520 



PHYSIOLOGY OF GONADS 



C. ROLE OF THE NERVOUS SYSTEM 

 IN OVULATION 



Historically, the fact of neural control of 

 reflex ovulation has been recognized in the 

 ral)bit for many years. The comparable role 

 of the nervous system in spontaneous ovu- 

 lation, on the other hand, has more recently 

 become apparent. It now seems justifiable 

 and useful to postulate in the hypothalamus 

 of reflex ovulators and spontaneous ovu- 

 lators alike the existence of a mechanism 

 that is peculiarly concerned with release 

 of ovulating hormone. Whether it consists 

 in a discrete anatomic entity is immaterial 

 for the present. 



The suggestion has been made that the 

 outstanding difference between reflex and 

 sjiontaneous ovulation may be in the kinds 

 of afferent impulses that most readily ex- 

 cite the hypothalamus (Sawyer, Everett 

 and Markee, 1949). The difference is not 

 absolute, for spontaneous ovulation has been 

 induced in rabbits by estrogen-progesterone 

 injection (see p. 519) and reflex ovulation 

 has been demonstrated under special cir- 

 cumstances in rats (Dempsey and Searles, 

 1943; Everett, 1952a) and cattle (Marion, 

 Smith, Wiley and Barrett, 1950). The ran- 

 dom distribution of reflex ovulators and 

 spontaneous ovulators among mammalian 

 orders becomes more understandable if one 

 assumes that dual controls are widely rep- 

 resented and that special adaptations favor 

 one or the other in given species. 



The ovulation reflex in rabbits is appar- 

 ently initiated by afferent impulses of mul- 

 tiple origin, among them not only impulses 

 from the genitalia, but also propriocejUive 

 impulses from muscles that are utilized in 

 coitus. Brooks (1937, 1938) found that, 

 although the sacral segments of the spinal 

 cord and the abdominal sympathetic chains 

 could be eliminated without jM'eventing ovu- 

 lation, the luml)ai' cord must remain. Only 

 by paralysis of the lower half of the body 

 so that the female could not take pai't in 

 coitus was ovulation pi'cvcntfMl. The neo- 

 cortex could be removed, together with the 

 olfactory bulbs, labyrinths, auditory ap- 

 paratus and eyes without impairing the 

 ovulation response. Even after complete de- 

 cortication, ovulation followecl coitus in 1 

 out of 3 j'abbits. It must l)e admitted, how- 



ever, that, although various parts of the 

 nervous system may thus be eliminated 

 without changing the end result, some of 

 them may normally play a considerable 

 role. With little cjuestion, direct stimuli from 

 the genitalia play a part in the natural 

 reflex. Under certain experimental condi- 

 tions detectable electrical activity in the 

 rabbit rhinencephalon is associated with the 

 induction of ovulation (Sawyer, 1955). 

 Electrical stimulation of the amygdala will 

 induce ovulation in rabbits and cats (Koi- 

 kegami, Yamada and Usei, 1954; Shealy 

 and Peele, 1957 ) . 



In rats, Davis (1939) found that re- 

 moval of the neocortex had no effect on the 

 estrous cycle and ovulation. Removal of 

 portions or of the entire sympathetic chains 

 of rats likewise did not interfere with ovu- 

 lation (Bacq, 1932). Bunn and Everett 

 (1957) reported ovulation in constant- 

 estrous rats after electrical stimulation of 

 the amygdala. 



The importance of the dorsal thalamus is 

 unknown. The reticular activating system 

 has been implicated as a component of the 

 ovulation mechanism (Sawyer, 1958), but 

 the manner of its contribution is not clear. 

 There is little cjuestion, on the other hand, 

 of the indispensability of the hypothalamus 

 and its neurovascular connection to the 

 adenoliyi:)ophysis through the median emi- 

 nence and the hypophyseal portal veins. 



Although the observation by ]\Iarshall 

 and Verney (1936) that ovulation can be 

 induced by passing an electric current 

 through the heads of estrous rabbits hardl}^ 

 limited the effect to the hypothalamus it- 

 self, it was later shown that more localized 

 electrical stimuli api)lied to certain hypo- 

 thalamic regions are consistently effective 

 (preoptic area, Haterius, 1937; Christian. 

 1956; posterior hypothalamus or tuber 

 cinereum, Harris, 1937, 19481); tuber ciner- 

 eum or adjacent hypothalamic areas, 

 Markee, Sawyer anirHollinshead. 1946; 

 medial hypothalamus fi'om ])i-eo])tic area 

 to mammillai'V bodies. Kui'otsiu Kurachi 

 and Ban, 195o"; Kuiotsu, Kurachi, Tabaya- 

 >hi and Ban, 1952). 



Ahliougli liypotliahimic lesions. l)oth 

 natural and expeiimental. hax'c frecjuently 

 been reported to interfere with normal 



