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The Spinal Cord · Chapter 4 of 7
Her numb band had an edge she could draw: the L5 zip code. This chapter wires it — rootlets into roots, two roots into a short mixed nerve, the nerve into rami, rami into plexuses and named nerves, and a chain of custody from the L5 root through the sciatic to the muscle that would not lift her foot. Three wrappings, a nerve and a ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS). under the microscope, and the map that named the level first.
The question you will answer
Sensation enters at the back, commands leave at the front, and everything outside the cord is mixed. How do roots become nerves and nerves become plexuses, why is every patch of skinThe body’s largest organ, providing protection and regulation. a return address, and how does a pinch at one root produce a numb patch, a burning stripe and a weak foot?
Opener · Zip Codes · about 1¾ minutes
The opener is the same narrated animation you will watch in Scene 1.
Watch the opener first. It sets the question; the chapter answers it.
1
Read the chart
Patient chart · Stina
2010–2015 · ages 35–40 · the skin map
Numb
Outer shin and top of the left foot, “wrapped in cotton wool”, a border she could draw: the L5 dermatomeA specific area of skin supplied by a single spinal nerve. (the Nervous Tissue module).
Burning
A hot stripe from the buttock down the back of the thigh and the outer calf — the sciatic’s path.
Weak
By 2014 the left foot catches on stairs; she cannot walk on that heel. Tibialis anteriorThe front of the body or toward the front when standing in the anatomical position. 4/5; the great toe is weak.
Both feet
Light touch and vibration reduced in both feet, stockings, since 2009 — the Nervous Tissue module’s second map, the anti-MAG neuropathy.
Told
“Sciatica.” “Pinched nerve.” Nobody said L5 out loud until 2015.
Chart clues this module: 1 · The picture · 2 · The paper towel · 3 · The forgiving basement · 4 · this chapter
Before you go on · your prediction
One root pressed by a disc. She has a numb patch, a burning stripe and a weak foot. How many roots does that take?
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
Three — one for each symptom
Ask what a spinal nerve carries, and whether sensation and motor leave the cord through the same root.
One mixed root can do all three: sensation in through the dorsal root, motor out through the ventral root, both squeezed at the same spot
Ask how the two roots merge, and what the numb patch and the weak muscle each tell you about the level.
None — this is the peripheral nerve in the leg, not a root
Ask how a root problem and a nerve problem differ on a dermatome mapA diagram showing which spinal nerves correspond to specific skin regions. and on a reflex.
From Stina’s diary
2
The story
Three short movements: the case, the anatomyThe study of the structure of the human body. and physiologyThe study of how the body functions., then the tie back to Stina. Each links to the scene where you’ll see it move.
I · Two roots, one short cable, and a bulge that says which way is back
The cord reaches the body by plumbing. Rootlets gather into roots; two roots merge into a spinal nerve; the nerve splits into rami. The dorsalRelating to the back side of the body. root carries sensation in, and its cell bodies sit outside the cord in the dorsal root ganglion — unipolar cellsThe basic structural and functional units of life., the cell bodyThe central part of a neuron containing the nucleus and organelles. hung off to one side so the signal never detours through it. The ventralRelating to the front or belly side of the body. root carries motor out from the anterior hornThe front section of the spinal cord’s gray matter, housing motor neurons., multipolar neuronsThe functional cells of the nervous system that transmit signals. whose axons run uninterrupted to a muscle. There is no ventral root ganglion: find the bulge and you have found dorsal. The two roots fuse into a mixed spinal nerve that is almost comically short — a length you could cover with a fingernail — trapped in the bony doorway of the intervertebral foramen. Step into a root and you have left the CNSComposed of the brain and spinal cord; integrates and processes information. : Schwann cellsPNS glial cells that form the myelin sheath around peripheral nerve fibers. wrap the myelin now, not oligodendrocytesGlial cells in the CNS that form the myelin sheath around axons.. A disc in the foramen sits close enough to the ganglion to squeeze it. Her L5 root carried both her foot’s sensation and its lift, so one pinched spot produced a numb dorsum — dorsal in — and a foot that drops — ventral out.
