A<unk>  is a type of intrafusal muscle<unk>  that lies in the center of a muscle spindle. Each has many nuclei concentrated in<unk> s and they cause excitation of the primary sensory<unk> s. There are two kinds of<unk><unk> s based upon contraction speed and motor innervation. #BAG2<unk> s are the largest. They have no striations in middle region and swell to enclose nuclei, hence their name. #BAG1<unk> s, smaller than<unk> 2. Both<unk>  types extend beyond the spindle capsule. These sense dynamic length of the muscle. They are sensitive to length and velocity.
Nuclear bag fiber
Polysome
Cubosome
Polymersome
Trabectome
Droplet nucleus
Cementicle
Dendrosome