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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



there are other prol)lems as yet unsolved. Langley 

 (268) had already pointed out the peculiarity that 

 relatively few preganglionic fibers were distributed to 

 such large territories as the esophageal and enteric 

 plexuses. It was not fully understood, however, what 

 a discrepancy exists in the proportion between the 

 preganglionic fibers and the postulated postganglionic 

 neurons, until Evans & Murray (136) obtained more 

 reliable quantitative data for the rai)bit vagus. Only 

 about 3000 efferent fibers from both vagi together 

 seem to enter the abdominal cavity, that is to say 

 about as many as the preganglionic fibers supplying 

 the ciliary ganglion in the cat (442). This stands in 

 sharp contrast to the number of enteric nerve cells 

 which probably amount to several million (cf. 223, 



375)- 



Several neurohistological studies have clearly shown 



that preganglionic vagal axons enter the intramural 

 ganglia in the gastrointestinal canal probably making 

 synaptic connections with at least some of the nerve 

 cells (177, 180, 205, 251, 281, 402). It is often claimed 

 that the vagal fibers synapse with only one of the 

 cell types and that the other neurons are some kind 

 of associative elements connected with each other via 

 dendrites and axon collaterals. Even more detailed 

 constructions of the relations between the different 

 elements can be found in papers concerning gastro- 

 intestinal innervation (226). However, these state- 

 ments are based on interpretations of histological 

 structures. Many of the observed structures have not 

 as yet been conclusively shown to have either func- 

 tional significance or real existence in the living 

 tissues. Others may be accepted as having both at- 

 tributes but the interpretations concerning their func- 

 tions cannot be accepted as conclusive without further 

 evidence. (The difficulties inherent in the neuro- 

 histological methods are discussed later in the section 

 on autonomic effector junctions.) There does not exist 

 any view, generally accepted in neurohistology, of 

 the general organization of the pre- and postgangli- 

 onic elements constituting the enteric plexuses. The 

 neurohistological observations show, however, that 

 very complicated arrangements may exist. In physi- 

 ology it is generally assumed that the vagal pre- 

 ganglionic fibers synapse with enteric neurons which 

 give rise to postganglionic fibers innervating the 

 smooth musculature and digestive glands. It has been 

 suggested that .some of the postganglionic neurons 

 may be adrenergic (7, 323) or that they may have an 

 atropine-resistant transmitter of unknown nature 

 (144, 421). No evidence is as yet available that gives 

 any clue to an understanding of the intraganglionic 



connections. There are rea.sons to believe, howe\er, 

 thai intrinsic nerv-ous mechanisms may play a role in 

 coordinating the more complex inovements of the 



stomach and intestines (see the later section on auto- 



I 

 nomic activities independent of the central nervous) 



system). 



The sacral parasympathetic emerges from cells in 

 the intermedial region of the upper or middle sacral 

 segments (Si to S3 or S2 to S4, table i ) and the pre- 

 ganglionic fibers run via the anterior primary division 

 of the corresponding sacral nerv-es to the pelvic 

 plexuses. They relay in ganglia in the pelvic plexuses 

 or in the intramural ganglia of the bladder and distal 

 colon. As for the vagal parasympathetic system, the 

 connections between the pre- and postganglionic 

 elements and the intrinsic nervous mechanisms are 

 on the whole unexplored. 



It has generally been assumed that the pelvic 

 ganglia, except those supplying the internal repro- 

 ductive organs, belong to the sacral autonomic out- 

 flow only. An experimental analysis in the cat em- 

 ploying neurohistological methods (252, 322) has 

 given at least suggestive evidence, however, that not 

 only the pelvic but also the intramural ganglia of 

 the bladder receive some of their preganglionic 

 fibers from the sympathetic division. This may be 

 taken as an illustration of the difficulties and com- 

 plications met with in investigations of the nervous 

 mechanisms regulating visceral functions. 



Many of the essential proi)lems concerning the 

 anatomy and physiology of the autonomic nervous 

 system which are impossible to consider in a brief 

 survey are extensively discussed in recent mono- 

 graphs (251, 317, 318, 439). 



The peripheral distribution of an axon from a single 

 postganglionic neuron is but little known. Histo- 

 logical studies (103) show that the postganglionic 

 fibers may branch soon after leaving the ganglia. 

 In view of the fact that relatively few fibers must give 

 innervation to a large number of cells, it is reasonable 

 to assume that a further and probably extensive 

 branching occurs at the periphery so that a single 

 neuron may activate a large number of effector cells. 

 Studies of postganglionic axon reflexes (see the section 

 on activities of the peripheral autonomic nervous 

 system independent of the central nervous system) 

 do indeed give evidence for the existence of a terminal 

 branching axon system supplying several sweat glands 

 within skin areas up to a size of 2 cm. It is of interest 

 that Downman (iio) in his study of the distribution 

 of inesenteric vasoconstrictor and inhibitorv fibers 



