Motor innervation of intrafusal fibers in rat muscle spindles: incomplete separation of dynamic and static systems.

Distributions of 53 motor axons to different types of intrafusal fibers were reconstructed from serial 1-micron-thick transverse sections of 13 poles of spindles in the rat soleus muscle. The mean number of motor axons that innervated a spindle pole was 4.1. Approximately 60% of motor axons lost their myelination prior to or shortly after entry into the periaxial fluid space of spindles. Motor innervation to the juxtaequatorial portion of nuclear bag fibers (particularly the bag1) consisted of groups of short, synaptic contacts that were terminations of thin, unmyelinated axons. In contrast, motor endings on both the bag1 and bag2 fibers were platelike in the polar intracapsular region. Chain fibers had a single midpolar platelike ending. The ratio of motor axons that innervated the bag1 fiber exclusively to axons that innervated bag2 and/or chain fibers was 1:1. However, one-fourth of motor axons coinnervated the dynamic bag1 fiber in conjunction with static bag2 and/or chain fibers. Thus the complete separation of motor control of the dynamic bag1 and static bag2 intrafusal systems observed in cat tenuissimus spindles is neither representative of the pattern of motor innervation in all other species of mammals nor essential to normal spindle function.