ioo8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



drew attention to the fact that the sexual rhythms of 

 many birds and mammals are conditioned by clianges 

 in the external environment which in all probability 

 afifect anterior pituitary activity through the central 

 nervous system. "... and it would appear certain 

 that many external factors which regulate the cycle 

 act through the intermediation of the central nervous 

 system upon the anterior pituitary, this gland playing 

 the part of a liaison organ between the nervous 

 system wiiich is affected by stimuli from without and 

 the endocrine system ..." (222). The basis for such 

 a view was the study of the reproductive cycles of 

 man>' animals in relationship to a variety of environ- 

 mental factors such as food, light, temperature, 

 presence of a mate and so on. It was observed (221) 

 that many birds and mammals will show a reversal of 

 the time of the breeding season after tran.sference 

 across the equator. One of the clearest examples of 

 environmental influences on gonadal activity is the 

 efifect of variation in environmental lighting. Among 

 the common mammals the reproductive rhythm of 

 the mouse (13, 135), rat (45, 74, 102, 265, 332), 

 ferret (28), hedgehog (i), cotton-tailed rabbit (32), 

 cat (70), raccoon (31), goat (30) and sheep (166, 

 366) is sensitive to changes in environmental lighting. 

 Of these animals the response of the ferret has been 

 most subjected to experimental study. The optimum 

 light wavelengths (224) and duration of light/dark 

 ratios (145, 167), the necessity of the optic nerves 

 (29, 55), of the hypophysial portal vessels (81), and 

 of the pituitary gland (176) for this response have all 

 been demonstrated. The anestrous state of the female 

 ferret during the months of September to February 

 is most easily changed to estrus if the animals are 

 exposed to 16 hr. of light per day of a wavelength 

 between 6500X to 3650X. It seems that the stimulus 

 of light initiates a nervous reflex through the eyes and 

 optic nerves which by some unknown neural pathway 

 excites the adenohypophysis via the pituitary stalk 

 and the resultant discharge of gonadotrophic hormone 

 results in ovarian activity. Many examples could be 

 quoted of similar reactions in birds. There is evidence 

 that the visual and auditory display of other members 

 of a flock may be indispensable to the reproductive 

 activity of individual birds (69). Further the number 

 of eggs laid in a clutch seems to be regulated through 

 either visual stimuli or proprioceptive stimuli Irom 

 the ventral body surface (see 220). In most birds and 

 some mammals (rabbit, ferret, cat, ground squirrel, 

 short-tailed shrew and mink), the occurrence of 

 ovulation is dependent on coitus or on some form of 

 sexual excitation. For example, the isolated female 



pigeon does not ovulate, but if the bird is placed in 

 view of a male bird, another female or a mirror, 

 ovulation follows (230). Similarly, in the mammals 

 mentioned abo\e, ovulation is dependent on some 

 stimulus emanating from the male, another female, 

 or mechanical or electrical excitation applied to the 

 \agina or cer\ix uteri. The inechanism invoked is a 

 nervous reflex excitation of gonadotrophic discharge 

 from the pars distalis. 



The data relating adrenal cortical activation to 

 environmental change are too well known and 

 numerous to list. The important work of Selyc (310, 

 311) first drew attention to the fact that a constant 

 pattern of response follows the application of many 

 difTerent types of stimuli, the common denominator 

 of which is that they tend to damage, or destroy the 

 homeostasis of, the organism. One element of this 

 'response to stress' is adrenal cortical activation. For 

 detailed references to the presently accumulated data 

 Selye's publications (312) may be consulted. In the 

 present context it may be mentioned that emotional 

 stress is a potent factor in evoking adrenal cortical 

 discharge, and observations on many forms, including 

 the mou.se (80), rat (343), rabbit (57) and especially 

 the human (33, 109, 175, 178, 181, 182, 248), form 

 strong evidence that in some way the central nervous 

 system e.xerts a controlling influence over the discharge 

 of adrenocorticotrophic hormone (ACTH) from the 

 adencjhypophvsis. 



In the case of the secretion of thyrotrophic hormone 

 (TSH) the evidence is less complete. There is evidence 

 that the overacti\'ity of the thyroid in cases of Graves' 

 disease is secondary to emotional disturbances (40, 

 59, 77, 122, 144, 210-212, 216, 217, 246, 255), and 

 the idea is often implicit in clinical publications that 

 this thyroid hyperfunction is due to increased secre- 

 tion of TSH from the pituitary (328). Many workers 

 (38, 46, 242, 256, 356) have reported that emotional 

 and physical stress inhibits thyroid function. It seems 

 clear that in the normal experimental animal emo- 

 tional stress results in decreased thyroid activity (46). 



The aboxe data form strong evidence that the 

 external environment, acting through the central 

 nervous system, may profoundly influence the se- 

 cretion of the follicle-stimulating, luteinizing, luteo- 

 trophic, adrenocorticotrophic and, possibly, thyro- 

 trophic hormones. 



Armlnnn of HypDlhalainoadenohypophysial System 



The nomenclature of Rioch et al. (284) will be 

 used in discussing anatomical details relating to the 



