Dendrodendritic synapses of cells that have axons: The fine structure of the Golgi type II cell in the medial geniculate body of the cat

Summary This study provides a combined analysis with the Golgi method and electron microscopy of the Golgi type II cells of the thalamus in the cat. In the ventral nucleus of the medial geniculate body these cells constitute a large, morphologically homogeneous population of neurons. They are clearly distinguished from the thalamo-cortical neurons by their size, shape, kinds of dendritic appendages, and cytoplasmic structure. The axon of the Golgi type II cell is exceptionally short and forms a small number of lumpy endings in the vicinity of its origin. The dendrites are often longer and much more elaborately branched than the axon. The shafts of these dendrites bear spiculated appendages, while the distal ends of the dendrites form clusters of very large endings. The appendages and terminal clusters participate in the nests of axonal endings formed by the afferent auditory axons and the dendritic branches of thalamo-cortical neurons. These axonal nests are the synaptic nests observed in electron micrographs. Within the synaptic nests the endings of Golgi type II neurons form dendrodendritic synapses on the dendrites of the thalamocortical neurons. The dendritic endings of Golgi type II neurons also receive synapses from the afferent axons. The dendrodendritic synapses may involve the Golgi type II neurons in an inhibitory role in the thalamo-cortical transformation of auditory signals. The dendrodendritic endings of the Golgi type II neurons continue to grow in the adult cat. Possibly these cells are involved in the evolution of cortical functions and in the plastic changes of neural activities that modify behavior.Supported by U. S. Public Health Service Research Grant NS 06115 and GRS Grant 5S01FR05381-08 to Harvard University.