A nuclear chain fiber is a specialized sensory organ contained within a muscle.

The afferent pathway resembles a spring wrapping around the nuclear chain fiber and connecting to one of its ends away from the bone.

Again, depending on the stress and strain the muscles sustains, this afferent and efferent coordination will measure the "stretch of the spring" and communicate the results to the central nervous system.

Stimulation of γ neurons causes the nuclear chain to shorten along with the extrafusal muscle fibers.

This shortening allows the nuclear chain fiber to be sensitive to changes in length while its corresponding muscle is contracted.