A nuclear bag fiber is a type of intrafusal muscle fiber that lies in the center of a muscle spindle. 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.
- Bag2 fibers are the largest. They have no striations in middle region and swell to enclose nuclei, hence their name.
- Bag1 fibers, smaller than bag2.
Both bag types extend beyond the spindle capsule.