Histological study of an unusual cat muscle spindle deficient in motor innervation

Summary An unusual muscle spindle innervated by only one somatic motor axon and one primary sensory axon was encountered in a cat tenuissimus muscle cut in serial transverse sections and examined by light and electron microscopes. The motor axon branched to supply the distal poles of the nuclear bag₂ and nuclear chain intrafusal muscle fibers through motor terminals of several types. The proximal poles of the bag₂ and chain fibers and both poles of the nuclear bag₁ fiber were devoid of motor endings. In spite of the limited motor nerve supply the bag₁, bag₂ and chain fibers exhibited the usual morphologic characteristics in terms of the appearance of the equatorial region, relative fiber lengths and diameters, and the number of associated elastic fibers. It appears that motor axons may play only a limited role, if any, in the differentiation and maintenance of the three types of intrafusal muscle fiber in the cat.