Nuclear bag fibers

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.