See it move: Scene 1 · Zip Codes · Scene 2 · In and Out
II · A cable of cables, and the branches it makes
In the periphery an axon becomes a fiber, and a nerve is a cable of cables. Endoneurium sleeves each fiber; perineuriumA connective tissue sheath that surrounds a bundle (fascicle) of nerve fibers. bundles fibers into a fascicle; epineuriumConnective tissue sheath surrounding an entire nerve. wraps all the fasciclesBundles of nerve fibers within a nerve.
or
Bundles of nerve fibers within a muscle. — what you would actually grab if you held a sciatic nerveThe largest nerve in the body, arising from the sacral plexus.. The parallel with muscle is exact: endomysium, perimysium, epimysium — connective tissue solving the same problem twice. On the site’s slide 068 a nerve in cross-section is a fistful of straws seen end-on: each circle a pink axon in a clear ring of washed-out myelin, a Schwann nucleusThe control center of the cell that contains DNA and directs cellular activities. on the rim, a dense band of perineurium round a fascicle. Beside it the ganglion: big round cell bodies with a pale nucleus and one dark nucleolus, each ringed by satellite cells like a necklace. Rings with holes means nerve; big round bodies with necklaces means ganglion. Just past the foramen the short nerve branches: one dorsal ramus to the muscles and skin of the back, a larger ventral ramus to the limbs and the front, and small rami communicantes to the sympathetic chain — the Autonomic Nervous System module. Roots are medialToward the midline of the body, rami lateralAway from the midline of the body. — two different splits.
See it move: Scene 3 · Under the Microscope: Nerve and Ganglion
III · Switchboards, a chain of custody, and the map
Ventral rami do not run straight to a limb; they braid through plexuses — cervical, brachial, lumbar, sacral — where roots are remixed into named nerves. One root feeds several nerves; one nerve carries several roots; the plexuses are named for the tissue they serve, not the cord level, and they are mixed, whatever the site’s roots-and-rami page says — a plexus that carried only motor fibers could never make a leg numb. Cervical plexus to the phrenic nerveA nerve from the cervical plexus that controls the diaphragm., C3–C5, keeps the diaphragm alive. Brachial to median, ulnar and radial — the ulnar is the funny bone. Lumbar to the femoral. Sacral to the sciatic, thumb-thick, drawing from both plexuses. Her chain of custody: L5 fibers through the lumbosacral plexus into the sciatic, down to the common fibular nerve wrapping the headRounded proximal end that fits into the acetabulum of the hip bone. of the fibulaSlender lateral leg bone; stabilizes ankle but bears little weight., into the deepAway from the surface of the body. fibular nerve, ending at the tibialis anteriorDorsiflexor Front of lower leg; lifts and inverts your foot when you walk. — crush the root at the top and the muscle goes quiet at the bottom. A dermatome is the skin of one root, a myotome its muscle; nipple line T4, umbilicus T10, top of the foot L5, little toe and sole S1, thumb C6; neighbors overlap by about half a segment. Her cotton-wool band was the L5 zip code.
See it move: Scene 4 · Switchboards · Scene 5 · The Case in the Skin
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
Zip Codes
Every patch of skin has a return address; hers was L5, and she could draw its border.
Animation · narrated, with captions
Zip Codes
Module 9 · 9.4 · Opener and Scene 1 · 8 scenes · 2 minutes · narrated. Press Play to start; the words appear in the caption box under the picture.
Scene 1
Paused
Keyboard: when the picture has focus, Space plays or pauses, the left/right arrow keys move between scenes, and M mutes or unmutes.
Text description and full narration
Eight-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender dermatome maps (the front and back figures without labels), her asset-library pictures of the brain and segmented spinal cord, a skeletal muscle, a lower leg and foot and a fingertip, and drawn shapes: wires, an envelope, a stair, a scanner, overlapping strips, a speech bubble, an eye and a timeline.
Scene 1 · Patches of skin (0:00–0:09)
On screen: Left: a picture of the brain and spinal cord, the cord banded into colored segments and labeled “cervical”, “thoracic”, “lumbar” and “sacral”. Right: a figure seen from the front with its skin divided by dashed lines into colored strips (dermatomes) in the same colors. Five colored wires draw themselves from five cord segments to five strips: the thumb, the chest, the belly, the top of the foot and the little toe. Chips: “nearly every spinal nerve → skin” and “the patches: a map”.
Narration: Every spinal nerve sends a branch into a patch of skin. And those patches, those are kind of like a map.
Scene 2 · Dermatome and myotome (0:09–0:25)
On screen: The belly strip that holds the umbilicus fills gold, and its wire stays bright while the other wires dim; the wire’s cord end is labeled “T10”. Right: a teal chip, “dermatome”, with “the strip of skin served by one root”. A drawn envelope appears with a return address, “from: the T10 root”, and the label “a zip code, a return address”. Then a red chip, “myotome”, beside a red skeletal muscle: “the same idea for muscle: motor, not sensory”.
