Time To Read
Date Last Modified
The Muscular System · Chapter 4 of 7
The calcium her bones were turning over in the Skeletal module is the ion that switches muscle on. This chapter couples excitation to contraction — the voltage sensor, the release channel, troponin and tropomyosin, the cross-bridge cycle with its ATPThe energy currency of cells used for muscle contraction. in the right place — then runs the sliding filaments, the relaxation that costs ATP too, and the two famous failures. Her 1989 cramps belong here.
The question you will answer
A signal has reached every T tubule. Calcium must flood the fiber, myosin must be allowed to touch actin, and ATP must be there to let go. Which fails in a cramp, which in rigor mortis — and which would a febrile body put under pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.?
Opener · Excitation–Contraction Coupling · 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
1989 · age 14 · leg cramps on day one
Cramps
Day one of the period: calf and thigh cramps that wake her; hard, painful, a minute or two each; Tylenol; refuses to see a doctor.
With fevers
The ache, not cramps — a different sensation: deepAway from the surface of the body., diffuse, slow, with the heaviness.
Calcium thread
Skeletal module: bone turning over fast under IL-1β/IL-6; serum calcium normal on every later draw — the fiber’s own store, the SR, never in question.
Not drawn
A CK the morning after a cramp; magnesium(Mg²⁺): Cofactor for many enzymes; important in muscle and nerve function.; nothing.
Told
“Cramps. Drink waterThe universal solvent essential for life., eat a banana.”
Chart clues this module: 1–3 · earlier chapters · 4 · this chapter
Before you go on · your prediction
Hard calf cramps on day one of the period, and a slow diffuse ache with every fever — two different feelings in the same muscles. They most likely share…
Open the one you lean toward. Nothing is recorded and nothing is revealed — you come back to it at the end.
A calcium problem — the mineral the Skeletal module watched her bones lose
Ask where a fiber’s calcium actually comes from, whether serum calcium ever reached it, and what a true calcium shortage does to a contraction.
Nothing — cramps are cramps, aches are aches, and both are common in teenage girls
Ask what a cramp is at the level of the sarcomere, what an ache is, and whether “common” is a mechanism.
An ATP problem — a fiber that cannot let go is a cramp; a fiber that cannot keep up is an ache
Ask which step of the cross-bridge cycle needs ATP, what happens when it is missing, and what a fever does to the supply.
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 feelings in one muscle
In 1989 she worked out the monthly cycle: on day one of her period her calves and thighs cramped — hard, sudden, a knot you could see, a minute or two each, enough to wake her — and she took Tylenol and refused to see a doctor, because by then the ER had told her what it thought of her. The fever ache was a different thing: slow, deep, diffuse, with the heaviness, and it did not knot. Two feelings in the same muscles, both filed under common. They are not the same event at the sarcomere. A cramp is a contraction that will not stop — units firing unasked, or cross-bridges that cannot let go. An ache is a tissue being charged: inflamed, short of fuel, breaking protein down. This chapter builds the machinery both use, from the T tubule to the moment the fiber relaxes — starting where the Skeletal module left off, with calcium, the ion her bones were spending and her muscles cannot move without.
See it move: Scene 1 · Excitation–Contraction Coupling · Scene 5 · Letting Go
II · Excitation–contraction coupling, and the key in the lock
The notes set three requirements: the muscle must be stimulated so its membrane potential changes; it must propagate the action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission.; and calcium must rise inside and cause contraction. Chapter 3 did the first two. The third happens at the triad. In the T-tubule wall sits a voltage-sensitive protein that changes shape as the action potential passes; pressed against it in the terminal cisterna is a calcium release channel that senses the change and opens; calcium floods the sarcoplasm. Now the thin filament. Each actin subunit has an active siteThe specific region of an enzyme where a substrate binds and a reaction occurs.; tropomyosin lies over the sites like a strap; troponin holds the strap. Calcium binds troponin — the lecture’s key in a lock — troponin shifts, tropomyosin rolls off, the sites are exposed. That is coupling: a voltage at the surface has become a shape on a filament deep inside, and the whole fiber has done it at once because every myofibrilCylindrical structures within muscle fibers that contain myofilaments. has a triad. Smooth muscle, in Chapter 7, has no troponin and uses calcium differently; cardiac borrows much of its calcium from outside. Skeletal muscle keeps its own store and spends it on demand.
