The different types of synapses in the thalamic nucleus ventralis anterior (VA) of Saimiri sciureus and their degeneration after pallidum coagulation.
In the squirrel monkey (Saimiri sciureus) the nucleus VA or lateropolaris (L. po), occupying the rostro-lateral pole of the thalamus, contains 7 principal types of synapses, two of which have axo-spinous variants, i.e., type I LR (spi) and type V SO (spi). The synapses of type I LR (Large boutons, Round to ovoid vesicles) and type II EF (Encapsulated bouton, elongated to Flat vesicles) contain large vesicles sometimes attached by neurofilaments. Because the type IV MR (Medium size, Round vesicles) synapses have virtually the same kind of axo-dendritic synaptic contacts as type I, either with or without aggregations of presynaptic vesicles, they belong to the same category of afferent boutons, differing only in size and in the number of mitochondria. The small axo-dendritic type III SR synapses (Small boutons, Round vesicles) are densely packed with vesicles having a relatively dense center, and undergo markedly asymmetric contacts with dendrites. The type V SO (Small bouton, Ovoid vesicles) synapses, the most frequent type of synapses, have larger pale ovoid vesicles in a bell-shaped bouton that is in asymmetric contact with a small dendrite. The subtype V St, which likewise contains pale ovoid vesicles has a stiletto-shaped rather than bell-shaped bouton which undergoes an asymmetric contact. In parallel to the other ventro-oral (or VL) nuclei, the nucleus VA possesses two types of presynaptic dendrites: type VI DE (Dendritic terminal with sparse, Evenly distributed vesicles) suspended among the neurofilaments; and type VII DC (Dendritic terminal in which many ovoid vesicles are Clustered). In the type VI junctions, the synaptic membrane specializations are difficult to detect. After circumscribed pallidum externum lesions in the squirrel monkey, many type I LR and type IV MR synapses, and a lesser number of type II EF synapses, undergo dark degeneration. In contrast, after such lesions the type VI DE and type VII DC dendritic terminals never show signs of degeneration. Also the bell-shaped type V SO terminal occasionally undergoes degeneration with darkening of the axoplasm; the subtype V St terminal, however, does not. Thus, it may be assumed that the type I LR, type IV MR, type II EF and type V SO boutons represent terminals of direct neuronal connections between the pallidum and thalamic nucleus VA.
D000344:Afferent Pathways / Q000033:anatomy & histology; D000818:Animals; D002529:Cerebellar Nuclei / Q000033:anatomy & histology; D003712:Dendrites / Q000648:ultrastructure; D005917:Globus Pallidus / Q000033:anatomy & histology*; D008854:Microscopy, Electron; D009044:Motor Cortex / Q000033:anatomy & histology; D009410:Nerve Degeneration; D009474:Neurons / Q000145:classification / Q000648:ultrastructure; D012453:Saimiri; D013569:Synapses / Q000648:ultrastructure*; D013572:Synaptic Vesicles / Q000648:ultrastructure; D013787:Thalamic Nuclei / Q000033:anatomy & histology*