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. This lesson follows the signal from the neuromuscular junction down the T-tubules to the calcium store, then through the cross-bridge cycle.
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.
Fig. 1: one sarcomere relaxed and contracted; the actin slides between the myosin so the Z lines move closer
Section 01
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. Then choose the cross-bridge cycle view and press Next stage of the cycle to step through all six stages, from excitation to the head re-cocking.
Region
What it contains
In an electron micrograph
On contraction
Z line
where actin filaments are anchored; ends of the sarcomere
thin dark line in the middle of each I band
Z lines move closer together
I band
actin only
light band
shorter
A band
the whole length of the myosin, overlapping actin at each end
dark band
same length
H zone
myosin only, in the middle of the A band
paler region in the centre of the A band
shorter (may disappear)
M line
proteins holding the myosin filaments together
line in the centre of the H zone
stays at the centre
Muscle fibre: a single, very long cell formed from many fused cells, so it is multinucleate. Its membrane is the sarcolemma; its cytoplasm is the sarcoplasm, with many mitochondria.
Myofibrils: run the length of the fibre; their sarcomeres line up side by side, giving the striped (striated) appearance.
Thick filaments: myosin molecules with heads that stick out towards the surrounding actin. Thin filaments: actin with tropomyosin wound round it and troponin attached at intervals.
Examiner trap
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.
Stage 1 · Learn
Check what the sim just showed you
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As the Contraction slider is increased in the sarcomere view, which change do you see?
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In stage 2 of the cross-bridge cycle, why can the myosin head now bind to actin?
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Which stage of the simulator's cycle is the power stroke linked to?
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In the sarcomere view, which region contains only myosin?
Section 02
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.
Neuromuscular junction: a synapse between a motor neurone and a muscle fibre. ACh is released from the motor end plate, binds to receptors on the sarcolemma and opens Na+ channels, depolarising it; acetylcholinesterase then breaks the ACh down.
Sarcolemma: the action potential spreads along the fibre's surface membrane.
T-tubules (transverse tubules): infoldings of the sarcolemma that carry the action potential deep into the fibre, so all the myofibrils are stimulated at the same moment.
Sarcoplasmic reticulum (SR): a specialised ER wrapped round the myofibrils, storing Ca2+. Depolarisation of the T-tubules opens Ca2+ channels in the SR, and Ca2+ floods into the sarcoplasm.
Relaxation: when stimulation stops, Ca2+ is pumped back into the SR by active transport (using ATP), so tropomyosin covers the binding sites again.
Section 03
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.
1. Uncovering the sites: Ca2+ binds to troponin; troponin changes shape and pulls tropomyosin away from the myosin-binding sites on actin.
2. Cross-bridge: a myosin head, carrying ADP and Pi, binds to an exposed site on actin.
3. Power stroke: ADP and Pi are released and the head tilts, pulling the actin filament towards the M line.
4. Detachment: a new ATP molecule binds to the myosin head, which detaches from actin.
5. Re-cocking: the myosin head acts as an ATPase, hydrolysing ATP to ADP and Pi; the energy released returns the head to its upright position, ready to bind further along.
Repeated cycles: continue as long as Ca2+ and ATP are present. Many heads working out of step keep the filaments sliding smoothly.
Examiner trap
ATP is needed to detach the myosin head as well as to re-cock it. The power stroke itself happens when ADP and Pi are released. This is why muscles lock in rigor mortis after death: with no ATP, the heads cannot let go of actin.
Stage 2 · Exam
Exam-style questions
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An electron micrograph shows a sarcomere before and after contraction. Which pair of measurements would be unchanged?
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What is the role of the T-tubules in a muscle fibre?
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After death, ATP is no longer made and muscles become rigid. Which step explains this?
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A sarcomere is 2.4 μm long when relaxed and 1.8 μm when contracted. The A band is 1.6 μm. What is the total length of the I band in the contracted sarcomere (both halves)?
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