Module 02 — Support and Movement · quiz worth 20 pts · 2026-09-29
The sliding-filament sequence is manageable when every step answers what calcium, actin, myosin, and ATP are doing.
It moves the regulatory barrier away from binding sites on actin.
An energized head binds actin and performs a power stroke.
ATP binding detaches myosin; ATP hydrolysis re-cocks it.
ATP-powered transport returns calcium to the sarcoplasmic reticulum.
Do the actin and myosin filaments themselves get shorter?
No. They slide past one another, increasing overlap and shortening the sarcomere.
Got it now? Scroll back up and re-read the section — it should land differently.
Myofibrils are built from repeating sarcomeres. Thin actin filaments and thick myosin filaments overlap inside each sarcomere. The filaments do not become shorter; instead, increased overlap pulls the sarcomere's ends closer together.
A motor-neuron signal reaches the muscle fiber, changes the sarcolemma, and triggers the sarcoplasmic reticulum to release calcium. Calcium binds regulatory proteins on the thin filament, shifting tropomyosin away from myosin-binding sites on actin.
An energized myosin head binds exposed actin to form a cross-bridge. Release of phosphate and then ADP accompanies the power stroke that pulls actin. A new ATP binds myosin so it detaches. ATP hydrolysis re-cocks the head, ready for another cycle while calcium and ATP remain available.
When stimulation stops, calcium is actively pumped back into the sarcoplasmic reticulum. The binding sites become covered, cross-bridge cycling stops, and the fiber relaxes. ATP is needed both to detach myosin and to restore the calcium gradient. Without ATP after death, cross-bridges remain attached, contributing to rigor mortis.
Skeletal muscle is usually under voluntary somatic control. Cardiac and smooth muscle contract involuntarily and are regulated by intrinsic activity, the autonomic nervous system, hormones, local conditions, or a combination. The timing and regulation differ among tissues.
Slow-twitch fibers contract more slowly, resist fatigue, and support activities such as posture and endurance. Fast-twitch fibers develop force quickly but fatigue sooner, fitting rapid or powerful movement. Most muscles contain a mixture whose proportions suit their usual work.
Track what calcium exposes, what ATP changes, and what moves during each step.
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