IOI2 HANDBOOK OF PHYSIOLOGY ^ NEUROPHYSIOLOC;Y II 



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FIG. 3. To compare different systems in which humoral 

 transmission of stimuli may occur: /) autonomic nervous 

 system, 2) neuromuscular ending, j) sympathoadrenal medul- 

 lary ending. In these three systems it is established that a 

 cholinergic (A.C.) or adrenergic (A) substance is liberated from 

 the nerve terminal and acts directly on the effector cell. 4) 

 Hypothalamoadenohypophysial system — in which the evidence 

 indicates that a short \ascular pathway intervenes between the 

 nerve terminal and effector cell, situated in the anterior pitui- 

 tary. 5) Hypothalamoneurohypophysial system — in which a 

 long vascular path ithe systemic circulation) intervenes be- 

 tween the nerve terminal in the neural lobe and the effector 

 cells in the kidney, breast or uterus. 



the nerve fibers of the liypothalanius liberate some 

 humoral suljstance or substances into the primary 

 plexus of the portal vessels in the median eminence 

 which is carried to the pars distalis of the pituitary 

 vvhere it exerts a controllinE; action. However, as has 

 been stated (157), ■"... the neurohumoral view 

 . . . will only be established if it is possible to firstly 

 identify a particular substance which exerts a direct 

 action on anterior pituitary cells; secondly, to show 

 this substance is present in the Ijlood in the hy- 

 pophysial portal ves.sels in greater amount than in 

 systemic blood; thirdly, to show that the concen- 

 tration of this substance in the blood of the hy- 

 pophysial portal vessels varies according to electrical 

 or reflex activation of hypothalamic nerve tracts; 

 and fourthly, to demonstrate that the activity of the 

 adenohypophysis is correlated witli this varying 

 concentration." None of the substances that have so 

 far been investigated may be said to fulfil these 

 criteria. However, the possibility that such sub- 

 stances exist is rendered more likely from what is 

 known regarding chemical transmission in the auto- 

 nomic nervous system and at the neuromuscular 

 junction, and from present theories regarding neuro- 



secretion and the posterior pituitary gland (see fig. 3). 

 If the above view is correct, then the hypothalamo- 

 adenohypophysial unit would seem to occupy an 

 intermediarv position between these other two 

 mechanisms. 



HUMOR.AL STIMUL.iiTION OF GONADOTROPHIC SECRETION. 



Taubenhaus & Soskin (325) were the first to suggest 

 that the release of luteinizing hormone (LH) from 

 the pituitary was controlled bv a humoral mechanism 

 acting through the hypophysial portal vessels, Markee 

 ('/ al. (219) proposed that an adrenergic mechanism 

 controlled LH secretion. They found that injection of 

 epinephrine into the pituitary gland of rabbits, by 

 means of a parapharyngeal operative approach, 

 resulted in ovulation in a proportion of cases — in 5 

 out of 10 rabbits following injection of about 100 

 Hg epinephrine (three injections of 40 /jl each of 

 I ' 1000 solution over the course of 30 min.). Donovan 

 & Harris (82) repeated this work, injecting the gland 

 through a hypodermic needle orientated stereo- 

 tactically and using a very slow rate of injection 

 (0.0002 ml per min. for 50 to 100 min.). They found 

 3 out of 16 rabbits ovulated when 60 to 120 /ag 

 epinephrine (bitartrate) was injected into the pars 

 distalis, and o out of g when 60 to 120 /ig norepi- 

 nephrine bitartrate was injected. In an attempt to 

 perfuse the pars distalis more completely with the 

 injected solution, injections were made into the 

 median eminence of the tuber cinereum on the 

 grounds that the solution might difTuse into the 

 primary plexus of the portal vessels and thereby be 

 widely and rapidly distributed in the anterior pitui- 

 tary. The striking feature of the results of injecting 

 epinephrine and norepinephrine solutions at this site 

 was the progressive reduction in number of positive 

 responses as the rate of injection of the solution was 

 decreased and as the pH of the solution was adjusted 

 to near neutrality. Control injection of isotonic sodium 

 chloride, tartaric acid and acetylcholine also resulted 

 in ovulation in 7 out of 25 cases. It is clear that the 

 correlation of the ovulation response with the chemi- 

 cal structure of substances injected into the tuber 

 cinereum must be made with caution. 



Pharmacological blockade of reflexly induced, or 

 spontaneous, o\ulation has been studied in an at- 

 tempt to define a humoral excitant of the anterior 

 pituitarv gland. Markee and his co-workers found 

 that various sympatholytic agents blocked coitus- 

 induced ovulation in the rabliit providing they were 

 injected within 90 sec. of the mating act (295, 296). 

 Atropine was also efTectixe in this respect if injected 



