Both roots of the olivo-cochlear nerve bundle to one ear were transected in the brain stems of 12 chinchillas. The animals were sacrificed at times ranging from 2 to 35 days after surgery. The normal olivo-cochlear terminals on the external hair cells in the cochleas of the control ears contained many mitochondria and small vesicles of constant size. The earliest evidence for degeneration was the presence of fine 100 A filaments in the proximal parts of the terminals. These were visible at 2 days. Animals sacrificed at later times showed a greater number of filaments and fewer vesicles, but few mitochondrial changes. After 1 week, disintegration of the terminals was more prominent. A few terminals showed mitochondrial swelling and lysis of the plasma membrane but few or no filaments within the first week. These latter terminals were interpreted as representing a more rapid process of disintegration than those terminals characterized by numerous filaments and seemingly unchanged mitochondria.

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