CENTRAL CONTROL OF DIGESTIVE FUNCTION 



I 165 



proprioceptive reflexes would then cause the rhyth- 

 micity in the movement. That it is possible to produce 

 rhythmical chewing on stimulation of subcortical 

 areas was shown by Hess & Magnus (47). The regions 

 stimulated by these authors were localized in the 

 \entral nucleus of the thalamus, in the hypothalamus 

 and in the septal areas. 



In addition to these observations wiiich may per- 

 tain to different parts of the same pathway, Rioch & 

 Brenner (77) showed that irregular licking, chewing 

 and swallowing could be elicited by electrical stimu- 

 latitjii of the region of the tuberculum olfactorium 

 and the lobus pyriformis. Similar obser\ations had 

 been made earlier by .Schaltenbrand & Cbbb (81). 

 Gastaut et al. (37), Vigourou.x et al. (91) and Kaada 

 (50) found that the effect was mainly due to acti\a- 

 tion of the amygdaloid nuclei or efferent fibers from 

 this region. Kaada et al. (51 ) localized the responsive 

 region to the anteromedial group of the amygdaloid 

 nuclei. 



In almost all of the ob.servations pertaining to this 

 second .system, it has been noted that there is a long 

 latent period before the onset of the chewing respon.se 

 (36). This is of considerable interest when we compare 

 the results of similar stimulation in man. It was ob- 

 served by Jack.son (89) that there is a type of epilepsy 

 characterized by masticatory seizures. These seizures 

 have their focus in the region of the temporal lobe as 

 described by Penfield & Jasper (72) and by Magnus 

 el al. (59). Mastication is not a part of the initial 

 seizure but belongs rather to the accompanying autom- 

 atisms and does not occur when the patient is con- 

 scious. The most likely region involved would be the 

 amygdaloid nuclei and these have connections with 

 the septal areas, the hypothalamus and the mid- 

 brain (61). The masticatory aspect of the seizure may 

 then be due to secondary activation of more deeply 

 situated regions, possibly the same as those Hess & 

 Magnus (47) and Magnus (58) have found to re- 

 spond to electrical stimulation in the septal and hypo- 

 thalamic areas. It is interesting to note that it has 

 not been possible in man, in spite of frequent at- 

 tempts, to reproduce the constant activation of rhyth- 

 mic chewing that occurs in animals. Removal of the 

 cortical areas implicated either in man or experi- 

 mental animals does not normally seem to interfere 

 with masticatory mechanisms. Schaltenbrand & 

 Cobb (81) ob.served no deviation from the normal 

 feeding pattern after bilateral hemispherectomy in 

 cats. If, on the other hand, the animals are decere- 

 brated, mastication is severely impaired (13). Food 

 placed in the mouth of the animal will stay there 



indefinitely. Destruction of the temporal areas does 

 not affect digestive function in animals (73). 



It must then be considered highly questionable if 

 the amygdaloid areas normally have anything at all 

 to do with the chewing mechanisms. The role of the 

 motor cortex is doubtful in man. As first suggested 

 by Magnus et al. (59), mastication, like walking, may 

 be one of those automatic functions which in man 

 have become localized in subcortical regions. 



SW.IlLLOWING 



The act of swallowing is a mechanically compli- 

 cated act wliich in the higher mammals involves some 

 20 muscles which act together in groups. The process 

 does not depend on activation of these muscles in a 

 certain order, as shown by the fact that destruction 

 of one muscle does not affect the act of swallowing 

 appreciably (25). Nor is there then reason to believe 

 that proprioceptive fibers from one muscle facilitate 

 the activation of the next muscle in the pattern. 

 Meltzer (63) postulated in 1899 that the orderly 

 progress of deglutition must be of central origin. He 

 thought that afferent impulses arriving at the center 

 of deglutition traveled through this in orderly 

 fashion, activating cell groups innervating the pharyn- 

 geal muscles and the esophagus. It had been es- 

 tablished by Pommerenke (74) that swallowing was 

 impossible without an afferent stimulus and Meltzer 

 added the observation that, once initiated, the swal- 

 lowing act did not require further afferent stimula- 

 tion. In order to explain the central mechanism of 

 the swallowing act we would have to assume that the 

 afferent stimuli set up an excitatory process in the 

 motor nuclei, either through specific pathways or 

 through a diffuse interneuronal system (25). The 

 small changes that occur from swallow to swallow 

 which were observed by Doty & Bosma with electro- 

 myography make it unlikely that specific pathways 

 could exist. We would still have to assume that there 

 is some regulating factor which gov-erns the delay in 

 the interneuronal system so that the impulses arrive 

 at the appropriate motor nuclei in orderly fashion. 



The nature of this regulating factor remains un- 

 known. Stimulation of the posterior part of the 

 pharynx does not evoke only swallowing responses. 

 Retching, gagging and salivation can also occur and 

 it is conceivable that psychic factors or .sensory stimu- 

 lation from taste and smell receptors may be neces- 

 sary to determine the type of response to afferent 

 stimulation. We have some experimental evidence 



