part of part of neural cell skeletal muscle fiber intrafusal muscle fiber 2020-09-26T21:07:01Z TH:H3.03.00.0.00012 myofibra intrafusalis 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). muscle spindle