Motor innervation of the cat muscle spindle studied by the cholinesterase technique.

Muscle spindles were traced in serial transverse sections of cat tenuissimus muscles. "Myofibrillar" adenosine triphosphatase staining reaction was used to identify nuclear bag1, nuclear bag2 and nuclear chain intrafusal muscle fibers. Typical chain fibers and long chain fibers were distinguished, the latter extending for more than 1,000 micron beyong the termination of the spindle capsule. Simple "rim" and more elaborate "plate" deposits were demonstrated histochemically along the poles of the typical chain fibers in staining for cholinesterases. They were considered to correspond, respectively, to the trail and plate motor nerve terminals. Most long chain fibers and the majority of nuclear bag fibers had their motor innervation limitd to "plate"-type endings. In addition, faint diffuse cholinesterase staining occurred along the spindle capsule and the surface of some intrafusal fibers. These histochemical observations are discussed with regard to the current concepts concerning the morphological and functional organization of the motor innervation of the cat muscle spindle.