The forms of non-pyramidal neurons in the visual cortex of the rat

Rapid Golgi preparations from area 17 of young adult rats have been studied to determine the morphology and distribution of non-pyramidal neurons. Such cells were observed in all of the cellular laminae of the cortex, but were particularly prevalent in layers IV and V. Non-pyramidal neurons were categorized according to two features: (1) dendritic projection pattern, and (2) abundance of dendritic spines. Dendritic patterns were classified as multipolar, bitufted, and bipolar, and spine patterns as spinous, sparsely spinous, and spine-free. Spinous dendrites were associated only with multipolar neurons, while sparsely spinous and spine-free dendrites were each associated with cells of all three non-pyramidal dendritic patterns. The most frequently observed non-pyramidal cell types were multipolar cells of the spine-free and sparsely spinous varieties. All of the general cell types encountered have been described in the literature on non-pyramidal neurons, indicating the lack of any unique forms in rat area 17. An analysis of the dendritic projections of individual non-pyramidal neurons through particular cortical laminae made possible an evaluation of common sources of dendrites present in the neuropil of each layer. Non-pyramidal cell axons were impregnated only in small numbers. Spinous multipolar axons invariably exhibited a descending main branch, while the axons of bipolar neurons were distributed in a narrow vertical field. Axonal patterns of remaining cell types, including Golgi type II arborizations, did not appear to correlate consistently with dendritic morphology. Axons of the basket cell type and "horsetail" axons associated with double bouquet cells of Cajal's original type were not impregnated.