See it move: Scene 1 · Excitation–Contraction Coupling · Scene 2 · Troponin, Tropomyosin, and the Key
III · The cycle, the slide, and letting go
With the sites exposed the cycle runs. Twelve o’clock: the myosin headRounded proximal end that fits into the acetabulum of the hip bone. is cocked, carrying ADPA molecule produced when ATP releases energy. and phosphate from an ATP it already split. It binds the site — a cross-bridge. It releases the phosphate and pivots — the power stroke, pulling the thin filament about ten nanometres toward the M lineThe middle line of a sarcomere that stabilizes thick filaments. — and drops the ADP. The stroke’s energyThe capacity to do work or cause change. came from the earlier hydrolysis, so the lecture’s “you use no energy” is half right: the ATP was paid in advance. A fresh ATP binds and the bridge detaches; the head splits it and re-cocks; round again, a million bridges, as long as calcium stays on troponin and ATP keeps arriving. Filaments slide, they do not shorten: Z discs move together, I bandsThe part of the sarcomere containing only thin filaments and H zoneThe middle of the sarcomere where only thick filaments are present. shrink, the A bandThe part of the sarcomere containing thick filaments. keeps its width, and the pull runs out through every wrapping to the tendon. Relaxation costs ATP too — pumps drag calcium home, tropomyosin re-covers the sites, titin recoils the sarcomere. Take the ATP away and the heads cannot let go: writer’s cramp in a hand built for sprints; rigor mortis after death, the last bridges ever made. Her cramps were fibers that would not let go; her ache was a fiber being charged. Chapter 5 presents the bill.
See it move: Scene 3 · The Cross-Bridge Cycle · Scene 4 · What Actually Shortens
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
Excitation–Contraction Coupling
From the T tubule to the troponin: voltage becomes calcium, calcium becomes an exposed site.
Animation · in production · 1 min 50 s
Excitation–Contraction Coupling
The action potential dives into a T tubule. The voltage-sensitive protein in the tubule wall changes shape; the calcium release channel in the cisterna beside it senses the change and opens; Ca²⁺ pours into the sarcoplasm. Zoom to a thin filament: troponin catches calcium, shifts, and rolls tropomyosin off the active sites. The notes’ three requirements ticked off one by one; the Skeletal module’s calcium icon arriving as a key.
Prefer the full lecture? Sliding Filaments (original video on the site)
Explore
Trace the coupling
A triad and a thin filament on one screen. Fire an action potential, then start removing pieces. Predict before anything moves.
Text description of this explore
A drawing of a triad above a thin filament. At the top, a band of blood outside the fiber with a few calcium ions in it. The sarcolemma runs across and dives down as a T tubule between two terminal cisternae of the sarcoplasmic reticulum, which are full of calcium ions. Teal voltage sensors sit in the T-tubule wall, pressed against purple calcium release channels in the cisterna walls. Below, a thin filament of 21 actin beads, each with an active site, is covered by a dark tropomyosin strap held by three troponin complexes. A legend names the voltage sensor, release channel, Ca²⁺, troponin and tropomyosin. Step 1: before an action potential is fired, decide for six parts whether each changes shape, opens, moves to a new place or stays put: the voltage-sensitive protein, the calcium release channel, the calcium stored in the cisternae, troponin, tropomyosin, and calcium in the blood. After you commit, the drawing shows the fired state: sensors tilted, channels open, calcium in the sarcoplasm and on troponin, and the strap rolled off the sites. Step 2: choose a piece to remove (nothing, the voltage sensor, a jammed release channel, an emptied reticulum, troponin, or low blood calcium), predict whether the active sites open on cue, stay covered, or receive calcium that can no longer switch them, then fire it; a table lists the sensor, channel, sarcoplasmic calcium, troponin and the sites. Step 3: the three requirements for a contraction as a checklist, with the third split into its parts plus a line for relaxing; for each, decide whether a fever would put it under strain or not.
Think · not graded
Skeletal muscle keeps its own calcium in the reticulum; serum calcium never enters the fiber to start a contraction. So the bone loss the Skeletal module tracked and the cramps of 1989 cannot share a “low calcium” cause in the simple sense. What could they share instead — and which chapter of this module holds it?
Spot-check your thinking: true or false?
Check · Drag the Words
Drop the steps of excitation–contraction coupling into order.
Evidence card 1
Add it to your evidence board
Calcium comes from the fiber’s own reticulum, released by a voltage sensor — the Skeletal module’s ion, used a new way.
Scene 2 of 5
Troponin, Tropomyosin, and the Key
Three proteinsLarge molecules made of amino acids with various functions in the body. on the thin filament and one ion that decides whether myosin may touch.
