Time To Read
Date Last Modified
The Spinal Cord · Chapter 5 of 7
September 2015: a tap on her left Achilles — nothing — while the right snaps back; the great toe is weak. Two bedside findings pin S1 and L5 before the MRI loads; a discectomy follows. Then, in 2016, a reflex nobody warned her about stops: she cannot tell when her bladderA muscular organ that stores urine before excretion. is full. This chapter builds the reflex arc, the hammer’s level key, upper versus lower motor neuronA neuron that directly stimulates a muscle., and the sacral loop.
The question you will answer
A reflex runs in and out through one segment without consulting the brain, so a failed reflex names a level — and cannot be faked. What are the five parts of an arc, how does “brisk means high, absent means low” sort a lesion, and what happens when the bladder’s arc fails?
Opener · The Silent Ankle · 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
Sep 2015 · age 40 · the exam — and 2016 · 41 · the bladder
Reflexes
Right ankle jerk 2+; left 0 — tapped, nothing. Knees 2+, symmetric.
Strength
Left tibialis anteriorDorsiflexor Front of lower leg; lifts and inverts your foot when you walk. and great toe 4/5; cannot walk on the left heel. No spasticity; plantars down. Straight-leg raise reproduces the stripe at 40°. “L5 radiculopathy, S1 root irritation — matches the MRI.”
Decision
Discectomy, 12 October 2015. “The disc is fixed; your pain should be gone.”
2016 · 41
She cannot tell when her bladder is full; 350 mL left behind; an areflexic detrusor. A catheter four times a day — the “urologist” of the ten folders.
Told
“Objective findings.” Then: “Bruised sacral nerves — they may or may not recover.”
Chart clues this module: 1 · The picture · 2 · The paper towel · 3 · The forgiving basement · 4 · The L5 zip code · 5 · this chapter
Before you go on · your prediction
A tapped Achilles tendon produces nothing on the left and a brisk kick on the right. What does that prove?
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
That she is not trying — reflexes can be suppressed
Ask what a reflex arc passes through, and whether the brain is in it.
That the S1 segment’s arc — root, synapse, root, muscle — is not conducting on that side, a finding she cannot influence
Ask what a reflex tests in one go, and why side-to-side comparison comes before the textbook.
That the spinal cord is damaged above the ankle
Ask what “brisk means high, absent means low” predicts for a lesion in the cord versus at a root.
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 · Five players, one arc
Step on a tack and you withdraw first, feel second. A reflex arc has five parts: a receptorA structure that detects stimuli. (a spindle feeling a stretch, a nociceptor meeting a hot mug), a sensory neuron (unipolar, cell bodyThe central part of a neuron containing the nucleus and organelles. in the dorsalRelating to the back side of the body. root ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS).), an integration center in the gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses., a motor neuron out through the ventralRelating to the front or belly side of the body. root, and an effector, whose job is to reverse the stimulus. The stretch reflexA reflex that prevents excessive muscle stretching. is the simplest circuit: a spindle in the quadriceps senses a stretch, one sensory neuron synapses directly onto an alpha motor neuron in the anterior hornThe front section of the spinal cord’s gray matter, housing motor neurons., and the muscle contracts. Monosynaptic, one level, lightning fast because there is no committee in the middle — the knee kick, and the corrections that keep you standing. The site’s reflexesAutomatic responses to stimuli. page starts it at a Golgi tendon organ — wrong organ: The tendon organ measures tension, not length, and drives a separate polysynaptic reflex that inhibits the muscle to protect its tendon. Every contraction comes with an order to relax the antagonist — reciprocal inhibitionA reflex that prevents opposing muscles from contracting simultaneously. — or the limb would stiffen.
See it move: Scene 1 · The Silent Ankle · Scene 2 · Five Players, One Arc
II · Polysynaptic, crossed, and a key to the levels
Most reflexes are polysynaptic — at least three neuronsThe functional cells of the nervous system that transmit signals., two synapses — and the interneuron is what concludes you should let go of the mug. The withdrawal reflex is one, whatever the site’s page says. The crossed-extensor reflex is reciprocal inhibition doubled and sent across the midline: step on glass and the injured side’s flexors contract while its extensors relax; on the other side, through interneuronsNeurons that process information and connect sensory and motor neurons, found only in the CNS. crossing the gray commissureThe bridge of gray matter connecting the left and right sides of the spinal cord., the extensors stiffen so that leg takes your weight — ipsilateralReferring to the same side of the body. and contralateralReferring to the opposite side of the body. in one circuit (the site’s crawl space, the rabid raccoon and the broken wrist). Then the key, learned cold: biceps C5–C6, patellar L2–L4, Achilles S1. Each jerk interrogates one segment — sensory root, synapseThe junction between two neurons where communication occurs., motor root, muscle — and the patient cannot influence it. The descending tractsPathways that carry motor signals from the brain to the spinal cord. restrain the arcs: cut above and they run unsupervised — hyperreflexia, clonus, a Babinski; cut at the root and the circuit is gone — flaccid, wasting, a dead reflex. Brisk means high; absent means low.