Narration: A dermatome, that's the strip of skin that is served by one root. So think of it as a zip code, or like a return address. A myotome is the same idea for muscle, not sensory.
Scene 3 · Five landmarks (0:25–0:41)
On screen: The front-view dermatome figure moves to the middle, larger. Landmarks light one at a time with gold rings and labels on white plates: a gold dashed line across the chest, “T4 · nipple line”; a ring on the navel, “T10 · umbilicus”; a ring on the thumb, “C6 · thumb”; then the strip running down the outer side of her left leg onto the top of the foot fills gold, “L5 · top of the foot”; and a ring on the outer edge of the foot and the little toe, “S1 · little toe and sole”. Chip: “a few landmarks do most of the work”.
Narration: So a few landmarks do most of the work here. The nipple line is about T4, the umbilicus about T10, thumb C6. Top of the foot, L5. And the little toe and the sole of the foot are S1.
Scene 4 · Her L5 zip code (0:41–0:51)
On screen: Close-up of the two legs from the front, labeled “right” and “her left” (her left leg is on the viewer’s right). On her left leg a band over the outer shin and the top of the foot turns white and soft, like cotton wool: “numb: like cotton wool”, “outer shin + top of the left foot”. A gold chip: “the L5 zip code”. Then a fingertip traces the band’s border all the way around as a dark dashed edge draws behind it: “a border she could draw with a fingertip”.
Narration: Stina's band of cotton wool sat on the outer shin on the top of her left foot, the L5 zip code. And she could draw its border with a fingertip.
Scene 5 · A stripe and a stair (0:51–1:11)
On screen: Left: the legs from behind, with the dermatome strips. A red, glowing stripe draws itself down the back of her left thigh from the buttock to below the knee: “hot, burning stripe: back of the thigh”, “along the sciatic nerve’s path”. Right: a chip “2014”; a drawn stair; a lower leg and foot seen from the side moves toward the step and its toes catch on the edge, with a red burst: “her left foot catching on the stairs”. A red muscle appears down the front of the shin, its tendon running to the top of the foot: “tibialis anterior (L4–L5)”, chip “myotome”, and “too weak: the toes don’t clear the step”.
Narration: There was this hot, burning stripe that ran down the back of her thigh along the sciatic path. And by 2014, her left foot was kind of catching on stairways. The L5 myotome, the tibialis anterior, was becoming too feeble to lift the toes.
Scene 6 · Two maps agree (1:11–1:22)
On screen: Two cards. Teal: “numb patch” — “outer shin, top of the foot” → “L5 dermatome”. Red: “weak muscle” — “tibialis anterior” → “myotome L4–L5”. Arrows from both cards meet at a large gold circle: “L5”, “one level”. Right: a drawn scanner with a patient table, crossed out with a red line: “localized without a CT or an MRI in the building”.
Narration: Numb patch, weak muscles agreeing on one level, that's localization without a CT or an MRI anywhere in the building.
Scene 7 · Two cautions (1:22–1:41)
On screen: Chip: “two cautions”. Caution 1: three overlapping translucent strips, “L4”, “L5” and “S1”, each overlapping its neighbor by half: “dermatomes overlap by about half a segment”. The L5 strip fades and only a pale, fuzzy patch is left: “silence one root → a hazy patch”. Then the L5 strip returns bright red with a crisp outline: “irritate it → a sharp one”. Caution 2: a speech bubble, “it burns!”, with “pain: a symptom the patient reports”; and a drawn eye that a red line strikes through: “not a sign you can observe”.
Narration: So two cautions, though. Dermatomes overlap by about half a segment. So silencing one root gives a hazy patch, while irritating it gives you a very sharp one. And pain, it's a symptom that the patient reports. It's not a sign that you can observe.
Scene 8 · A clue, not a verdict (1:41–1:52)
On screen: A large teal chip: “the map: a clue”, and beside it “a verdict”, struck through in red. Below, a timeline: “2010 · the map points to L5” at the left and “2015 · the MRI” at the right, with a bracket between them: “five years”. The chip “a clue” lights again.
Narration: But this map is a clue. It's not a verdict. It's a clue. A verdict will be developed from it five years before the MRI.
Prefer the full lecture? Roots and Rami (original video on the site)
Explore · in production
Draw her band
An unlabeled lower-limb dermatome map. Shade where her cotton-wool numbness sits, then name the root. Drag five landmark pins — nipple line, umbilicus, top of the foot, little toe, thumb — to their segmentsThe subdivisions of lung lobes, supplied by tertiary bronchi.. Widen each strip by half a segment and predict why silencing a root gives a hazy patch while irritating it gives a sharp one. Last: a numb sole and little toe — predict the root and the reflex to test.