Slide · a still to study
The Thin Filament and the Lock
The thin filament
- Actin: G-actin beads strung into two twisted F-actin strands. Each bead has an active site where a myosin head can bind.
- Tropomyosin: the strap lying along the strands, covering the active sites at rest.
- Troponin: the lock, a three-part protein that holds tropomyosin in place and binds calcium.
- Nebulin runs along the core and sets the length of the filament.
- Troponin and tropomyosin are the regulatory proteins; actin and myosin are the contractile ones.
Turning the key
- Calcium binds troponin.
- Troponin changes shape and rolls tropomyosin off the active sites.
- A cocked myosin head binds an exposed site: a cross-bridge.
Calcium is the key, troponin the lock, tropomyosin the door, and the door moves only when the key is in.
Prefer the full lecture? Actin and Myosin (original video on the site)
Game
Key, lock, door
Calcium is the key, troponin the lock, tropomyosin the door. Turn the key, then start swapping parts.
Text description of this game
A thin filament of 21 actin beads, each with an active site, drawn under a dark tropomyosin strap that carries three troponin locks, each lock with a stretch of seven sites. A thick filament with myosin heads lies below, and a tray of calcium ions (the keys) waits above. A legend names the calcium key, troponin lock, tropomyosin door, active site and myosin head. Step 1: predict how many calcium keys it takes to expose a site: one per site, one lock’s worth that opens a whole stretch of about seven, keys in every lock first, or none at all. After you commit, two keys drop onto the middle troponin, its stretch of strap rolls off, its seven sites open and the heads beneath them bind. Step 2: two panels of the filament, with calcium released and after the pumps take it back. Swap one part (nothing, troponin removed, tropomyosin removed, or calcium doubled with every lock already full), predict whether the sites open and close as normal, stay covered, stay open, or open with a stronger pull, then run it; a table gives the result. Step 3: choose which of four observations from Stina’s chart shows her calcium switch was working. Step 4: a five-box chain for a smooth muscle cell with no troponin; predict how it could contract, then the chain fills in: calcium rises, binds calmodulin, switches on myosin light-chain kinase, which puts a phosphate on the myosin head, and the head binds actin.
Think · not graded
Tropomyosin is a brake, not an engine: take it away and the muscle cannot relax. If a tissue is inflamed for years, which is the more worrying failure — a key that is slow to turn, or a strap that is slow to come back? Which would feel like a cramp, and which like weakness?
Spot-check your thinking: true or false?
Check · Drag the Words
Name each part of the cross-bridge: actin, active site, tropomyosin, troponin, calcium, myosin head, thick filament.
Evidence card 2
Add it to your evidence board
The site opens only with calcium on troponin — a switch, not a dial; whatever made her weak did not jam the switch.
Scene 3 of 5
The Cross-Bridge Cycle
Cocked, bound, stroked, released, re-cocked — and where the ATP actually goes.
Animation · in production · 2 min
The Cross-Bridge Cycle
Twelve o’clock: a cocked head carrying ADP + Pi, sites covered. Calcium arrives; the head binds — cross-bridge. Pi leaves; the head pivots — power stroke, thin filament pulled ~10 nm toward the M line; ADP leaves. A new ATP binds; the head lets go. ATP is split; the head re-cocks — the recovery stroke. Round again, a million bridges. The correction on screen: the stroke spends energy paid earlier; ATP is needed to detach AND to re-cock. The OpenStax rowing boat.
Prefer the full lecture? Sliding Filaments (original video on the site)
Game
Describe, then interpret
Five pictures of the cross-bridge cycle, no captions. Describe only what you see and have the description checked before you interpret anything.
