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10.3 Cables and Islands

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

14–21 minutes

Date Last Modified

The Brain · Chapter 3 of 7

Underneath the cortex, filling most of each hemisphere, is white matter: myelinated axons and almost nothing else, sorted into three families by where they end up. Stranded among them are islands of gray — the basal nuclei — that start, stop and scale every movement and never touch a muscle. This chapter opens the wiring the spots sat in, meets the site’s second patient (Russell, seventy, Parkinson’s), looks at the cortex under the microscope, and files a memory in the right drawer.

The question you will answer

The 2020 report put her hyperintensities in the white matter — periventricular and subcortical. What runs there, where does it go, who makes its myelin, what gray islands sit among it, and what does a slowed cable feel like compared with a dead region?

Opener · Three Kinds of Fiber · about 2 minutes

The opener is the same narrated animation you will watch in Scene 1.

Watch it in Scene 1 ↓

Watch the opener first. It sets the question; the chapter answers it.

1

Read the chart

Patient chart · Stina

2020 · age 45 · where the spots sat

2020 · 45
MRI: hyperintensities “periventricular and subcortical” — beside the lateral ventricles and just under the cortex, in white matter. None in cortex, none in the deep gray, none in the brainstem or cerebellum.

Standing
No tremor, no rigidity, no shuffling, no slowness of movement — ever. Finger-to-nose and gait normal (2017).

2017–20
Memory: three-word recall normal; the complaint is retrieval — the drawer is there, the folder is slow to come.

Russell
The site’s brain case: 70, stage 3 Parkinson’s, resting tremor that began on one side, “trouble starting and stopping,” stone face, no thirst, no sweat. A disease of the islands, not the cables — kept as the contrast.

Chart clues this module: #10 — A few white spots, more than expected for age

Before you go on · your prediction

Spots beside the ventricles and under the cortex, a fog of delays, and no tremor, rigidity or weakness. The tissue most likely injured is…

Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.

The cortex — the thinking surface

Ask what a cortical injury does at the bedside, and whether “periventricular” is anywhere near the cortex.

The basal nuclei — the deep gray that governs movement

Ask what failing basal nuclei look like — the site’s Russell is the worked example — and whether she had any of it.

The cables — myelinated projection and association fibers running past the ventricles

Ask what lies beside the lateral ventricles, who makes the myelin there, and what slowed conduction in a cable feels like.

From Stina’s diary

2026 · age 51 · “Every page”

2

The story

Three short movements: the case, the anatomy and physiology, then the tie back to Stina. Each links to the scene where you’ll see it move.

I · Periventricular and subcortical

The two location words on the 2020 report are the most useful words on it. <em>Periventricular</em> means beside the lateral ventricles, where projection fibers funnel toward the internal capsule and the corona radiata fans out above them; <em>subcortical</em> means just under the cortex, where the short association fibers hop between neighboring gyri. Both are white matter. Nothing was in the cortex, nothing in the deep gray, nothing in the brainstem or cerebellum — and the exam agreed: no weakness, no tremor, no rigidity, no staggering. The site’s own brain case is the contrast. Russell Johnson, seventy, stage 3 Parkinson’s, began with a resting tremor on one side, now has it in both hands, walks hunched and shuffling, has “trouble starting and stopping,” shows the stone face his grandchildren noticed, and is often dehydrated without feeling thirsty. His disease is in the islands — the substantia nigra and the basal nuclei it feeds — and every one of his signs is the signature of an island failing. Stina had none of them. Her disease, if the spots were a disease, was in the cables.

