[Ultrastructural organization of the geniculo-cortical neurons demonstrated by retrograde axonal transport of horseradish peroxidase].

The ultrastructural organization of geniculo-cortical relay neurons projecting to the primary auditory cortex (field 22) was studied in the cat by the method of retrograde axonal transport of horseradish peroxidase. The labeled neurons appear to be a medium-sized cells containing a large amount of profiles of vacuolar systems as well as organelles involved in catabolic processes. These cells differ from small unmarked neurons in having a narrow band scanty cytoplasm. Only a few axo-somatic synapses are found on the labeled neurons, the majority of afferent impulsation is perceived by their dendrites. The function of relay neurons is discussed.