The fine structure of neurons and synapses in the ventrobasal thalamus of the cat

Abstract

The ventrobasal complex (VB) of the cat thalamus has been studied by light and electron microscopy. The majority of the neurons in VB are about 25 μ in diameter, with dendrites that exhibit irregular appendages along their shafts. Several synaptic types are recognized large and small knobs containing rounded vesicles and forming asymmetrical junctions; knobs with a mixture of flattened and rounded vesicles which make symmetrical contacts; and infrequent axo-somatic synapses. A population of synapses arising from profiles resembling dendrites is also present. The large synaptic knobs usually synapse upon larger dendrites in the neuropil, and are also seen in segregated synaptic complexes called synaptic islands. Small knobs containing round vesicles and knobs with flattened vesicles synapse upon medium to small dendritic profiles. It is likely that the various synaptic types represent differences in the sites of origin of the parent axons giving rise to these synapses. When axo-axonal complexes are seen, the postsynaptic element is always a profile containing flattened vesicles. Neurofilamentous rings are present in some dendrites, and probably correspond to neurofibrillar rings seen by light microscopy. Extensive desmosome-like contact regions are found between dendrites of all sizes.