See it move: Scene 3 · Reciprocal and Crossed
III · The silent ankle, the fix, and the loop nobody mentioned
The neurologist tapped her left Achilles — nothing — while the right snapped back: S1 not conducting. Her left great toe was weak: the L5 myotome. Two adjacent levels, reflexes diminished rather than brisk, no Babinski: lower motor neuron, damage at the exit. Hammer and magnet agreed; in October 2015 the surgeon lifted the fragment off the root. “The disc is fixed. Your pain should be gone.” Then, in 2016, the cost nobody had booked. Voiding is a reflex through S2–S4: stretch receptorsProteins located on the surface or inside cells that bind specific molecules (e.g., neurotransmitter report filling, the sacral cord orders the detrusor to squeeze through parasympathetic fibers, and the brain vetoes it until a polite moment — a spinal reflex with an adult supervisor. The sacral segmentsThe subdivisions of lung lobes, supplied by tertiary bronchi. sit around T12–L1, so their roots run the length of the cauda equinaA bundle of nerve roots extending from the lower spinal cord, resembling a horse’s tail.; hers had been stretched by the fragment and the surgery that chased it. She stopped feeling her bladder fill; 350 mL stayed behind; the detrusor would not contract. A flaccid, lower-motor-neuron neurogenic bladder — the same class of injury as her silent ankle, in an autonomic circuit — managed four times a day with a catheter: of everything the spineProminent ridge on the posterior scapula dividing it into supraspinous and infraspinous fossae. had taken, the most private loss.
See it move: Scene 4 · The Sacral Loop · Scene 5 · Losing the Bladder
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
The Silent Ankle
Tap, nothing; tap, a kick. Four seconds, five dollars, and two roots pinned before the scan loads.
Animation · narrated, with captions
The Silent Ankle
Module 9 · 9.5 · Opener and Scene 1 · 6 scenes · 1¾ 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
Six-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender cross-section of the spinal cord with its roots and spinal nerves; her asset-library pictures of a calf and foot, a reflex hammer, a muscle spindle and a brain; and the site’s drawing of a herniated lumbar disc; with a drawn calf muscle, nerve, volley, disc fragment, stopwatch, five-dollar bill, column of cord segments, apartment building and screen.
Scene 1 · The right ankle kicks (0:00–0:25)
On screen: Left: a lower leg and foot seen from the side, with a red calf muscle drawn over the back of the calf. A chip: “right Achilles tendon”. A reflex hammer swings and taps the tendon at the back of the heel. Red arrows at the top and bottom of the calf: “the tendon stretches the calf”. A white inset shows a muscle spindle coiled around fibers inside a muscle: “a spindle fires”. Right: a cross-section of the spinal cord, “spinal cord, cut across”, with its roots, dorsal root ganglia and spinal nerves; a drawn nerve runs from the calf up to it. A gold dot, the volley, runs from the spindle up the nerve, drawing a blue line through the dorsal root into the cord (“in: dorsal root”) to a purple dot in the anterior horn (“one synapse in the anterior horn”); a second dot runs back out along a red line through the ventral root (“out: ventral root”) and down to the calf. The foot jerks downward with motion lines: “the calf kicks”. Bottom right: a stopwatch, “four seconds”, and a five-dollar bill, “five dollars”.
Narration: The neurologist tapped Stina's right Achilles tendon. The tendon stretched the calf. A spindle in the muscle fired. The volley kind of ran through the dorsal root and made a single synapse in the anterior horn, ran out the ventral root, and the calf kicked. Four seconds, five dollars. Then he left. I'm kidding.
