BIOSCI107 · Biology for Biomedical Science: Cellular Processes and Development
Skeletal Muscle: Contraction & Mechanics
Topic 7 (exam Section D) opens with skeletal muscle: the sarcomere and the sliding-filament model, the four-step cross-bridge cycle with its Ca²⁺/troponin/tropomyosin trigger, excitation–contraction coupling from the neuromuscular junction to SR Ca²⁺ release, the length–tension relationship, muscle metabolism and motor units. Examined in the 40% final exam (paper Teleform MCQ) — reliably as items on the cross-bridge steps, the role of ATP and Ca²⁺, which sarcomere bands change, and how force is graded.
What this chapter covers
- 01Sarcomere anatomy: thick (myosin) A band, thin (actin) I band, H zone, Z-disc, M line, titin; T-tubules and the sarcoplasmic reticulum (SR) as Ca²⁺ store
- 02Thin-filament regulation: troponin (binds Ca²⁺) + tropomyosin (blocks myosin-binding sites on actin)
- 03Sliding-filament theory: Z-lines pulled toward the M-line; I band and H zone narrow; the A band stays constant
- 04The four-step cross-bridge cycle: cross-bridge formation → power stroke → detachment (needs new ATP) → re-energisation (ATP hydrolysed)
- 05Role of Ca²⁺: binds troponin → tropomyosin shifts → exposes actin's myosin-binding sites; no ATP → rigor
- 06Excitation–contraction coupling: motor AP → ACh → end-plate potential → muscle AP → T-tubule → SR Ca²⁺ release → contraction; Ca²⁺-ATPase pumps Ca²⁺ back to relax
- 07Length–tension relationship (maximal active force at ~2.0–2.2 µm overlap); total = active + passive tension
- 08Muscle metabolism (creatine phosphate, anaerobic glycolysis, aerobic) and force grading (rate coding/tetanus + recruitment; motor unit)
The cross-bridge cycle — order, and the jobs of Ca²⁺ and ATP
- +1(a) Order the cycle: (1) cross-bridge formation — the energised myosin head binds actin (only possible once Ca²⁺ has exposed the binding site); (2) power stroke — ADP + Pi are released, the head pivots to ~45° and pulls the thin filament toward the M-line; (3) detachment — a NEW ATP binds myosin, which releases from actin; (4) re-energisation — that ATP is hydrolysed to ADP + Pi, re-cocking the head to ~90°, ready to bind again. [+1]
- +1(b) Role of Ca²⁺: cytosolic Ca²⁺ binds troponin, which shifts tropomyosin off the myosin-binding sites on actin. This uncovers the sites so the cycle can begin, and it continues only while Ca²⁺ stays above threshold — Ca²⁺ is the on-switch, not the fuel. [+1]
- +1(c) Rigor logic (step of the cycle): detachment requires a new ATP to bind the myosin head. With no ATP, the head cannot detach after its power stroke, so it stays locked onto actin. [+1]
- +1(c, conclusion) Every myosin head therefore stays bound, the muscle is stiff and cannot relax — this is rigor mortis. Note ATP is needed both to detach the head and (once hydrolysed) to re-cock it, so ATP appears twice in the cycle. [+1]
Key terms
- Sarcomere
- The contractile unit between two Z-discs: thick (myosin) filaments form the A band, thin (actin) filaments the I band, the H zone is thick-only, the M line anchors the middle, and elastic titin tethers myosin to the Z-disc. Shortening narrows the I band and H zone; the A band stays constant.
- Cross-bridge cycle
- The four-step ATP-driven cycle: cross-bridge formation (myosin binds actin), power stroke (head pivots, releasing ADP + Pi, pulling the thin filament), detachment (new ATP binds, myosin releases), and re-energisation (ATP hydrolysed, head re-cocks). It repeats while Ca²⁺ keeps the binding sites exposed.
