Nuclear chain fiber
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Part of the muscle fibers known as "extrafusial fibers" along with a similar structure known as a nuclear bag fiber.
There are 3-9 nuclear chain fibers per muscle spindle that are half the size of the nuclear bag fibers. Their nuclei are aligned in a chain and they excite the secondary nerve. They are static while the nuclear bag fibers are dynamic in comparison.
The secondary nerve association involves an efferent and afferent pathway that measure the stress an strain placed on the muslce (usually the extrafusial fibers connected from the muscle portion to a bone). 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.
Also note that a similar structure attaching one end to muscle and the other end to a tendon is known as a Golgi tendon.
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