The structural organization of the ventrobasal complex of the rat as revealed by the analysis of physiologically characterized neurons injected intracellularly with horseradish peroxidase.

The ventrobasal complex (VB) of the rat thalamus contains neurons responding to non-noxious somatic stimuli as well as neurons driven exclusively by noxious stimuli. This study presents a comparison of morphological features of these two kinds of neurons. Thirteen neurons electrophysiologically characterized were impaled with the micropipette used for the recordings and intracellularly injected with horseradish peroxidase. After revealing the marker and preparation for electron microscopic procedures, 3 out of the 13 neurons were carefully studied using both the light and the electron microscope. VB neurons are stellate cells with a central rounded cell body and 6 to 10 primary dendrites which branch rapidly, giving a 'tufted' appearance. Dendrites of all orders present various types of protrusions. At the electron microscope level, 3 main kinds of synaptic profiles were observed contacting the injected neurons: small terminals with round vesicles which make asymmetrical contacts with distal dendrites; medium-sized terminals with flattened vesicles which make symmetrical contacts with dendrites of all orders and the soma; and large terminals with round vesicles which make asymmetrical contacts with primary dendrites and the soma. This study failed to reveal obvious morphological differences between functionally different VB neurons. In addition, it showed that their synaptology was apparently equivalent.