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Nervous Tissue & the Senses · Chapter 2 of 7
Neurons never work alone. Around every axon is a crew of support cellsThe basic structural and functional units of life., and in the peripheral nervesNerves that extend beyond the brain and spinal cord. one of them wraps the wire in the insulation this whole module turns on.
The question you will answer
Light touch and vibration faded in both feet, but strength and pinprick held. Would you suspect the neuronsThe functional cells of the nervous system that transmit signals. themselves, or the cells that wrap them?
About 45 minutes. Work the steps in order.
Animation · in production · 1 min 20 s
The Support Crew
A lone neuron flickers on an empty stage. The CNSComposed of the brain and spinal cord; integrates and processes information. crew arrives: astrocytesStar-shaped glial cells in the CNS that support neurons and maintain the blood-brain barrier. plant their feet on a capillary, oligodendrocytesGlial cells in the CNS that form the myelin sheath around axons. reach out arms to wrap several axons, microgliaSmall CNS glial cells that act as macrophages, cleaning up debris and pathogens. patrol, ependymal cellsGlial cells that produce and circulate cerebrospinal fluid. line a fluid space with beating ciliaHair-like projections on the surface of some cells that move fluids or particles..
Watch the opener first. It sets the question; the chapter answers it.
1
Read the chart
Patient chart · Stina
2010, age 35 · neurology, months after the crash
| Strength | Normal — pushes, pulls, stands on her toes and heels. |
|---|---|
| Light touch | Reduced in both feet, fading upward toward mid-shin. |
| Vibration | Tuning fork reduced at both big toes; normal at the knees and fingers. |
| Pinprick | Close to normal. |
| Neurologist’s note | “Likely positional, accident-related.” |
This chapter writes line 2 of Chart Clue #8.
Your prediction
Sick neurons, or sick wrapping? Open the one you’d bet on — nothing is graded, and the answer isn’t here; the scenes will test your bet.
The neurons themselves are failing
You’re betting on the neurons. Fair: the loss begins at the far ends of the longest axons, and a dying axon gives out there first. In Scene 3, take a nerve apart layer by layer — and ask whether the wire and its wrapping can fail separately.
The wrapping around them (myelin) is failing
You’re betting on the insulation. Fair: she lost some sensations and kept others, so something is being choosy. In Scene 4, find out which fibers each bedside test really checks — and ask whether losing axons could make the same pattern.
Can’t say yet — I’d need to know more first
An honest bet. Decide what would settle it — which fibers failed? whether an axon can survive without its myelin? how fast her signals travel? — and look for it as you go. Some of it is in these scenes, and some waits for Chapter 3.
From Stina’s diary
“The numbness in my feet is on both sides. The neurologist wrote that down and never wondered about it.”
2
The story
Three short movements. Read them once now; each links to the scene where you’ll see it move.
I · Symmetric Numbness After a One-Sided Crash
When a neurologist finally tested Stina months later, her strength was fine — she could push, pull, and stand on her toes. But light touch and the buzz of a tuning fork had faded in both feet, not just the leg that hurt, while a pin still felt sharp. The note read “likely positional, accident-related,” and the visit ended. Nobody lingered on the strangest detail: a car crash hits one side of a body, yet her sensory loss was symmetric, climbing from both feet like waterThe universal solvent essential for life. filling a glass. That symmetry was a fingerprint, written down and ignored.
See it move: Scene 4 · Thick Wires, Thin Wires
II · The Support Crew and the Insulation
Neurons don’t work alone. In the CNS, astrocytes support neurons and help form the blood–brain barrier, microglia patrol as immune cells, ependymal cells line the fluid spaces, and oligodendrocytes wrap segmentsThe subdivisions of lung lobes, supplied by tertiary bronchi. of many axons at once. In the PNS, satellite cells surround cell bodies in ganglia and Schwann cellsPNS glial cells that form the myelin sheath around peripheral nerve fibers. each wrap one segment of one axon. That wrapping, myelin, is a fatty sheath rolled around the axon many times. It insulates the wire and leaves bare gaps, the nodes of Ranvier, where the signal is renewed. Gripping the innermost wrap of peripheral myelin to the axon is a sticky anchor protein called MAG, myelin-associated glycoproteinA protein with a carbohydrate chain attached, important for cell signaling.. Remember it.
See it move: Scene 1 · Glia of the CNS · Scene 2 · The Schwann Cell and MAG · Scene 3 · White Matter, Gray Matter
III · Which Cells Would You Suspect?
