PERIPHERAL AUTONOMIC MECHANISMS 



997 



This inability to produce exciting junctions in spite 

 of very favoraljle conditions for regeneration (myeli- 

 nated postganglionic fibers and a ratio of pregan- 

 glionic fibers to postganglionic neurons of about i :2) 

 needs explanation (cf. 206, 300). 



STRUCTURE AND FUNCTIONAL ORGANIZATION OF 



AUrONOMIC EFFECTOR JUNCTIONS 



The neurohistological investigations of the last 

 twenty years have given rise to many highly diver- 

 gent opinions on the structure of the autonomic 

 innervation apparatus. The nonspecificity of silver 

 impregnations is vs-ell-known and it has been clearly 

 shown that the fixation procedure and other treat- 

 ments of the tissues when using these methods give 

 rise to misleading artefacts (203, 206, 210, 243, 280, 

 435). In spite of this, many prominent neurohis- 

 tologists, for instance Stohr, Boeke and their students 

 (see e.g. 30, 403), claim that the peripheral exten- 

 sions of the autonomic nerve fibers have a syncytial 

 construction and a continuous neurofibrillar con- 

 nection with a network located within the innervated 

 cells, a network with fibrils of such fine dimensions 

 that they are near the limit for the resolving power of 

 the microscope. It is obvious that the claims made 

 by Stohr and Boeke cannot be accepted until very 

 strong evidence has been furnished that the struc- 

 tures demonstrated are both of nervous nature and 

 existent in the living tissues. Xo such e\idence has 

 yet been presented, however, neither in their earlier 

 (cf. 206) nor in their more recent papers (33, 355, 

 404-406). Furthermore, their results concerning the 

 microscopic appearance of the nervous syncytia are 

 quite different and partly incompatible in spite of 

 the fact that both Stohr and Boeke use silver im- 

 pregnations and that they both argue along the 

 same lines for the validity of their views. This gives a 

 good illustration of the difficulties inherent in their 

 methods. A detailed criticism of their neurohis- 

 tological studies has been published (206). In the 

 discussion of the 'interstitial cells' (see below), an- 

 other example will be given illustrating the general 

 tendency to draw far-reaching conclusions con- 

 cerning the presence of histological structures, the 

 nature and function of which ha\e been interpreted 

 without rigid criteria for their characterization. 



Up to 1932 the autonomic effector cells were con- 

 sidered to be innervated by postganglionic fibers 

 terminating in extra- or intracellular nerve endings. 

 Recently the existence of far more extensive nerve 



structures has been claimed. Stohr described a 

 'terminal reticulum' composed of sympathetic and 

 parasympathetic endings which anastomose with a 

 syncytialK constructed network partly embedded 

 in the cytoplasm of the innervated cells. Boeke pro- 

 posed a 'sympathetic ground plexus", arising from 

 coarser nerve fibers which connect with a fine- 

 meshed protoplasmic network containing extensively 

 anastomosing neurofibrils. This network is distally 

 in protoplasmic continuit\- with a system of 'inter- 

 stitial cells', interpreted as nerve cells, which in turn 

 make synapse-like connections with the effector 

 cells (33). Boeke's descriptions of the proposed 

 structures are unconventional and coiifusing (27, 29). 



The postulation that the interstitial cells of Ramon 

 y Cajal constitute third neuron links in peripheral 

 autonomic innervation (30-32, 224-227, 287, 312, 

 314, 315, 326, 327) deserves critical comment as 

 this concept may become of importance in physi- 

 ology (cf. 6, 152). The evidence claimed for the 

 view that the interstitial cells are in fact nerve cells 

 and not sheath cells is as follows (32, 33, 224-226, 

 287, 312-314): a) their cytological characteristics 

 are the same since they are vitally stained by methy- 

 lene blue, contain Nissl granules and neurofibrils, 

 and show positive oxidase and peroxidase reactions; 

 and h) they do not degenerate following postgan- 

 glionic section. However, much of this is disputed, 

 both as to the findings and as to their interpretation 

 (21, 206, 290, 331, 396). 



In view of the inconclusiveness of the neurohis- 

 tological evidence, the concept of a terminal syn- 

 cytium of nerve cells interposed between the post- 

 ganglionic nerve endings and the effector cells cannot 

 be accepted. There is no reason to suppose that the 

 cells are anything but neurilemma cells, as suggested 

 long ago by Lawrentjew, Stohr, de Castro, Schaba- 

 dasch, Nageotte and others, and strongly supported 

 by the recent careful studies of Greving (181, 182) 

 and Herzog (203). 



However, certain physiological and pharmacologi- 

 cal findings have been believed (6, 148, 417, 418) 

 to indicate the existence of a terminal nervous 

 syncytium, e.g. the diffuse character of the responses 

 obtained on stimulation of a small proportion of the 

 nerves supplying the smooth muscle of an organ. 

 Strictly localized responses are often found in smooth 

 muscle (8, iio, 127, 169, 211, 266, 300, 321), and 

 the diffuse responses usually obtained were ade- 

 quately explained by Langley (266) as being due to 

 "the intermingling of the postganglionic fibers which 

 occurs in the preterminal plexus on the way to the 



