PERIPHERAL AUTONOMIC MECHANISMS 



983 



along the small intestine observed a vasoconstriction 

 in territories not more than q cm in length on stimu- 

 lation of small mesenteric nerve Ijundles. He con- 

 cluded that the innervation area of the terminal 

 ramifications from an individual axon is at the most 

 of this size. This conclusion seems to be valid as it 

 follows from the experiments that no spatial summa- 

 tion effects produced by fillers in adjacent territories 

 are necessary to give an observable vasoconstriction. 



As the cholinergic and the adrenergic mediation 

 seem to be inherent properties of two different types 

 of postganglionic neurons, it is improbable that an 

 indi\idual postganglionic fiber might give both a 

 cholinergic innervation to one structure and an 

 adrenergic supply to another. As a matter of fact, 

 there is not even any convincing evidence as yet that 

 a single postganglionic axon does directly activate 

 two anatomically different structures. (The problem 

 of transmitter diffusion is discussed in the later section 

 on the construction and functional organization of 

 the autonomic innervation apparatus.) On the 

 contrary, several observations suggest that func- 

 tionally and anatomically different effectors within 

 the same organ are subserved by independent and 

 separate fibers (see e.g. no, 163, 320). 



There are only a few studies directlv concerned 

 with the question of the inner\ation of structures 

 supplied bilaterally with autonomic nerves. In ex- 

 periments on vagal inhibition of the heart, Brown & 

 Eccles (52 j showed that there was no refractory period 

 effect on simultaneous stimulation of the right and 

 left \'agi. This indicates that the preganglionic fibers 

 from the two sides have no appreciable convergence 

 to the .same group of postganglionics. Another ap- 

 proach to this problem was used l^y Dye (115, 116). 

 He found that pre\ious exhaustion of a given bi- 

 laterally innervated structure ijv a unilateral stimu- 

 lation did not affect the response obtainable from 

 stimulation of the corresponding nerves from the 

 opposite side and that summation of responses was 

 linear on brief simultaneous stimulation of the pre- 

 and postganglionic nerves from both sides. This 

 clearly supports the conclusion drawn by Brown & 

 Eccles. It is not so obv'ious that the two groups of 

 postganglionic neurons, each acti\ated by pre- 

 ganglionic filjers from one side only, distribute their 

 axons to different effector cells within the same ana- 

 tomical structure, a view championed by Dye. Un- 

 fortunately, however, his interpretation is based on 

 a.ssumptions concerning the local production and the 

 local and remote effects of the liberated chemical 

 mediator which are not necessarih valid. As regards 



T.\BLE 3. Number and Extent of Myelination of 

 Preganglionic Fibers in the Cervical Sympathetic 

 Trunk oj Various Species 



the bladder it has been clearly shown by Langley & 

 Anderson (270) that the pelvic ner\e supply is strictly 

 unilateral. This has been confirmed by recording the 

 tonic acti\it\' in the postganglionic fibers (140). 



FIBER TYPES AND THEIR FUNCTIONAL SIGNIFICANCE 



Gaskell (159) stated that the fibers going to the 

 autonomic ganglia are mcduUated (1.8 to 3.6 n) 

 and that the fibers leaving the ganglia are non- 

 medullated and of very fine caliber. This classical 

 rule was shown, especially by Langley (cf. 267), to 

 have a vast number of exceptions. Since then it has 

 been found that there are not only great differences 

 between animals of different species but also that 

 there are very great variations i^etween animals of 

 the same species (table 3). 



In mammals all peripheral autonomic fibers are 

 either small meduUated (i to 3.5 ix and occasionally 

 up to 5 ix) or small nonmeduUated (up to 2 fi). As 

 yet no differences in morphology (size and myelina- 

 tion) or in neurophysiological properties (conduction 

 velocity, etc.) have been shown to exist between the 

 sympathetic and the parasympathetic system or be- 

 tween adrenergic and cholinergic fibers. 



Quantitatixe data concerning the number and 

 size of the autonomic fibers in the \entral roots in 

 several mammals and in man have been given, es- 

 pecially by Haggqvist and his associates (cf. 356). 

 Caliber spectra of the ventral roots show that the 

 medullated fibers belonging to the thoracolumbar 

 and sacral autonomic outflows give rise to a separate 

 thin-fiber group (i to 4 /x) showing a high and dis- 

 tinct peak at 2 to 3 /J and constituting 60 to 75 per 

 cent of the total number of medullated fibers (356). 

 The size of the small medullated fibers in these out- 

 flows does not \'ary to any appreciable extent in cat. 



