I 02t 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



animal is due to lack of development of some neural 

 (possibly a hypothalamic) mechanism. As an inter- 

 esting speculation the pituitary-adrenocortical func- 

 tion of the newborn animal may be compared with 

 that of the adult animal with a cut pituitary stalk. 



The data regarding the onset of activity of the 

 gonads are similar to that gi\cn for the adrenal cortex. 

 A certain endocrine activity of the ovary and testis is 

 established before birth. Also it is clear that the gonads 

 are capable of responding to gonadotrophic hormone 

 before puberty occurs, and that the anterior pituitary 

 contains and is capable of liberating gonadotrophic 

 hormone before puberty. The evidence indicates that 

 the central nervous system plays an important part in 

 triggering and maintaining the activity of the system 

 at and after puberty. The question has been more 

 fully discussed (156) and more direct data brought 

 forward in support of this view since it has been 

 found that hypothalamic lesions may result in ad- 

 vancement in the date of puberty in the rabbit (119) 

 and rat (84). 



Conclusions Regarding Central Control 0/ Aihnohvpophysis 



There can be little doubt that changes in the exter- 

 nal environment, acting through the central nervous 

 system, can exert a major regulating influence upon 

 endocrine function. This widespread influence is 

 brought about, in the main, through the intermedia- 

 tion of the hypothalamus and hypopliysis. In discus- 

 sion of the anatomical pathway from the hypothala- 

 mus to the hypophysis, attention has been drawn to 

 the absence or scarcitv of nerve fibers passing from 

 the former to the latter structure. The only constant 

 anatomical link between the diencephalon and 

 anterior pituitary gland lies in the neural pathways of 

 the hypothalamus to the median eminence of the 

 tuber cinereum and the vascular path, the hypophy- 

 sial portal vessels, passing from the median eminence 

 to the pars distalis. There is strong evidence that this 

 neurovascular path forms the route by which the 

 hypothalamus influences anterior pituitary activity. 

 How this control is mediated is unknown. The most 

 likely view, based on general data regarding humoral 

 transmission of nerve impulses and current views on 

 the mode of formation of posterior pituitary hormones, 

 is that a humoral substance or substances are liberated 

 by nerve endings into the primary plexus of the portal 

 vessels and transmitted by these vessels to effect 

 changes in activity in anterior pituitary cells. Indirect 

 evidence in support of this view is availaiile but 



direct evidence, according to various criteria laid 

 down for the study of this problem, is lacking. 



In order to estimate the degree of dependence of 

 the endocrine system on the central nervous system, it 

 is necessary to know the intrinsic autonomous activity 

 of the anterior pituitary gland and its target organs 

 when separated from the central nervous system (by 

 pituitary stalk section or pituitary transplantation). 

 Much data are available on this point. It seems that 

 the endocrine activity of the gonads almost entirely 

 ceases, although thyroid and adrenocortical activity 

 continues at a reduced level under these conditions. 

 Thyroid function is still modified by an increase in 

 the blood concentration of thyroid hormone (indi- 

 cating that such a change exerts an eff"ect directly on 

 anterior pituitary cells), and the activity of the thyroid 

 and adrenal cortex seems to be still influenced by 

 'systemic' stresses (in Fortier's sense of stresses involv- 

 ing tissue trauma or metabolic disturbance). 



The feed-back mechanism, by which the blood con- 

 centration of target organ hormones affects the lib- 

 eration of anterior pituitary hormones, appears to 

 perform a stabilizing function by which the pitui- 

 tary-target organ system maintains endocrine activity 

 at a constant le\el under conditions of a constant en- 

 vironment. Although data are accumulating, it can- 

 not yet be stated clearly what part the hypothalamus 

 plays in this feed-back mechanism. 



The responsibilities of the central nervous system, 

 with regard to endocrine activity, may be discussed 

 under three main headings. First, it must initiate in the 

 newborn animal or in the prepul^ertal animal the 

 adult pattern of endocrine function and maintain this 

 level of function. The gonads are the target organs 

 most wholly dependent on some neural stimulus to 

 the adenohypophysis. Not only the onset of gonadal 

 activity at puberty and the maintenance of this activ- 

 ity in the adult above the prepubertal level, but the 

 different patterns of gonadotrophin secretion seen in 

 the male and female animal appear dependent on the 

 nervous system. The secretion of TSH and ACTH at 

 a normal level is also dependent on the nervous sys- 

 tem, although apparently not to the same degree as is 

 secretion of cronadotrophic hormone. Recent data 

 indicate that the hypothalamus may mediate both 

 excitatory and iniiibitory influence over the secretion 

 of anterior pituitary hormones. Second, the nervous 

 system is responsil)le for mediating the effects of 

 stimuli arising from a changing external environment. 

 Changes in the external environment which arc sutii- 

 cientlv intense to result in tissue trauma or metabolic 

 disturbances are possibly capable of affectins? anterior 