Scene 2 · The left ankle: silence (0:25–0:46)
On screen: The same leg, hammer and cord. Chip: “left Achilles tendon”. The hammer taps again; a gold dot runs up the nerve on a blue line but stops and fades at a pale blue disc fragment pressing the root beside the cord: “a root crowded by a disc fragment”, tagged “S1”. The way out stays a gray dashed line with a red ✕ on the ventral root: “nothing came back”. A red ✕ by the still foot: “no kick, no flutter”. Chip: “the S1 root is not conducting”, and “a finding the patient can’t influence”. A small brain at the right is joined to the cord by a dashed line crossed with a red ✕: “the brain is not in this circuit”.
Narration: The tap went in and met a root the fragment had crowded. And nothing came back. There was no kick, no flutter for Stina. That silence said that the S1 root was not conducting action potential. And it's a finding the patient can't influence because the brain is not involved in that circuit at all.
Scene 3 · The great toe (0:46–1:00)
On screen: The leg alone. Chip: “lift your great toe”. A teal arrow points up from the great toe and the foot lifts only slightly; a teal band on the front of the shin: “muscles of the L5 myotome”, and “barely answered”. Right: a column of cord segments, “L3, L4, L5, S1, S2”, headed “cord segments”. L5 turns teal: “great toe: barely lifts”; then S1 turns red: “ankle jerk: silent”, under the chip “two bedside findings”. A purple bracket joins L5 and S1: “two adjacent levels”, and a chip: “one low lesion”.
Narration: Then she was asked to lift her great toe and the muscles of the L5 myotome, they barely answered. Two bedside findings, two adjacent levels, one low.
Scene 4 · Low: the exit (1:00–1:15)
On screen: Chip: “lumbosacral lesion”, and beside the bracket “one low lesion”. Chip: “ankle reflex: absent, not brisk”. A thin probe strokes along the sole from the heel toward the toes, and a curved purple arrow shows the great toe curling down: “sole stroked: toe goes down (no Babinski)”. Right: a drawn apartment building. The penthouse on top is crossed out in gray: “not the penthouse (brain)”. An arrow runs from the S1 box to the red door at the bottom: “damage at the exit: the lower motor neuron”.
Narration: Lumbosacral lesion. And the reflexes diminished pretty briskly with the toe going down, not up. Damage at the exit, the lower motor neuron, not the penthouse.
Scene 5 · The second opinion (1:15–1:25)
On screen: The column of segments, with L5 and S1 still lit, moves to the left. A dark screen slides in: “the MRI, drawn: what it showed”. Inside, the site’s drawing of a lumbar vertebra seen from above, its disc bulging backward into the canal. Labels: “left L5 root, pressed by the L4/L5 disc” and “sacral roots (S1) crowded”. Chips: “it agreed with the hammer” and “imaging: a second opinion”. Below, a small reflex hammer: “the hammer had already localized the lesion”.
Narration: When the MRI was finally put on the screen, it agreed with that. Imaging was kind of like a second opinion. The hammer had already been localized. The lesion.
Scene 6 · A failed arc names its level (1:25–1:41)
On screen: Chip: “the whole reason we teach reflexes”, with the reflex hammer. The spinal cord cross-section returns; a blue line runs in through the dorsal root (“in”) and a red line runs back out through the ventral root (“out”); a purple bracket over the cord: “one segment”. A red ✕ lands on the root where the fragment sat. Chips: “a reflex arc: in and out through one segment”, “so a failed arc names its level” and “her failed arc: S1”.
Narration: And that's the whole reason we teach reflexes. A reflex arc runs in and out through one segment. So a failed arc names its level.
Prefer the full lecture? Spinal Reflexes (original video on the site)
Explore · in production
Run the arc
Her two ankles with a hammer you swing. Predict each kick, watch the volley travel the arc, and on the left mark where it stops. Drag biceps, patellar and Achilles onto their levels. Sort hyperreflexia, clonus, Babinski, flaccid, wasting and absent reflex into “high” and “low”. Last: write the exam line that pinned her two roots.
Think · not graded
A reflex bypasses the brain for speed but does not hide from it. If her left ankle arc was dead, what did the brain still hear from that tap — and what would a brisk arc with an upgoing toe have meant?
Spot-check your thinking: true or false?
Check · Multiple Choice
Her left ankle jerk is absent, her reflexes are not brisk, and her plantar response is downgoing. The lesion is…
Evidence card 1
Add it to your evidence board
Absent left S1 reflex, weak L5 muscles, flaccid not spastic: two roots pinned by a hammer, confirmed by the magnet.