See it move: Scene 1 · Three Kinds of Fiber · Scene 4 · Russell’s Tremor

II · Three families of cable and a cluster of islands

White matter is myelinated axons, white because myelin is largely lipid and fast for the same reason; in the central nervous system the myelin is made by oligodendrocytes, each wrapping segments of many axons at once (the Nervous Tissue module). The fibers sort into three families by destination. <strong>Commissural fibers</strong> cross between hemispheres — the corpus callosum is the great one, the anterior and posterior commissures the small ones; separate the hemispheres in the lab and you see them. <strong>Association fibers</strong> stay within a hemisphere: short ones connect adjacent gyri and are lightly myelinated, long ones connect distant lobes and are wrapped heavily, because distance costs time. <strong>Projection fibers</strong> run vertically — sensory up, motor down — tying the cerebrum to the brainstem and cord; they funnel through the internal capsule, a dense band between the thalamus and the lentiform nucleus, and fan out above as the corona radiata (crown, radiating: a shape name and nothing more — the site’s habit of calling it part of the reticular activating system is a slip). Stranded in that white matter are the <strong>basal nuclei</strong>: the caudate nucleus arcing with the lateral ventricle, and the lentiform nucleus — putamen outside, globus pallidus inside — lateral to the thalamus. A nucleus is a cluster of cell bodies inside the CNS, a ganglion is one outside it, so “basal ganglia” breaks a rule the same books teach two chapters earlier. Their four jobs, from the packet: influence movement indirectly (through the thalamus, never a motor neuron), attention and cognition, inhibit unnecessary movement, and govern the start, stop and intensity of movement. The cortex excites them with glutamate; the substantia nigra of the midbrain shapes the answer with dopamine. Lose that dopamine and parts of the circuit are under-stimulated, others over-stimulated: the tremor, the rigidity, the akinesia and postural instability of Parkinson’s; lose other cells and the unwanted movements of Huntington’s appear. Under the microscope the cortex shows its folding at the tissue edge, a darker cellular rind over paler fibrous white, and a density that changes band by band — the six layers, with pyramidal cells in the deeper ones. And memory, which the temporal lobe consolidates rather than stores: short-term (working) memory holds a few items for seconds to minutes; long-term is effectively unlimited; emotion, rehearsal, association and automatic memory decide what crosses; consolidation is the filing — finding the drawer and then the folder; declarative facts are one category, and procedural, motor and emotional memory are the nondeclarative ones that need practice and are hard to unlearn.

See it move: Scene 1 · Three Kinds of Fiber · Scene 2 · Under the Microscope: Cortex · Scene 3 · Islands in the White · Scene 5 · Filing the Folder

III · A slowed cable is not a dead region

Here is why “nothing focal” and “fog” were the same finding seen two ways. A lesion in a region removes one function and leaves a mark the exam finds: a weak hand, a lost word, a tremor. Injury to the cables removes nothing; it slows everything that must travel. Demyelinated axons conduct slowly and unevenly, so regions that are individually intact take longer to reach one another — the word stalls on its way from Wernicke’s to Broca’s, the thread of a sentence waits on a long association fiber, the morning takes an hour because the prefrontal cortex is a long way from everywhere. The memory complaint fits the same shape: the drawer and the folder are there (three-word recall was normal); the retrieval is slow. The cables are also where this module’s last chapter will end — because white matter is fed by small vessels and maintained by oligodendrocytes, and both are what a tide of cytokines reaches first.

See it move: Scene 1 · Three Kinds of Fiber · Scene 5 · Filing the Folder

3

Watch · explore · think · check

5 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, a diagram 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

Three Kinds of Fiber

Sorted by destination: across, within, up-and-down — and all of them white because of myelin.

Animation · in production · 1 min 55 s

Three Kinds of Fiber

The frontal section; three colored families; corpus callosum, internal capsule, corona radiata; the oligodendrocyte wrapping several axons. Caption: cables sorted by destination.

Prefer the full lecture? Cerebral White Matter (original video on the site)

Explore · in production

Route the message

You are about to cross a street. The premotor cortex needs the curb’s distance (visual association area, same hemisphere), the crosswalk beep (auditory association, same hemisphere), what the other hemisphere knows, and a command sent to the cord. For each leg of the journey, choose the fiber family before the reveal — association, commissural or projection — and the structure it passes through (corpus callosum, internal capsule, corona radiata). A wrong pick explains itself. Last: mark on the frontal section where “periventricular” is, and name the family that runs there.

Think · not graded

Short association fibers are lightly myelinated and long ones heavily, “because distance costs time.” If myelin is thinned along a long fiber, what changes — the message, or its arrival time? And which would a bedside exam notice?

The true/false spot-check for this prompt is in production.

Check · Multiple Choice

Which structure is made of projection fibers?

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

Three families of cable, and the ones beside the ventricles are projection fibers funneling to the internal capsule — the spots sat on the main trunk line.

Scene 2 of 5

Under the Microscope: Cortex

Six layers you can actually see, and a gray/white boundary you can see without a microscope at all.

Slide · in production

The Edge of the Tissue

Two site micrographs side by side with the gyrus, sulcus, gray band, white matter and a pyramidal cell ringed; the stain caveat in the caption.

