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 2 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 - are smaller than bag2.
Both bag types extend beyond the spindle capsule.
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