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

Abstract 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 fromn 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 roatated 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 esseentially confined to layer II, is the other sending long (up to 1 mm) branches antero‐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 axone 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 cends into lalyer 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. Conversely the same features both in these cells and in the spiny stellate cells of the visual cortex (which were also examined) differ markedly fromn those of small pyramidal cells with somata of similar dimensions. The three remaining non‐pyramidal 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 in layer IV and in the third it is bush‐like in layers II–IV. There are morphological changes in certain of the cell types associated with the thinning of the cortex as it is folded into the central sulcus. For examplle, the spiny cells of layer IV become much more stellate‐like in the floor of the sulcus. It is thought that the changes in the shape of these cells can be correlated with the concomitant changes in the superificial to deep extent of the thalamic afferent plexus. Certain differences were also detected between the juvenile and adult animals. Certain differences were also detectex between the thalamic afferent plxus. Certain differences were also detecd between the In the juvenile, many more dendritic spines were seen on cells which in the adults normally possess few or none and the axonal plexusesscs of some cell were were less oraganized. In discussing the reusults, attempts are made toof correlate the these findings with those of other workers and to suggest possible morphological correlatessates of the elecotrophysiologically identifiable “cell column” of the cortex.