Think · not graded
Dermatomes are a clue, not a verdict: they overlap, and pain is reported rather than observed. What made her band usable anyway — the edge, the side, the match with the weak muscle — and what would a two-sided, fading, stocking-shaped numbness say instead?
Spot-check your thinking: true or false?
Check · Drag and Drop
On the dermatome map, drag each root to the box whose line points to its band: L2, L3, L4, L5 and S1. L1 is done for you.
Evidence card 1
Add it to your evidence board
A one-sided band with an edge, on the top of the foot: the L5 zip code, written on her skin from day two.
Scene 2 of 5
In and Out
Dorsal root in, ventral root out, one short mixed nerve — and the bulge that says which way is back.
Animation · narrated, with captions
In and Out
Module 9 · 9.4 · Scene 2 · 8 scenes · 2¼ minutes · narrated. Press Play to start; the words appear in the caption box under the picture.
Scene 1
Paused
Keyboard: when the picture has focus, Space plays or pauses, the left/right arrow keys move between scenes, and M mutes or unmutes.
Text description and full narration
Eight-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender cross-sections of the spinal cord with its roots and rami, the site’s micrographs of a dorsal root ganglion (site slide 065-2) and of the cord with its ganglion (site slide 065-1a), asset-library pictures of a unipolar neuron, a multipolar neuron, a skeletal muscle, a myelin spiral and a lower leg and foot, and drawn rootlets, arrows, a fingertip and a clamp.
Scene 1 · Plumbing (0:00–0:20)
On screen: Chip: “the cord reaches the body by plumbing”. A BioRender cross-section of the spinal cord (back at the top) with a pair of roots on each side: on each side a dorsal root with a bulge and a ventral root join into a short spinal nerve, which splits again at the outer end. Fans of thin rootlets draw themselves where the roots meet the cord: “rootlets”. Labels follow the narration: “dorsal root” and “ventral root” (right side); “spinal nerve” where the two roots meet, and “dorsal ramus” and “ventral ramus” where it splits (left side). Along the bottom a chain builds: “rootlets → roots → spinal nerve → rami (one: ramus)”.
Narration: The cord reaches the body by, you gotta think of it like plumbing. There are little rootlets that gather into roots. There are two roots that merge into a spinal nerve. And then the nerve splits into rami, singular ramus.
Scene 2 · Dorsal root: sensory in (0:20–0:39)
On screen: The cross-section moves up. A blue arrow, “sensation in”, runs from the outer end along the dorsal root into the back of the cord. A gold ring circles the bulge on the dorsal root: “cell bodies sit outside the cord, in a bulge”, then “the bulge: the dorsal root ganglion”. Below left, the site’s micrograph of a dorsal root ganglion (site slide 065-2) with large round cell bodies: “sensory cell bodies”. Below right, a drawing of a unipolar (pseudounipolar) neuron: a round cell body on a short stalk off one long myelinated fiber, “unipolar sensory cell”, “cell body hung off to one side”. A blue dot runs along the fiber from right to left, straight past the stalk: “the signal never detours through it”.
Narration: The dorsal root carries sensation in and its cell bodies sit outside the cord in a bulge. The bulge is called the dorsal root ganglion. Those are unipolar sensory cells. The cell body is like hung off to one side so the signal never has to actually detour through the cell body.
Scene 3 · Ventral root: motor out (0:39–0:54)
On screen: A red dot cluster marks the anterior horn at the front of the cord’s gray matter: “anterior horn”. A red arrow, “ventral root: motor out”, runs from the anterior horn out along the ventral root to the outer end. Below, a drawing of a multipolar neuron lying on its side, “multipolar motor neuron”, with its long axon ending on a red skeletal muscle, “neuromuscular junction”. A red dot runs the whole length of the axon to the muscle: “axon runs uninterrupted”.
Narration: The ventral root carries motor axons out from the anterior horn. Multi-polar motor neurons whose axons run uninterrupted to the neuromuscular junction.
Scene 4 · Find the bulge (0:54–1:05)
On screen: A dashed circle on the ventral root, crossed out in red: “no ventral root ganglion”. The gold ring returns to the bulge on the dorsal root. Chip: “find the bulge → you’ve found dorsal”. Below, the site’s Masson’s trichrome micrograph of the spinal cord with a dorsal root ganglion attached at its side (site slide 065-1a), labeled “the bulge: dorsal root ganglion” and, at the top of the cord, “dorsal side”.