A single blow lands on one side. Something that reaches both feet equally is reaching every long nerve at once — the neurons, their wrapping, or both. Vibration and light touch ride on one kind of fiber; pinprick rides on another. Which fibers failed, and what wraps them, is the clue. Collect the evidence.
See it move: Scene 4 · Thick Wires, Thin Wires · Scene 5 · The Blood–Brain Barrier: a Preview
3
Watch · explore · think · check
Five scenes. Each one has a short animation or a slide, something to explore, a true/false spot-check of your thinking, and a quick check. Every scene gives you an evidence card for your board.
How to read the scenes
- Animation (dark teal) — a short animated explainer, one idea each
- Slide (dark teal) — a still to study: a micrograph or a map
- Explore (purple) — something to push on while you watch
- Game (purple) — a quick challenge
- Think (gold) — reason it out, then spot-check yourself with a few true/false questions; not graded
- Check (gold) — a quick H5P that scores itself
Scene 1 of 5
Glia of the CNS
Four support cells inside the brain and cord, each with a headline job.
Animation · in production · 1 min 50 s
Four Jobs Inside the Walls
Astrocytes: end-feet on a capillary, mopping up extra K⁺, recycling neurotransmitterChemicals that transmit signals across synapses., sealing a wound with a scar. Oligodendrocytes: one cell, many arms, a myelin segment on each of several axons.
Explore
Neuroglia of the CNS
Open each marker to meet a glial cell of the brain and spinal cordThe central nervous system structure that relays signals between the brain and body. and the job it does.
Think · not graded
Microglia are the CNS’s resident immune cells. Given Stina’s lifelong IL-1β problem, what might microglia that never switch off do to the neurons and myelin around them? (You’ll meet this again in the Brain module.)
Spot-check your thinking: true or false?
Check · Drag the Words
Drag each CNS glial cell to the job it does.
Evidence card 1
Add it to your evidence board
Chronically activated microglia can injure both neurons and myelin.
Scene 2 of 5
The Schwann Cell and MAG
Watch a Schwann cell roll a myelin sheathA fatty covering around axons that increases conduction speed. — and find MAG on its innermost wrap.
Animation · in production · 2 min
Rolling a Myelin Sheath
A Schwann cell settles on one segment of an axon and begins to wrap, like a jelly roll. Cytoplasm squeezes out; the layers compact into myelin; the Schwann cell’s nucleusThe control center of the cell that contains DNA and directs cellular activities. and cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. stay outside as the neurilemma.
Game
Sort the Neuroglia
Drag six glial cells, each with its job, to where they work: the CNS or the PNS.
Think · not graded
One Schwann cell myelinates one segment of one axon; one oligodendrocyte myelinates segments of many axons. If an antibody attacked a myelin-making cell, which arrangement would damage more axons per injured cell — and which would be easier to repair?
Spot-check your thinking: true or false?
Check · Fill in the Blanks
Fill in what myelin is made of, where the signal renews itself, and what MAG does.
Evidence card 2
Add it to your evidence board
One Schwann cell wraps one axon segment; MAG glues the wrap to the axon.
Scene 3 of 5
White Matter, Gray Matter
See why myelin makes tissue white, and find the myelin rings in a real nerve.
Slide · a still to study
Under the Microscope: Myelin
Open each marker to tour axons and the myelin wrapped around them in real tissue.
Game
Zoom the Nerve
Put the layers of a peripheral nerve in order, from its tough outer coat in to one axon.
Think · not graded
Myelin is mostly lipid, which is why it looks white. If a biopsy of Stina’s nerve showed myelin rings that were thin and loose but the axons still there, which hypothesis would that support?
Spot-check your thinking: true or false?
Check · Mark the Words
Mark every term that belongs to white matterThe outer portion of the spinal cord made of myelinated nerve fibers that transmit signals., and leave the gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses. terms unmarked.
Evidence card 3
Add it to your evidence board
Myelin can be lost or loosened while the axon underneath survives.
Scene 4 of 5
Thick Wires, Thin Wires
Match each bedside test to the fibers it really tests.
Animation · in production · 1 min 30 s
Which Fibers Carry What
A nerve in cross-section, fibers colored by size: large and heavily myelinated, medium, small and bare. A tuning fork touches the big toe; only the large myelinated fibers light up. A wisp of cotton: the large fibers again.
Explore
Bedside Exam Sorter
Sort each bedside test by the fibers it really checks. Then find the two tests Stina failed.
Think · not graded
Stina’s vibration and light touch failed, but pinprick held. Which fiber population failed, and what do those fibers have that the others don’t?
Spot-check your thinking: true or false?
