440 DOMINICK P. PURPURA 



A B 



Fig. 14. Adult cat anesthetized with 15mg/K pentobarbital sodium. Upper 

 channel records in a and b, s.c.r. recorded 1-5 mm from site of stimulation. 

 Stimulus sub-threshold for late negative component in s.c.r. Lower channel 

 records in a, s.c.r. recorded at 3-0 mm and b, at 4-5 mm from stimulating 

 electrodes. (1) Control responses; (2) 20 sec after topical application of Ce to all 

 recording sites; (3) recovery after flushing cortex with Ringer's. Increase in 

 early and late components of s.c.r.'s recorded at 1-5 and 30 mm is clearly 

 shown. Early component of s.c.r. at 4-5 mm is unaffected by Ce. Late activity 

 in 4-5 mm distant response represents propagation of activity initiated by Ce 

 at site of stimulation. Cal. 100 c/s; 0-1 mV. 



surface-negativity in the near s.c.r. (Fig. 14). Cortical "excitability" was 

 reduced by administration of sub-anesthetic amounts of sodium pento- 

 barbital (15 mg/K). Under these conditions, Ce augmented early and "acti- 

 vated" late components of s.c.r.'s recorded up to 3 mm from the site of 

 stimulation (Fig. 14a (2)). Early components of s.c.r.'s recorded 4-5 mm 

 from the stimulating electrodes were unaltered, whereas a late component 

 developed that appeared to represent propagation of the late component in 

 near responses. Experiments similar to those illustrated in Fig. 14 were also 

 designed to determine the distance at which initial components in s.c.r.'s 

 differed in their response to Ce- Such studies revealed that the action of Ce 

 was confined to s.c.r.'s recorded within 3-3-5 mm of the site of stimulation. 

 The different effects of Ce on near and distant responses were reproduced by 

 topical strychnine (0-1 %). 



COMMENT 



The foregoing survey of the ontogenetic changes in the morphophysio- 

 logical properties of elements involved in the generation and spread of evoked 

 superficial responses in cat neocortex bears on two aspects of the organization 

 of axodendritic synaptic pathways in sub-pial neuropil : (1) the degree to 



