The muscle spindle in cortisone-induced muscular atrophy

The muscle spindles in serially sectioned rat extensor digitorum longus (EDL) and soleus (SOL) muscles were studied histologically and histochemically after chronic cortisone administration. Nuclear chain fibers were found to be significantly atrophied in the EDL but not in the SOL. Nuclear bag fibers were not significantly affected in either muscle. Nuclear chain fibers all stained darkly with the pH 9.4 myofibrillar adenosine triphosphatase (ATPase) reaction (type II) and also stained darkly with the succine dehydrogenase (SDH) reaction. Nuclear bag fibers showed mostly light ATPase staining (type I) and dark, light, or intermediate SDH staining. Extrafusal fibers showed preferential type II fiber atrophy in EDL with relative sparing of type II fibers in SOL. The results suggest that histochemical characteristics that we have evaluated are alone insufficient to explain the differential susceptibility of skeletal muscle fibers to corticosteroids and, therefore, do not in themselves define either a myopathic or neurogenic etiology for the observed muscle wasting.