Text description of this game
Five BioRender pictures of a myosin head under a thin filament with troponin and tropomyosin, labelled A to E, with no captions. Picture A: a myosin head rises from the thick filament and touches the thin filament above it. A circle labelled ATP sits at the base of the head. Small teal balls sit on both troponin complexes. Picture B: a myosin head leans back toward the thin filament without touching it; a dashed outline and a curved arrow show it has swung over from standing upright. A circle labelled ADP sits on the head. The troponin complexes carry no teal balls. Picture C: a myosin head touches the thin filament, bent over, with a dashed outline and a curved arrow showing where it leaned before. A circle labelled ADP sits on the head. An arrow above points along the thin filament: it has moved. Teal balls sit on both troponins. Picture D: a myosin head stands clear of the thin filament, not touching it. A circle labelled ATP sits at its base. Teal balls sit on both troponins. Picture E: orange calcium ions, labelled Ca²⁺, fall onto the thin filament, and teal balls sit on both troponins. A myosin head touches the thin filament. A circle labelled ADP sits on the head. There are no arrows. Step 1: for each picture, describe the head and the sites (head on actin with sites open, head on actin with sites covered, head off with sites open, or head off with sites covered; a teal ball on troponin means calcium is bound and the sites are open) and choose what sits on the head (ADP, ATP or nothing). Step 2: the five pictures appear as a strip above a list; put them in order, starting where calcium has just arrived and ending with the head cocked and waiting, calcium gone. Step 3: with the strip above, choose the picture for four events: where a fresh ATP binds, where the ATP is split and the head re-cocks, where the power stroke happens, and where the energy for that stroke was paid. Step 4: with no ATP left and calcium still on troponin, predict which picture the cycle freezes on (A, B, C, D, or none of the five: head on, pocket empty). After you commit, a sixth picture, F, joins the strip: a myosin head touches the thin filament, with an empty circle where ATP or ADP would sit. Teal balls sit on both troponins.
Think · not graded
ATP is needed to detach the head and to re-cock it; without ATP the cycle freezes with the bridges attached. Which step freezes in rigor mortis, which in writer’s cramp, and what would a fiber do if ATP were merely scarce rather than absent — would it cramp, weaken, or ache?
Spot-check your thinking: true or false?
Check · Drag the Words
Order the cross-bridge cycle, starting with the cocked head.
Evidence card 3
Add it to your evidence board
Every bridge costs an ATP to release — the cycle runs only as fast as the fuel arrives.
Scene 4 of 5
What Actually Shortens
Sliding, not shrinking: Z discs closer, I bands and H zone smaller, the A band unchanged.
Slide · a still to study
Sliding Filaments
What the theoryA well-tested and widely accepted explanation. says
- Myosin heads pull the thin filaments toward the M line.
- The Z discs move closer together, so each sarcomere shortens.
- Neither the thick nor the thin filaments get shorter. They slide past each other, and the overlap grows.
- Every sarcomere along a myofibril shortens at once, so the myofibril, and then the fiber, shortens.
Electron micrographs of relaxed and contracted muscle show the band pattern changing just as sliding predicts.
The theory nobody has watched directly: we have seen shadows, and the shadows say the filaments slide.
Prefer the full lecture? Sliding Filaments (original video on the site)
Explore
Measure the bands
A sarcomere drawn to scale over a ruler. Measure it relaxed, predict it contracted, then test the claim that trips people up.
Text description of this explore
A sarcomere drawn to scale over a ruler marked from 0 to 5.8 micrometers in 0.2-micrometer steps. Z discs are zigzag lines; red thin filaments are anchored at each Z disc; dark thick filaments sit in the middle, forming the shaded A band, with a paler H zone in its center and pale I bands straddling each Z disc. Brackets above name the I bands and the A band; letters below mark the Z discs and the H zone. Caliper readings appear under the ruler after you commit. Step 1: in the relaxed sarcomere, measure four widths with the ruler and choose 0.2, 0.6, 1.0, 1.6 or 2.6 µm for each: Z disc to Z disc, the A band, the I band and the H zone. Step 2: the sarcomere contracts to 2.2 µm; predict whether the I band, the H zone, the thin filament, the thick filament and the zone of overlap get narrower, stay the same or get wider, then the drawing contracts and the calipers read the new widths. Step 3: a slider sets the sarcomere from 2.0 to 3.6 µm; predict whether the A band will read narrower than, the same as, or wider than 1.6 µm, then measure; a table lists the sarcomere, A band, I band, H zone and overlap. Step 4: a 10-centimeter myofibril of about 38,000 sarcomeres each shortens 0.4 µm; choose how far its end moves. Step 5: put the stops of the pull in order, from the sliding thin filaments to the bone.
Think · not graded
If filaments slid until the thick ones hit the Z discs, the sarcomere could shorten no further; if they were pulled apart until nothing overlapped, no bridge could form. Where between those two does a muscle want to rest — and what does that predict about a muscle stretched by a twenty-pound backpack all day?
Spot-check your thinking: true or false?
Check · Dialog Cards
Flip each band and zone to check what it does during a contraction.
Evidence card 4
Add it to your evidence board
The sarcomere shortens by sliding, with an optimum overlap — Chapter 6 shows what a stretched muscle can and cannot do.
Scene 5 of 5
Letting Go
Relaxation costs ATP too — and when the ATP is gone, the last contraction is the one you keep.