Narration: There is no ventral root ganglion. So the bulge and you have, so if you find the bulge then you have found the dorsal side.
Scene 5 · A short, mixed nerve (1:05–1:25)
On screen: Close-up on the place where the two roots meet. A dashed oval circles the junction: “in the intervertebral foramen”. Inside the short spinal nerve a blue arrow points toward the cord and a red arrow points away: chip “spinal nerve: mixed, sensory in, motor out”. A bracket over it: “comically short”. A drawn fingertip slides in and its nail covers the whole spinal nerve: “a length you could cover with a fingernail”. Then a dashed purple outline traces the surface of the cord, “CNS: the cord”, and the roots and nerve are labeled “PNS: roots, spinal nerve, rami”, with a purple spiral of myelin: “Schwann cells make the myelin”.
Narration: The two roots, they fuse in the intervertebral foramen into a mixed spinal nerve that is almost comically short. A length you could cover with a fingernail. That's it. Step into a root and you have left kind of like the central nervous system, Schwann cells wrap around the myelin now.
Scene 6 · The nerve splits (1:25–1:47)
On screen: A BioRender cross-section, back at the top: from the cord a root swells into its ganglion and continues as the spinal nerve, which splits. Chip: “just past the foramen, the nerve splits”. A purple line traces the branch curving up and back: “dorsal ramus → muscles and skin of the back”. A blue line traces the larger branch running out and forward: “ventral ramus (larger) → the limbs and the front”. Green lines trace two thin branches down to a small round ganglion: “rami communicantes → sympathetic chain”, with “more in another module”. Side labels: “back” at the top, “front” at the bottom.
Narration: But just past the foramen the nerve splits. So one dorsal ramus serves the muscles and skin of the back. A larger ventral ramus serves the limb of the front. Small rami communicantes wire into the sympathetic chain. And we'll talk about that more in another module.
Scene 7 · Medial and lateral (1:47–1:58)
On screen: The roots-and-rami cross-section again. Two brackets over the left side: over the roots, “roots: medial to the spinal nerve”; over the outer branches, “rami: lateral to the spinal nerve”; the short stretch between is labeled “spinal nerve”. Rings mark two forks — “split 1: dorsal root / ventral root” and “split 2: dorsal ramus / ventral ramus” — and a chip: “the two splits are not the same”.
Narration: Roots are medial to the spinal nerve. Rami are lateral to the spinal nerve. And the two splits, they're not equal.
Scene 8 · One pinch, two deficits (1:58–2:22)
On screen: The cross-section moves up. A gray clamp closes on the roots on one side, with a red burst: “Stina’s L5 root, squeezed at one spot”. The blue sensory arrow and the red motor arrow are both stopped at the clamp. Below left, a lower leg and foot with a gray patch on the top of the foot: “sensation in goes quiet: a numb dorsum”. Below right, the same leg with a short, dashed red arrow curving only a little way up from the toes: “motor out goes weak: a foot that lifts weakly”. Chips: “dorsal = sensory”, “ventral = motor”, and “one pinch, two deficits”.
Narration: Alright, now squeeze Stina's L5 root at one spot. Sensation goes quiet, a numb, dorsum. Motor goes out but it's very weak, a foot that drops. So one pinch can actually create two deficits, dorsal, sensory, ventral, motor.
Prefer the full lecture? Roots and Rami (original video on the site)
Explore · in production
Build a spinal nerve
Rootlets, two roots, a ganglion, a nerve, three rami — loose parts to assemble before the diagram appears. Drag “in” and “out” onto the right roots and say where each cell body lives. Then a flipped section A cut or slice of the body or an organ for study. with one ganglion — commit to dorsal. Last: pinch the nerve at the root and predict which of her symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). it produces; pinch it past the plexus and predict what changes.
Think · not graded
A dorsal root ganglion sits in the foramen, close enough for a disc to squeeze. Why does squeezing a sensory cell body hurt differently from squeezing an axon — and why is the motor side “the last mile of the corticospinal tract” rather than a cell body at all?
Spot-check your thinking: true or false?
Check · Fill in the Blanks
Name the parts from cord to branches.
Evidence card 2
Add it to your evidence board
One root carried her foot’s sensation and its lift — one pinch, two deficits, dorsal in and ventral out.
Scene 3 of 5
Under the Microscope: Nerve and Ganglion
Slide 068 and slide 065-2: straws seen end-on, and round bodies wearing necklaces.
Slide · in production
Slides 068 and 065-2
Rings with holes means nerve; big round bodies with necklaces means ganglion. The file the site uses for the longitudinal nerve is named for a ganglion — a credit to fix.
