432 DOMINICK P. PURPURA 



"reinforced" responses coincides with a change in activity cycles, thereby 

 facilitating temporal summation of distant s.c.r.'s. The dual nature of the 

 processes of spread and generation of the s.c.r. in immature cortex is shown by 

 the appearance of distant "reinforced" responses and the fact that eHmination 

 of near s.c.r.'s does not alter distant responses (Fig. 5). The latter finding 

 confirms previous observations made in adult animals by Jasper et al. (1958). 



The foregoing data, presented in greater detail elsewhere (Purpura et al., 

 1960a) when considered in relation to histological information on the density 

 and distribution of elements in superficial regions of neocortex provide strong 

 support for the hypothesis that s.c.r.'s are generated postsynaptically in 

 superficial dendrites by conductile pathways of variable length and trajectory 

 (Eccles, 1951; Purpura and Grundfest, 1956). Various lines of evidence 

 compel rejection of alternative hypotheses concerning the nature and origin 

 of the s.c.r. (cf. Purpura, 1959). The fact that basilar dendrites are absent and 

 apical dendrites of pyramidal neurons do not have tangential branches at a 

 time when s.c.r.'s can be detected up to 5-7 mm from the site of stimulation 

 would appear to eliminate the possibility that the s.c.r. is conducted along 

 dendrites (Chang, 1951). Also, the demonstration of variations in s.c.r. 

 ampHtude at different loci and the finding that abolition of an s.c.r. at one 

 locus does not alter responses at other loci precludes the possibility that the 

 s.c.r. is both generated in and conducted along a single species of tangential 

 fibers (Burns, 1958). 



The relatively long duration of s.c.r.'s recorded in immature cortex at 

 distances (> 1 mm) comparable to those at which 15-20 msec responses are 

 observed in mature cortex indicates that in neonatal kittens a single brief 

 weak stimulus to the cortical surface initiates postsynaptic events in super- 

 ficial dendrites and perhaps other elements in superficial regions of cortex. 

 In the second and third postnatal weeks, distant responses of considerable 

 magnitude are observed indicating that the capacity for temporo-spatial 

 summation of p.s.p.'s in distant elements has been markedly facifitated. 



It is abundantly clear from the foregoing brief analysis of the generation 

 and spread of postsynaptic activities in superficial cortex that the sub-pial 

 neuropil is potentially capable of participating in the elaboration of electro- 

 cortical events, even in the near-term fetus. With advancing age changes 

 in its organization are reflected, in part, in the changing electrographic 

 characteristics of s.c.r.'s at varying distances. More importantly, however, 

 such alterations are also apparent in the different effects which selectively 

 acting pharmacological agents produce on superficial axodendritic organi- 

 zations at different developmental stages. 



Ontogenetic Changes in the Pharmacological Properties of Superficial Axo- 

 dendritic Synaptic Pathways 



The use of aliphatic w-amino carboxyhc acids as pharmacological tools for 



