i 5 i6 



IIWDBOOK OF l'IIVSIiilii[;V 



NEUROPHYSIOLOGY III 



ing, micturition and a normal pattern of defecation, 

 including crouching and covering; a normal sequence 

 of responses leading to and including sleep; rage and 

 defensive reactions, flight and changes in feeding. 

 While some of these responses may he produced upon 

 stimulation of other parts of the central nervous 

 system, the importance of the dicncephalon is striking. 



While Hess has emphasized the view that func- 

 tional areas of the dicncephalon overlap and thus are 

 poorly localized, the investigations of others offer 

 evidence for much more discrete localization of 

 function. Two general conclusions seem clear from 

 these studies, a) There are discrete regions within the 

 hypothalamus where marked changes can be experi- 

 mentally produced in different types of motivated 

 behavior, b) Some of these regions are excitatory in 

 that they yield increases in motivated behavior upon 

 stimulation and decreases in motivated behavior 

 upon ablation; others, by the same criteria, are in- 

 hibitory. 



Both excitatory and inhibitory regions are found 

 in studies of hunger and sleep. In hunger, bilateral 

 lesions in the vicinity of the ventromedial nucleus 

 of the hypothalamus produce marked hyperphagia, 

 a doubling or a tripling of food intake (30); similar 

 lesions more lateral in the lateral hypothalamus re- 

 sult in starvation (2, 157). Confirmation of the con- 

 clusion that the medial region is inhibitory and the 

 lateral excitatory is brought by the fact that stimula- 

 tion of the medial region through chronically im- 

 planted electrodes in the rat markedly depresses food 

 intake whereas lateral stimulation elevates it sig- 

 nificantly (85, 145). (See Brobeck's presentation of 

 this matter in Chapter XIA'Il of this work.) In the 

 case of sleep, discrete bilateral lesions in the pos- 

 lerior hypothalamus in the region of the mammillary 

 bodies cause somnolence (112, 121) while anterior 

 hypothalamic lesions in the preoptic region yield 

 persistent wakefulness (112). Thus, there is a pos- 

 terior excitatory mechanism and an anterior in- 

 hibitory mechanism for wakefulness. Stimulation 

 studies so far have revealed successful induction of 

 sleep only when electrodes are located in the massa 

 intermedia of the thalamus (70), suggesting an addi- 

 tional inhibitory area or another route for reaching 

 the inhibitor) mechanism physiologic-lily.- (This sub- 



• It is interesting to note, however, tli.it stimulation of the 

 medial reticulai formation in tli<- lower medulla of the 1 .ii 

 will elicit the postural adjustments <>l sleep without any sign 

 1li.1i i is ,K tually asleep I 1 171 Thus il sccrns possible 

 to eparati thi responsi mechanism ft the mechanism me- 

 diating the arousal and satiation of motivated behavioi in ex- 

 pei imental prot edures of this 1 n 1 



ject is also discussed by Lindslcy in Chapter LXIY 

 of this Handhook.) 



Several additional points of interest have come 

 from further studies of these excitatory and inhibitory 

 mechanisms, a) In both sleep and hunger, the at- 

 tempt has been made to ablate the excitatory and 

 inhibitory regions in one preparation (2, 112); the 

 result in each case was that the symptoms were the 

 same as those produced by destruction of the excita- 

 tory mechanism alone, namely somnolence and 

 starvation. These findings have suggested the possi- 

 bility that the main influence of the inhibitory area is 

 upon the excitatory mechanism, but they do not rule 

 out the possibility that both the excitatory and in- 

 hibitory mechanisms function through a common 

 structure below the dicncephalon. h) The hyper- 

 phagia produced by ventromedial hypothalamic 

 lesions is permanent whereas there is recovery from 

 the starvation produced by lateral lesions if the ani- 

 mals are maintained for several weeks by forced 

 feeding (157). In the case of somnolence following 

 posterior hypothalamic lesions, there is also recovery, 

 with some capacity to maintain wakefulness return- 

 ing gradually over a period of several weeks (121). 

 These findings suggest additional mechanisms for 

 maintaining wakefulness and feeding behavior. 



So far only an excitatory mechanism has been re- 

 vealed in the study of thirst. Stimulating the dicn- 

 cephalon of the goat electrically or with minute 

 quantities of hypertonic saline directly injected 

 through an implanted pipette will promptly produce 

 great drinking in water-satiated goats (3, 4, 5). The 

 critical area is dorsal to the infundibulum between the 

 fornix and the mammillothalamic tract, just lateral 

 to the paraventricular nucleus, midway between the 

 dorsal and ventral hypothalamus. Similar results 

 have been reported in two investigations using the 

 1. u Hi 3, in-,). In addition, ablation of the same area 

 in the dog produced adipsia from which there was 

 recovery after 14 da\ s (hi. 



In the case of sexual behavior, bilateral lesions in 

 the ventral portion of the anterior hypothalamus be- 

 tween the optic chiasm and the st.dk of the pituitary 

 will abolish sexual behavior in the female guinea pig, 

 even following the injection of gonadal hormones 



, 52). Similar data have been reported on the 

 male guinea pig (34) and on rats ol both sexes (42). 



Furthermore, upon stimulation of the lateral pre- 

 optic area in male and female rats l>\ injection of 

 minute amounts nl appropriate sex hormones through 



.111 implanted pipette, long-lasting sexu.il beh.i\ioi 



w.is elicited (56). Thus, there is evidence for an ex- 

 citatory mechanism fot sexual behavior in the hypo- 



