The morphology and synaptic connections of spiny stellate neurons in monkey visual cortex (area 17): a Golgi-electron microscopic study.

Based on a gold-toning, Golgi-electron microscope examination of 12 small and medium-sized spiny stellate neurons in laminae 4A, 4B, and 4C of the monkey visual cortex (area 17), the ultrastructure of the cell somata, dendrites, and axons of these neurons is described. Particular attention is paid to the synapses involving the surface of different parts of these neurons. Only symmetric synapses occur on the somata of spiny stellate neurons, and these occur with a frequency of 11.0-15.9 synapses/100 microns2 perikaryal surface. Symmetric synapses also occur on dendritic shafts and, occasionally, on dendritic spines. Asymmetric synapses are occasionally present along the dendritic shafts of spiny stellate neurons, but the majority of asymmetric synapses (75-95%) occur on their dendritic spines. The initial axon segments of the smallest spiny stellate neurons possess no axo-axonal synapses, but several symmetric synapses are present along the initial segment of a medium-sized, spiny stellate neuron in layer 4B. Fifty-three synapses made by boutons of the axons of these spiny stellate neurons have been identified, and all are asymmetric. Sixty per cent of the synapses are formed by boutons en passant and the remainder by the terminal swellings of spine-like axonal appendages, boutons terminaux. Of the synapses formed by the axons of spiny stellate cells, axo-spinous synapses outnumber axo-dendritic synapses two to one, and axo-dendritic synapses involve both spinous and aspinous dendrites. Evidence is presented which suggests that many of the axon terminals forming asymmetric synapses with the dendritic shafts and spines of spiny stellate neurons are derived from other spiny stellate neurons.