CENTRAL CONTROL OF PITUITARY SECRETION 



1029 



pituitary secretion throu?;h the general systemic circu- 

 lation, but the majority of environmental stimuli 

 (arising from such factors as changing conditions of 

 light, temperature, sound, presence or absence of a 

 mate and so on) exert their effects through the nervous 

 system. And third, it appears likely that the hypo- 

 thalamus acts as an integrating mechanism whereh\' 

 the effects of afferent nerxous impulses derived from 

 sensory stimuli, changes in the emotional or psycho- 

 logical state and perhaps from changes in the concen- 

 tration of target-organ hormones in the blood are 

 correlated and coordinated and the resultant 'stimu- 

 lus' to the adenohvpophysis transmitted down the 

 'final common path' of the pituitary stalk. The 

 hypothalamus seems to be in a key position for inte- 

 grating not only patterns of endocrine acti\'ity but 

 also patterns of emotional behavior, and it is possible 

 that these two functions are closely linked (cf. the 

 effect of estrogens on patterns of sexual behavior and 

 discharge of gonadotrophic hormone, or the effect of 

 emotional exciteinent on the discharge of anterior 

 pituitary hormones and adrenal medullary hormones) 

 at this forebrain level. 



NEUROHYPOPHYSIS 



Recent and major advances in the field of central 

 control of the neurohypophysis come from work on 

 neurosecretory mechanisms, and from studies on 

 blood osmotic pressure in relation to posterior pitui- 

 tary activity and on nervous reflex release of oxytocic 

 hormone in relationship to lactation. The question of 

 neurosecretion is dealt with in Chapter XL by Ort- 

 mann in this Handbook and will not receive direct 

 attention here. The remainder of this chapter will be 

 dex'oted to an account of the anatomy of the hypo- 

 thalamoneurohypophysial system and of the changes 

 in the internal and external environment which 

 reflexly modify the rate of secretion of posterior 

 pituitary hormone or hormones by acting through 

 this system. 



Anatomy of Hvpol/ialamoneiirohvJ'o/i/n'iifil System 



According to the terminology of Rioch et al. (284), 

 which is accepted as the standard nomenclature for 

 the hypopliNsis and its various subdivisions, the 

 neurohypophysis consists of three parts : the infun- 

 dibular process (lobus nervosus or neural lobe), the 

 infundibular stem and the median eminence of the 

 tuber cinereum. There are manv grounds for believ- 



ing that the neural lobe, the infundibular stem and 

 the expanded upper end of the stalk or median emi- 

 nence are composed of tissue of a uniform type and 

 different from that of the hypothalamus proper. 

 Evidence for vital staining (361), vascular supply 

 (126, 358, 360), embryology (330) and cytology (120, 

 346) may be cited in support of this view. There is 

 present then a 'gland' which, from the viewpoint of 

 the naked eye, consists of the neural lobe in the sella 

 turcica, the neural tissue of the pituitary stalk and the 

 expanded upper end of the stalk (median eminence). 

 All this tissue is innervated by the supraopticohypo- 

 physial tract, originating in the para\entricular and 

 supraoptic nuclei in the hypothalamus. According to 

 the neurosecretory theorv, the hormones of the pos- 

 terior pituitary gland are formed in these two nuclear 

 groups, transported down the axons of the tract and 

 stored, or liberated into the blood as required, in the 

 three parts of the neurohypophysis. There can be no 

 doubt that the rate of hormonal liberation is con- 

 trolled by ner\e impulses in the supraopticohypo- 

 physial tract, the fibers of which then may possess the 

 dual function of a transport system and a secretomotor 

 innerx'ation. If the above views are correct, the ques- 

 tion may be asked as to why signs of glandular 

 deficiency occur (diabetes insipidus, failure of the 

 milk-ejection reflex, as described below) when the 

 supraopticohypophysial tract is sectioned between the 

 levels of the nuclei or origin of the tract and the 

 neurohypophysis (^4 in fig. 8). It might be argued 

 that if the hormones are formed in the cells of the 

 paraventricular and supraoptic nuclei, liberation 

 could occur directly from these cells into the surround- 

 ing capillaries, especially since these two nuclei are 

 ainong the most highly vascularized in the brain. 

 Two possible reasons suggest themselves. First, section 

 of the supraopticohypophysial tract is followed by 

 retrograde degeneration and loss of many cells in the 

 supraoptic and paraventricular nuclei (see 156; 275, 

 p. 193). Possibly too few secreting cells are left to 

 pre\'ent hormonal deficiency. And second, the blood 

 vessels of the hypothalamus proper may be imperme- 

 able to the polypeptide (or larger) molecules com- 

 prising the hormones. The permeability of the hypo- 

 physial vessels seems to be very different from that 

 of the hypothalainus (361) and it is possible that, 

 even if sufficient hormone is formed in hypothalamic 

 neurones, it would be unable to pass into the blood 

 stream unless transport to the neurohypophysis is 

 possible. 



The anatomy of the nerve supply to the neuro- 

 hypophysis was first described by Ramon )■ Cajal 



