Synaptic glomeruli in the nucleus submedius of the rat thalamus.

Synaptic glomeruli in the nucleus submedius of the rat are described and the source of some of the component terminals identified. The glomeruli consist of large terminals with round synaptic vesicles establishing Gray type I contacts with dendrites and surrounded by layers of astrocyte derived membranes. The astrocyte processes may be composed of cell membranes with minimal interventing cytoplasm or, less frequently, contain larger amounts of cytoplasm. Horseradish peroxidase injected into the trigeminal nucleus caudalis labels some of the large astrocyte-enclosed terminals in nucleus submedius.