Prefer the full lecture? Cerebral Gray Matter (original video on the site)

Explore · in production

Drive the slide

The 076b micrograph with a magnifier you drag. Four targets in order — the tissue edge, the gray/white boundary, a band where cell density changes, a pyramidal cell — and for each the student must click before the ring appears and then pick the one feature that decided it. Then the stain trap: the same field in H&E and in TB&E, and the question “which is the gray matter?” answered twice. Nothing is recorded; every miss explains itself.

Think · not graded

On one slide the gray matter is darker; on another it is lighter. The Guided Journey says “it’s not about what you see, it’s about what you know.” What do you know that decides which band is gray on any brain slide, regardless of stain?

The true/false spot-check for this prompt is in production.

Check · Drag the Words

Label the cortex slide.

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

On the slide the cables are the pale fibrous depth under the cellular rind — the tissue the scan lit up, seen with a stain instead of a magnet.

Scene 3 of 5

Islands in the White

Gray matter buried deep: a nucleus in the CNS, a ganglion outside it — and four jobs that never touch a muscle.

Slide · in production

The Basal Nuclei

Transverse section with caudate, putamen, globus pallidus, thalamus and internal capsule labeled; the four jobs and the vocabulary rule beside it.

Prefer the full lecture? Cerebral Gray Matter (original video on the site)

Explore · in production

Start, stop, scale

A walking figure and three dials — start, stop, intensity — wired to a basal-nuclei circuit with a dopamine supply slider. Before touching anything, predict what each dial does with dopamine at 100%, then at 20% (the site’s Russell: stage 3 ≈ 20% of normal dopamine). Run it and watch: with low dopamine the figure cannot start, cannot stop, and shakes at rest. Then a vocabulary gate: five clusters of cell bodies in the nervous system — drag each to “nucleus” or “ganglion” by location. Last: tick which of Stina’s findings (none) belong to this circuit.

Think · not graded

The basal nuclei “never contact a motor neuron” — they work through the thalamus and back up to the cortex. Then why does losing their dopamine supply produce a tremor you can see, and why can a Parkinson’s tremor sometimes be damped on purpose but not unconsciously?

The true/false spot-check for this prompt is in production.

Check · Flip Cards

The basal nuclei — name and place.

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

Islands of gray that start, stop and scale movement — and nothing about her movement ever failed to start, stop or scale.

Scene 4 of 5

Russell’s Tremor

The site’s second patient: a disease of the islands, kept as the contrast to a disease of the cables.

Animation · in production · 2 min 00 s

A Disease of the Islands

The midbrain in transverse section; the substantia nigra dark with neuromelanin. Its dopamine axons climb to the basal nuclei. Over three years the dark band pales and the dopamine thins: the circuit tips — thalamus over-inhibited (rigidity), globus pallidus overactive (tremor let through). Russell appears as a line drawing: one-sided resting tremor, then both hands; the hunched shuffle; the stone face; a glass of water he does not feel thirsty for. TRAP stamps: tremor, rigidity, akinesia, postural instability. Then the drugs: levodopa crosses the barrier and becomes dopamine; the others conserve what is left. Caption: the islands failing, one sign at a time.

Prefer the full lecture? Midbrain (original video on the site)

Explore · in production

Two patients, one map

Russell’s chart and Stina’s chart on facing pages, a brain between them. Drag each finding — resting tremor, stone face, no thirst, shuffling gait, slow word-finding, dropped threads, periventricular spots, a normal finger-to-nose — onto the tissue it belongs to: substantia nigra, basal nuclei, hypothalamus, white matter, or “nowhere on this map.” Before the reveal, answer: which patient has a disease of regions and which of connections? Then the pharmacology gate: five Parkinson’s drug classes to sort by “crosses the blood-brain barrier” or “works outside it” — a first look at the wall Chapter 7 builds.

Think · not graded

Levodopa works because it crosses the blood-brain barrier and dopamine does not. Hold that sentence: a barrier that stops a small neurotransmitter is the same barrier this module says cytokines got past. What would have to be different about the barrier — or about the exposure — for both to be true?

The true/false spot-check for this prompt is in production.

Check · Summary

What the basal nuclei do — and what happens when they fail.

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

Russell’s every sign is an island failing; Stina has none of them — two patients, two maps, and hers is the cables.

Scene 5 of 5

Filing the Folder

Short-term, long-term, the four things that decide what crosses — and a cabinet with drawers and folders.

