BIO 304 . Human Anatomy & Physiology . American River College
Skeletal Muscle Microanatomy
Module 6 . Muscular System
From whole muscle down to the sarcomere: the structural ladder of contractile tissue. Watch the video, then complete the retrieval check below to unlock your spaced-recall cards.
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- Trace the connective tissue hierarchy from epimysium down to a single myofibril.
- Label every band, line, and zone of the sarcomere and state which filament dominates each.
- Identify T-tubules and sarcoplasmic reticulum and explain why they sit where they sit.
Structural hierarchy (big to small)
- Whole muscleWrapped in epimysium (dense irregular CT).
- FascicleBundle of muscle fibers wrapped in perimysium.
- Muscle fiber (cell)Single multinucleate cell wrapped in endomysium. · Plasma membrane = sarcolemma. Cytoplasm = sarcoplasm.
- MyofibrilRod-shaped contractile organelle inside the fiber. Hundreds per fiber.
- SarcomereFunctional unit. Z-line to Z-line. Hundreds end-to-end down each myofibril.
The sarcomere
- Z-lineSarcomere boundary. Thin filaments anchor here.
- I bandThin filaments only (light under microscopy). Shortens during contraction.
- A bandFull length of thick filament (dark band). Width never changes during contraction.
- H zoneThick filaments only, no overlap. Shortens during contraction.
- M lineCenter of sarcomere. Thick filaments anchor here.
Excitation machinery
- T-tubulesInvaginations of sarcolemma. Carry the action potential deep into the fiber.
- Sarcoplasmic reticulumSmooth ER specialized for Ca2+ storage. Releases Ca2+ when the T-tubule signal arrives.
Step 3 . Retrieval check
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