1202 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



feeding reflexes directly or through the reticuhTr 

 formation. It is conceivable that the lateral hypo- 

 thalamus or appetite mechanism serves to facilitate 

 the feeding reflexes, while the medial hypothalamus 

 or satiety mechanism acts to inhibit the reflexes. In 

 a fasting condition the lateral portion would be active, 

 the medial one quiet, with a resulting high appetite 

 and low satiety. But after feeding the lateral portion 

 would be quiet and the medial one active — low ap- 

 petite and high satiety. When the medial mechanism 

 is injured and hyperphagia follows, any remaining 

 regulation probably occurs through variations in 

 activity of the lateral portion, that is, through changes 

 only in appetite. This may explain why food intake 

 is so labile in animals with hyperphagia, as Kennedy 

 (49), Stevenson (63) and Teitelbaum (85) have ob- 

 served. 



The highest ie\el of the brain, the cortex, is also 

 involved in feeding responses, as the early paper by 

 Paget (71) and the later review by Kirschbaum (51) 

 emphasize. There are, however, not many experi- 

 mental data relating to this subject with the excep- 

 tion of certain p.sychological studies on animals, es- 

 pecially primates. Ob.servations by Pribram & 

 Bagshaw (72) using monkeys imply that the cortex is 

 necessary for recognition of objects suitable for food 

 and for feeding responses determined by social en- 

 vironment. Since the work of Goltz (38) it has been 

 known that decorticate animals are restless at feeding 

 time and quiet after they are fed (76). Similar ob- 

 servations have been made upon infants born with 

 cerebral lesions or an imperfectly developed brain. 

 If the lesions are confined to the cortex, the babies 

 seem to have little difficulty in nursing. Indeed, even 

 in normal infants there is no reason to suppo.se that 

 feeding requires the highest levels of the nervous 

 system. Later, as more complicated behavior patterns 

 appear, the cerebral cortex is more definitely im- 

 plicated. 



CENTR.\L PERCEPTION 



All of the prominent hypotheses regarding regula- 

 tion of feeding and drinking are alike in that they 

 assume that the brain, probably the hypothalamus, 

 contains sensitive or 'sensory' elements capable of 

 responding to the 'qualities of the circulating blood' 

 mentioned by .Sherrington. Among the qualities or 

 'signals' proposed are water concentrations (9, 48), 

 availability of glucose (59, 60), metabolites related 

 in concentration to the size of bodilv reserves of fat 



(50) and thermal gradients (18, 83). .Although certain 

 authors have been inclined to look upon .some one of 

 these as the basis of the regulation of, for example, 

 feeding, others have decided that multiple factors are 

 responsible for the transition from appetite to satiety 

 (5, 18, 46). Of the changes mentioned, there is little 

 question that animals are hungry when the brain is 

 lacking in carbohydrate supply, as it is during insulin 

 hypoglycemia; but if a lack of available glucose is to 

 be used to explain the enhanced food intake of diabetes 

 mellitus (59), then the "feeding centers' of the hypo- 

 thalamus must differ from the rest of the brain in that 

 they require insulin as muscle does for normal utiliza- 

 tion of carbohydrate. Similarly, it is certain that most 

 animals cannot eat when they are dehydrated, and 

 also that after feeding there is a movement of fluid 

 out of the rest of the body into the digestive tract (40, 

 55). But whether this movement can provide a signal 

 that eating has taken place is not established, although 

 it seems plausible. Again, food intake is responsive to 

 changes in environmental temperature and to the 

 circumstances of temperature regulation within the 

 body (18); that the heat released during assimilation 

 of food is a signal to the hypothalamus, however, is 

 not accepted by all investigators. Finally, the main- 

 tenance of a constant depot of fat within the body 

 takes place only under certain limited conditions of 

 feeding and environment (45). In at least two species 

 of animals the fat stores depend upon genetic strain 

 and upon fat concentration in the diet (34, 61). One 

 cannot state that any one of these possible 'signals' 

 is not important; but it does appear that certain of 

 them are more suitable for regulation than others. 

 Shifts of water occur during digestion, for example, 

 no matter what the composition of the diet, and there 

 are also definite water requirements associated with 

 regulation of body temperature and with activity, 

 as well as with storage of protein and, to a lesser 

 extent, of fat. Water might, therefore, pro\ide a 

 common denominator necessary for interrelationships 

 in control of feeding, drinking, body temperature, 

 activity, body size and energy intake (28). The extra 

 heat of metabolism of food, the specific dynamic 

 action (S.D.A.), ofl"ers similar advantages, in addition 

 to being responsive to the metabolic state of the body 

 in a fashion that could explain alterations in food 

 intake following starvation and during growth, and 

 when the composition of the diet is changed. High 

 protein diets have a high S.D.A. and animals rarely 

 become obese on such a diet, while high fat diets pre- 

 dispose to obesity in some animals and have a lower 

 S.D.A. It is not necessary to inquire further into this 



