The complex architecture and innervation of intrafusal muscle fibers in the sensory region of a muscle spindle in the adult Chinese hamster.

A particular muscle spindle was encountered which consisted of intrafusal muscle fibers taking a complex three-dimensional architecture and innervation pattern along the equatorial to the juxtaequatorial region in the tenuissimus muscle of the adult Chinese hamster. These intrafusal fibers formed pairings not only between nuclear bag fibers but also between nuclear chain fibers, and decreased in number from the equatorial region toward the juxtaequatorial region in order that the chain fibers might freely terminate within the inner capsule without a cellular attachment. In addition, a bag fiber and a chain fiber were innervated by multiaxonal nerve endings located adjacent to the annulospiral sensory endings and considered to be autonomic in nature. These findings suggest that the intrafusal fibers may fail to accomplish differentiation at an early stage of development, retaining immature features into adulthood.