🔄 Recap of Lesson 2 – Please watch the video before starting!

In Lesson 2, we met the neuron, the tiny messenger cell of the nervous system. We learned its parts:

• Dendrites = receivers of information.

• Cell Body (Soma) = the decision center.

• Axon = the carrier of messages.

• Myelin sheath = insulation that speeds up the message.

• Axon terminals = the delivery points.

We also discovered that neurons communicate using electricity and chemicals. Today, we zoom in on the electricity part—a fascinating process called the action potential.

⚡ Step 1: Electricity in Your Body?

When we say there is “electricity” in your body, we don’t mean the kind that comes from plugs or lightning. In your body, electricity comes from tiny charged particles (called ions) that move in and out of neurons.

Think of it like a crowd of kids holding balloons – some balloons have a “+” sign, some a “–” sign.

When the kids move in or out of the classroom, the total number of “+” and “–” balloons inside changes, and that creates a little spark of energy.

Another way to imagine it is with a spring.

When a spring is pressed down and held, it is full of energy but hasn’t moved yet.

That’s what a resting neuron is like—loaded with potential energy, waiting. When the neuron “fires,” it’s like letting go of the spring: the energy is released quickly, and the signal shoots down the axon.

So, neurons don’t use big bolts of lightning ⚡ like in a storm.

Instead, they use millions of tiny “sparkles” created by the movement of these charged particles, just enough to carry messages fast and safely through your body.

These sparks are a special kind of electricity created by charged particles inside your cells.

These particles are called ions (tiny charged atoms or molecules).

• Sodium ions (Na⁺): positively charged, like little sparks.

• Potassium ions (K⁺): also positively charged.

• Chloride ions (Cl⁻): negatively charged.

These ions move in and out of neurons through tiny “doors” called ion channels. When they move, they create electrical signals.

🎵 Step 2: Resting Potential – The Neuron at Rest

You might see words like volt or millivolt when people talk about electricity in the body.

A volt is just a way scientists measure how strong electricity is – like we measure distance in meters or weight in kilograms.

Now, the electricity inside your neurons is very tiny. Instead of whole volts, it is usually measured in millivolts.

“Milli” means one-thousandth. So 1 millivolt is one-thousandth of a volt. That’s like saying if 1 volt was a full pizza 🍕, then 1 millivolt is just a tiny crumb.

So when we say a neuron has –70 millivolts, it just means there’s a small difference in charge inside versus outside the cell—enough to keep the neuron ready, like a spring that is pressed down and waiting to bounce.

Even when a neuron is not sending a message, it is not totally quiet. It maintains a balance of charges across its membrane (outer wall).

Think of it like a charged battery 🔋 waiting to be used.

• Inside the neuron: more negative ions.

• Outside the neuron: more positive ions.

This difference creates a voltage, called the resting potential (about –70 millivolts).

So, the neuron is like a loaded spring or a coiled-up slinky—ready to fire when needed.

🚦 Step 3: What is an Action Potential?

An action potential is simply a brief electrical signal that travels down the axon.

Imagine pressing a light switch. ⚡ The current flows instantly through the wire to turn on the bulb. Similarly, when a neuron is triggered, an electrical wave runs down its axon.

🧩 Step 4: How Does an Action Potential Happen?

The process goes in four steps, like opening and closing gates:

1. Trigger (Threshold Reached):
   If the signal from dendrites is strong enough, the neuron decides: “Yes, let’s fire!”

2. Depolarization (Signal Starts):
   Sodium gates open → sodium ions rush into the cell → inside becomes more positive.

3. Repolarization (Reset):
   Potassium gates open → potassium ions flow out → inside becomes negative again.

4. Refractory Period (Rest):
   The neuron takes a short break 💤 so it doesn’t fire again too quickly.

Once started, the action potential travels all the way down the axon—like a wave rolling through a stadium crowd 🌊.

🎢 Step 5: All-or-None Law

Here’s an important rule: an action potential is all-or-none.

That means:

• If the signal is strong enough, the neuron fires at full strength.

• If not, it doesn’t fire at all.

It’s like flipping a switch—either the light is ON 💡 or OFF. There’s no halfway.

🚀 Step 6: Speeding Things Up – Myelin Sheath and Saltatory Conduction

Remember the myelin sheath we talked about in Lesson 2? This fatty coating wraps around the axon in segments, leaving tiny gaps called nodes of Ranvier.

Instead of the electrical signal crawling slowly, it “jumps” from node to node. This jumping is called saltatory conduction (from the Latin word “saltare,” meaning “to jump”).

This makes the signal travel much faster—like skipping steps instead of walking one by one.

🎼 Step 7: Why Action Potentials Matter

Without action potentials:

• Your muscles wouldn’t move.

• Your eyes wouldn’t see.

• Your ears wouldn’t hear.

• Your brain wouldn’t think.

Every thought, every memory, every heartbeat—all depend on billions of action potentials firing every second.

🔬 Step 8: A Real-Life Example

Imagine you step on something hard barefoot 🦶🧱.

• Sensory neurons in your foot fire action potentials.

• The signals rush up to your spinal cord and brain.

• Your brain screams: “Ouch!”

• Motor neurons fire action potentials to make you lift your foot.

This all happens in a flash—thanks to action potentials.

🧠 Fun Fact

Your brain generates about 20 watts of electrical power—enough to power a dim light bulb! 💡

📝 Recap of Lesson 3

• Neurons use ions to create electricity.

• Resting potential keeps them ready, like a loaded spring.

• Action potentials are electrical signals that travel down the axon.

• They work in four steps: depolarization, repolarization, refractory period, reset.

• Action potentials follow the all-or-none law.

• Myelin sheath speeds them up using saltatory conduction.

• Without action potentials, the nervous system simply could not function.