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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



on intense reflex activation of the vasomotor center 

 could be obtained by an increase of constrictor fiber 

 discharge from 2 per sec. to 5 to 6 per sec. Other 

 experiments concerning the local elimination and 

 exhaustion of the vasoconstrictor transmitter support 

 Folkow's conclusion that the upper limit of the physio- 

 logical discharge range is 6 to 8 impulses per sec. 

 The validity of this conclusion has been confirmed 

 by experiments using several other eflfectors of a very 

 diH'erent nature (81, 82, 154). The study of Celander 

 (8i) in particular shows that most efTector systems 

 may give very pronounced responses at remarkably 

 slow firing rates of the autonomic nerves (0.25 to 2 

 impulses per sec). (See Chapter VII on autonomic 

 neuroeffector mechanisms by von Euler in this work 

 for further discussion of this topic.) 



ACTIVITIES OF PERIPHERAL AUTONOMIC NERVOUS SYSTEM 

 INDEPENDENT OF CENTRAL NERVOUS SYSTEM 



^Spontaneous' Activity 



Since the work of Bronk and his associates, it is 

 well known that the autononiic nerves exert a tonic 

 activity by a more or less continuous, usually char- 

 acteristically g rouped discharge, evoked particularly 

 by various afferent reflex mechanisms. If this afferent 

 driving is cut off, usually no activity is found in the 

 pre- and postganglionic neurons. Some exceptions 

 have been observed, however. 



Alexander (4) recorded a persistent residual tonic 

 activity in the inferior cardiac nerve of the cat when 

 all afferent impulses to the preganglionic cells had 

 been excluded. The activity showed a definite reduc- 

 tion when the arterial pressure was increased, thus 

 imitating the behavior in animals with intact affer- 

 ents. The experiments indicated, however, that the 

 phenomenon was caused by lowered oxygen and in- 

 creased carbon dioxide tension in the spinal cord. An 

 observation more difficult to explain was made by 

 Keller (238). In cats with deafferented oculomotor 

 nuclei the pupils were constricted for several weeks, 

 supposedly owing to persistent constrictor tone as the 

 pupils dilated on local application of atropine. How- 

 ever, this assumption was not controlled by nerve 

 section and no further analysis of the mechanisms was 

 performed. 



It has been claimed that denervated autonomic 

 ganglia may have some autonomous activity (35, 

 140, 172-175, 198). There is -no evidence that the 

 phenomena observed have any physiological signifi- 



cance and they may well be explained on the basis of 

 two facts. A_utonomic gangli^ develop supersensitivity 

 at denervation (see the section on degeneration and 

 regeneration in the peripheral autonomic nervous 

 system) and there may be a slow continuous release of 

 the chemical mediators from the postganglionic nerve 

 terminals similar to that found in motor end plates 

 (141, 143). In the studies of Govaerts (172-175) and 

 Evans (140), for instance, the recorded postganglionic 

 discharge from isolated ganglia may have been due to 

 supersensitivity as the discharge did not develop or 

 was not observed until several days after the denerva- 

 tion. The observations of Tower & Richter (415), 

 assumed to give evidence of some independent activ- 

 ity in sympathetic ganglia, have been shown by Hare 

 (190) to be due to incomplete denervation. Hare and 

 others (3, 4, 50, 200) have not been able to find any 

 such activity. The discharge occasionally seen when 

 recording from postganglionic fibers in acute experi- 

 ments is most probai^ly produced by injury to the 

 ganglia (3, 440). 



Axon Reflexes 



Some curious reflexes mediated through decentral- 

 ized autonomic ganglia were shown by Langley 

 (263, 269) to be caused by preganglionic collaterals 

 to different ganglia. Such a branching system is, as 

 Langley demonstrated, a common arrangement in the 

 preganglionic neuron. Stimulation of the distal part 

 of a preganglionic fiber thus generally activates 

 ganglia at the proximal part of the fiber. The Sokow- 

 nin crossed bladder reflex, contraction of the bladder 

 on stimulation of the central end of one cut hxEp- 

 gastric nerve, could be explained on this basis as 

 evoked by impulses traveling up in preganglionic 

 fibers destined to more distally located ganglia and 

 giving off c^laterals_tj3_J;he contralateral inferior 

 mesenteric ganglion. The correctness of this explana- 

 tion has been proved in neurophysiological investiga- 

 tions demonstrating that preganglionic B fibers tra- 

 versing the ipsilateral ganglion are concerned in the 

 reflex (230, 231, 293). Other examples of such pre- 

 ganglionic axon reflexes have been recorded by 

 Langley (263) and others (109, 231, 330). The only 

 evidence that pseudoreflexes of this type play any 

 role except under experimental conditions comes from 

 Hilton's studies of v-asojugtor axon reflexes which 

 may participate in causing the postcontraction hy- 

 peremia of skeletal muscle. 



Evidence for the view that axon reflexes may also 

 be evoked in the terminal ramifications of postgan- 



