Characterstics of motor innervation of muscle spindles in the monkey

The motor nerve supplies of four whole muscle spindles and 16 half spindles (equator and one pole) from lumbrical muscles of the monkey were reconstructed by light microscopy of serial, 1-microm-thick transverse sections.The 24 poles of spindle were innervated by 37 fusimotor (gamma) axons and 18 skeletofusimotor (beta) axons. Sixty-seven percent of spindle poles received gamma axons only, 25% were supplied by both gamma and beta axons or beta axons only, and 8%were not innervated by motor axons. All beta axons except one and 35% of the gamma axons innervated one type of intrafusal fiber only. The other 65% of gamma axons coinnervated two or three different types of intrafusal fiber. Gamma axons innervated the (dynamic) bag1, intrafusal fiber together with the (static) bag2,and or chain fibers in 65% of the poles of spindle in which the bag1 fiber received motor supply. The bag2 fiber shared gamma-innervation with either the bag1 or chain fibers in nearly every spindle pole. The chain fibers were usually coinnervated with the bag2 fiber by gamma axons. Compared to spindIes in the cat tenuissimus muscle, the monkey spindle received fewer gamma axons and had a higher incidence of shared gamma-innervation between the dynamic and static intrafusal fibers. Unlike cat tenuissimus spindles, the monkey spindles lacked gamma axons selective to the bag2 fiber. However, the beta motor system within the monkey lumbrical muscle was organized in a manner similar to the cat tenuissimus muscle. The significance of these observations is discussed relative to the general motor organization and function of spindles in different muscles and species of mammals.