Varieties and distribution of non-pyramidal cells in the somatic sensory cortex of the squirrel monkey

The morphology and distribution of cells which do not conform to the conventional pyramidal pattern have been investigated in rapid Golgi, Golgi-Kopsch and Golgi-Cox preparations from cortical areas 3, 1 and 2 of juvenile and mature squirrel monkeys. The material has been analyzed qualitatively and quantitatively by means of a computer program which permits cells to be rotated so as to display their three-dimensional architecture. Nine non-pyramidal types are identified of which one is a rare giant cell and another, forming a major proportion of the cells in layer VI, is considered to be a modified form of pyramidal cell. Of the other seven types, two have horizontally distributed axons, one essentially confined to layer II, the other sending long (up to 1 mm) branches anter-posteriorly through all layers. Two types have vertical axons. One, corresponding to the "double bouquet dendritique" cell of Cajal, is mainly situated in layer II or the upper part of layer III and has a cluster of large axon branches which descend to layers IV and V and which enclose and terminate on the apical dendrites of pyramidal cells. The other type is the only non-pyramidal cell which has a relatively high concentration of dendritic spines in the adult animal. Its soma lies in layer IV and it has several strongly recurrent, thick axonal branches ascending to layer II, also enclosing the apical dendrites of pyramidal cells. The dendritic field is not truly stellate but is drawn out into a pronounced ascending tuft which ascends into layer IIIb. The cell thus resembles a "star-pyramid" of Lorente de Nó. Nevertheless such cells have many features, notably the distribution of their axons and the distribution of dendritic spines which are identical to those of the well-known "spiny stellate" cell of the visual cortex. Conversely the same features both in these cells and in the spiny stellate cells of the visual cortex (which were also eamined) differ markedly from those of small pyramidal cells with somata of similar dimensions. The three remaining non-pyramida cell types have locally ramifying axons which appear to terminate predominantly on pyramidal cells. In one, the axon forms smoothly curving arcades in layer III, in another it is intensely tangled in layer IV and in the third it is bush-like in layers II-IV. continued.