Ultrastructural effects of immobilization on rat muscle spindles

The ultrastructure of rat muscle spindles was examined after the anterior tibial muscles had been immobilized in a plaster cast. There was an increase in the number of collagen fibrils and external laminae around the outer capsules and in the intracapsular space 2 weeks after immobilization. The changes in the intrafusal muscle fibers within 4 weeks included disorientation of myofilaments. After 6 weeks, Z bands had become disarranged, and there was vacuolar degeneration of the sarcoplasmic reticulum in some fibers. Myelin sheaths of many of the myelinated nerve fibers (especially the thick ones, which were probably sensory nerve fibers) had degenerated within 2 weeks. These results indicate that immobilization of skeletal muscles affects not only extrafusal muscle fibers but also the structure of the muscle spindle.