PERIPHERAL AUTONOMIC MECHANISMS 



987 



ganglionic axons. This conclusion is based on the 

 postulate of Langley that the preganglionic fibers 

 make synaptic connections directly with the post- 

 ganglionic neurons. The validity of this postulate is 

 discussed below. 



As the morphological construction and the localiza- 

 tion of the synaptic structures in autonomic ganglia 

 are of importance to an understanding of the internal 

 ganglion organization, some critical comments may 

 be necessary. It must be stressed, however, that no 

 generally accepted view exists concerning the struc- 

 ture of the ganglionic synapse and that the various 

 neurohistological schools have given very divergent 

 interpretations. This unfortunate state of affairs has 

 two principal causes. On the one hand, there are very 

 great difficulties inherent in the neurohistological 

 methods commonly used which often make it im- 

 possible to decide whether a microscopically de- 

 monstrable structure is of a ner\ous nature or not. 

 Frequently there is not e\en any evidence that the 

 observed structures are structures really existing in 

 the living tissue. On the other hand, in spite of these 

 difficulties, there is a general tendency in neuro- 

 histology to ascribe synaptic functions to various 

 histological structures without rigid criteria or with- 

 out any evidence at all for such an interpretation. 

 The facts and reasons on which this critical view is 

 based are discussed in a study of the ganglionic 

 synapse (206). The neurohistological work of the 

 last decade has not revealed any new evidence con- 

 cerning the methodological difficulties in neuro- 

 histology as may he seen from recent investigations 

 (33. 84, 103, 179, 226, 242, 405). 



The earlier investigations into the morphology of 

 the ganglionic synap.se have been reviewed in several 

 papers (26, 206, 329, 403). Recent studies have not 

 revealed any new aspects of the picture (see e.g. 

 103, 179, 226, 242). 



The structures most commonly described as syn- 

 aptic are small ring or club-like endings or reticu- 

 lated enlargements in the vicinity of cell body or 

 dendrites (103, 168, 226, 242, 244, 245J and different 

 types of pericellular arborizations or arborizations 

 in dendritic tracts and glomeruli (103, 248, 249). 

 Silver impregnation techniques, as their defenders 

 admit (e.g. 103, 226), reveal structures assumed to be 

 real preganglionic endings only with great difficulty. 

 Whether these structures, found very spar.sely in 

 autonomic ganglia, are more than fragments ol 

 terminal axon ramifications may be questioned (cf. 

 206). The school of Stohr (355, 404) denies the 

 existence of such free nerve endings and believes the 



pre-postganglionic junction to be a syncytial asynaptic 

 structure with a terminal reticulum. 



Intravital staining of autonomic ganglia with 

 methylene blue by Hillarp (206) consistently re- 

 vealed a dense plexus of very fine nerve fibers di- 

 rectly on the surface of the ganglion cells, enclosing 

 them. The plexus is formed by the terminal ramifica- 

 tions of one or more preganglionic axons. The fact 

 that the impulse-transmitting ability of a ganglion 

 reappears after nerve section when this intracapsular 

 pericellular structure regenerates is strong evidence 

 that it is synaptic. No such evidence has been adduced 

 for the other structures claimed to have synaptic 

 function. However, it cannot be assumed that the 

 pericellular apparatus of Hillarp is the only form of 

 synapse in autonomic ganglia. The dendrites pos- 

 sessed by certain postganglionic neurons ma\' well 

 make synaptic connections. 



The nerve fibers enclosing the cell bod\' of auto- 

 nomic ganglion cells have recently been obser\ed by 

 electron microscopy, and the pre- and postsynaptic 

 elements have been shown to possess a memijrane 



o 



70 to 100 A thick, separated from each other by a 

 space 100 to 150 A wide (79, 104). 



Interneurons with processes wholly confined witiiin 

 a ganglion and synaptic connections between post- 

 ganglionic neurons via axon collaterals have been 

 claimed to exist by neurohistologists (107, 215, 278, 

 343) but denied by others (233, 234, 266, 374). No 

 pericellular synaptic structures were observed in the 

 superior cervical ganglion of rats after preganglionic 

 denervation (206). The histological evidence for the 

 existence of such connectional elements is inconclusive, 

 and the many neurophysiological studies made on 

 autonomic ganglia have failed to reveal neurons of 

 this nature. 



Many different arrangements of the short and long 

 processes emerging from the postganglionic cell bodies 

 have been described (see 98, 103). Although a clear 

 differentiation between axons and dendrites is difficult 

 and often impossible (177, 178), the processes are 

 usually assumed to be dendrites. Stohr with his ex- 

 tensive experience in the neurohistology of autonomic 

 ganglia denies the po.ssiisility of such a distinction 

 (403). Although many interpretations of the func- 

 tional significance of the neuronal processes ha\e been 

 presented, it may be questioned whether the neuro- 

 histological observations as yet permit any statement 

 to be made. The presence of an extensive system of 

 cell processes indicates the possibility, however, that 

 the autonomic ganglia may have a more complicated 

 construction than was postulated by Langley. 



