Computer-assisted analyses of barrel neuron axons and their putative synaptic contacts

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Abstract
The “barrels” in layer IVof rodent SmI neocortex receive inputs from individual whiskers on the contralateral face. Previous analyses of neuronal morphology in mouse and rat barrel cortex, as revealed by Golgi impregnations, have focused on the dendritic patterns of the stellate cells. The cells can be classified into two groups: Class I cells with spiny dendrites and Class i I cells with smooth, beaded dendrites. These classes can be subdivided further according to somal position and spatial distribution of dendrites with respect to barrel cytoarchitectonic boundaries. In the present study the axons of these cells were examined and the locations of close appositions to dendrites of other impregnated neurons were mapped. All data are taken from Golgi‐Cox preparations, cut parallel to layer IV at 140 μm, counter‐stained with Nissl to reveal the barrels, and measured with a computer‐microscope. Axons which had extensive branching within the section (present on 10% of all impregnated cells) were chosen for measurement. The analysis of the axons revealed: (1) Class I axons are thin and directed to the white matter with recurrent collaterals in the barrels, while Class II axons are thick, frequently beaded, and directed toward the pia before cascading down into the barrels; (2) in layer IV, the axons of both cell classes tend to be as restricted to a barrel as the dendrites of the same cell are (i.e., most axons are confined to one barrel); (3) within layer IV, the Class II cell axons have a total length about three times that of Class I cell axons, and about four times as many branch points. The analysis of the appositions of these axons to impregnated dendrites of other cells revealed: (1) A maj ority of “contacts” tended to be made by terminal branches of the axonal trees. (2) For the Class I neurons, a greater number of appositions occur near the distal ends of complete dendritic segments. As measured from the “contacted” cell soma, appositions are more or less uniformly distributed along dendritic trees. (3) No striking patterns are found, such as an obvious propensity for axons of one cell type to prefer or avoid another cell type. These results show that the axons of barrel cells of each class are as consistent and distinctive as their dendritic trees. Specifically, the cells in each class can be distinguished by their axonal patterns on purely numerical bases. The new computer programs which were developed to systematically search for sites of apposition between axons and dendrites ‐a necessary prerequisite for the existence of synaptic contacts‐greatly facilitated the collection of these data: The data may be of value for defining “probable” sites of interaction between particular cell types. In particular, these light microscopic observations of the distribution of appositions along dendritic segments are similar in some ways to White and Rock's ('80, '81) electron microscopic mapping of synapses on single‐barrel neurons.