952 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



The autonomic manifestations of an emotion are ex- 

 ceedingly important since they may enable the in- 

 dividual to live with that emotion as well as to support 

 and maintain it. The relationships of an indi\ idual 

 with his environment and with his fellow beings are 

 based upon a proper emotional orientation and 

 suitable modulation of autonomic activity. 



It would be impossible to discuss autonomic activi- 

 ties alone. These are so commingled with somatic as 

 to require a comprehensive viewpoint. Both auto- 

 nomic and somatic reflex outflows may be evoked by 

 common somatic or visceral inputs, and central 

 autonomic centers may control or modulate both 

 autonomic and somatic peripheral actis'ities. 



SPIN.AL .AUTONOMIC MECHANISMS 



Certain autonomic functions proceed in mammals 

 which have been experimentally deprived of higher 

 regulatory influences. These functions are relatively 

 simple, for there is no douijt that loss of connections 

 with the brain eliminates or greatly modifies the more 

 complex sorts of autonomic activities. However, study 

 of these remaining activities in animals which have 

 been subjected to decapitation or section of the spinal 

 cord have confirmed that the basic functional organi- 

 zation of the autonomic portion of the nervous system, 

 as for the somatic, lies in its reflex arcs. The organiza- 

 tion of these arcs varies greatly, from simple mono- 

 synaptic connections between visceral afferent and 

 visceral efferent neurons to very complex mechanisms 

 involving many intermediary neurons as well as the 

 primary afferent and efferent ones. When sufficient 

 numbers of such units which subserve a common type 

 of response are grouped at a certain level of the 

 nervous system such groupings are customarily re- 

 ferred to as 'centers.' Certainly this term has been of 

 considerable value in the physiologist's notation, but 

 as the complexities and vagaries of neural organiza- 

 tion have revealed themselves, it hardly seems proper 

 to hold to such a simple concept (74). 



Nevertheless, groupings of efferent neurons devoted 

 to specific autonomic responses do occur in the 

 spinal cord, and from these the notion of spinal 

 autonomic centers is derived. These have a fairly 

 uniform segmental arrangement, similar to somatic 

 reflex patterns, and correspond roughh' to the seg- 

 ments of outflow. The visceral afferent neurons, the 

 cell bodies of which lie in the dorsal root ganglia, are 

 arranged similarly to the somatic afferents and their 

 peripheral distribution shows evidence of segmenta- 

 tion, allowing for developmentally induced displace- 



ments of the deep viscera. It is no longer held that 

 afferent neurons are present in autonomic ganglia, 

 although a few aberrant ones may at times be found 

 elsewhere in the ner\e roots and trunks. 



Analnmical Considerations 



An historical survey of the development of knowl- 

 edge of the autonomic outflow is not properly a part of 

 tills chapter; such surveys have been written by 

 Sheehan (151, 152), Mitchell (121) and Gaskell (59). 

 However, recognition of the division into thoracic and 

 sacral outflows contributed much to the knowledge of 

 central autonomic mechanisms. Workers prior to 

 1885 were handicapped by Bichat's concept of ani- 

 malic (concerned with somatic) and organic (con- 

 cerned with nutritional regulation) systems which 

 implied an independence of the sympathetic nervous 

 system in spite of the fact that the rami communicantes 

 hax'e been kno\vn for many \ears. A number oi 

 histological and developmental studies culminated in 

 Gaskelfs work of 1886 (58) in which, as a result of 

 anatomical and physiological observations, he recog- 

 nized that the white rami communicantes were re- 

 stricted in the dog to the thoracic and upper lumbar 

 regions. Gaskell studied the fiber composition of the 

 white rami and traced fine myelinated fibers from the 

 ventral roots through these rami into the sympa- 

 thetic chain. The association between these fibers and 

 the unmyelinated fibers emerging from the ganglia 

 was recognized on physiological grounds, since cardiac 

 acceleration could be produced by stimulation ol 

 either. Gaskell found similar fibers in the central roots 

 of the second and third .sacral nerves and discerned 

 that these connected with collateral ganglia of tlie 

 pelvis rather than with the chain ganglia. None of 

 these preganglionic fibers was found in the cervical 

 nerve roots. Thus the concept of thoracicolumbar 

 and sacral divisions was born. Similar preganglionic 

 fibers were found and traced in certain cranial nerves, 

 beginning with the spinal portion of the accessory 

 nerve. Further experimental work by Gaskell and his 

 co-workers and bv others established the details of 

 structural and functional differences between the 

 cranio.sacral and thoracicolumbar divisions of the 

 autonomic system. Nearly all of this has been more 

 recently confirmed. Sheehan (152) has examined the 

 spinal autonomic outflows in man and found that, if 

 any cervical contribution to the thoracicolumbar 

 system exists, it must Ije \ery rare indeed. The human 

 sacral outflow is usually from S3 and S4, with occa- 

 sional small contributions from S2 or S5. 



Our particular interest here lies in the central sta- 



