PERIPHERAL AUTONOMIC MECHANISMS 



985 



ganglion of the rabbit and tlie cat (54, 293) where 

 the C type is the main or only component. 



The B group, on the other hand, consists in many 

 instances of several components which often overlap 

 concerning conduction velocity but which are more 

 or less distinctly separable on the basis of other prop- 

 erties (threshold, synaptic delay, distribution of fibers, 

 etc.). This has most clearly been shown for the cervi- 

 cal sympathetic in the cat. Bishop & Heinbecker 

 (25) recorded in the preganglionic stretch four spike 

 potential waves. Mi to M3 and U, the first three hav- 

 ing properties of the B type and the last arising from 

 C fibers. This was confirmed by Eccles (11 7-1 19, 

 121 ) who inade an extensive analysis of the properties 

 of the four fiber groups, labeled Si to S4. The conduc- 

 tion velocities were observed to be appro.xiniately 

 18 to 26, 10 to 12, 7 to 10 and 1.3 to 5 m per sec. 

 The four preganglionic groups were further shown 

 to make synaptic connections with ganglion cells giv- 

 ing rise to four postganglionic fiber groups with con- 

 siderably slower velocities (approx. 5 to 8, 1.7 to 5 

 and I m per .sec.). The postganglionic Si, proi^ably 

 produced by the medullated fibers found in the cat, 

 was distributed to the eye, S.> to S4 were found in 

 all postganglionic branches. The So wave was always 

 the largest. It was assumed by Eccles that the Si to S3 

 groups in the cervical trunk correspond to medul- 

 lated fibers presumably of descending order of size 

 and the S4 to nonmedullated fibers. In some in- 

 stances with long conduction paths available. Si 

 was found to separate out in subsidiary waves, in- 

 dicating a composite nature. Rosenblueth & Simeone 

 (371) have reported similar findings concerning fiber 

 groups in the cervical sympathetic. 



It does not seem unlikely that the four fiber groups 

 present in the cer\-ical sympathetic trunk of the cat 

 and also found in the rabbit and rhesus monkey (25) 

 subserve different functions. In fact, Bishop & Hein- 

 becker (25) have made observations supporting this 

 \'iew. According to them the Mi fibers activate the 

 sphincter of the pupil, the nictitating membrane and 

 the muscles of Miiller; the Mo group gives vasocon- 

 strictors to the ear and conjuncti\a and pilomotors 

 to the hairs between ear and eye. Further esidence 

 was given by Eccles (i 17) and Brown (51 ) supporting 

 the view that the preganglionic and postganglionic 

 fibers of highest conduction \elocit\ and of lowest 

 threshold (cat) are distributed to the orbit. The 

 functions of the other groups ha\e not ijeen studied. 

 Howc\er, no effects are observed on stimulation of 

 the postganglionic fibers in the raljidt until fibers 

 of the C type are activated (25). 



By using other autonomic nerves Bishop & Hein- 

 becker have tried to provide further evidence for the 

 assumption that there exists a correlation between 

 functional fiber groups and groups obtained on a 

 neurophysiological and anatomical basis. In a study 

 of the vagal efferents to the heart of the turtle, Hein- 

 becker (194) claimed that myelinated axons pri- 

 marily influence an inotropic mechanism whereas 

 chronotropic effects are produced by nonmyelinated 

 fibers. In a later paper (196), however, it was re- 

 ported that the fibers responsible for both the inotropic 

 and the chronotropic effects belong to both B and 

 C types. 



In the cat and monkey Sheehan & Marrazzi (386) 

 have recorded large C waves (with conduction ve- 

 locity of I to 2m per sec.) and small B waves (8 to 

 20 m per sec.) from peripheral nerves to the limbs on 

 stimulation of ventral roots. Both waves were shown 

 to originate from sympathetic postganglionic fibers. 

 It is not known, however, whether the B fibers have 

 as their destination eff"ectors functionally different 

 from those supplied by the C group. 



It may thus be concluded that attempts, on the 

 basis of morphological characteristics, to divide the 

 peripheral autonomic fibers into separate groups other 

 than the two groups based on the presence or absence 

 of myelin and corresponding to B and C fibers have 

 so far been unsuccessful but that the medullated B 

 fibers can in some instances be subdivided on the 

 basis of neurophysiological properties into two or 

 three distinct types. From the studies referred to 

 aijove it may further be concluded that, with one or 

 two exceptions, there is no experimental e\idence 

 which makes it po.ssible to correlate functionally dif- 

 ferent types of fibers with filjer groups demonstrable 

 by morphological or neurophysiological methods. 



In contrast to somatic ner\e fibers, the autonomic 

 B and above all C fibers have considerably increased 

 oxygen uptake at low frequency activity and high 

 susceptibility to lack of glucose (277). 



STRUCTURE OF AUTONOMIC GANGLIA 



Types of Ganglion Cells 



No obvious morphological differences have been 

 found between the nerve cells in sympathetic and in 

 parasympathetic ganglia. Although uni- and bipolar 

 cells are sometimes observed, it is evident that the 

 great majority of the postganglionic neurons are 

 multipolar. Great variations in their morphology 



