title: Types of synapses and degeneration in the thalamic nucleus ventralis oralis posterior after cerebellar lesions in the squirrel monkey.
abstract: The thalamic nucleus ventralis oralis posterior (V.o.p) of Saimiri sciureus corresponds to the posterior basal part of the ventral lateral nucleus and is characterized by medium-sized nerve cells. Electron microscopic study revealed seven different types of synapses and the size of vesicles in each has been quantitatively determined. The synapse of type I LR (Large with Rounded vesicles) is considerably larger than the similar type IV MR (Medium-sized with Rounded vesicles) both contain round to ovoid vesicles and a large amount of mitochondria. Their contacts are asymmetric with presynaptic aggregations of synaptic vesicles. The type I LR synapses may also form button-like contacts (adhering junctions) without presynaptic accumulation of synaptic vesicles. The type II F (large with Flattened vesicles) symmetric synapses are formed by large boutons containing flattened vesicles and are partially encircled by astrocytic processes. The type III SR (Small with Rounded vesicles) bouton possesses small, nearly round till ovoid dense vesicles and forms a strongly asymmetric contact. In the type V SO (Small with Ovoid vesicles) bouton, the pleomorphic vesicles are larger than in type III and form asymmetric contacts with relatively small dendrites. Finally, two types of dendritic terminals were distinguished (VI and VII), which can be differentiated chiefly by the distribution of synaptic vesicles; VI DE (Dendritic terminal with Evenly distributed vesicles) and VII DC (Dendritic terminal with Clustered vesicles). Following hemicerebellectomy (4 experiments), only the type I LR and type IV MR synapses were found to be degenerated in the thalamic nucleus V.o.p. Similar degeneration was found in the ventral intermediate nucleus (V.im) which corresponds to the most rostral part of the VPL nucleus (VPLo). After three days survival, the forms of degeneration were largely neurofilamentous or pale, becoming mostly dark after four days. Degeneration of these same types of synapses appeared also in the nucleus ventralis oralis internus (V.o.i) and in the nucleus dorsalis intermedius (D.im), which lies above the ventral intermediate and ventrocaudal nuclei of the thalamus. These results are in good agreement with the findings in the light microscopic investigations if one considers the cyto- and myeloarchitectonic boundaries between the V.o.p and V.im.
mesh_terms: D000818:Animals; D002531:Cerebellum / Q000033:anatomy & histology*; D003712:Dendrites / Q000648:ultrastructure; D008854:Microscopy, Electron; D009434:Neural Pathways / Q000033:anatomy & histology; D012453:Saimiri; D013569:Synapses / Q000648:ultrastructure; D013572:Synaptic Vesicles / Q000648:ultrastructure; D013787:Thalamic Nuclei / Q000033:anatomy & histology* / Q000648:ultrastructure