Prefer the full lecture? PNS Nerves (original video on the site) · histology page: Peripheral Nerve · Dorsal Root Ganglia
Explore · in production
Straws or necklaces?
Six unlabeled fields, shuffled: nerve cross-section, nerve longitudinal, ganglion, muscle cross-section (the trap), a nerve inside a gland, a node of RanvierGaps between segments of the myelin sheath where action potentials are regenerated.. Commit to each before the label, naming the one feature that decided it. Then slide 068 at 40×: mark one axon, its myelin ring, a Schwann nucleus, the endoneuriumA thin layer of connective tissue surrounding each nerve fiber., a perineurium band — before the overlay. Last: predict what a demyelinating neuropathy (the Nervous Tissue module’s stockings) would change on this slide, and what a root compression would not.
Think · not graded
Three wrappings in a nerve, three in a muscle — connective tissue solving the same problem twice. What would a nerve be like with no perineurium, and why do the wrappings matter to a nerve that has to survive being stretched when you bend?
Spot-check your thinking: true or false?
Check · Question set
On slide 068, name the epineurium, a fascicle and its perineurium, an axon, a Schwann cell nucleus and the endoneurium.
Evidence card 3
Add it to your evidence board
The stockings on both feet are a myelin story on this slide (the Nervous Tissue module); the band on one foot is a root story that this slide cannot show.
Scene 4 of 5
Switchboards
Four plexuses, named nerves, mixed traffic — and a chain of custody from L5 to the tibialis anterior.
Animation · narrated, with captions
Switchboards
Module 9 · 9.4 · Scene 4 · 9 scenes · 2¼ minutes · narrated. Press Play to start; the words appear in the caption box under the picture.
Scene 1
Paused
Keyboard: when the picture has focus, Space plays or pauses, the left/right arrow keys move between scenes, and M mutes or unmutes.
Text description and full narration
Nine-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender plexus drawings (the lumbar and sacral plexuses, the trunk with its four plexuses) and front-view dermatome map, her asset-library pictures of a head and neck with the cervical plexus, head and shoulders with the brachial plexus, an arm and a seated leg with its knee bones, and drawn tracks, trains, a diaphragm, nerve routes, a parcel, a tracking card and a cast.
Scene 1 · Not a straight line (0:00–0:21)
On screen: Top: a chip “cord”, a dashed straight arrow to a chip “limb”, crossed out in red: “too easy”. Left: a BioRender drawing of the lumbar plexus — four nerve roots entering at the upper right, braiding together and leaving as thick nerves at the lower left: “plexuses: braided networks”. Right: a drawn switchboard. Four colored tracks, “roots in”, enter a gold box, “switching station”; three cables leave it, “named nerves out”, each made of strands of two or three of the input colors. Colored dots ride like trains along the tracks into the station and out along the strands.
Narration: Ventral rami, they don't run like straight out to a limb. That would be too easy. They braid through networks, plexuses. And as they braid through it, it's kind of like trains going through stations and switching stations. And this is where roots are remixed into named nerves.
Scene 2 · Four plexuses (0:21–0:31)
On screen: A back view of the trunk with the spinal nerves and four brackets beside the spine; four empty boxes on the left fill in, top to bottom: “cervical”, “brachial”, “lumbar”, “sacral”. Right: a chip, “named for body regions”, and a list: “cervical · neck”, “brachial · arm”, “lumbar · lower back → thigh”, “sacral · sacrum → leg”, under the heading “four plexuses”.
Narration: Plexuses are named for the tissues that they serve, not the cord level from which they come. There are four plexuses.
Scene 3 · Cervical: breathing (0:31–0:46)
On screen: Left: a head and neck in profile with the nerves of the cervical plexus: “cervical plexus”. Right: a drawn segment of cord with boxes “C3”, “C4”, “C5”; an amber line, “phrenic nerve”, runs from them down to a red dome, “diaphragm”, which rises and falls as if breathing: “for breathing”. A gold chip: “C3, C4, C5 keep the diaphragm alive”, and “(the rhyme is on purpose)”. Then a red burst on the cord above C3: “a high neck injury can take away breathing”, and the diaphragm stops moving.
Narration: There's the cervical plexus, which houses the phrenic nerve for breathing. C3, C4, C5, they keep the diaphragm alive. That rhyming was intentional. And this is why a high neck injury can take away your breathing.
