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

Summary 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 m beyond 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 limited 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.