Specialized, skeletal muscle fibers found within muscle spindles that detect muscle stretch and become innervated by gamma motoneurons during early development (see figure below).
Intrafusal fibers in muscle spindles relative to extrafusal fibers and gamma motoneurons that innervate infrafusal fibers.
See Actin, Alpha (α) motoneuron, Extrafusal muscle fibers, Gamma (γ) motoneuron, Hypotonia, Motor cortex, Muscle fiber, Muscle spindle