Scene 4 · Brachial: three nerves (0:46–0:59)
On screen: Left: head and shoulders with yellow nerves spreading from the neck into the shoulder: “brachial plexus”. Right: an arm hanging at the side, seen from the front, thumb out to the right. Three colored lines draw down it: purple “median” down the middle to the palm; green “ulnar” down the inner side, behind the elbow, to the little-finger side; blue “radial” down the outer side to the thumb. A gold star at the inner elbow: “funny bone: the ulnar nerve at the elbow”. A red dot pulses at the thumb side of the wrist: “radial pulse (radial artery)”, “near the radial nerve”.
Narration: The brachial plexus gives the median, the ulnar, and the radial nerves. The ulnar at the elbow is the funny bone. Usually we take your radial pulse near the radial nerve.
Scene 5 · Lumbar and sacral (0:59–1:20)
On screen: A BioRender drawing of the lumbar plexus (roots L1–L4) and the sacral plexus (roots L4–S4) side by side, their roots braiding into thick nerves. A dashed teal ring circles the lumbar plexus: “lumbar plexus → femoral nerve: front of the thigh”; then a gold ring on the sacral plexus’s thick round cable: “sacral plexus → sciatic nerve”, “thumb-thick: the main cable of the leg”. A curved arrow joins the lumbar roots to the sacral roots: “draws from both plexuses”. Chip: “why low back pain is hard to pin on one level”.
Narration: The lumbar plexus gives us the femoral nerve to the front of the thigh. And the sacral plexus gives us the sciatic, the thumb-thick main cable of the leg, which actually draws from both plexuses at once. Which is why low back pain is hard to pin on one level.
Scene 6 · A parcel with tracking (1:20–1:32)
On screen: Left: a tracking card, “Stina’s chain of custody”, with six stops and empty check circles: “1 L5 root”, “2 lumbosacral plexus”, “3 sciatic nerve”, “4 common fibular nerve, at the fibular head”, “5 deep fibular nerve”, “6 tibialis anterior”. Chip: “a parcel with tracking”. Right: the BioRender sacral plexus, labeled “L5 root”, “lumbosacral plexus” and “sciatic nerve”. A drawn parcel enters at the L5 root, travels through the braid and out along the thick sciatic cable; stops 1, 2 and 3 tick.
Narration: Here's Stina's like chain of custody. It's like a parcel with tracking. L5 fibers enter the lumbosacral plexus and they travel in the sciatic.
Scene 7 · Down to the shin (1:32–1:53)
On screen: The tracking card stays. Right: a seated leg seen from the side, knee bent, its knee bones showing. A teal route draws itself with the parcel riding along it: down the back of the thigh, “sciatic nerve”; around the head of the fibula just below the knee, “common fibular nerve wraps the head of the fibula” (stop 4 ticks); down the front of the shin, “deep fibular nerve” (stop 5); to a red muscle on the front of the shin, “tibialis anterior: lifts the foot” (stop 6). Then a red burst on stop 1, “crush the root at the top”, the route turns gray and the muscle grays out: “the muscle goes quiet at the very bottom”.
Narration: They branch into the common fibular nerve wrapping around the head of the fibula, just below the knee. And then the deep fibular nerve and at the end the tibialis anterior, the muscle that lifts the foot. So if you crush that root at the top, the muscle goes quiet at the very bottom.
Scene 8 · One caution (1:53–2:12)
On screen: Chip: “one caution”. A gold ring circles the head of the fibula: “the common fibular nerve is superficial at the fibular head”, and stop 4 on the card is outlined in red. A drawn cast wraps the leg just below the knee: “a tight cast”, then a chip “a long cross-legged afternoon”. A red burst at the fibular head; the muscle grays out: “→ foot drop on its own”.
Narration: So one caution. The common fibular nerve is superficial at the fibular head. So a tight cast or a long cross-legged afternoon can actually cause foot drop on its own.
Scene 9 · Check the map (2:12–2:21)
On screen: The BioRender front-view dermatome figure, with the strip down the outer side of the left leg onto the top of the foot filled gold: “L5”. Chips: “confirm the level with your map” and “before you blame the root”.
Narration: And this is why, you know, you should always confirm the level with your map of your patient before you blame whatever root.
Prefer the full lecture? Roots and Rami (original video on the site)
Game · in production
Root to nerve to muscle
Four plexus cards and eight named nerves, matched before the key; then the roots: drag C3–C5 onto the phrenic and predict what a high neckNarrow region just below the head; common fracture site. injury does to breathing. Then the chain: order six cards — L5 root, lumbosacral plexus, sciatic, common fibular, deep fibular, tibialis anterior — and cut the chain at two places (root; fibular head) and predict what each cut does to the dermatome, the reflex and the foot. Last: mark the one site where a tight cast could fake her root.