Check · Mark the Words
A preview of Chapter 3: mark every term that describes a Group A fiber — the kind a tuning fork tests.
Evidence card 4
Add it to your evidence board
Lost sensations ride the most heavily myelinated fibers; pinprick holds.
Scene 5 of 5
The Blood–Brain Barrier: a Preview
See what astrocytes guard, and why antibodies reach nerves more easily than the brain.
Animation · in production · 1 min
Astrocyte End-Feet
A brain capillary: endothelial cells stitched by tight junctions, wrapped by astrocyte end-feet. Oxygen and alcohol slip through; glucoseA simple sugar that is the main source of energy for cells. rides a transporter; a large antibody bounces off.
Game
What Gets Through?
Predict which moleculesGroups of atoms bonded together. cross into the brain and which the barrier keeps out, then check.
Think · not graded
Antibodies are large proteinsLarge molecules made of amino acids with various functions in the body. that cross an intact blood–brain barrier poorly, while the barrier around peripheral nerves is leakier. If Stina’s own antibodies were attacking myelin, where would you expect the damage to show first?
Spot-check your thinking: true or false?
Check · Summary
In each set, choose the statement about the blood–brain barrier that holds up.
Evidence card 5
Add it to your evidence board
Antibodies reach peripheral nerves far more easily than the brain.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds two more. Drag each card to the side it supports — some can go in more than one place, and the feedback tells you which ones decide the case.
5
Your verdict
Time to decide. Build your verdict from statements that hold up — no writing needed. At the end you’ll see your whole argument in one place.
Look back at your prediction
If you bet on the neurons
A fair bet, and not a dead one: loss that starts at the ends of the longest axons fits dying axons too. But the sensations that failed ride on the most heavily myelinated fibers, and myelin can fail while the axon survives. The wrapping now leads, and Chapter 3’s nerve study gets the final say.
If you bet on the wrapping
Your bet leads for now. Check that it leads for the right reasons: the loss is symmetric, and vibration and light touch ride on the thickest, most heavily myelinated fibers while pinprick held. Large-fiber axon loss could still look the same at the bedside, so hold the bet loosely until Chapter 3.
If you said you couldn’t say yet
That was honest, and it is still partly true. You now know what narrowed it: which fibers failed, and that myelin can be lost while the axon survives. What settles it is a test of how fast her signals travel — exactly what Chapter 3 reads.
6
The finding
Added to Stina’s chart · Chart Clue #8, line 2 of 7
Symmetric loss of light touch and vibration — the large myelinated fibers are failing
Strength normal, pinprick nearly normal, but vibration and light touch faded in both feet. The fibers that failed are the thickest and most heavily myelinated, and the loss is symmetric — something reaching every long nerve, not a one-sided blow. The wrapping is the leading suspect, and MAG, the glue on its innermost layer, is worth remembering. Only a conductionThe transmission of nerve impulses along neurons. study can tell a stripped wire from a dead one.
Next · Chapter 3
The Spark
A well-wrapped wire is useless until something runs through it. Next: the voltage a neuron holds at rest, the spike that fires it, and a nerve study that reads how fast the spike arrives.
Reliable information on rare diseases
NORD · NIH GARD · Orphanet · MedlinePlus Genetics: FMF · FMF & AID Global Association · Autoinflammatory Alliance · Global Genes · Amyloidosis Foundation
Stina has one rare disease. In the United States, about 1 in 10 people live with one of the more than 7,000 known rare diseases. These organizations are where patients, families and clinicians go for trustworthy information.
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All Modules
- Anatomical Language, Membranes & Homeostasis
- Just Enough Chemistry
- The Cell & Its Transport
- Making Cells & Proteins
- The Integumentary System
- The Skeletal System
- The Muscular System
- Nervous Tissue & the Senses
- The Spinal Cord
- The Brain & the Blood–Brain Barrier
- The Autonomic Nervous System
- Special Senses (in developmentThe process of growth and differentiation.)
- The Endocrine System
- Blood
- The Heart
- Blood Vessels
- The Digestive System
- The Respiratory System
- The Urinary System
- Fluids, Electrolytes & Acid–Base Balance
- The Reproductive System
- The Immune System
List of terms
- cells
- peripheral nerves
- neurons
- CNS
- astrocytes
- oligodendrocytes
- microglia
- ependymal cells
- cilia
- water
- segments
- Schwann cells
- glycoprotein
- neurotransmitter
- spinal cord
- myelin sheath
- nucleus
- cytoplasm
- white matter
- gray matter
- glucose
- molecules
- proteins
- conduction
- development