Most Things Biology

7.7 Three Muscles, One Fire

Case home  ·  Start at the beginning  ·  The Reveal

Time To Read

13–20 minutes

Date Last Modified

The Muscular System · Chapter 7 of 7

A racing heart on fever nights and a cramping gut — the other two muscles in the same surge. This chapter builds cardiac muscle on slides 098-1 and 057 and smooth muscle on slides 212 and 155, adds how muscle develops and repairs, then lays the muscular chart on one page and names Chart Clue #7 — with the statin at fifty held apart.

The question you will answer

Three tissues, one trigger, one bloodstream. If the same inflammatory signal reaches all three, what would each one report — and how do you tell a tissue that is sick from a tissue that is being charged?

Opener · Spindles in Sheets · about 1¾ minutes

The opener is the same narrated animation you will watch in Scene 2.

Watch it in Scene 2 ↓

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

1

Read the chart

Patient chart · Stina

1988 → 2026 · the other two muscles, and the chart on one page

Heart
Fever nights at 13: “my heart races and I can feel it in my ears”; pericarditis with a rub at 33 and 44 (Heart module) — the sac.

Gut
Crampy abdominal pain with every attack since the ER at 9; the appendix at 15 (Module 1) — smooth muscle squeezing an inflamed wall.

Skeletal
Febrile myalgia; exertional pain; days of exhaustion; day-one cramps; CK normal, ESR high.

Red herring
Mar 2026, age 50: a statin — legs, headaches, eyes; stopped. Real aches from a drug; colchicine and statins together carry a known myopathy risk.

Told, 1988
“Out of shape.” “Growing pains.” “Nervous stomach.” “A heart that races is just a fever.”

Chart clues this module: 1–6 · earlier chapters · 7 · this chapter · Chart Clue #7

Before you go on · your prediction

A racing heart, a cramping gut and aching thighs on the same fever nights — three complaints, or one process reaching three muscles?

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

Three separate, ordinary things: fever raises the pulse, nerves cramp the gut, and a tired girl aches

Ask what the fever is doing to the pacemaker, what an inflamed wall does to a smooth-muscle sheet, and whether “ordinary” is a mechanism.

One systemic inflammation reaching three tissues that share a trigger — calcium — and a blood supply

Ask what each tissue uses calcium for, how each one is reached by what circulates, and which of the three can be examined for it.

A primary muscle disease affecting all three types

Ask what a myopathy looks like (CK, weakness between attacks, a biopsy) and whether a heart and a gut that work perfectly between fevers fit it.

From Stina’s diary

1988 · age 13 · “The backpack”

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 · A heart that raced and a gut that cramped

On fever nights at thirteen her heart raced — she could hear it in her ears on the pillow — and her stomach cramped, as it had in the ER at nine and as it would at fifteen when a surgeon took out an unremarkable appendix. The chart filed each with its own word: a fast heart is just a fever; a cramping gut is nerves; aching thighs are growing pains. All three are muscle. The heart is cardiac muscle, beating on its own, driven faster by the signals that raise a temperature; years later it would be the sac around it — pericarditis, twice — that the fire reached. The gut is smooth muscle in two sheets around an inflamed wall. The thighs are skeletal muscle being charged. Three tissues, one trigger, one bloodstream, one fire. This chapter builds the two it has not built, says what each would report if inflamed, and lays the whole chart on one page.

See it move: Scene 1 · Cardiac on a Slide · Scene 5 · Muscles That Were Never Out of Shape

II · Cardiac: a tissue that beats on its own

Cardiac muscle is found in one place, the myocardium, and is striated because its actin and myosin sit in sarcomeres. But the cells are short — about a hundred micrometres — branched into Y and T shapes, with one central nucleus, joined end to end by intercalated discs: dark step-like lines across the fiber, the highest-yield cue on slide 098-1 and the one feature no other tissue has. A disc holds two junctions doing two jobs: desmosomes anchor the cells so they do not pull apart; gap junctions pass the depolarising current straight into the neighbour, so the heart contracts as one unit — a syncytium — squeezing blood out like the last of the toothpaste. It is involuntary, driven by pacemaker cells with autorhythmicity, only sped up or slowed by autonomic nerves and hormones. Its action potential is long, with a plateau carried by calcium entering through the sarcolemma — much of the calcium that starts a beat comes from outside and triggers the reticulum to release the rest. Many mitochondria, much myoglobin, never resting; its myoblasts do not fuse, and damaged heart scars. A fever speeds the pacemaker — all her racing heart was.

