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We are studying the response to injury within the brainstem trigeminal nucleus following trigeminal nerve lesions. We have previously shown with light microscopy and reduced silver stains that unilateral retrogasserian rhizotomy results not only in massive degeneration throughout the ipsilateral spinal trigeminal nucleus; in addition, degeneration is seen in a ventral position at the periobex region (involving caudal pars interpolaris and rostral pars caudalis) in the contralateral spinal trigeminal nucleus. In the present study, we have used electron microscopy to identify the source of the degenerating elements seen bilaterally after unilateral retrogasserian rhizotomy in eight adult felines with survival times ranging from 3 to 20 days. At short survival times (3-7 days) degenerating terminals with round synaptic vesicles (R terminals) and type 1, asymmetric contacts predominate bilaterally, while fewer degenerating terminals with flattened synaptic vesicles (F terminals) and type 2, symmetric contacts are seen. At longer survival times more F terminal degeneration is seen, especially on the contralateral side. Postsynaptic sites and dendrites show minimal alterations. These findings suggest that the degenerating R terminals seen on the contralateral side originate from primary afferents while the degenerating F terminals seen on the contralateral side originate from intrinsic sources involving a crossed internuclear pathway. In addition, the finding of degenerating F terminals may represent a novel form of selective transynaptic change of intrinsic neurons, associated with minimal dendritic or somatic alterations. [ABSTRACT FROM AUTHOR]