Scene 2 of 5
Five Players, One Arc
Receptor, sensory neuron, integration center, motor neuron, effector — and the one circuit with no committee.
Animation · narrated, with captions
Five Players, One Arc
Module 9 · 9.5 · Scene 2 · 7 scenes · 1¾ 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
Seven-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender pictures — the thirteen-frame hot-mug reflex arc, cross-faded so the spark travels, and the cross-section of the spinal cord with its roots — and her asset-library pictures of a foot, a muscle spindle, a man seated with his leg down and kicked out, and a man walking; with a drawn thumbtack, nerve, volley, quadriceps, player list and neuron chains.
Scene 1 · Withdraw first (0:00–0:17)
On screen: Left: a bare foot comes down on a red thumbtack on the floor: “step on a tack”. It lifts straight away, with a blue arrow up: “1st: withdraw”; then a gold pain burst flashes on the tack: “2nd: feel it”. Chip: “the part of the nervous system that doesn’t consult you”. Right: “a reflex arc”, then “five players”: five empty numbered boxes, 1 to 5, appear one under another, and a purple arrow runs down beside them: “always in this order”.
Narration: If you step on a tack, you withdraw first and kinda like feel it second. And that's the part of the nervous system that doesn't consult you. A reflex arc. It actually has five players to make that happen. And they always fire or have action potentials in this order.
Scene 2 · The receptor (0:17–0:30)
On screen: Left, on a white card, a BioRender reflex arc: a hand touches a hot mug at the top; a blue sensory neuron runs from the fingertip to a spinal cord cross-section, where a teal interneuron links it to an orange motor neuron that runs back to a second hand pulling away from a mug below. A spark sits at the fingertip: “nociceptor”, “meets a hot mug”, “or the tack you stepped on”. An inset shows a muscle spindle coiled around fibers inside a muscle: “or a muscle spindle, feeling a stretch”. Right, box 1 of the list fills in, ringed in orange: “receptor: nociceptor or muscle spindle”.
Narration: There was always some kind of receptor. In this case, a nociceptor, meeting a hot mug that you picked up, meeting the tack that you stepped on. It could also be a muscle spindle, feeling a stretch.
Scene 3 · The sensory neuron (0:30–0:39)
On screen: The spark moves frame by frame along the blue sensory neuron toward the cord: “sensory neuron”, “unipolar”; it reaches the swelling on the dorsal root: “cell body: dorsal root ganglion”, then the root beside the cord: “in through the dorsal root”. The orange ring moves to box 2: “sensory neuron: in through the dorsal root”.
Narration: There's also a sensory neuron. It's probably unipolar. Its cell body is in the dorsal root ganglion and its axons run in through the dorsal root.
Scene 4 · Center, motor neuron, effector (0:39–0:56)
On screen: The spark enters the gray matter: “integration center: gray matter”; crosses to the motor neuron and runs out along the orange axon: “motor neuron: out through the ventral root”; and arrives at the lower hand as it pulls away from the mug: “effector: pulls the hand away”. Beside it: “same word as negative feedback, same job: reverse the stimulus”. The ring steps through boxes 3, 4 and 5: “integration center: in the gray matter”, “motor neuron: out through the ventral root”, “effector: reverses the stimulus”.
Narration: There's an integration center in gray matter. There's a motor neuron that runs out through the ventral root and an effector, the same word that we used with negative feedback and with the same job, reverse the stimulus.
Scene 5 · The interneuron (0:56–1:08)
On screen: The spark sits at the start of the teal interneuron in the cord, circled in teal: “interneuron”. Chip: “most arcs: an interneuron in the middle”, and a speech chip: “you are burning your hand — let it go”. The ring returns to box 3, the integration center.
Narration: Most arcs have an interneuron in the middle, and it's that interneuron that concludes you are burning your hand and that you should let it go.
Scene 6 · The stretch reflex (1:08–1:29)
On screen: Chips: “the stretch reflex: no interneuron” and “directly wired: sensory neuron → motor neuron”. Left: a man sitting on a chair, a red quadriceps drawn on his thigh; an inset shows a muscle spindle: “a spindle in the quadriceps senses a sudden stretch”. A drawn nerve runs from the thigh to a spinal cord cross-section. A gold dot draws a blue line in through the dorsal root (“one sensory neuron”) to a purple dot in the anterior horn (“a single synapse in the anterior horn”); a second dot runs back out on a red line (“motor neuron”) to the quadriceps, and the lower leg kicks out straight: “the muscle contracts”.
