A skeletal muscle fiber that is part of a muscle spindle. These are specialized muscle fibers that serve as proprioceptors, detecting the amount and rate of change in length of a muscle. They are innervated by both sensory neurons and motor neurons (gamma and beta motorneurons, collectively referred to as fusimotor neurons). [ http://dx.doi.org/10.1016/B978-0-443-10321-6.00004-7 http://en.wikipedia.org/wiki/Intrafusal_muscle_fiber&oldid=937508784 ]