- Troponin / tropomyosin
- The thin-filament regulatory proteins. At rest tropomyosin covers the myosin-binding sites on actin; when Ca²⁺ binds troponin, troponin pulls tropomyosin aside, exposing the sites and allowing the cross-bridge cycle. Ca²⁺ acts on troponin — it is the switch that starts contraction.
- Excitation–contraction coupling
- The chain linking a motor-neuron action potential to contraction: ACh at the NMJ → end-plate potential → muscle action potential → propagation into the T-tubules → opening of SR Ca²⁺-release channels → Ca²⁺ triggers the cross-bridge cycle. Relaxation follows when the SR Ca²⁺-ATPase pumps Ca²⁺ back.
- Length–tension relationship
- Active force depends on filament overlap: it is maximal at an optimal sarcomere length (~2.0–2.2 µm) and falls when the muscle is too short (filaments collide) or too long (too few cross-bridges). Total tension = active (cross-bridge) force + passive (elastic) force.
- Motor unit
- One motor neuron plus all the muscle fibres it innervates. Whole-muscle force is graded two ways: by the rate of stimulation (single twitch → unfused → fused/complete tetanus, via temporal summation) and by recruitment of more motor units (small, fatigue-resistant units first).
Skeletal Muscle: Contraction & Mechanics FAQ
Where exactly does ATP act in the cross-bridge cycle?
In two places, and neither is the power stroke itself. First, ATP binding to the myosin head causes it to DETACH from actin after the power stroke. Second, hydrolysis of that ATP to ADP + Pi RE-ENERGISES (re-cocks) the head so it can bind again. The power stroke is powered by releasing the ADP + Pi already stored on a cocked head. This is why no ATP means no detachment — the heads stay locked on actin, producing rigor.
What actually triggers a skeletal muscle to contract?
Ca²⁺ binding to troponin. When a muscle action potential reaches the T-tubules it opens SR Ca²⁺-release channels; the released Ca²⁺ binds troponin, which shifts tropomyosin off the myosin-binding sites on actin, allowing cross-bridges to form. Contraction continues while cytosolic Ca²⁺ is high, and stops when the SR Ca²⁺-ATPase pumps Ca²⁺ back and tropomyosin re-covers the sites. Note the skeletal trigger is voltage-driven (the muscle's own AP), which contrasts with cardiac calcium-induced calcium release.
How does a muscle produce more force?
Two mechanisms. Rate coding: stimulate a fibre faster and the twitches fuse — single twitch → unfused (incomplete) tetanus → fused (complete) tetanus — because a twitch outlasts its action potential, so summation builds tension. Recruitment: activate more motor units, with small fatigue-resistant units recruited first and large powerful ones added for maximal effort. Length also matters — force is greatest near the optimal overlap of the length–tension curve.
Can AI help me with skeletal muscle in BIOSCI 107?
Yes, for study. Sia can order the cross-bridge steps, pinpoint where Ca²⁺ and ATP act, explain which sarcomere bands change, and drill the force-grading mechanisms. Use it to prepare for the final exam — it does not sit the exam for you, and the exam is an AI-free lane under the course's academic-integrity policy. Confirm the rules on Canvas.
Exam move
Draw the sarcomere once and label everything (A band, I band, H zone, Z-disc, M line, titin), then be able to say which bands narrow on shortening — the A band never does, a favourite false-statement item. Rehearse the four-step cross-bridge cycle out loud until the order and the two ATP steps (detachment + re-cocking) are automatic, and separate ATP's job (detach/re-energise) from Ca²⁺'s job (bind troponin, expose sites) — mixing them up is the classic error, and it is what makes rigor an ATP-absence state. Trace excitation–contraction coupling end to end (NMJ → muscle AP → T-tubule → SR Ca²⁺ → contraction; Ca²⁺-ATPase → relaxation), noting the skeletal trigger is voltage-driven. Finish with the length–tension curve, the three metabolic ATP sources, and the two force-grading routes (rate/tetanus and recruitment). This is exam material (Topic 7, Section D); confirm the exam date and Teleform format on Canvas.
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