A nuclear bag fiber is a type of intrafusal muscle fiber

Intrafusal muscle fiber

Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...

 that lies in the center of a muscle spindle

Muscle spindle

Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...

. Each has a large number of nuclei

Cell nucleus

In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...

 concentrated in bags and they cause excitation of both the primary and secondary nerve fiber

Nerve fiber

A nerve fiber is a threadlike extension of a nerve cell and consists of an axon and myelin sheath in the nervous system. There are nerve fibers in the central nervous system and peripheral nervous system. A nerve fiber may be myelinated and/or unmyelinated. In the central nervous system , myelin...

s.

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.

External links

• http://www.unmc.edu/Physiology/Mann/mann11.html