A reassessment of the forms of nonpyramidal neurons in area 17 of cat visual cortex

The nonpyramidal neurons in area 17 of cat visual cortex have been examined in Golgi preparations. From their dendritic patterns, neurons are classified as being multipolar, bitufted, or bipolar, and on the basis of the abundance of dendritic spines as spinous, sparsely spinous, or smooth. When neurons are so classified seven different types of nonpyramidal neurons are encountered in layers II through V. Three of the types of multipolar neurons in layers II through V have spherical dendritic trees. The small multipolar cells have smooth dendrites and are the smallest neurons in the cortex. They have short dendrites and dense local axonal plexuses and occur throughout layers II to V The sparsely spinous stellate cells have longer dendrites, are confined to layer II/III, and have local axonal arborizations, whereas the spinous stellate cells are limited to layer IV. A fourth type of multipolar neuron in layers II through V is the basket cell. Such neurons have elongate dendritic trees and either smooth or sparsely spinous dendrites. Depending upon the orientation of the neurons in the sections, their axons appear to form arcades or long, horizontally extended branches, or a mixture of these two axonal patterns. The terminal portions of the axons of these basket cells pass around the cell bodies of adjacent neurons. The two types of bitufted neurons in layers II through V have vertically oriented dendritic trees. One type, the chandelier cell, has smooth dendrites and a characteristic axon forming vertical strings of terminals. The other sparsely spinous bitufted neurons have axons producing vertically oriented plexuses. The remaining type of neuron encountered in layers II through V is a bipolar cell. The bipolar cell has a single major dendritic trunk arising from each pole of the cell body, and each of these gives rise to a very narrow, long, and vertically oriented dendritic tree. The axon usually takes origin from one of the primary dendrites. In layer I are horizontally oriented, bitufted cells with smooth dendrites. The axons of these horizontal cells of layer I arise from one of the primary dendritic trunks and appear to form a plexus confined to layer I. Horizontally oriented neurons are also present in deep layer VI, but the horizontal cells of layer VI are bipolar. The other two neuronal types in layer VI are multipolar cells with sparsely spinous dendrites. The larger of these two types resembles the basket cells in layers II through V, the only important difference between them being that in addition to the long horizontal branches, the axons of the basket cells of layer VI have a long ascending branch which reaches at least as far as layer IV. The other sparsely spinous cells of layer VI are medium sized. Their axons take a descending and oblique course before elaborating a locally distributed plexus. The various types of neurons defined in this study are compared with neurons described by previous authors who have examined the populations of nonpyramidal cells in area 17 of cat visual cortex and in other visual and nonvisual cortical areas of cats, monkeys, and rodents. In some cases it has been possible to postulate the functional roles that particular types of neurons might play within cat visual cortex.