HANDBOOK OF PHVSIOLOGV 



NEURGPHySIOLOCi' 11 



pansions but must have a considerable length. This 

 is in good agreement with the assumption that the 

 apcon ramifications running in the autonomic ground 

 plexus are real terminals releasing the transmitters 

 when nervous impulses travel along them and thus 

 acting on the effector cells with very short diffusion 

 distances. This hypothesis makes it possible to explain 

 the dual innervation of an effector cell without post- 

 ulating the existence of two independent innervation 

 structures for which there is no morphological evi- 

 dence. It does not seem unreasonable that axon termi- 

 nals from both sympathetic and para.sympathetic 

 neurons may be enclosed in the same ground plexus. 

 On the contrary, all observations of the morphological 

 construction of the plexus point in this direction. 

 In this way both adrenergic and cholinergic medi- 

 ators may be released from an inner\ation structure 

 common to both autonomic svstems. 



FUNCTIONAL SIGNIFICANCE OF AUTONOMIC NERVOUS 

 SYSTEM AND ITS SUBDIVISIONS 



Our present knowledge of the basic coordinating 

 functions of the autonomic nervous system is founded 

 on the classical work of Cannon whose fascinating 

 ideas and brilliant capacity for systematization have 

 been of the utmost importance to the understanding 

 of this system. Any attempts to give a brief summary 

 of his contributions are doomed to failure but, fortu- 

 nately, his work and views have been summed up in 

 four monographs well-known today (70, 72, 76, 77). 

 According to Cannon, the sympathetic and para- 

 sympathetic divisions are organized quite differently. 

 The parasympathetic has a restricted distribution 

 with more or less local functions, the cranial division 

 as a 'conserver of bodily resources' and the sacral as a 

 'mechanism for emptying.' In contrast to this, the 

 s;^pathetic has a widespread activity with a diffuse 

 distribution of nerve impulses which makes it pos- 

 sible to call many different effectors into simultaneous 

 play whereby a variety of functions serve to maintain 

 homeostasis. Furthermore, the system subserves its 

 general functions in intimate cooperation with the 

 adrenal medulla, the hormone secretion of which 

 supports the sympathetic activities. This cooperation 

 is seen in many conditions of stres s, above all in 

 states of emergency, and is of such importance that 

 the sympathetic division and the adrenal medulla 

 may be considered as a sympathicoadrenal system. 



In general, in organs innervated both by the 



sympathetic and parasympathetic, activation of these 

 systems produces responses opposite in direction. 



The \alidity of the concept of the sympathetic as a 

 system maintaining homeostasis was elegantly demon- 

 strated in experiments on sympathectomized animals 

 (74, 376) which were shown not to differ markedly 

 from normal animals under protected conditions but 

 to have lost the ability to make acute adjustments 

 when subjected to stress. The problems of autonomic 

 regulations with regard to horneostasis have been ex- 

 tensively reviewed by Gellhorn (163). The concept 

 of the sympathicoadrenal system as a cooperating 

 unit has in a way received strong support from the 

 more recent investigations into the role of the adrenal 

 cortex in conditions of stress (cf 422). 



It is evident, however, that a sharp distinction 

 between the sympathetic and parasympathetic 

 cannot be made from a functional point of view. 

 In certain effectors, such as the salivary glands, the 

 two divisions show synergistic effects and this may be 

 said to hold to some extent for the sphincter mech- 

 anisms in the gastrointestinal and urinary system 

 also. Other effectors, such as the heart, have their 

 activities delicately balanced by cooperation of the 

 sympathetic and parasympathetic in a strictly 

 reciprocal manner. The widespread diffusion of 

 sympathetic impulses may in a sense be considered to 

 be due not to an organization of the sympathetic 

 system different from that of the parasympathetic 

 but to the fact that the effector systems with the 

 same general functions, e.g. the skin vessels, have a 

 widespread distribution, and the diffuse character of 

 some parts of the sympathetic may be considered to 

 have its equivalent in the vagus system. There is, 

 furthermore, good evidence that some parts of the 

 parasympathetic division are activated and cooperate 

 with the sympathicoadrenal system in conditions of 

 stress (cf 163). This has been shown to hold above all 

 for the vagoinsulin system, the activation of which 

 gives an insulin secretion interpreted to be an im- 

 portant adjustment enabling the striated muscles to 

 utilize more fully the blood sugar which is increased 

 by the acti\iiy of the sympathicoadrenal system 

 (cf 164). 



The concept of the svmpathicoadrenal system has 

 unfortunately not been adjusted to the demands of 

 the new evidence concerning the differences in 

 actions and functions of epinephrine as a hormone 

 with mainly metabolic effects and of norepinephrine 

 as an adrenergic transmitter. Indiscriminate use of 

 the concept has been criticized on this basis by von 

 Euler (cf 423-425). This suljject is, ho\vc\'er, dealt 



