Skeletal Muscle Anatomy and Ultrastructure

1. Connective-tissue hierarchy and the fascicle

The epimysium is the dense connective-tissue sheath around the entire muscle; it merges with tendons. The perimysium partitions the muscle into fascicles, bundles of fibers that share a nerve and blood supply. The endomysium is the delicate sheath around each individual muscle fiber (one multinucleated cell). Together these layers hold the muscle together and transmit the force a fiber generates out to the tendon and bone.

2. The muscle fiber and its myofibrils

A muscle fiber is a single, very long, multinucleated cell. Its plasma membrane is the sarcolemma. Packed inside are rod-like myofibrils, each a chain of repeating sarcomeres laid end to end. Because myofibrils fill the fiber, their repeating striations give the whole fiber its striped (striated) appearance under a microscope.

3. The sarcomere and its protein filaments

A sarcomere is the segment from one Z disc (Z line) to the next; it is the contractile unit. The dark A band is the full length of the thick filament, made of the motor protein myosin, whose heads project outward. The light I band contains only thin filament — actin, plus the regulatory proteins tropomyosin and troponin. The H zone is the central, lighter region of the A band where thin filaments do not overlap the thick filaments, and the M line runs down the middle of the sarcomere, anchoring the thick filaments in register.

How it works

1. The epimysium wraps the whole muscle, and tendons anchor it to bone.
2. Perimysium divides the muscle into fascicles; endomysium wraps each muscle fiber.
3. Inside each fiber, myofibrils are built from sarcomeres arranged in series.
4. Each sarcomere is bounded by Z discs, with an A band (thick myosin) and I bands (thin actin) between them.
5. The M line centers the thick filaments; the H zone is the region of thick filaments without thin-filament overlap.
6. The sarcolemma folds inward as T tubules that lie beside terminal cisternae of the sarcoplasmic reticulum, forming triads.
7. A signal down the T tubule triggers calcium release from the SR, and the sarcomere shortens.

Common confusions

Memory aids

"Z-I-A-H-M" reads inward from each end: Z disc → I band → A band → H zone → M line. Or: "Zebras In Africa Have Muscles" (Z disc, I band, A band, H zone, M line). Remember that during contraction the A band stays the same, while the I band and H zone shrink.

Quick review

Topic Recap

Skeletal muscle is organized from whole muscle (epimysium) to fascicle (perimysium) to fiber (endomysium). Each fiber is packed with myofibrils made of sarcomeres, the contractile units bounded by Z discs and containing overlapping thick (myosin) and thin (actin, tropomyosin, troponin) filaments that define the A band, I band, H zone, and M line. The sarcolemma's T tubules and the sarcoplasmic reticulum's terminal cisternae form triads that translate surface signals into calcium release and contraction.

Knowledge Check

1. Which connective-tissue layer wraps an individual muscle fiber?
2. Which sarcomere band(s) shorten during contraction, and which stays constant?
3. What three proteins make up the thin filament, and what does each do?
4. What is the triad, and why does its position matter?
5. Where are the sarcolemma, T tubules, and sarcoplasmic reticulum relative to each other?

Answers and Rationales

1. The endomysium wraps each individual muscle fiber. Rationale: epimysium wraps the whole muscle and perimysium wraps fascicles, leaving endomysium for the single fiber.
2. The I band and H zone shorten; the A band stays constant. Rationale: the A band is the length of the thick filament, which does not change, while thin filaments slide into the A band, shrinking the I band and H zone.
3. Actin (the track with myosin-binding sites), tropomyosin (blocks those sites at rest), and troponin (binds calcium and moves tropomyosin). Rationale: these three proteins explain both the structure and the regulation of contraction.
4. The triad is one T tubule flanked by two terminal cisternae of the sarcoplasmic reticulum. Its position lets the surface signal trigger calcium release right beside each sarcomere. Rationale: proximity between membrane signal and calcium store is what makes coupling fast.
5. The sarcolemma is the outer plasma membrane; T tubules are its inward folds; the sarcoplasmic reticulum surrounds myofibrils with terminal cisternae abutting the T tubules. Rationale: this arrangement carries surface excitation to the deep contractile machinery.