Skeletal Muscle Anatomy

Muscle is organized as nested bundles. A skeletal muscle is a hierarchy, each level wrapped in connective tissue:

Whole muscle
  → fascicles (bundles of fibers)
    → muscle fibers (single cells)
      → myofibrils (rods inside each fiber)
        → sarcomeres (contractile units)
          → actin and myosin filaments

Each muscle fiber is a long, cylindrical, multinucleated cell packed with myofibrils. The myofibrils are made of repeating sarcomeres, and the sarcomeres contain the two proteins that do the contracting — actin and myosin.

Connective tissue coverings. Three sheaths organize and toughen the muscle: the endomysium wraps each individual fiber, the perimysium wraps each fascicle, and the epimysium wraps the whole muscle. At the muscle's ends, these sheaths merge and continue as a tendon (or a flat sheet, an aponeurosis) that anchors muscle to bone. Because the connective tissue is continuous with the tendon and bone, a muscle's pull is transmitted directly to the skeleton.

The sarcomere — the contractile unit. Within each myofibril, sarcomeres line up end to end, giving skeletal muscle its striated (striped) appearance. A sarcomere runs between two Z discs. Anchored to the Z discs are the thin filaments (actin); overlapping them in the center are the thick filaments (myosin), whose "heads" can grab actin. During contraction, myosin pulls the actin filaments toward the sarcomere's center, so the Z discs move closer and the sarcomere shortens — the essence of the sliding-filament mechanism detailed next. Two other proteins, troponin and tropomyosin, sit on the actin and act as a switch that controls when myosin can bind (calcium flips this switch).

The neuromuscular junction (NMJ). Skeletal muscle contracts only when told to by a motor neuron. Where the neuron's axon meets the muscle fiber is the neuromuscular junction. When the nerve signal arrives, the neuron releases the neurotransmitter acetylcholine (ACh) into the tiny gap. ACh binds receptors on the muscle fiber, triggering an electrical impulse that spreads across the fiber and into its interior (via T-tubules), ultimately releasing calcium to start contraction. This junction is where several diseases and drugs act — for example, myasthenia gravis impairs ACh receptors, and certain paralytic drugs block this junction.

How it works

From nerve signal to a shortened muscle (overview):

Motor neuron fires → releases acetylcholine at the NMJ
   → muscle fiber membrane depolarizes
   → signal spreads inward → calcium released inside the fiber
   → calcium exposes actin → myosin pulls actin → sarcomeres shorten
   → the whole muscle contracts and pulls on its tendon/bone

Comparisons

Common confusions

• Muscle vs muscle fiber vs myofibril. Muscle = organ; fiber = one cell; myofibril = a rod inside the cell; sarcomere = the unit inside the myofibril.
• Actin vs myosin. Actin = thin filament (pulled); myosin = thick filament (has the pulling heads).
• Tendon vs the muscle sheaths. The sheaths (endo/peri/epimysium) merge to form the tendon.
• NMJ transmitter is acetylcholine for skeletal muscle specifically.

Memory aids

• Coverings: "Endo (each fiber), Peri (perimeter of a fascicle), Epi (entire muscle)."
• "Thin actin, thick myosin."
• ACh = the "action chemical" at the muscle.

Quick review

• Skeletal muscle is nested: muscle → fascicle → fiber → myofibril → sarcomere → actin/myosin.
• Connective sheaths (endomysium, perimysium, epimysium) merge into the tendon that anchors muscle to bone.
• The sarcomere (Z disc to Z disc) contains actin (thin) and myosin (thick); troponin/tropomyosin form a calcium-controlled switch.
• The neuromuscular junction uses acetylcholine to start contraction — a key site for disease and drugs.