426 DOMINICK P. PURPURA 



actions of topically applied oj-amino acid drugs were similar to those described 

 previously (Purpura and Grundfest, 1956, 1957; Purpura et al, 1959b, 1960a). 



Postnatal Development of Superficial Neocortical Neuropil 



The following survey of the salient morphological features of the major 

 constituents of superficial neuropil is intended to facilitate interpretation of 

 physiological data on the characteristics of superficial cortical responses at 

 different developmental stages. For present purposes useful information may 

 be obtained from Golgi-Cox material on the overall dimensions of dendrites, 

 their general distribution and the origin and distribution of axons in superficial 

 regions of cortex. 



Sub-pial neocortical neuropil is extraordinarily well developed in the neo- 

 natal and near-term fetal cat. Dendritic components are derived from small, 

 medium, and large pyramidal neurons in all layers (Fig. 1a), and the extensive 

 ramifications of relatively numerous "embryonic" Cajal-Retzius cells (Fig. 

 1b, c). The apical dendrites of pyramidal neurons emerge from cell bodies 

 whose diameters range from 10-25 /x. Proximal portions of apical dendrites 

 are of considerable thickness relative to cell body diameter (Fig. Id). Branch- 

 ing of middle or distal segments is minimal, but when this occurs, radial 

 orientation of branches is maintained. Emphasis is to be placed on the fact 

 that tangential branches of apical dendrites in their proximal or middle seg- 

 ments are not observed in the perinatal cat, although a few delicate processes 

 less than 50 /x are occasionally discerned in small superficially located pyra- 

 midal neurons. Apical dendrites are densely packed in the molecular layer and 

 terminate within 25 /x of the pial surface or in contact with the latter. Tangen- 

 tial spread in the molecular layer of apical dendrites of medium and giant 

 pyramidal cells is rarely greater than 100 jn, but some small superficially 

 located neurons give rise to apical dendrites whose tangential branching in 

 the molecular layer may approach 150-200 /n (ShoU, 1956). The most con- 

 spicuous feature of the pyramidal cell dendritic system in the neonatal period 

 is the relatively poor development of basilar dendrites of medium and large 

 pyramids. Some small pyramidal neurons in the sub-molecular layer may 

 have basilar dendrites three to four times the diameter of their cell bodies. 

 Dendrites of superficially located stellate neurons are poorly developed, but 

 some terminal branches ramify in the molecular layer. 



Descending axons of pyramidal neurons of all sizes are well developed but 

 relatively devoid of collaterals in the neonatal cat. Some ascending axons 

 from deep lying pyramidal neurons and others of unknown origin often appear 

 to be intimately related with apical dendrites throughout their entire course. 

 Tangential axons in the molecular layer arise from three sources ; the embry- 

 onic Cajal-Retzius neurons, pyramidal neurons, and sub-cortical elements. 

 Some of the identifiable axons of Cajal-Retzius cells intertwine with densely 



