Ultrastructural changes in rabbit extraocular muscles after oculomotor nerve section.

Ultrastructural changes in rabbit superior recti have been studied from 24 hours to 21 days after intracranial section of the oculomotor nerve. Earliest changes were observed in a few myelinated axons within the muscle 24 hours after nerve section. Within 48 hours, degenerative changes in the myelinated axons became manifest and fragmentation of the myelin sheaths started. During the second and third weeks, almost all of the myelinated fibers were completely degenerated, and these degenerated axons and myelin sheaths were always observed inside the Schwann cells or bounded with plasma membranes of the Schivann cells. The unmyelinated fibers, however, preserved apparently normal fine structure throughout the period of this study. Earliest changes in the neuromuscular junctions were detected in their terminal axons on the third day, and within 5 days these axons disappeared from the junctions. In the muscle fibers, earliest changes occurred during the fourth and fifth days in the form of a splitting or fragmentation of the Z line. Occasionally there were also focal accumulations of X-line material which were considered to be the same as that described in a human congenital myopathy, "nemaline myopathy." Degeneration in some restricted areas of the muscle fibers progressed so that the cross-striation became disorganized by the second week and had disappeared by the third week. The significance of the early changes with special reference to the Z-line changes has been discussed.