Animation · in production · 1 min 50 s

Drawer, Then Folder

A desk: a few items float in working memory for seconds — then fade unless something acts. Four stamps decide what crosses: emotional state (an item arriving with adrenaline is filed first), rehearsal (repetition), association (a hook onto something stored), automatic memory (the “oh yeah” that surfaces later). A filing cabinet: consolidation finds the drawer, then the folder (Marieb 12.22 — losing the file vs failing to find it). Two categories: declarative (facts, the association cortex and hippocampus) and nondeclarative — procedural, motor, emotional — that need practice and are hard to unlearn (basal nuclei, cerebellum). The temporal lobe consolidates; it does not store. Caption: read it, say it, draw it — working with it is what files it.

Prefer the full lecture? Lobes of the Cerebrum (original video on the site)

Explore · in production

Lost, or slow to find

A cabinet with six drawers. Ten memory moments (a phone number held for a minute, a birthday, how to ride a bike, the smell of a hospital, a fact learned by rote, a word on the tip of the tongue…). Drag each to declarative or nondeclarative, then to the structure that files it — hippocampus and association cortex, basal nuclei, cerebellum, amygdala. Then the retrieval test: for each of Stina’s complaints, choose “the folder is gone” or “the folder is slow to come” — and say which one the 2017 three-word recall could tell apart. Reveal: it could not; both pass at five minutes.

Think · not graded

Your site says every time you retrieve a memory “it changes slightly and is put back incorrectly,” and the packet says consolidation is like finding the drawer and then the folder. If the drawers and folders are intact but the corridor between them is slow, what does the person experience — amnesia, or a delay?

The true/false spot-check for this prompt is in production.

Check · Single Choice

What moves a memory from short-term to long-term?

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

Her memory complaint was retrieval that was slow, not folders that were gone — the shape of a slowed corridor, not a lost drawer.

4

Your evidence board

Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: A disease of the cables — slowed connections between intact regions, Neither on its own, or A disease of the islands, or of the cortex. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.

Evidence board · drag and drop · in production

The sorting board for this chapter’s cards is being built. Until it arrives, sort your evidence cards on paper into the three columns above.

5

Your verdict

Time to decide. Look back at your bet, then build your verdict from statements that hold up — no writing needed.

Look back at your prediction

Spots beside the ventricles and under the cortex, a fog of delays, and no tremor, rigidity or weakness — the tissue most likely injured is…

Find the one you chose at the top. Does the evidence support it, refute it, or revise it?

  • The cortex — the thinking surface
  • The basal nuclei — the deep gray that governs movement
  • The cables — myelinated projection and association fibers running past the ventricles

Build your verdict · no writing

Choose the statement that holds in each set. The ones you keep become your argument.

This activity is in production.

6

The finding

Finding · the cables

Spots on the Trunk Line

The 2020 hyperintensities sit periventricular and subcortical — in white matter, where projection fibers funnel toward the internal capsule and short association fibers hop between gyri; cortex, basal nuclei, brainstem and cerebellum are clean, and so is her movement. Chart entry: a disease of the cables, not the islands — delays, not losses. Held open: cables are fed by small vessels and maintained by oligodendrocytes, and both are reachable from the blood.

Next · Chapter 4

The Thermostat and the Relay

Beneath the hat, in a few cubic centimeters nobody can draw a crisp boundary around, sits the relay for every sensation, the clock, and the thermostat that set her first fever at two — and that her mother’s surgeon had to work beside.

Continue to Chapter 4 →

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.

Hop to:

← Familial Mediterranean Fever — case home

All pages for Module 10 — The Brain

10.0 Module Overview
10.1 Spots on the Scan
10.2 The Big Brain Hat
10.3 Cables and Islands
10.4 The Thermostat and the Relay
10.5 The Stalk and the Little Brain
10.6 Cushions and Currents
10.7 The Wall and the Tide
10.8 Conclusion and Assessment
10.9 Evidence Behind the Case

All Modules
  1. Anatomical Language, Membranes & Homeostasis
  2. Just Enough Chemistry
  3. The Cell & Its Transport
  4. Making Cells & Proteins
  5. The Integumentary System
  6. The Skeletal System
  7. The Muscular System
  8. Nervous Tissue & the Senses
  9. The Spinal Cord
  10. The Brain & the Blood–Brain Barrier
  11. The Autonomic Nervous System
  12. Special Senses (in development)
  13. The Endocrine System
  14. Blood
  15. The Heart
  16. Blood Vessels
  17. The Digestive System
  18. The Respiratory System
  19. The Urinary System
  20. Fluids, Electrolytes & Acid–Base Balance
  21. The Reproductive System
  22. The Immune System

List of terms