See it move: Scene 1 · Cardiac on a Slide · Scene 1 · Cardiac on a Slide

III · Smooth: spindles in sheets, calmodulin’s switch — and the chart on one page

Smooth muscle lines the squishy organs — gut, bladder, uterus, airways, every vessel, the iris. A fiber is a spindle, thirty to two hundred micrometres, one cigar nucleus, no sarcomeres: actin and myosin run diagonally between dense bodies, so a contraction twists the cell like a shoelace pulled tight. Calcium comes mostly from outside through the sarcolemma and caveolae, and there is no troponin: calcium binds calmodulin, calmodulin activates myosin light-chain kinase, the kinase phosphorylates the myosin heads, then bridges form — the notes’ five steps. Slow, prolonged, cheap, able to latch for hours; no optimum length, working from thirty to a hundred and fifty percent; it stretches and settles — stress–relaxation — so a bladder can fill; and it can divide — hyperplasia. Two sheets around a tube alternate into peristalsis. Single-unit, visceral muscle is coupled by gap junctions and answers hormones and stretch; multiunit — iris, airways, big vessels, arrector pili — answers nerves only, through varicosities. It regenerates best, from pericytes. Now the page: a pacemaker sped by a thermometer; a sheet squeezing an inflamed wall; a skeletal muscle charged and dismantled, CK normal, ESR never. Three tissues, one fire. The statin at fifty: real aches, set aside. Chart Clue #7: never out of shape.

See it move: Scene 2 · Spindles in Sheets · Scene 5 · Muscles That Were Never Out of Shape

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

Cardiac on a Slide

Branching cylinders, one central nucleus, and the dark step across the fiber that only cardiac has.

Slide · in production

Slides 098-1, 057 and 098-N

The Journey’s nucleus slip (the synthesis table says “against sarcolemma” for cardiac) is corrected here: central, oval, one or two.

Prefer the full lecture? Types of Muscle (original video on the site) · histology page: Cardiac Muscle

Explore · in production

Find the disc

Slides 098-1 and 057 with a stage. Before the overlay, mark three intercalated discs and three striations and say how you told them apart; find a branch point and a central nucleus. Then zoom to a disc and sort its two jobs: which junction stops the cells pulling apart, which passes the current. Then fire one cell and predict how far the depolarisation travels. Last: set a fever and predict what the pacemaker does — and what the fiber itself does.

Think · not graded

A cardiac cell contracts when its neighbour depolarises, through a gap junction, with no nerve required; a skeletal fiber waits for its own motor neuron. Which tissue can a fever speed up without touching a single junction — and which one reports a fever as pain rather than pace?

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

Check · Drag the Words

Build the cardiac syncytium.

This self-scoring check is in production.

Evidence card 1

Add it to your evidence board

A racing heart on a fever night is a pacemaker sped up by circulating signals — the muscle itself was fine; the fire found the sac later.

Scene 2 of 5

Spindles in Sheets

Flame-shaped cells, two layers around every tube, peristalsis — and nerves that end in beads, not junctions.

Animation · in production · 1 min 50 s

Spindles in Sheets

A spindle — the holiday-candle flame — with one cigar nucleus; a sheet of them; two sheets around a tube, circular inside narrowing it, longitudinal outside shortening it like an inchworm; the alternation that is peristalsis. The autonomic nerve laid over the sheet with varicosities, beads that release transmitter onto the whole sheet. Slide 155 and slide 212: cut across, the spindles become circles of different sizes — the epithelium trap, with no lumen. The detrusor’s three-layer interlace.

