The shape of the lesson
By the end of the lesson, learners can
- Inside a sarcomere: In the sarcomere view, drag the Contraction slider and watch the Z lines move together while the blue actin and pink myosin stay the same length.
- From nerve to calcium: Go back to stage 1 (excitation) in the cross-bridge view and read how the signal reaches the inside of the fibre.
- The sliding filament model: Step through the cross-bridge cycle in the lab again and say each event out loud before you press Next stage of the cycle.
- A band stays the same; I band and H zone shorten.
- NMJ → sarcolemma → T-tubules → SR releases Ca2+.
- Ca2+ + troponin moves tropomyosin off the binding sites.
- ATP detaches the head, then is hydrolysed to re-cock it.
One picture every idea returns to
A striated muscle fibre is a long, multinucleate cell packed with myofibrils, each made of repeating sarcomeres from one Z line to the next. Thin actin filaments attach to the Z lines; thick myosin filaments are held at the M line. The A band is the length of the myosin (dark), the I band has actin only (light), and the H zone has myosin only. At the neuromuscular junction ACh depolarises the sarcolemma; the action potential spreads down the T-tubules and makes the sarcoplasmic reticulum release Ca2+. Ca2+ binds to troponin, which moves tropomyosin off the myosin-binding sites on actin. Myosin heads bind, forming cross-bridges, and tilt (the power stroke), pulling actin towards the M line. ATP binds to detach each head and is hydrolysed to re-cock it. The filaments do not change length: the I band and H zone shorten, the A band stays the same.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Filaments that slide.. Skeletal muscle shortens because thin actin filaments are pulled past thick myosin filaments, a few nanometres at a time, by myosin heads that bind, pull, let go and reset. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Inside a sarcomere | In the sarcomere view, drag the Contraction slider and watch the Z lines move together while the blue actin and pink myosin stay the same length. Project tab 7 of the lab (Control and coordination lab, striated muscle) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: From nerve to calcium | Go back to stage 1 (excitation) in the cross-bridge view and read how the signal reaches the inside of the fibre. | Pairs |
| 21 to 30 min | Teach and check: The sliding filament model | Step through the cross-bridge cycle in the lab again and say each event out loud before you press Next stage of the cycle. | Anticipation / Reaction Guide |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 21 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in The sliding filament model, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Anticipation / Reaction Guide
Before the lesson, students mark five statements true or false. After the teaching they mark them again and justify any change of mind in one sentence each. The statements come from the examiner traps below.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| The filaments themselves do not shorten. | Only the overlap changes. If a question asks which band stays the same length, the answer is the A band, because it equals the length of the myosin filaments. |
| ATP is needed to detach the myosin head as well as to re-cock it. | The power stroke itself happens when ADP and P i are released. This is why muscles lock in rigor mortis after death: with no ATP, the heads cannot let go of actin. |
Striated muscle and contraction: worksheet
6 questions, 21 marks. Written in the style of the papers; no past paper question is reproduced.
Describe the ultrastructure of a sarcomere as seen in an electron micrograph.
Describe how an impulse in a motor neurone leads to the release of calcium ions inside a muscle fibre.
Explain the sliding filament model of muscle contraction, including the roles of troponin, tropomyosin, calcium ions and ATP.
A relaxed sarcomere is 2.6 μm long, with an A band of 1.6 μm and an H zone of 0.6 μm. After contraction the sarcomere is 2.2 μm long. Calculate the new total I band length and the new H zone length.
Explain why a muscle fibre relaxes when stimulation by its motor neurone stops.
Suggest why muscle fibres contain large numbers of mitochondria.
Answer key and marking notes
- A sarcomere runs from one Z line to the next.
- Light I bands contain thin actin filaments only, attached to the Z lines.
- The dark A band is the length of the thick myosin filaments and includes regions where actin and myosin overlap.
- The paler H zone in the centre of the A band contains myosin only, with the M line at its centre.
- At the neuromuscular junction, ACh is released and binds to receptors on the sarcolemma.
- Na+ channels open and the sarcolemma depolarises; an action potential spreads along it.
- The action potential travels down the T-tubules into the fibre.
- This causes the sarcoplasmic reticulum to release Ca2+ into the sarcoplasm (Ca2+ channels in the SR open).
- Ca2+ binds to troponin, which changes shape.
- This moves tropomyosin away from the myosin-binding sites on the actin filaments.
- Myosin heads (with ADP and Pi attached) bind to actin, forming cross-bridges.
- ADP and Pi are released; the heads tilt (power stroke), pulling actin towards the M line.
- ATP binds to the myosin heads, causing them to detach from actin.
- Myosin ATPase hydrolyses ATP, re-cocking the heads, which bind further along; the cycle repeats while Ca2+ and ATP are present.
- The A band stays at 1.6 μm, so new I band (both halves) = 2.2 - 1.6 = 0.6 μm.
- The sarcomere shortened by 2.6 - 2.2 = 0.4 μm, and the H zone shortens by the same amount as the sarcomere.
- New H zone = 0.6 - 0.4 = 0.2 μm.