Think · not graded
A plexus remixes roots, so one nerve carries several roots and one root feeds several nerves. What does that predict about how a root lesion and a nerve lesion look on the skin — and why does “the plexuses are all motor” fail the moment someone’s leg goes numb?
Spot-check your thinking: true or false?
Check · Drag the Words
Trace L5 to the muscle that lifts the foot.
Evidence card 4
Add it to your evidence board
A chain of custody from one root to one muscle — and one site at the knee that would have to be ruled out before blaming the root.
Scene 5 of 5
The Case in the Skin
One root, three lanes: numb dorsum, burning stripe, foot too weak to clear a stair — and two maps on one patient.
Slide · in production
From Level to Symptom
Dermatomes are a clue, not a verdict — but a band with an edge, a stripe along one nerve’s path and a weak muscle from one root all pointing at L5 is a verdict the map can give before any scan.
Prefer the full lecture? Sensory Spinal Pathways (original video on the site)
Explore · in production
Two maps, one patient
Her two numbness patterns on one figure: shade the L5 band and the stockings before the overlay; then for each, pick root or long-nerve, one-sided or two-sided, sharp or fading, and predict which the crash can honestly claim. Then the three lanes: drag numb, burning and weak onto dorsal columns, spinothalamic and corticospinal and predict which direction each travels. Last: list what she does not have — spasticity, brisk reflexesAutomatic responses to stimuli., a Babinski — and say what that rules out.
Think · not graded
Her stockings were on both feet a year before the crash; her band was on one foot two days after. Reading the skin alone, which could a disc at L4/L5 produce, which could it not, and what did the chart do with the one it could not explain?
Spot-check your thinking: true or false?
Check · Multiple Choice
Which of her findings could the L4/L5 disc NOT produce?
Evidence card 5
Add it to your evidence board
One root explains a band, a stripe and a weak foot on the left; nothing at L4/L5 explains both feet — the chart blamed the crash for that too.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: One root, three lanes — a localisable injury, Neither on its own, or A muscle problem, or the whole nerve, or both feet. Some cards can honestly go in more than one place — that’s allowed, and the feedback tells you which cards decide the case.
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
A bandThe part of the sarcomere containing thick filaments., a stripe and a weak foot on one side, and stockings on both — the reading that fits best is…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- Three — one for each symptom
- One mixed root can do all three: sensation in through the dorsal root, motor out through the ventral root, both squeezed at the same spot
- None — this is the peripheral nerve in the leg, not a root
Build your verdict · no writing
Choose the statement that holds in each set. The ones you keep become your argument.
6
The finding
Finding · the map
The L5 Zip Code
2010–2015: a cotton-wool band on the outer shin and top of the left foot, a hot stripe down the back of the leg, a foot that catches on stairs — the L5 dermatome, the sciatic’s path, the L5 myotome — beside stockings on both feet that began a year before the crash. Chart entry: dorsal root in, ventral root out, one short mixed nerve, rami to plexuses, L5 → lumbosacral plexus → sciatic → common fibular → deep fibular → tibialis anterior. The skin named the level in 2010. Held open: the stockings are the Nervous Tissue module’s map, and the back pain on fever nights is on neither map.
Next · Chapter 5
The Five-Dollar Localizer
A rubber hammer, four seconds, and a reflex the patient cannot fake. The 2015 exam that pinned two roots before the MRI agreed — and then, in 2016, the reflex nobody warned her about: the bladderA muscular organ that stores urine before excretion. that stopped reporting for duty.
Reliable information on rare diseases
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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 pages for Module 9 — The Spinal Cord
9.0 Module Overview
9.1 The Disc That Finally Had a Picture
9.2 Inside the Cable
9.3 Three Wrappers and a Needle
9.4 Roots to Skin
9.5 The Five-Dollar Localizer
9.6 Highways With Names
9.7 The Gate and the Generator
9.8 Conclusion and Assessment
9.9 Evidence Behind the Case
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
- ganglion
- skin
- dermatome
- anterior
- dermatome map
- anatomy
- physiology
- dorsal
- cells
- cell body
- ventral
- anterior horn
- neurons
- CNS
- Schwann cells
- oligodendrocytes
- perineurium
- epineurium
- fascicles
- sciatic nerve
- nucleus
- medial
- lateral
- phrenic nerve
- head
- fibula
- deep
- tibialis anterior
- segments
- section
- symptoms
- node of Ranvier
- endoneurium
- neck
- reflexes
- A band
- bladder
- development