Narration: The stretch reflex, though, it does not. It is a directly wired sensory neuron to motor neuron. There's a spindle inside your quadriceps that senses a sudden stretch, and one sensory neuron makes a single synapse directly onto a motor neuron in the anterior horn, and the muscle contracts.
Scene 7 · No committee in the middle (1:29–1:50)
On screen: Chips: “monosynaptic”, “one level”, “one synapse”, “two neurons”. Two chains of circles: “most arcs:” S → I → M, with “a committee in the middle”; “stretch reflex:” S → M, “lightning fast”; key: “S = sensory neuron · I = interneuron · M = motor neuron”. The seated man kicks again: “the knee kick: monosynaptic”; a walking man appears: “and a thousand small corrections that keep you upright”.
Narration: That's monosynaptic, one level, one synapse, two neurons, and it's lightning fast because there's no committee in the middle. So the knee kick and the thousand small corrections that keep you upright during the day are monosynaptic reflexes.
Prefer the full lecture? Spinal Reflexes (original video on the site)
Game · in production
Order the five, then count the synapses
Five cards ordered before the arrow appears. Then stretch and withdrawal: count the neurons and synapses before the number shows, and say which has an interneuron and why. Fix the site’s slip — a patellar reflexThe knee-jerk reflex. drawn from a Golgi tendon organ. Last: cut the arc at each part and predict the exam.
Think · not graded
The stretch reflex has no interneuron; withdrawal needs one. What does the interneuron add — decision, spread to several muscles, inhibition of the antagonist — and what does it cost in time?
Spot-check your thinking: true or false?
Check · Drag the Words
Build the arc in order.
Evidence card 2
Add it to your evidence board
A reflex is a circuit, not a decision — which is why a dead one is evidence and not a mood.
Scene 3 of 5
Reciprocal and Crossed
Every contraction comes with an order to relax — and stepping on glass braces the other leg before you know it.
Slide · in production
Reciprocal Inhibition and the Crossed-Extensor Reflex
Inhibition does as much work as excitation: a reflex that only contracted would turn a limb into a plank.
Prefer the full lecture? Spinal Reflexes (original video on the site)
Explore · in production
Step on the glass
A figure stepping on glass with both legs wired to one cross-section. Before the circuit draws, predict for each of four muscle groups — injured-side flexors and extensors, far-side flexors and extensors — on or off. Then mark the interneurons that cross the gray commissure. Then the raccoon: one arm pinned, a fall from a crawl space — predict which limb the reflex used and what it cost. Last: remove reciprocal inhibition and predict what the leg does.
Think · not graded
A crossed-extensor reflex has to be polysynaptic — list the jobs it does in one movementA fundamental property of life involving motion of the body or its parts. and say why none of them could happen with a single synapse. Then: where in the cord does the “crossed” part happen?
Spot-check your thinking: true or false?
Check · Summary
Choose the correct statement in each set about these circuits.
Evidence card 3
Add it to your evidence board
Her arcs were not the problem — one of them was simply disconnected at its root.
Scene 4 of 5
The Sacral Loop
How a bladder empties: stretch, S2–S4, the detrusor — and a brain that vetoes until it is polite.
Animation · narrated, with captions
The Sacral Loop
Module 9 · 9.5 · Scene 4 · 7 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
Seven-scene animation, narrated by Amy Fenwick, with captions. Built from Amy’s BioRender pictures — a bladder cut open, the cross-section of the spinal cord with its roots, and the lower lumbar spine with the cauda equina seen from behind — and her asset-library pictures of the brain, the vertebral column with its cord colored by region, and a scalpel; with drawn sphincters, nerves, a volley, a measuring gauge, cord segments, a clock and a disc fragment.
Scene 1 · A reflex through S2–S4 (0:00–0:09)
On screen: Left: a BioRender bladder cut open, small and partly filled, with two ureters above and the urethra below; two small orange blocks sit at its neck and two red blocks lower on the urethra. Chip: “voiding: a reflex”, and “clearing your bladder”. Right: a column of “sacral cord segments”, S1 to S5; S2, S3 and S4 turn purple and a bracket joins them: “runs through S2–S4”.
Narration: Voiding, or clearing your bladder, is a reflex, and it runs through S2 to S4.
