Observe the action potential, the glowing signal traveling down the healthy axon. It is wrapped in myelin sheaths, the yellow segments.
The signal jumps rapidly between the gaps. This is called saltatory conduction, and it is what makes healthy neural signaling so fast.
Zooming out: gray vs white matter
The axon segment you're watching lives in White Matter. White matter is the brain's wiring, bundles of myelinated axons that carry signals from place to place. Gray Matter is where the cell bodies and synapses live, the thinking and processing centers. Myelin is what makes white matter look white.
✎Sketch the axon in your handbook. Label the myelin sheaths, the gaps (Nodes of Ranvier), and the signal. Add a quick side note: is this slice from gray or white matter?
Step 2: Hypothesis
Multiple Sclerosis is an autoimmune disease. The body's own immune system attacks and destroys the myelin sheaths on neurons in the brain and spinal cord.
Where MS strikes
Because MS attacks myelin, it primarily damages White Matter, the wiring highways. Early in the disease, Gray Matter is largely spared. That's why MS is often called a white matter disease.
Prediction: What will happen to the speed of the action potential if the myelin is removed?
✎Write your prediction in your handbook and one sentence explaining your reasoning.
Step 3: Intervention
Your cursor is now simulating the autoimmune attack. Press and drag across the yellow myelin sheaths to scrub the myelin away.
You can also hover over a sheath and scroll to thin it or thicken it back, or click the ↺ Restore button that appears under each damaged sheath to snap it back to full health. Watch the signal speed respond in real time, both ways.
What you're doing
Every sheath you strip is a small patch of White Matter becoming a lesion. You're simulating, in one axon, what's happening across millions of axons inside a real MS lesion.
✎Record three data points of your choice. Any three lesion volumes and the matching velocities. You can also scroll up on a segment to watch the signal recover.
Check your thinking before moving on
MS primarily attacks White Matter (myelinated wiring), while Gray Matter (cell bodies and synapses) is largely spared in classic presentations. What does this predict about typical early MS symptoms?
Step 4: Conclusion
Without myelin insulation, neural signals degrade and slow down drastically. On the T2-weighted MRI scan, this damage appears as white matter lesions.
These lesions represent areas of demyelination and inflammation. They are the physical basis for the sensory, motor, and cognitive symptoms that characterize MS.
Tying it together
The bright MRI spots cluster in White Matter because that's where the myelin is. Gray Matter can also be affected as MS progresses, but the defining signature of the disease is white matter damage. Symptoms match which wiring pathway got hit.
✎Write a 2-sentence conclusion comparing the healthy and demyelinated axon. Include the term white matter in your answer. Compare your prediction from Step 2 to what you observed.