CHARLES H. SAWYER 173 



the eslrus cycle. He has observed in a few animals, during the critical 2-4 p.m. 

 period on the day of proestrus, marked localized electrical changes characterized 

 by an amiilitude of 100-200 fA', sl frequency of approximately 15 cycles per 

 second and a duration of about 20 minutes (fig. 7). A limited area in the lateral 

 preoptic region and lateral anterior hypothalamus is the only site thus far ob- 

 served to display these electrical changes. The work is still in progress, but the 

 results suggest that this rather discrete 'spontaneous' electrical activity may 

 represent part of the central nervous i)rocess responsible for activation of the 

 adenohypophysis and the resultant release of ovulating hormone. 



SUMMARY 



In three different species with varied sexual behavior patterns and two funda- 

 mentally different afferent mechanisms controlling release of pituitary gonado- 

 trophin, similar EEG changes have been recorded during periods in which the 

 adenohypophysis is triggered by the central nervous system. Artificial stimula- 

 tion and blocking experiments suggest that the rhinencephalon and the reticular 

 activating system may contribute directly to the triggering process. The ventro- 

 medial nucleus of the hypothalamus and its afferent projections, the medial 

 forebrain bundle (21) and the stria terminalis from rhinencephalic centers, have 

 all been implicated in one or more species to play important roles in the pituitary 

 activation sequence. However, in the process of unearthing the whole chain of 

 events from periphery to pituitary, the missing links probably far outnumber 

 those so far described; their discovery and clarification offer challenging 

 problems for future research. 



The present approach to the trigger problem in these examples of higher 

 nervous initiation of behavioral and endocrine activities consists of localizing 

 the most directly involved structures and placing them in functional sequence. 

 Recent work may be said to represent significant progress in this area, and it is 

 perhaps a fair sample of the type of analytical advance now occurring in the 

 central neurophysiology of behavioral and neuroendocrine triggers. 



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