Nuclear bag fiber
A nuclear bag fiber is one of two types of intrafusal muscle fibers in muscle spindles, so named because its muscle cell nuclei are congregated into a bulged section at the middle portion of the fiber. It is innervated solely by type Ia sensory fibers and responds to dynamic changes in muscle stretch.
The diameter of nuclear bag fibers is about twice that of nuclear chain fibers. Each muscle spindle contains 1–3 nuclear bag fibers.
The tautness of nuclear chain fibers is adjusted mainly by dynamic γ motor neurons to optimise the dynamic response of the muscle spindle.
Types
There are two kinds of bag fibers based upon contraction speed and motor innervation:- BAG2 fibers are the largest. They have no striations in middle region and swell to enclose nuclei.
- BAG1 fibers, smaller than BAG2.