PERIPHERAL AUTONOMIC MECHANISMS 



989 



mechanisms within autonomic gangha do not gi\'e 

 large and easily detectable effects may be inferred 

 from the fact that the characteristically grouped dis- 

 charges from the cardiac sympathetic centers are not 

 altered in their form and frequency by passing through 

 the stellate ganglion (45). 



The old problem of the role played by the short 

 and long cell processes assumed to be dendrites in 

 the autonomic ganglia has not as yet been elucidated 

 in neurophysiological studies. The evidence brought 

 forth by Lorcnte de No & Laporte (296), indicating 

 that the presynaptic fibers exert excitatory and in- 

 hibitory actions on different parts of the ganglion 

 cells, may have some bearing on this problem. In an\ 

 case the intricate histology and the complicated syn- 

 aptic events seem to justify the conclusion that the 

 conventional diagrams of the synaptic connections 

 in autonomic ganglia can no longer be assumed to 

 give true pictures of the function and organization 

 of the ganglia. 



A new feature in the picture of the autonomic 

 ganglia was introduced by tiie discovery by Marrazzi 

 (306) that epinephrine has an inhibitor)- effect on 

 ganglionic transmission. The effect can be produced 

 by epinephrine liljerated from the adrenal medulla 

 and may thus be of physiological significance (305, 

 308, 346). Marrazzi has interpreted this as a "self- 

 limiting mechanism in sympathetic homeostatic ad- 

 justment." It has been found that an epinephrine-like 

 substance is liberated in ganglia on preganglionic 

 stimulation (58) which, according to Marrazzi (307), 

 suggests that epinephrine may be a humoral inhibitor 

 at ganglionic synapses. The existence of inhibitory 

 fibers in autonomic ganglia, once postulated i^y 

 Eccles (i 18), and the importance of localh' liberated 

 epinephrine as inhibitor have been seriously ques- 

 tioned, however (232, 298). On the basis of recent 

 quantitative studies of the secretion rate of epinephrine 

 from the adrenal medulla on strong reflex excitation 

 (cf. 81, 152), it seems questionable whether epi- 

 nephrine from this source may play any important 

 role as a regulator of ganglionic transmission. 



PHYSIOLOGICAL DISCHARGE R.ATE IN PERIPHERAL 

 AUTONOMIC NERVOUS SYSTEM 



It has long been known that many autonomic 

 effectors, even under 'resting' conditions, are usualh 

 more or less influenced by a jtonic activity in the 

 nerves supplying them and that this tonic control 

 may be rapidly changed into strong excitation or 



inhibition of the efiectors by reflex stimulation of the 

 autonomic nervous centers. From .several points of 

 view it is of importance to know the discharge rate 

 in the different states of activity. A brief survey of 

 studies pertinent to this problem is therefore necessary. 



One of the early significant facts observed was that 

 the response of an autonomic effector on pregan- 

 glionic stimulation is not maintained if the stimulation 

 frequency is relatively high (>20 to 40 per sec.) 

 (25, 247, 256, 333, 365). Even at a rate of 10 per sec, 

 fatigue may set in rapidly (25). Recording of the 

 postganglionic action potentials showed, furthermore, 

 that at frequencies abo\e approximately 20 per sec. 

 an increasing_asynchronism of the postganglionic 

 discharge appears and that ganglion cells progres- 

 sively drop out of action (44, 47, 69, 276, 277). 



Another significant fact is that strong responses from 

 \arious effectors are obtained at low frequency stimu- 

 lation. Rosenblueth (361) found that a frequency 

 of 15 to 25 per sec. produced approximatelv maximal 

 responses in all the sympathetic effectors examined 

 by him. The same or often even lower values have 

 been found in other studies (37, 158, 246, 297, 332, 

 368, 369). Recent work on \asomotor fibers to various 

 tissues has demonstrated that almost maximal va.so- 

 constriction oi^vasodilatation can be obtained at dis- 

 charge rates of about or below 10 per sec. (cf. 81, 152). 



The studies referred to above suggest that the upper 

 limit for the ph>siological discharge rate does not 

 exceed about 20 per sec. However, there is evidence 

 from ol)servations of a more direct nature strongly 

 indicating that this limit is reached at considerably 

 lower frequencies and that tonic control is exerted 

 at \'ery low discharge rates. By recording from single 

 or a few sympathetic fibers Bronk and his associates 

 (43, 45, 46, 344, 345) were able to show that there 

 is a continuous discharge of impulses with frequencies 

 down to and. bejow i jDer^sec during 'resting' condi- 

 tions and that even in_tense reflex stimulation seldom 

 increased the rate above 10 to 15 per sec. In similar 

 experiments with parasympathetic fibers to the blad- 

 der Evans (140) observed a tonic discharge rate below 

 I per sec. With well-controlled quantitative methods 

 Folkow (150, 151) compared in cats the constrictor 

 tone in an isolated vascular area on reflex excitation 

 of the vasomotor center and on stimulation of the 

 constrictor fibers. The experiments clearly demon- 

 strated that the constrictor tone present at normal 

 arterial pressure values could be maintained by i to 2 

 impulses per sec, that almost maximal vasocon- 

 striction is obtained at frequencies of 6 to 8 per sec, 

 and that the very high pressor reactions observed 