Prefer the full lecture? Smooth Muscle (original video on the site) · histology page: Smooth Muscle

Explore · in production

Squeeze the tube

A tube wrapped in two sheets you can fire separately. Before each, predict what the tube does — narrower or shorter — then alternate them and predict the wave. Then cut the tube across and predict what the sheets look like at 40× (circles of different sizes; the trap that looks like epithelium — check for a lumen). Then lay a nerve over the sheet and predict how one varicosity reaches many cells. Last: inflame the wall under the sheets and predict what the squeeze feels like from inside.

Think · not graded

A smooth-muscle sheet squeezes whatever is inside the tube, inflamed or not; the pain of a cramping gut is the wall being squeezed, not the muscle being sick. If the wall of her gut was inflamed on fever nights — and years later held amyloid — what would the muscle report, and what would it never report?

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

Check · Fill in the Blanks

Smooth muscle anatomy.

This self-scoring check is in production.

Evidence card 2

Add it to your evidence board

A cramping gut is two sheets squeezing an inflamed wall — the muscle working, the wall the problem.

Scene 3 of 5

Calmodulin’s Switch

No troponin: calcium, calmodulin, a kinase, phosphorylated myosin — slow, cheap, and able to latch.

Animation · in production · 2 min

Calmodulin’s Switch

A spindle with actin anchored to dense bodies, myosin between — the shoelace. Calcium enters from outside through the sarcolemma and caveolae (and a little SR). Five steps: Ca²⁺ in · binds calmodulin · calmodulin activates the kinase · kinase phosphorylates myosin · cross-bridges form and the cell twists. Slow, prolonged, little ATP; the latch holding tension for hours; the stress–relaxation response as a bladder fills and settles; 30% to 150% of resting length; relaxation as calcium is pumped out and myosin dephosphorylated. ANS, hormones (gastrin), low O₂ / high CO₂ as the controls.

Prefer the full lecture? Smooth Muscle (original video on the site)

Explore · in production

Two switches side by side

A skeletal thin filament and a smooth-muscle cell on one screen, each with a calcium dial. Predict what calcium binds in each before the overlay (troponin / calmodulin), what moves next (tropomyosin / a kinase), and what finally lets myosin touch. Then run both for an hour and predict which one needs more ATP and which can latch. Then stretch the smooth cell to 150% and predict whether it still contracts. Last: fill a bladder slowly and predict the pressure trace — stress–relaxation.

Think · not graded

Skeletal muscle keeps its calcium inside and switches on a troponin; smooth muscle borrows calcium from outside and switches on an enzyme; cardiac does both. Three tissues, one ion, three locks. Which of the three can inflammation reach by changing what circulates — and which would show it first?

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

Check · Drag the Words

Order the five steps of smooth-muscle contraction.

This self-scoring check is in production.

Evidence card 3

Add it to your evidence board

Three locks for one key: troponin, calmodulin, and the heart’s outside calcium — every muscle switched on by the Skeletal module’s ion.

Scene 4 of 5

Single Unit, Multiunit, and Repair

Visceral sheets that answer stretch and hormones; scattered cells that answer only nerves — and which tissues can mend.

Slide · in production

Two Kinds of Smooth, and How Muscle Grows and Mends

The goosebumps on the back of your neck: arrector pili are the textbook multiunit example — scattered, separate, and all under the same command.

Prefer the full lecture? Smooth Muscle (original video on the site)

Game · in production

Sort the sheets

Eight tissues — gut wall, uterus, bladder, iris, airway, a large artery, arrector pili, ureter — and two bins: single-unit or multiunit. Sort before the reveal, then test each with three stimuli (a hormone, a stretch, a nerve) and predict which responds. Then the repair bench: injure skeletal, cardiac and smooth muscle and predict which heals with new fibers, which with scar, and which from pericytes — and which can grow by hyperplasia. Last: name the one smooth muscle in her chart that was squeezing an inflamed wall.

