444 DOMINICK P. PURPURA 



electron microscopical data are obtained on the ultrastructure of axoden- 

 dritic synapses during postnatal ontogenesis. 



The differential augmentation of s.c.r.'s recorded within 4 mm of the site 

 of stimulation is shown to be a final maturational event. Thus, although 

 s.c.r.'s recorded at various loci are initiated by fundamentally similar pro- 

 cesses (Fan and Feng, 1957; Purpura et aL, 1960a), it must be allowed that 

 s.c.r.'s differ with respect to their pharmacological responsiveness to com- 

 pounds which presumably exert convulsant effects through blockade of in- 

 hibitory p.s.p.'s. Viewed from the standpoint of the hypothesis (cf. Eccles, 

 1957, and elsewhere in this volume) that inhibitory activities in the mam- 

 malian CNS are exerted by pathways operating at relatively close range, it is 

 not unlikely that a weak, localized stimulus to the cortical surface in adult 

 animals initiates activity in superficial pathways capable of generating ex- 

 citatory p.s.p.'s in dendrites 8-10 mm from the site of stimulation and in- 

 hibitory p.s.p.'s in dendritic elements less than 3-4 mm from the stimulating 

 electrodes. 



If, as proposed here, functional maturation of some inhibitory axodendritic 

 synaptic pathway in sub-pial neuropil of cat neocortex proceeds at a con- 

 siderably slower rate than maturation of excitatory axodendritic pathways, the 

 question may be raised as to why sustained states of paroxysmal activity are 

 not more commonly observed in new-born cat cortex (Grossman, 1955). An 

 answer to this may be provided by histological observations on the relative 

 paucity of neuronal interconnections and the restrictions imposed by the 

 prolonged post-excitatory depression of processes operating at superficial 

 neocortical axodendritic synapses in the neonatal period (Purpura et aL, 

 1960a). Progressive development of neuronal interrelations results primarily 

 from proliferation of basilar dendritic systems and axon collaterals and is, 

 in part, reflected in the appearance of spines on basilar and apical dendrites 

 in Golgi-Cox preparations. In the cat, the phase of maximum neocortical 

 neuronal development occurs during the second postnatal week and is 

 heralded by a significant increase in the capacity for temporal and spatial 

 summation of excitatory p.s.p.'s in superficial dendrites. This period cor- 

 responds to the time during which inhibitory synaptic activities in the s.c.r. 

 become demonstrable with long-chain oj-amino acids. Thus, despite initial 

 differences in the temporal pattern of development of inhibitory and excitat- 

 ory processes, both appear to attain full expression by the third to fourth 

 postnatal week. 



The hypothesis that different cortical synaptic organizations exhibit 

 different rates of functional maturation is supported by observations on the 

 comparative development of some archicortical synaptic pathways (Purpura, 

 1960b). Hippocampal pyramidal neurons have morphological features in the 

 new-born cat that are not observed in neocortical pyramidal neurons until 

 the second postnatal week. The relatively advanced development of hippo- 



