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Nuclear bag fiber
Latinmyofibra sacculiformis
Anatomical terms of microanatomy

A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle.[1] Each has many nuclei concentrated in bags and they cause excitation of the primary sensory fibers.[2]

There are two kinds of bag fibers based upon contraction speed and motor innervation.

  1. BAG2 fibers are the largest. They have no striations in middle region and swell to enclose nuclei, hence their name.
  2. BAG1 fibers, smaller than BAG2.

Both bag types extend beyond the spindle capsule.

These sense dynamic length of the muscle. They are sensitive to length and velocity.

See also


  1. ^ Boyd, I A; Gladden, M H; McWilliam, P N; Ward, J (1 February 1977). "Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels". The Journal of Physiology. 265 (1): 133–162. doi:10.1113/jphysiol.1977.sp011709. PMC 1307812. PMID 139469.
  2. ^ Mihailoff, G.A.; Haines, D.E. (2018). "Motor System I". Fundamental Neuroscience for Basic and Clinical Applications. pp. 346–359.e1. doi:10.1016/B978-0-323-39632-5.00024-4. ISBN 978-0-323-39632-5.