Think · not graded

Skeletal muscle repairs slowly through satellite cells and cannot replace large losses; cardiac scars; smooth regenerates. If an inflammatory signal charged all three tissues for years, which would carry visible damage, which would scar, and which would look untouched — and what does that predict for a biopsy of each?

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

Check · Drag the Words

Single-unit or multiunit?

This self-scoring check is in production.

Evidence card 4

Add it to your evidence board

Smooth muscle mends and keeps working; the wall it squeezed was what carried the disease.

Scene 5 of 5

Muscles That Were Never Out of Shape

The muscular chart on one page — three tissues, one fire, one red herring — Chart Clue #7.

Animation · in production · 2 min

Muscles That Were Never Out of Shape

The 1988 chart on one page: the mile, the fevers, the hollowed-out days, the racing heart, the cramping gut, the squats. Three tissues on the figure, one cytokine hand reaching all three: the pacemaker sped; a sheet squeezing an inflamed wall; skeletal muscle charged and dismantled. Two labs from 2016 onward: CK normal, ESR high. The three words — deconditioned, growing pains, nervous stomach — struck through. Then a card from 2026: the statin at fifty, real aches, stopped — set beside the pile, not in it. The stamp: Chart Clue #7.

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

Explore · in production

The chart on one page

A three-column ledger — mechanics, the fire, and set aside — and nine finding cards from 1988 and 2026: the mile, the fever ache, the hollowed-out days, the day-one cramps, the racing heart, the cramping gut, twenty squats, the backpack shoulders, the statin at fifty. Sort each before the key appears, and for each card in the fire column name the tissue and what it was charged for. Then the honesty check: mark the one card whose link to FMF the Evidence page calls an extrapolation. Last: write Chart Clue #7 in one sentence.

Think · not graded

Her muscles passed every test, her CK was always normal, and her ESR never was. Say in one sentence what kind of problem that is — then say why “deconditioned”, “growing pains” and “nervous stomach” each described a real thing and still missed the only thing that connected them.

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

Check · Single Choice

Chart Clue #7 reads her muscular history as…

This self-scoring check is in production.

Evidence card 5

Add it to your evidence board

Three tissues reporting one fire, two labs that say charged-not-torn, and one drug set aside: Chart Clue #7.

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 systemic inflammation reaching three muscle tissues, Neither on its own, or Three separate ordinary complaints, or a muscle disease. 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

A racing heart, a cramping gut and aching thighs on the same fever nights — 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 separate, ordinary things: fever raises the pulse, nerves cramp the gut, and a tired girl aches
  • One systemic inflammation reaching three tissues that share a trigger — calcium — and a blood supply
  • A primary muscle disease affecting all three types

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 · Chart Clue #7

Three Muscles, One Fire

1988: a racing heart and a cramping gut on fever nights, aching thighs, days of exhaustion; later, pericarditis twice, an unremarkable appendix, and at fifty a statin that hurt and was stopped. Chart entry: cardiac muscle — branching cells joined by intercalated discs, autorhythmic, calcium partly from outside — was sped by the fever and never sick; smooth muscle — spindles in sheets, calmodulin’s switch, two layers in peristalsis — squeezed an inflamed wall; skeletal muscle was charged for a fever and dismantled to pay, CK normal, ESR never. Three tissues, one trigger, one fire. The statin is a red herring, named and set aside. Chart Clue #7: her muscles were never out of shape.

Next · Module 8

Nervous Tissue & the Senses

The calcium that fires every one of her muscles is the same pulse a neuron uses to release its transmitter. The next module follows that ion out of the muscle and into nervous tissue — and reads the car crash of 2010 and the headaches nobody counted.

Continue to the Conclusion →

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 7 — The Muscular System

7.0 Module Overview
7.1 Muscles That Were Never Out of Shape
7.2 Built to Pull: Tendon to Sarcomere
7.3 The Signal: Nerve Meets Muscle
7.4 Calcium Turns the Key
7.5 The Fuel Bill
7.6 Grading the Pull
7.7 Three Muscles, One Fire
7.8 Conclusion and Assessment
7.9 The 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