Scene 2 · The bladder fills (0:09–0:28)
On screen: The bladder grows as it fills. Gold stars light up around its wall: “stretch receptors in the wall report in”. Right: a measuring gauge marked 0, 150, 300, 400, 500 and 600 mL fills with yellow as she speaks: “first urge · 150 mL”, “full bladder · 300–400 mL” with a bracket, and “urgency · 500–600 mL” with a bracket.
Narration: As the bladder fills, stretch receptors in its wall kind of like report in. A first urge, around 150 milliliters. A full bladder, at 300 or 400 milliliters, and there's urgency at 500 or 600 milliliters.
Scene 3 · Squeeze and open (0:28–0:46)
On screen: The full bladder at left; at right a cross-section of the spinal cord with its roots: “sacral cord (S2–S4)”. A drawn nerve runs from the bladder wall to the cord’s root. A gold dot draws a blue line from a receptor in the bladder wall into the cord: “the volley enters the sacral root”. A second dot draws a purple line back out of the cord to the lower bladder wall: “parasympathetic fibers”, with a leader to the purple line. Purple arrows press in on the wall and the bladder squeezes: “squeeze”, “detrusor: the smooth muscle of the bladder wall”. The two orange blocks at the neck slide apart: “internal sphincter relaxes”.
Narration: The volley kind of enters the sacral root, and the cord orders the detrusor muscle, which is the smooth muscle of the bladder wall, it orders it to squeeze through parasympathetic fibers, while the internal sphincter relaxes.
Scene 4 · The adult supervisor (0:46–1:11)
On screen: A dashed gold arrow points down out of the urethra: “left alone, the loop would empty the bladder”, and a red ✕ lands beside it. A brain appears at the top right, and a dashed red line runs down from it into the cord: “a descending signal from the brain”. A red line runs out of the cord along a nerve to the red blocks on the urethra, which are boxed and pulse: “external sphincter: held”, and just under that label, “skeletal muscle · pudendal nerve · somatic”. A wall clock’s minute hand sweeps round: “until a socially acceptable moment”.
Narration: Left alone, that loop would empty the bladder on its own, but what stops it is a descending signal from the brain holding the external sphincter, which is skeletal muscle, pudendal nerve, somatic, until a socially acceptable moment, which is the only reason long meetings are survivable.
Scene 5 · The packet’s rule (1:11–1:30)
On screen: Chip: “a spinal reflex with an adult supervisor”, with the brain in the corner: “the supervisor”. The cord cross-section grows to fill the screen: “the packet’s rule applies here, too”. A blue ring on the dorsal horn: “visceral sensation enters the dorsal horn”; a purple ring on the gray matter between the horns: “visceral motor leaves from the visceral column”; a red ring on the somatic motor pools of the anterior horn, the ring set just below the purple one: “…deep to the somatic pools”.
Narration: A spinal reflex with an adult supervisor. The packet's rule applies here, too. Visceral sensation enters the dorsal horn. Visceral motor leaves from the visceral column deep to the somatic pools.
Scene 6 · Segments up high (1:30–1:45)
On screen: The vertebral column seen from the side, with the spinal cord inside colored by region: orange at the neck, green through the chest, then a short red and blue end, the blue sacral roots running far down to the sacrum. A blue ring on the short blue end: “the sacral segments”; a bracket beside the vertebrae there: “sit up around the T12 and L1 vertebrae”. A blue arrow runs down beside the long roots: “their roots: a long trip down the cauda equina”, “to reach the pelvis”.
Narration: Now, notice where the sacral segments sit, up around the T12 and L1 vertebra. Their roots have a long trip down the cauda equina to reach the pelvis.
Scene 7 · A long way down (1:45–2:01)
On screen: Beside the column, a BioRender view from behind of the lower lumbar spine and sacrum, the cauda equina roots running down its middle; one disc bulges to the left against a root. Chip: “a long exposure to anything pressing on the way”. Labels: “L4/L5 disc”; a pale blue fragment appears below it among the roots: “Stina: a migrated fragment at L4/L5”; “sacral roots passing on their way down”. A scalpel: “the surgery that tried to chase it”.
Narration: A long exposure to anything pressing on the way, and in Stina's case, a migrated fragment at L4, L5, and the surgery that kinda tried to chase it down.