Animation · in production · 1 min 50 s
Letting Go: Relaxation, Cramp, Rigor
The signal stops; acetylcholinesterase clears the cleft; the fiber repolarises; the release channelsProtein passages in the cell membrane that allow specific molecules to pass through. close. ATP-driven pumps haul calcium back into the reticulum; troponin lets go; tropomyosin re-covers the sites; titin recoils the sarcomere; the bands return. Then take the ATP away: a hand after an hour of writing, bridges attached, no reset — writer’s cramp. Then the body after death: the last cross-bridges, hours of stiffness, release only when the proteins decay — rigor mortis. Her 1989 cramp on the sarcomere.
Prefer the full lecture? Sliding Filaments (original video on the site)
Explore
Run it dry
A contracting sarcomere wired to two gauges, ATP and calcium. Drain them one at a time and predict what the heads do.
Text description of this explore
A sarcomere with zigzag Z discs, red thin filaments and dark thick filaments whose myosin heads are drawn either bound to the thin filaments or folded back, with calcium ions as gold dots around them. Beside it stand two gauges: ATP (green) and calcium in the sarcoplasm (gold), each with a reading under it. Step 1: the stimulus stops; predict which gauge must move first for the fiber to relax (calcium must fall, ATP must fall, neither because titin springs it back, or both together); after you commit, the calcium gauge falls, the ATP gauge dips as the pumps spend it, and the heads let go. Step 2: set the ATP gauge (full, low or empty) and the calcium gauge (high, falling or low), or load the two experiments with the buttons, predict what the heads do (keep cycling, let go, let go slowly, or lock on), then run it; a table lists both gauges, the heads and the sarcomere. Step 3: set the gauges for three cases: a writer whose hand knots after an hour of writing and lets go in a minute or two, a body at twelve hours that is stiff in every muscle, and the same body two days later, limp again. For each choose the ATP level and the calcium level; small gauges then show the answer. Step 4: Stina’s day-one cramps in 1989; predict which gauge was the problem; after you commit, the gauges read calcium high and ATP present. Step 5: write whether the advice “Cramps. Drink water, eat a banana.” holds up, then compare with a model answer.
Think · not graded
A cramp is bridges that will not release or units that will not stop firing; an ache is a fiber being charged for fuel it does not have. Both happened in the same calves a few weeks apart. Which gauge was low in each — and what single measurement on an attack morning would have told you the fiber was being broken down rather than merely tired?
Spot-check your thinking: true or false?
Check · Multiple Choice
Select every event that needs ATP, either binding it or splitting it.
Evidence card 5
Add it to your evidence board
Relaxation is paid for; a cramp is a fiber that cannot pay to let go — and an ache is a fiber charged for something else.
4
Your evidence board
Every scene gave you an evidence card, and the chart adds more. Sort each card into the column it supports: An ATP and fuel problem in the fiber, Neither on its own, or A calcium problem shared with her bones. 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
Day-one cramps and fever aches in the same calves — what they share is…
Find the one you chose at the top. Does the evidence support it, refute it, or revise it?
- A calcium problem — the mineral the Skeletal module watched her bones lose
- Nothing — cramps are cramps, aches are aches, and both are common in teenage girls
- An ATP problem — a fiber that cannot let go is a cramp; a fiber that cannot keep up is an ache
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 key worked; the question is the fuel
Calcium Turns the Key
1989, age 14: calf and thigh cramps on day one of the period, brief and hard; the fever ache slow and diffuse; serum calcium normal whenever drawn. Chart entry: the action potential reaches the triad, a voltage sensor opens the reticulum’s release channel, calcium binds troponin, tropomyosin rolls off, and the cross-bridge cycle runs — bind, stroke, ATP binds and the head lets go, ATP split and the head re-cocks — until the pumps drag calcium home, which also costs ATP. The fiber’s calcium is its own; the switch was never jammed. What a cramp and an ache share is the account that pays for letting go and keeping up: the fuel. Chapter 5 presents the bill.
Next · Chapter 5
The Fuel Bill
A million bridges, an ATP each. Chapter 5 follows the three ways a fiber makes ATP, the clock from four seconds to hours, the wall, the debt, the heat — and then adds the one item her chart never priced: a fever.
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
- 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
- ATP
- pressure
- deep
- magnesium
- water
- anatomy
- physiology
- action potential
- active site
- myofibril
- head
- ADP
- M line
- energy
- I bands
- H zone
- A band
- proteins
- theory
- channels
- development