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📘 How does a muscle contract?

Skeletal muscle contraction begins with a signal from a motor neuron, not with ATP acting by itself.

6
lessons
~30 min
to learn
Adults
level
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What you’ll learn

  1. A nerve signal starts the sequenceExplain how a motor-neuron signal reaches the sarcoplasmic reticulum through the muscle fiber membrane and T-tubules.An electrical message starts the calcium pulse that activates the contractile machinery.
  2. Inside the sarcomereIdentify sarcomeres, actin, myosin, Z discs, and the evidence for filament sliding.Sarcomeres shorten because thin and thick filaments slide with increasing overlap.
  3. Calcium opens the binding sitesDescribe how calcium, troponin, and tropomyosin regulate access to actin.Calcium changes a molecular gate so myosin can reach actin binding sites.
  4. The cross-bridge cycleTrace myosin attachment, the power stroke, ATP-driven detachment, and re-cocking.ATP-powered myosin heads pull actin in repeated asynchronous cycles.
  5. Relaxation resets the systemExplain calcium reuptake, regulatory-site blocking, and band changes during shortening.Relaxation actively restores low calcium and the resting arrangement of the sarcomere.
  6. Force depends on timing and overlapConnect calcium duration, motor-unit recruitment, filament overlap, mechanical load, and the full contraction chain.Whole-muscle force depends on neural recruitment, molecular geometry, and a controlled signal-to-pull sequence.

Questions this course answers

What event begins excitation-contraction coupling in a skeletal muscle fiber?

The electrical signal travels across the sarcolemma and into T-tubules before calcium release and cross-bridge cycling.

Put the broad sequence in order.

The neural signal is converted into an electrical wave, then calcium release, then molecular force.

What does the sliding-filament model say?

Relative sliding increases filament overlap and shortens the sarcomere.

How does calcium make cross-bridge formation possible?

Calcium is the regulatory permission signal for striated muscle; it does not directly power the myosin stroke.

What causes myosin to detach from actin?

ATP binding lowers myosin's affinity for actin and permits detachment.

Match each component to its role.

The parts cooperate but have different jobs in the cycle.

Grounded in trusted sources

  • OpenStax, Anatomy and Physiology 2e, 10.3 Muscle Fiber Contraction and Relaxation, https://openstax.org/books/anatomy-and-physiology-2e/pages/10-3-muscle-fiber-contraction-and-relaxation
  • NCBI Bookshelf, Physiology, Skeletal Muscle Contraction, https://www.ncbi.nlm.nih.gov/books/NBK559006/
  • NCBI Bookshelf, Physiology, Skeletal Muscle, https://www.ncbi.nlm.nih.gov/books/NBK537139/
  • Szent-Gyorgyi, Calcium regulation of muscle contraction, Biophysical Journal via PMC, https://pmc.ncbi.nlm.nih.gov/articles/PMC1334730/
  • Signaling in Muscle Contraction, Molecular Interventions via PMC, https://pmc.ncbi.nlm.nih.gov/articles/PMC4315934/
  • OpenStax, Biology 2e, 38.4 Muscle Contraction and Locomotion, https://openstax.org/books/biology/pages/38-4-muscle-contraction-and-locomotion
  • Wikimedia Commons MediaWiki API, image metadata and 960px thumbnails, https://commons.wikimedia.org/w/api.php

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