- Ca2+ is actively pumped back into the sarcoplasmic reticulum (using ATP).
- Troponin loses its Ca2+, so tropomyosin covers the binding sites again; no cross-bridges form and the filaments can slide apart.
- Contraction needs large amounts of ATP, for detaching and re-cocking myosin heads.
- ATP is also needed to pump Ca2+ back into the sarcoplasmic reticulum (and to make ACh at the junction); aerobic respiration in mitochondria supplies it.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is stored in the sarcoplasmic reticulum of a resting muscle fibre and released when the fibre is stimulated?
- Acalcium ions
- Bacetylcholine
- CATP
- Dsodium ions
To which protein do calcium ions bind to start the cross-bridge cycle?
- Atroponin
- Btropomyosin
- Cmyosin
- Dactin
The diagram shows a sarcomere. Which labelled region contains only actin filaments and shortens during contraction?
- AP
- BQ
- CR
- DS
What is the immediate effect of a molecule of ATP binding to a myosin head that is attached to actin?
- Athe myosin head detaches from actin
- Bthe power stroke pulls actin towards the M line
- Ctropomyosin moves to uncover the binding sites
- Dcalcium ions are released from the sarcoplasmic reticulum
Which describes the role of the myosin head in the cross-bridge cycle?
- Ait binds to actin and acts as an ATPase, hydrolysing ATP to re-cock itself
- Bit binds to calcium ions and pulls tropomyosin off actin
- Cit stores calcium ions and releases them on stimulation
- Dit anchors the thin filaments to the Z line
In an experiment, the T-tubules of a muscle fibre are detached from the sarcolemma. The sarcolemma is then stimulated. What is the most likely result?
- Aonly myofibrils near the surface contract, as the action potential cannot reach the SR deep in the fibre
- Bthe whole fibre contracts normally, as Ca2+ diffuses in from the tissue fluid
- Cthe fibre contracts more strongly, as the action potential stays on the surface
- Dno ACh is released at the neuromuscular junction
A muscle fibre contains 4000 sarcomeres in series along its length. During contraction each sarcomere shortens from 2.5 μm to 2.0 μm. By how much does the fibre shorten?
- A2.0 mm
- B0.2 mm
- C20 mm
- D8.0 mm
The graph shows the tension a muscle fibre can produce at different sarcomere lengths. Why is the tension zero at a sarcomere length of 3.6 μm?
- Athe actin and myosin filaments no longer overlap, so no cross-bridges can form
- Bthe myosin filaments have been stretched until they break
- Cthe sarcomere is too long for Ca2+ to reach troponin
- Dthe actin filaments overlap each other and block the binding sites
Quiz key
The sarcoplasmic reticulum actively takes up and stores Ca2+, releasing it into the sarcoplasm when the T-tubules depolarise. Acetylcholine is stored in vesicles in the motor neurone, not the muscle. ATP is made in mitochondria and used throughout the sarcoplasm. Na+ enters across the sarcolemma during depolarisation; it is not stored in the SR.
Ca2+ binds to troponin, which changes shape and moves tropomyosin off the myosin-binding sites on actin. Tropomyosin is moved, but Ca2+ does not bind to it. Myosin binds to actin, not Ca2+. Actin carries the binding sites for myosin heads, not for Ca2+.
P spans from a Z line to the end of the myosin, half of an I band: actin only, and it shortens as actin slides inwards. Q is the H zone, which shortens but contains myosin only. R is the A band, the length of the myosin, which does not change. S is the whole sarcomere from Z line to Z line, which contains both proteins.
ATP binding breaks the cross-bridge, so the head detaches; hydrolysis of the ATP then re-cocks the head. The power stroke happens when ADP and Pi are released, before ATP binds. Tropomyosin is moved by troponin when Ca2+ binds. Ca2+ release from the SR is triggered by the action potential in the T-tubules.
Myosin heads form cross-bridges with actin and hydrolyse ATP, using the energy to return to the cocked position. Ca2+ binds to troponin, not myosin. Ca2+ is stored in the sarcoplasmic reticulum. Thin filaments are anchored by the Z line proteins, not by myosin.
T-tubules carry the action potential into the fibre, so all the SR releases Ca2+ at once. Without them, only SR close to the sarcolemma would be affected, so contraction is weak and uneven. Ca2+ from the tissue fluid would diffuse far too slowly to activate the inner myofibrils. Keeping the action potential on the surface cannot strengthen contraction. ACh release depends on the motor neurone, which is unaffected.
Each sarcomere shortens by 0.5 μm; 0.5 × 4000 = 2000 μm = 2.0 mm. 0.2 mm and 20 mm come from errors converting μm to mm (1 mm = 1000 μm). 8.0 mm uses the contracted length (2.0 × 4000) instead of the change in length.
At 3.6 μm the Z lines are so far apart that the thin filaments are pulled clear of the thick ones; with no overlap, no myosin heads can bind actin and no force is produced. Filaments do not break on stretching in this experiment. Ca2+ release is not affected by sarcomere length. Actin filaments overlapping each other is the explanation for the fall in tension at short lengths, such as 1.6 μm, not at long lengths.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.