Extrafusal and Intrafusal Fibers

Clinical History

• This 1-year-old patient underwent muscle biopsy to evaluate for hypotonia.

What are the muscle fibers within the small circular structure called?

A. Intrafusal fibers

B. Atrophic fibers

C. Extrafusal fibers

D. Parasite

Frozen hematoxylin and eosin section

This higher magnification image demonstrates seven somewhat slender appearing muscle fibers in cross-section within a circular perineurium-like sheath (see arrow) of a muscle fiber spindle within a background of surrounding skeletal muscle tissue.

Note the several small intramuscular nerve twigs included in the biopsy sample (see asterix).

Correct Answer: A. Intrafusal fibers

• The muscle fibers within the circular perineurium-like capsule are “intrafusal” fibers (inside the spindle).  The skeletal muscle fibers outside of the spindle are “extrafusal” fibers.
• The muscle fiber spindle is a mechanoreceptor (stretch receptor) for muscle, and helps to coordinate agonist and antagonist muscle contraction/relaxation.
• Also see: https://www.arkanalabs.com/muscle-fiber-spindle/

References/Additional Readings

• Banks RW. The innervation of the muscle spindle: a personal history. J Anat. 2015 Aug;227(2):115-35. doi: 10.1111/joa.12297. Epub 2015 Jun 19. PMID: 26095428; PMCID: PMC4523316.
• Kröger S, Watkins B. Muscle spindle function in healthy and diseased muscle. Skelet Muscle. 2021 Jan 7;11(1):3. doi: 10.1186/s13395-020-00258-x. PMID: 33407830; PMCID: PMC7788844.
• Thornell LE, Carlsson L, Eriksson PO, Liu JX, Österlund C, Stål P, Pedrosa-Domellöf F. Fibre typing of intrafusal fibres. J Anat. 2015 Aug;227(2):136-56. doi: 10.1111/joa.12338. PMID: 26179023; PMCID: PMC4523317.
• Walkowski AD, Munakomi S. Monosynaptic Reflex. 2021 Feb 8. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021 Jan–. PMID: 31082072.