1032 



HANDBOOK OF PHVSIOLOOY 



NEUROPHYSIOLOGY II 



REFLEX ACTIVATION OF OXYTOCIC HORMONE SECRE- 

 TION. The effects of oxytocic secretion may be ob- 

 served on the iactating breast and on the uterus. 

 Such secretion appears to be evoked by direct stimu- 

 lation of the reproductive organs (nipple, external 

 genitalia or uterine cervix) and also by conditioned 

 reflexes. It has been established that nervous reflex 

 release of oxytocic hormone plays a necessary role in 

 milk ejection from the Iactating breast, and it appears 

 probable that a similar mechanism underlies the 

 processes of parturition and of sperm transport up 

 the female reproductive tract. 



Oxytocic hormone and milk-ejection reflex. It has long 

 been known that the transfer of milk from the Iactat- 

 ing Ijreast to the suckling young involves an active 

 expulsion of milk from the mammary gland. Gaines 

 (114) showed many years ago that suckling pups 

 cannot obtain milk from an anesthetized bitch. 

 Petersen and his co-workers (8g, 258, 259) first 

 brought forward evidence that secretion of oxytocic 

 hormone forms an essential part of the physiological 

 mechanism underlying milk ejection. They found that 

 cutting the motor nerves to one half of the udder of 

 the cow had no effect on the amount of milk obtained 

 from that half of the udder, and that milk ejection 

 could be stimulated in the isolated perfu.sed udder by 

 addition of oxytocin to the perfusing blood or by the 

 addition of blood withdrawn from the jugular vein of 

 a cow recently subjected to a milking stimulus. A few 

 years after these observations Richardson (279) dem- 

 onstrated clearly the myoepithelial cells which form 

 basketlike networks around individual alveoli and 

 around the ducts of the breast and which in all 

 probability represent the contractile tissue of the 

 organ. 



More direct evidence for the involvement of the 

 supraopticohypophysial tract and the neurohypophy- 

 sis in the mechanism of milk ejection came from the 

 independent researches of Cross & Harris (63) on 

 rabbits and Andersson (7, 8) on sheep and goats. 

 Cross & Harris found that electrical stimulation of 

 the supraopticohypophysial tract in the hypothala- 

 mus, infundibular stem or infundibular process in the 

 Iactating rabbit evoked ejection of milk from a can- 

 nulated teat duct. The time course of this response 

 had the character of one humorally mediated and 

 could be duplicated very closely by intravenous in- 

 jection of posterior pituitary extract. In further ex- 

 periments it was found that electrolytic lesions placed 

 in the supraopticohypophysial tract markedly reduced 

 the amount of milk obtained by a suckling litter, un- 

 less posterior pituitary extract was injected into the 



doe immediately before suckling. Andersson observed 

 a flow of milk from the teats of unanesthetized sheep 

 and goats following stimulation of the supraoptic 

 nuclei by the Hess technique. Denervation of the 

 udder or of the whole sacral region of the goat did 

 not affect the result, although a similar response was 

 obtained following the injection of blood of another 

 stimulated animal. These results have since been 

 extended by Cross (60, 62) who has investigated in 

 more detail the effects of hypothalamic stimulation, 

 by Cross & van Dyke (64) who found purified oxytocic 

 hormone to be aijout si.x times as effective in eliciting 

 milk ejection as purified antidiuretic hormone, by 

 Harris & Jacobsohn (161) who studied lactation after 

 pituitary transplantation in rats, and by Benson & 

 Cowie {24) who investigated the milk-ejection reflex 

 after removal of the infundibular process. Very little 

 is known regarding the reflex nervous pathways to 

 the supraoptic (and paraventricular) nuclei which 

 underlie the milk-ejection refle.x. There can be no 

 doubt that conditioned reflexes play a large part in 

 exciting the reflex in the normal animal, i)ut the 

 sensory path from the nipples to the hypothalamus 

 might profitably be investigated in the anesthetized 

 animal. 



Oxytocic hormone and parturition. The essential nature 

 of the hypothalamoneurohypophy.sial system for nor- 

 mal parturition has not yet been demonstrated. It 

 has been observed (79, loi) that the majority of cats 

 and guinea pigs in which hypothalamic lesions have 

 induced a state of diabetes insipidus are either unable 

 to deliver their young or deliver them after a pro- 

 longed and diflicult labor. However, a few animals 

 [slightly less than one third (79)] do appear to un- 

 dergo normal parturition in the presence of a dener- 

 vated neurohypophysis. The potentiality of the neuro- 

 hypophysis to evoke strong uterine contractions has 

 been made clear by studies in which the supraoptico- 

 hvpophysial tract has been stimulated electrically. 

 Haterius & Ferguson (170) and Ferguson (99) 

 found that electrical stimulation of the pituitary stalk 

 of post partum rabbits resulted in an increased fre- 

 quency, and sometimes increased amplitude, of uter- 

 ine contractions. Harris (149) studied the uterine 

 response of estrous or estrogenized rabbits to remote- 

 control stimulation of different areas in the hypothala- 

 mus or pituitary. Stimulation of the region of the 

 supraopticohypophysial tract, but not other areas, 

 was found to result in a well-marked uterine response 

 which had the characters of one humorally mediated 

 and which could be closely duplicated l)y injection of 



