a fusiform end organ in skeletal muscle in which afferent and a few efferent nerve fibers terminate; it contains from 3 to 10 modified striated muscle fibers (intrafusal fibers) that are much smaller than the ordinary muscle fibers, are separated from them by a capsule that encloses the organ, and are innervated by the thin axon of a gamma motoneuron (gamma motor fiber); the sensory endings that occur on the intrafusal fibers are either annulospiral or flower-spray endings; this sensory end organ is particularly sensitive to passive stretching of the muscle in which it is enclosed. SYN: Kühne spindle, muscle spindle.