Organization of neurons in the visual cortex, area 17, of the monkey (Macaca mulatta)

Abstract A study of neuron morphology in Golgi Rapid and Kopsch preparations of area 17 of the monkey has shown three basic cell groups— pyramidal neurons, stellate neurons with spinous dendrites and stellate neurons with spine‐free or sparsely spined dendrites. These three neuron groups show different distributions in depth from pia to white matter and differ in their relationship to zones of concentrated termination of geniculo‐cortical axons. The neuron type most closely related to the laminae receiving a heavy geniculo‐cortical projection is the spinous stellate cell. This cell type is restricted to lamina IV. Included in this zone is the broadest band of geniculo‐cortical axon projection (lamina IVC), the horizontal fiber band (lamina IVB) comprising the major portion of the stria of Gennari (receiving little or no thalamic projection) and the narrow band of thalamo‐cortical fiber termination which occurs superficial to the stria of Gennari (lamina IVA). Pyramidal neuron cell bodies are almost totally excluded from lamina IVC and the apical dendrites of lower pyramidal neurons bear many fewer spines in lamina IVC than in laminae V and VI. The basal dendrites of upper pyramidal cells spread superficial and deep to lamina IVA rather than within it. Spine‐free stellate neurons occur at all cortical levels and sparselyspined varieties have not been impregnated in lamina IV, but occur in the other laminae. Three groups of presumed thalamo‐cortical axons have been identified, two of which resemble each other in morphology (having long collaterals which appear to terminate by means of spine‐like projections of the axonal surface) but not in distribution. One group of these axons bearing spine‐like projections distributes in laminae IVC (principally in the deeper half, IVCβ) and IVA; the other is restricted in distribution to the upper half of lamina IVC (IVCα). The third group of thalamo‐cortical axons distributes to lamina I and appears to lack the spine‐like projections shown by the other two axon groups.