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.
Both bag types extend beyond the spindle capsule.
These sense dynamic length of the muscle. They are sensitive to length & velocity.
|Nuclear bag fiber|
|Anatomical terms of microanatomy|