Nuclear bag fiber

A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle.[1] Each has a large number of nuclei concentrated in bags and they cause excitation of both the primary and secondary nerve fibers.

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 & velocity.

Nuclear bag fiber
Latinmyofibra sacculiformis
Anatomical terms of microanatomy

See also


  1. ^ Boyd, I A; Gladden, M H; McWilliam, P N; Ward, J (February 1977). "Control of dynamic and static nuclear bag fibres and nuclear chain fibres by gamma and beta axons in isolated cat muscle spindels". Journal of Physiology. 265 (1): 133–162. PMID 139469.

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