No calcium is present. Tropomyosin covers the binding sites on actin, so myosin cannot attach and the muscle stays relaxed at its resting length.
It is tempting to picture a contracting muscle as fibers that simply get shorter, like a stretched rubber band snapping back. That is wrong. The filaments inside a muscle keep their length the whole time. What actually happens is more like a tug-of-war team hauling in a rope hand over hand: the muscle’s motor protein, myosin, reaches out, grabs the actin rope, and pulls it in — over and over — so the two sets of filaments slide past each other and the muscle as a whole gets shorter.
A muscle fiber is packed with identical repeating units called sarcomeres, lined up end to end. Each one runs from one Z-disc to the next. Thin filaments (actin) are anchored to the Z-discs and point inward; a thick filament (myosin) sits in the middle with hundreds of heads sticking out toward the actin. A resting sarcomere is about $2.2\ \mu\text{m}$ long. When it contracts, the Z-discs are pulled toward the center (the M-line), and because thousands of sarcomeres shorten together, the whole muscle shortens.
Each head runs a four-step cross-bridge cycle: it binds actin, does a power stroke (pivoting to drag the thin filament about $10\ \text{nm}$ and producing roughly $3\text{–}5\ \text{pN}$ of force), detaches when a new ATP binds, then splits that ATP to re-cock for another pull. Two switches gate the whole thing. Calcium is the on switch: $\text{Ca}^{2+}$ binds troponin, slides tropomyosin off actin, and exposes the binding sites. ATP is required to let go and reset — with no ATP the heads stay locked, which is exactly what causes rigor mortis.
Let it play on the Contracting preset and follow one head through attach → power stroke → detach → re-cock while the Z-discs creep inward and the sarcomere length readout drops. Now drag Calcium to low: tropomyosin re-covers the sites and contraction stalls — no $\text{Ca}^{2+}$, no pulling. Switch on the Rigor preset (ATP removed): a head completes its power stroke but cannot release, freezing the cross-bridge. Finally watch the marker on the length–tension curve slide left as the sarcomere shortens, dropping off the plateau as overlap is lost.
| Stage | What happens |
|---|---|
| 1. Relaxed | No $\text{Ca}^{2+}$. Tropomyosin covers actin’s binding sites; myosin cannot attach and the muscle rests. |
| 2. Calcium switch | A signal releases $\text{Ca}^{2+}$, which binds troponin, shifting tropomyosin aside to uncover the binding sites on actin. |
| 3. Cross-bridge forms | A cocked, energized myosin head binds an exposed actin site, forming a cross-bridge. Inorganic phosphate (Pₙ) is released. |
| 4. Power stroke | The head pivots, dragging the thin filament about 10 nm toward the M-line; ADP is then released. This is the force-producing step. |
| 5. Detachment | A new ATP binds the myosin head, which lets go of actin. With no ATP, the head stays locked (rigor). |
| 6. Re-cocking | Myosin splits the ATP into ADP + Pₙ, using the energy to re-cock the head to its high-energy position, ready to grab again. |
The classic evidence for sliding filaments comes from watching the sarcomere’s bands. The A-band equals the length of the thick filaments and stays constant during contraction. The I-band (thin filament only) and the central H-zone (thick filament only) both get narrower as the actin slides inward, and the Z-discs move closer together. If the filaments were actually shortening, the A-band would shrink too — it does not, which is why the sliding model won out.
Every power stroke costs one ATP, so contraction is expensive: a single myosin head can cycle several times per second, and a muscle holds enormous numbers of heads. ATP is spent not on the pull itself but on detaching and re-cocking the head. Calcium handling also costs ATP, because relaxing the muscle means actively pumping $\text{Ca}^{2+}$ back into the sarcoplasmic reticulum against its gradient. When ATP runs out entirely — as after death — heads cannot detach and the muscle stiffens into rigor mortis until the proteins themselves break down.
The sliding filament model explains that a muscle shortens when its two sets of protein filaments slide past one another, without the filaments themselves getting shorter. Inside each muscle fiber are repeating units called sarcomeres. Each sarcomere has thin filaments (actin) anchored at its ends to Z-discs, and thick filaments (myosin) in the middle. During contraction, the myosin heads grab the actin and pull it inward toward the center, so the Z-discs are dragged closer together and the whole sarcomere shortens. Because thousands of sarcomeres are lined up end to end, all shortening at once, the entire muscle shortens.
