992 



HANDBOOK OF PHYSKJLOGV 



NEUROPHYSIOLOGY II 



criticized by Freund & Sheehan (156) on the basis of 

 the extreme difficulty of obtaining a complete decen- 

 tralization of the abdominal ganglia. In animals 

 sympathectomized according to Cannon, they were 

 not able to obtain any intestinointestinal reflexes. 

 However, the experiments of Freund & Sheehan do 

 not give any explanation for the significant observa- 

 tion of Kuntz that intact nerve fibers remain in the 

 distal stump of the cut colonic or mesenteric nerves 

 after degeneration of th; fibers coming from the 

 ganglia. If regeneration can be excluded, as claimed, 

 the undegenerated fibers must arise from neurons in 

 the intestinal wall and may thus be the afferent link 

 in the reflexes observed. 



The view of Kuntz ihat there_a£e_afiJererLt neurons 

 in the intestinal wall synaptically connected to effer- 

 ents in the prevertebral ganglia has support from 

 recent heurophysiological studies. On stimulation of 

 the central stump of the cut hypogastric nerve in the 

 cat Job & Lundberg (230, '231) recorded reflexes in- 

 volving C fibers in both the contralateral and the 

 ipsilateral colonic and hypogastric nerves. By de- 

 generation experiments it was shown that the fibers 

 giving rise to the C reflexes must have their trophic 

 centers distalwards and that they terminate in the 

 inferior mesenteric ganglia. Brown & Pa.scoe (54) and 

 McLennan & Pascoe (310) have made similar ob- 

 servations in the rabbit. In the mesenteric nerves 

 leaving the inferior mesenteric ganglion they showed 

 the presence of a distinct group of nerve fibers with 

 cell bodies possibly located in the gut and terminating 

 in the ganglion. The fibers belonging to this group 

 have a remarkably low conduction velocity (0.25 m 

 per sec.) and are connected to cells with axons of the 

 common C type returning along the same nerves. 



The studies of the inferior mesenteric ganglia seem 

 to make it necessary to postulate the existence of re- 

 flex systems involving synaptically connected neurons 

 wholly confined to the peripheral autonomic ner\ous 

 system. 



The enteric plexuses have long been regarded as 

 holding a unique position in the autonomic nervous 

 system. It may be assumed that local reflexes can take 

 place here, but the nature of and the anatomical sub- 

 stratum for such reflexes are unknown. On the basis 

 of the old experiments made by Magnus on ganglion- 

 free intestinal walls it has often been concluded that 

 the spontaneous contractions of isolated intestinal 

 segments are of nervous origin. However, all the 

 subsequent work in this field has clearly shown this 

 conclusion to be invalid. In the recent careful study of 

 Evans & Schild (138), it was demonstrated that 



plexus-free circular muscle of cat jejunum exhibits 

 rhythmic activity in response to raising of the intra- 

 luminal pressure. It is very difficult to decide to what 

 extent the movements of the intestinal wall are de- 

 pendent on intraganglionic reflexes. Much work has 

 been done using nicotine to paralyze the intramural 

 ganglia but the results are controversial, and it is now 

 well established that this drug has effects on many 

 structures other than ganglia (cf. 131, 138, 144, 419). 

 From studies of the action of the ganglion-blocking 

 agent hexamethonium Feldberg (144) concluded 

 that some spike-like contractions and contractions of 

 composite character are ganglionic in origin. On the 

 other hand, hexamethonium did not affect the large, 

 rhythmic contractions of the longitudinal muscle in 

 rabbit ileum preparations. However, this drug may 

 have effects on smooth muscle also (139). 



EFFECTS OF DECENTRALIZATION OR DENERVATION 

 ON AUTONOMIC EFFECTORS 



Effects on Structure and Activity of Effectors 



No gross anatomical changes are seen after sym- 

 pathectomy even if denervation is performed early in 

 life (71, 309, 412) and the tissues show, with some 

 exceptions, a normal histological appearance (85). 

 Denervated smooth muscles do not atrophy or de- 

 generate, and no obvious cytological changes have 

 been found in the cells. Sweat glands have been re- 

 ported to atrophy (414) or to lae unaffected (85, 184, 

 229) after sympathectomy. Cytological changes, in- 

 considerable to very marked, and even a substantial 

 atrophy may develop in salivary glands with severed 

 secretory nerves (11, 128). The dener\ated adrenal 

 medulla, on the other hand, has quite normal cy- 

 tology (199, 208). 



It is not as yet possible to explain why some effector 

 cells atrophy after section of their autonomic nerves. 

 It seems rea.sonable, however, that the structural 

 changes found in some glands are wholly due to lack 

 of adequate secretory stimuli. This v'iew is supported 

 by the findings that it is possible to obtain the same 

 histological changes in the cat's submaxillary glands 

 both by atropine treatment and by severing the chorda 

 tympani, and that pilocarpine reverses the changes in 

 decentralized glands (132). 



Neither decentralization nor denervation seem to 

 alter to any appreciable extent the basic properties of 

 smooth muscle or gland cells, as chemical stimuli may 

 e\okc the same maximal contractile or secretory re- 



