely to fire
• Important in learning and memory

• GABA (gamma‑aminobutyric acid)
• Main inhibitory neurotransmitter in the brain
• Makes postsynaptic neurons less likely to fire
• Helps prevent over‑excitation and seizures

• Dopamine
• Involved in motivation, reward, movement, and learning
• Imbalances are linked to conditions like Parkinson’s disease and some forms of psychosis (for example, in schizophrenia)

• Serotonin
• Involved in mood, sleep, appetite, and pain
• Many modern antidepressant medications (like SSRIs) act on serotonin reuptake at synapses

• Acetylcholine
• Important at neuromuscular junctions (where nerves meet muscles)
• Also involved in attention and memory in the brain

Synapses can be:

• Excitatory – increase the chance the postsynaptic neuron will fire
• Inhibitory – decrease the chance it will fire

Each neuron can receive input from thousands of synapses. The neuron adds up (integrates) all excitatory and inhibitory inputs. If the total crosses threshold, it fires an action potential.

10. Thought Exercise: Balancing Excitation and Inhibition

Imagine a neuron sitting at its cell body as a decision point.

• It receives excitatory inputs (like people shouting “GO!”)
• It receives inhibitory inputs (like people shouting “STOP!”)

Try this mental model:

1. Picture 10 people around the neuron.
2. 6 are excitatory synapses (GO), 4 are inhibitory synapses (STOP).
3. At a given moment:

• 4 GO synapses are active
• 3 STOP synapses are active

Questions (answer in your head or on paper):

1. Does the neuron get more GO or more STOP at that moment?
2. Would that make it more likely or less likely to reach threshold and fire?
3. Now imagine a medicine that increases GABA activity (more inhibition). How might that affect the neuron’s chance of firing—increase or decrease?

Reflect on your answers, then compare:

• More active GO than STOP → neuron is more likely to fire.
• If GABA‑like inhibition increases, the neuron becomes less likely to fire.

This balance between excitation and inhibition is crucial. Too much excitation can lead to seizures; too much inhibition can slow thinking or movement.

11. Neural Networks: How Many Neurons Make a Thought?

One neuron alone cannot create a complex thought or behavior. Networks of neurons working together do.

Key ideas about neural networks:

• Pathways
• Neurons are wired in circuits: sensory → processing → motor
• Example: visual pathways from the eye to visual cortex to decision areas

• Parallel processing
• Many networks work at the same time
• Example: when you see a face, some neurons process shape, others process color, others process movement—simultaneously

• Plasticity (neuroplasticity)
• Connections between neurons can strengthen, weaken, form, or disappear over time
• This underlies learning, memory, and recovery after injury

• Hebb’s rule (often summarized as “cells that fire together wire together”)
• When two neurons are active at the same time, their connection tends to strengthen
• Over time, this builds stable patterns of activity that represent memories or skills

Modern brain research (including up‑to‑date imaging and recording techniques as of 2026) shows that:

• Even a simple decision (like choosing tea vs. coffee) involves multiple brain regions and millions of neurons firing in specific patterns.

So when you:

• Remember a phone number
• Learn a new language
• Practice a musical instrument

…you are literally reshaping the strength and pattern of synapses in these networks.

12. Checkpoint Quiz: Putting It All Together

Choose the best answer based on what you’ve learned.

13. Flashcard Review: Key Terms

Use these flashcards to quickly review the main ideas from this module.