Key takeaway: muscles contract because actin slides past myosin and pulls the Z-discs together, shortening each sarcomere while the filaments stay the same length.The cross-bridge cycle is the repeating set of steps a myosin head goes through to pull on actin. First, a myosin head that has already split ATP into ADP and phosphate is in a cocked, high-energy position and binds to a site on actin, forming a cross-bridge. The phosphate is released, which triggers the power stroke: the head pivots and drags the thin filament about 10 nanometers toward the center of the sarcomere, then ADP is released. The head is now locked tightly to actin in the rigor state. A new ATP binds, which makes myosin let go of the actin. Finally the head splits that ATP, using the energy to re-cock back to its high-energy position.
Key takeaway: the cross-bridge cycle is attach, power stroke, detach, and re-cock, repeated over and over, with each cycle using one ATP to pull the thin filament further.At rest, a long protein called tropomyosin lies over the binding sites on actin, so myosin cannot attach and the muscle stays relaxed. When a nerve signal reaches the muscle, calcium ions are released from a storage network called the sarcoplasmic reticulum. The calcium binds troponin, a protein attached to tropomyosin. This changes troponin’s shape and pulls tropomyosin off the actin, uncovering the binding sites. Now myosin can attach and the cross-bridge cycle can run. When the signal stops, calcium is pumped back into storage, tropomyosin slides back over the sites, and the muscle relaxes.
Key takeaway: calcium switches contraction on by binding troponin and moving tropomyosin off the actin binding sites, and contraction switches off when calcium is pumped away.ATP is needed at two points in every cross-bridge cycle. First, a fresh ATP must bind the myosin head to make it release actin after the power stroke; without ATP the head stays stuck. Second, splitting that ATP into ADP and phosphate provides the energy to re-cock the head into its high-energy position so it can pull again. So ATP is not used to make the head bind or to power the stroke directly; instead it powers detachment and re-cocking. This is also why rigor mortis happens after death: ATP production stops, myosin cannot detach, and the muscles become stiff.
Key takeaway: ATP lets myosin release actin and then re-cock for the next stroke, which is why muscles stiffen into rigor mortis when ATP runs out.A sarcomere has named regions visible under a microscope. The A-band is the full length of the thick filaments and does not change during contraction. The I-band contains only thin filaments and the H-zone in the middle contains only thick filaments. As the sarcomere contracts, the thin filaments slide deeper into the thick filaments, so the I-band gets narrower and the H-zone gets narrower or disappears, while the A-band stays exactly the same width. The Z-discs move closer together. Seeing the A-band stay constant while the I-band and H-zone shrink is the classic evidence for the sliding filament model.
Key takeaway: during contraction the I-band and H-zone shrink and the Z-discs move closer, but the A-band stays the same, proving the filaments slide rather than shorten.The length-tension relationship describes how the force a muscle can produce depends on the length of its sarcomeres, because force depends on how well the thin and thick filaments overlap. At an intermediate, optimal sarcomere length of roughly 2.0 to 2.25 micrometers, the maximum number of myosin heads can reach actin, so the muscle generates its greatest force, giving a plateau on the curve. If the sarcomere is stretched too long, the filaments barely overlap, fewer cross-bridges can form, and force falls along the descending limb. If the sarcomere is too short, the thin filaments collide and the thick filaments hit the Z-discs, which also reduces force along the ascending limb.
Key takeaway: muscle force is greatest at an optimal sarcomere length where filament overlap is ideal, and it drops when the muscle is too stretched or too shortened.Excitation-contraction coupling is the chain of events that turns an electrical nerve signal into a mechanical contraction. A motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction, which starts an action potential across the muscle fiber’s membrane. The impulse travels down deep infoldings called T-tubules into the interior of the fiber. There it triggers the sarcoplasmic reticulum to release stored calcium. The calcium binds troponin, moves tropomyosin off actin, and lets the cross-bridge cycle run. When the signal ends, calcium is pumped back and the muscle relaxes.
Key takeaway: a nerve signal becomes contraction by way of an action potential that spreads down T-tubules and makes the sarcoplasmic reticulum release calcium, which unlocks actin for myosin.