# Troponin and heart-muscle injury

Source: saved session `01a155db-c943-70ca-b8c8-2b2936783c63`, October 19, 2026.

The original detailed write-up is preserved verbatim below. Desktop paths and verification claims refer to that original session, not this import. Educational material only; not a clinical protocol or medical device. Clinical statements, diagnostic thresholds, and treatment recommendations have not been independently verified during import. Original model assumptions and limitations are preserved; independent clinical review is required before teaching or patient-care use.

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I read "tropin" as **troponin**. The interactive figure is built and works in both light and dark themes.

**Files** (in `~/Desktop/02_projects/anatomy-troponin/`): `troponin.html` and `troponin-light.html`, built by `build.mjs`, plus `preview-dark.png` and `preview-light.png`.

## The figure: a bench model of one heart-muscle filament

| Part of the machine | What it stands for |
|---|---|
| Row of beads on an axle, fixed to a plate on the left | Actin, anchored to the Z-disc |
| Thin rod on rocker links, lying across a white dot on each bead | Tropomyosin covering the myosin-binding sites |
| Collar clamped on the rod | TnT, which ties troponin to tropomyosin |
| Two-lobed dumbbell above the collar; the lobe with a cup is the one highlighted part | TnC; the cup is site II, the only regulatory calcium site in heart muscle |
| Arm running from the dumbbell down onto a bead | TnI, the latch that holds the system "off" |
| Reservoir overhead that drops a ball each beat | The sarcoplasmic reticulum releasing Ca²⁺ |
| Levers with ball heads in front of the beads | Myosin heads |
| Tray holding everything | The heart-muscle cell (membrane) |
| Spout and valve → vial with a tap → log scale with a red tick | Troponin leaking into blood, being cleared, and the 99th percentile cutoff |
| Loose pellets on the tray floor | The small free pool of troponin in the cell's cytoplasm |

**Controls:** on the left of the figure the heart is healthy and beating. Hover to the right to scroll through 0 h → 14 days after a coronary artery blockage. Click, or press Home, to go back to healthy. If you leave it alone, it plays a 30-second loop.

## How troponin works

1. **At rest:** TnI holds tropomyosin over the binding sites on actin, so myosin can't attach.
2. **Systole:** Ca²⁺ goes from about 0.1 to about 1 µM and binds TnC's site II. TnC grabs TnI, TnI lets go of actin, and tropomyosin rolls off the sites. Myosin heads bind, pull, and the muscle shortens.
3. **Diastole:** the SERCA pump takes Ca²⁺ back into storage. The ball rises back into the reservoir, tropomyosin rolls back, and the muscle relaxes.

Troponin is the calcium switch that turns each heartbeat on and off.

## Why it is the test for a heart attack (MI)

- **Specific to heart:** cardiac TnI is made only in heart muscle, and cardiac TnT almost only there. Older markers such as CK-MB also come from skeletal muscle.
- **It only gets out when cells die.** In the model:
  - Contraction stops within about a minute of the blockage.
  - After about 30 minutes the damage can't be undone and the membrane fails (the valve opens).
  - The small free pool in the cytoplasm leaks first: the pellets disappear.
  - Then the much larger filament-bound pool leaks over days as the filaments break down.
- **Sensitive, and stays up a long time.** In the model's moderate infarct, high-sensitivity troponin T (hs-cTnT):

  | Time after blockage | hs-cTnT |
  |---|---|
  | ~2 h | passes 14 ng/L |
  | ~3 h | ~50 ng/L |
  | 13 h | 1,315 ng/L (first peak) |
  | day 3–4 | second rise |
  | day 12 | still above 14 ng/L |

  The long tail lets you catch a heart attack days after it happened.
- **How it is used clinically:** the Fourth Universal Definition of MI requires:
  - a **rise and/or fall** in troponin,
  - with at least one value above the 99th percentile of healthy people (14 ng/L for hs-cTnT),
  - **plus** signs of ischaemia: symptoms, ECG changes, or imaging.

  The ESC 0/1-hour hs-cTnT pathway:
  - **Rule out:** below 5 ng/L at arrival (if chest pain began more than 3 h ago), or below 12 ng/L with less than a 3 ng/L rise at 1 h.
  - **Rule in:** 52 ng/L or more at arrival, or a rise of 5 ng/L or more at 1 h.
- **Why the change over time matters:** troponin also rises in myocarditis, pulmonary embolism, sepsis, heart failure and chronic kidney disease. One high value means heart injury; a rising-and-falling pattern plus a matching clinical picture means MI.

The curve's shape, timing and 14 ng/L cutoff follow published values. The calcium level and how strongly it switches the filament on are rounded textbook figures I tuned for teaching, so this is not for clinical use. The 99th-percentile cutoff depends on the assay, and is sex-specific for hs-cTnI.

**Checks run:**
- The line-end check found 0 loose line ends in both themes, and the browser console showed no errors.
- I captured and looked at the healthy beat (calcium in TnC, sites uncovered, heads bound), the blocked heart at 4 h, 7 h, 13 h, day 5 and day 14, a phone-width layout, and reduced motion.
- I didn't run the skill's geometry audit, which proves no parts collide. The figure has no pipes or cables (the case that audit covers), so I checked clearances by hand instead.