HANDBOOK OF PHVSIOLOG^• 



NEUROPHYSIOLOGY II 



brain most definitely involved is the reticular forma- 

 tion of the brain stem and the hypothalamus. In their 

 representation within the brain, hunger and thirst 

 are related to other phenomena such as posture and 

 locomotion, control of pulmonary ventilation, the 

 sleep-waking cycle and regulation of body temper- 

 ature. All of these appear to be regulated by mecha- 

 nisms of the general class of controls systems — systems 

 which are analyzed by engineers using techniques de- 

 rived from information theory. This type of study is 

 difficult in biological systems and has not as yet been 

 developed fully for any one regulation. As a way of 

 thinking about these phenomena, it seems to offer 

 the advantage of a hope of mathematical analysis, 

 and it also encourages the study of a regulation by 

 means of analogy with a system better understood. 

 As Sherrington mentioned, a useful analogy is con- 

 trol of pulmonary ventilation, since it has many char- 

 acteristics in common with regulation of eating and 

 drinking (77). Like breathing, feeding and drinking 

 are periodic and rhythmic phenomena, subjected to 

 reflex control, with integration and possibly 'motiva- 

 tion' from the brain stem. All levels of the nervous 

 system, including the cerebral cortex, take part in the 

 regulation. The quantities regulated — minute volume 

 and food or water intake, respectively — are in each 

 case the product of a frequency multiplied by a quan- 

 tity. Respiratory minute volume is the product of 

 tidal air multiplied by respiratory rate, while food 

 intake is a product of frequency of feeding and size 

 of meals. In the brain stem the inspiratory and ap- 

 petite centers are analogous, while the expiratory and 

 satiety mechanisms appear to be similar. Both regula- 

 tions have their central mechanisms of control, yet 

 they are each affected by specific reflexes from par- 

 ticular organs. For example, reflexes from chemocep- 

 tors and from stretch receptors in the respiratory 

 tract are important in respiration, while reflexes be- 

 ginning with olfaction and taste, together with those 

 from receptors in the wall of the digestive system, 

 take part in the control of feeding. The inhibition of 

 feeding when the stomach is distended may be analo- 

 gous to the inhibition of inspiration when the lungs 

 are stretched, the Hering-Breuer reflex. And finally, 

 both of these regulations are associated with painful 

 sensations designated as hunger, e.g. air hunger. It 

 is necessary to point out that study of pulmonary 

 ventilation has gone forward with little attention to 

 air hunger. As painful and as dramatic as this sensa- 

 tion may be, it does not appear in the present-day 

 schemes of the control of respiration; its function may 

 be limited to periods of exceptional respiratorv need. 



Whether gastric hunger is similar in its significance is 

 not known. It is possibly most important in babies 

 where the crying it causes helps to secure for the 

 infant his mother's attention when food is needed. 



In control of pulmonary ventilation the respiratory 

 minute volume is regulated by way of the arterial 

 tension of carbon dioxide, the pH of arterial blood 

 and, apparently under unusual circumstances, by 

 the oxygen tension. Students are often surprised to 

 learn that a process so essential to life as is the supply 

 of oxygen should be regulated, almost incidentally 

 as it were, through control of carbon dioxide export. 

 A similar situation exists for feeding mechanisms; 

 although the end result of regulation is control of 

 energy intake, animals apparently have no mecha- 

 nisms for measuring energy per se. They do not meter 

 calories ingested but attain energy balance indirectly 

 through reactions that are related to or proportional 

 to energy need. The several hypotheses put forward 

 to explain this are reviewed later. 



Adolph (3) seems to have introduced into the liter- 

 ature of this field the idea that animals show priori- 

 ties, competition and compromises in their regulation 

 of the several variables contributing to homeostasis. 

 Considering the regulations where the tegmentum 

 of the brain stem plays a part, one can say that pul- 

 monary ventilation has first priority, then body tem- 

 perature, body water and energy intake, in that order. 

 The exchange of respiratory gases must be regulated 

 almost from moment to moment, and errors, es- 

 pecially for oxygen, cannot be tolerated for the con- 

 venience of some other regulation; there is little inertia 

 in the system for oxygen because of small stores within 

 the body. Heat exchange is less demanding, especially 

 in larger animals where heat content of the body 

 offers inertia in the direction of both gain and loss; 

 errors are corrected in minutes or hours, in small or 

 large animals, respectively, and rate of production 

 can be balanced against rate of loss to compensate for 

 limitation of one or the other. For water exchange 

 the correction may require up to a day or longer; in 

 man a load or deficit can be tolerated for days, and 

 panting animals incur dehydration to prev-ent over- 

 heating. With respect to food intake and energy 

 balance even longer intervals are po.ssible, and balance 

 is readily sacrificed to maintain either body temper- 

 ature or pulmonary ventilation, or even to accomplish 

 muscular exercise. These are not independent vari- 

 ables in a rigid hierarchy, as one can see by observing 

 changes in ventilation accompanying or even neces- 

 sary for eating and drinking. This competition among 

 regulations and the influence of one regulation upon 



