Electron microscopy of synaptic glomeruli in the thalamic relay nuclei of the cat

The normal fine structure of the ventroposterior, and medial and lateral geniculate nuclei of the thalamus has been examined in the cat, particular attention being paid to localized aggregations of pre- and postsynaptic profiles which have been termed ‘glomeruli’, and which are common to these and certain other thalamic nuclei. The ventroposterior and medial geniculate nuclei show many similarities. Their glial-ensheathed glomeruli have one (or occasionally two) central dendrites which are usually bulbous side branches of main stem dendrites arising from neurons in the nuclei. These side branches commonly contain bundles, rings and whorls of neurofilaments. Two types of axon terminal complete the glomerulus: every glomerulus contains one (or at most two) large dark terminals; these have a heavy concentration of synaptic vesicles and a dense background cytoplasm and are derived from axons of medium diameter. They make asymmetrical synaptic contacts with the central dendrite and (axo-axonically) with the second type of terminal. This second type is a pale terminal with a clear cytoplasm and contains fewer synaptic vesicles than the large dark type; several pale terminals occur in a single glomerulus and they make short symmetrical synaptic contacts with the central dendrite only. Both types of axon terminal are also joined to the central dendrite by multiple adhesion plaques. The pale axon terminals are bulbous enlargements of long beaded processes; these come off very large unmyelinated profiles which resemble axon hillocks and may, therefore, be recurrent collaterals of thalamo-cortical relay cells or axons of interneurons. The commonest type of axon terminal outside the glomeruli is a small dark type which makes asymmetrical synaptic contacts with dendrites and the long beaded parent processes of the pale axon terminals, each of which may enter glomeruli close to or at some distance from this synapse. Dendrites beyond the point at which they enter glomeruli may be completely surrounded by small dark terminals.