Two kinds of motor nerve endings on mammalian intrafusal muscle fibers revealed by the cholinesterase technique

There have been several recent significant advances concerning the innervation of muscle, particularly the destination of the small-nerve motor supply and the efferent innervation of the intrafusal fibers of the muscle spindle. The small motor nerves in frogs innervate special extrafusal fibers (Kuffler and Vaughan Williams, '53a, b). The endings on these fibers are supposed to be diffuse or multiterminal in nature, in contrast to the more discrete endings on ordinary extrafusal fibers. In the frog, the intrafusal fibers are innervated by both large and small nerve fibers. The large fibers could be branches of nerves destined to innervate ordinary extrafusal fibers (Katz, '49), and Gray ('57) has shown that some intrafusal fibers can receive the type of nerve ending corresponding to the small motor nerve endings of the special extrafusal fibers. In mammals, the small motor nerve supply goes exclusively to intrafusal fibers (Kuffler and Hunt, '52). Very recently, investigators have found evidence that there are two kinds of intrafusal muscle fibers in mammalian spindles, which have two different kinds of motor nerve endings. The morphological and physiological evidence for this has been presented by Boyd ('58, '59) and Boyd and Davey ('59) in a series of three abstracts. Diete-Spiff and Pascoe ('59) have recently also presented physiological evidence that two distinct types of nerve fibers innervate mammalian intrafusal muscle fibers. The present investigation was undertaken to present more detailed morphological evidence for the presence of two different kinds of motor endings on mammalian intrafusal muscle fibers. The cholinesterase histochemical technique was used. As will be shown, this method stains only the motor nerve terminations on intrafusal muscle fibers and thus has several advantages over the gold, silver or methylene blue techniques which stain all nerve fibers and perhaps thin connective tissue fibers and hence might cause confusion because of the great number of sensory nerve terminations and connective tissue on and surrounding intrafusal muscle fibers which would be revealed by these latter techniques.