Prefer the full lecture? Spinal Gray Matter (original video on the site)
Explore · in production
Fill the bladder
A bladder with a volume slider and the sacral cord beside it. Fill it and predict, before the signal draws, when the stretch receptors first report, which segments receive them, which fibers order the detrusor, and which sphincter the brain holds. Then the packet’s rule: visceral sensorySensory input from internal organs to the CNS. into the dorsal horn, visceral motor out of the lateralAway from the midline of the body./sacral column — place both. Then the long trip: drag the S2–S4 roots down the cauda equina past her L4/L5 disc and predict how exposed they are. Last: cut the sensory side and predict what the gauge does.
Think · not graded
Voiding is a reflex with a supervisor. If the sensory side of the loop is cut, what does the bladder stop doing, what does it keep doing, and why does the supervisor never get the message — and what is the mirror image if the cut is above the sacral cord instead?
Spot-check your thinking: true or false?
Check · Fill in the Blanks
Wire the sacral loop.
Evidence card 4
Add it to your evidence board
The bladder is a reflex arc through S2–S4 whose roots passed her disc on the way down.
Scene 5 of 5
Losing the Bladder
Flaccid, lower-motor, insensate: the same class of injury as her silent ankle, in an autonomic circuit.
Slide · in production
Two Kinds of Neurogenic Bladder
Neurogenic bladder by itself tells you almost nothing; flaccid or spastic tells you where the lesion is — the mirror image of the reflex rule, applied to an organ instead of an ankle.
Prefer the full lecture? Spinal Gray Matter (original video on the site)
Game · in production
Flaccid or spastic?
Two bladders, one cut at the sacral roots and one cut high in the cord, with six feature cards — insensate, overactive, large capacity, high pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange., overflow, high residual — sorted before the key. Then her chart: residual 350 mL, areflexic detrusor, no sense of filling — commit to flaccid or spastic and to the lesion level. Then the emergency: saddle numbness, both legs, retention — predict the diagnosis and the clock. Last: match her silent ankle and her silent bladder to the same rule.
Think · not graded
Her ankle arc and her bladder arc failed the same way — at the root, flaccid, lower-motor. Say why one is somatic and the other autonomic, why both are “absent means low”, and why a spastic bladder would have pointed the clinician somewhere else entirely.
Spot-check your thinking: true or false?
Check · Multiple Choice
No sensation of filling, an areflexic detrusor, 350 mL left behind — the bladder is…
Evidence card 5
Add it to your evidence board
A flaccid neurogenic bladder from stretched sacral roots — a fix and a cost, booked on the same admission.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: Lower-motor-neuron injuries at the roots — ankle and bladder, one rule, Neither on its own, or A cord injury, a muscle problem, or not trying. 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 dead ankle jerk, a weak great toe, flaccid not spastic, and a year later a bladder that cannot feel or squeeze — the reading that fits best is…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- That she is not trying — reflexes can be suppressed
- That the S1 segment’s arc — root, synapse, root, muscle — is not conducting on that side, a finding she cannot influence
- That the spinal cordThe central nervous system structure that relays signals between the brain and body. is damaged above the ankle
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 hammer
Absent Means Low
September 2015: left Achilles silent, great toe weak, flaccid, plantars down — S1 and L5 pinned at the bedside; October: the fragment removed, “your pain should be gone”. 2016: a bladder that cannot feel or squeeze, 350 mL behind, a catheter four times a day — a flaccid neurogenic bladder from stretched sacral roots. Chart entry: five parts to an arc, one synapse for stretch and many for everything else, a level key, and one rule for an ankle and a bladder alike. Held open: the surgery fixed the pinch; whether it fixed the pain is a separate question — and the pain that came with the fevers never left.
Next · Chapter 6
Highways With Names
With the disc fixed, which lanes still carry what? Three ascending tractsBundles of axons that carry sensory information from the spinal cord to the brain., two kinds of descending ones, four rules every tract obeys, and the full circuit from her sole to her cortex and back — the white matterThe outer portion of the spinal cord made of myelinated nerve fibers that transmit signals. that explains where a signal travels, and, soon, where a device can sit.
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 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
- bladder
- lower motor neuron
- tibialis anterior
- anatomy
- physiology
- receptor
- cell body
- dorsal
- ganglion
- gray matter
- ventral
- stretch reflex
- anterior horn
- reflexes
- reciprocal inhibition
- neurons
- interneurons
- gray commissure
- ipsilateral
- contralateral
- synapse
- descending tracts
- receptors
- segments
- cauda equina
- spine
- patellar reflex
- movement
- visceral sensory
- lateral
- pressure
- spinal cord
- ascending tracts
- white matter
- development