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Extrafusal Muscle Fiber
Skeletal Muscle

Extrafusal Muscle Fiber

Myofibra extrafusalis

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Quick Facts

Extrafusal muscle fibers are ordinary muscle fibers, as opposed to the intrafusal fibers of the muscle spindle (Dorland, 2011).

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Extrafusal muscle fibers are long, cylindrical multinucleated cells. The cytoplasm of a muscle fiber, the sarcoplasm, is enclosed by a plasma membrane called the sarcolemma. The sarcoplasm of the cell contains chains of long cylindrical structure called myofibrils that extend the length of the muscle fiber. Numerous nuclei occupy the space between adjacent myofibrils and the sarcolemma. Extrafusal muscle fibers receive motor innervation.

It is important not to confuse extrafusal muscle fibers with intrafusal muscle fibers. Intrafusal muscle fibers are found within muscle spindles (stretch receptors found within muscle) and are innervated by both sensory and motor nerve fibers. The muscle spindles signal the length of extrafusal muscle fibers at rest, or during relaxation and contraction, and the velocity at which it occurs (Mescher, 2013).

Key Features/Anatomical Relations

Each extrafusal muscle fiber is embedded in a delicate connective tissue layer called the endomysium. Muscle fibers are arranged in bundles, surrounded by another connective tissue layer called the perimysium. These bundles of muscle fibers are called fascicles (Martini et al., 2017).


Muscle fibers are composed of myofibrils. Myofibrils contain numerous sarcomeres, the functional contractile unit of the muscle.

List of Clinical Correlates

—Muscular atrophy

—Muscular dystrophy





Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Martini, F. H., Nath, J. L. and Bartholomew, E. F. (2017) Fundamentals of Anatomy & Physiology. Pearson Education.

Mescher, A. (2013) Junqueira's Basic Histology: Text and Atlas. 13th edn.: McGraw-Hill Education.

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