CENTRAL AUTONOMIC MECHANISMS 



955 



FIG. I. Viscerosomatic reflexes. Maximal single-shocls stim- 

 ulation of splanchnic nerve evokes reflex \olleys in body wall 

 nerves, shown as they were recorded off the central crushed 

 end of each nerve. Volleys recorded consecutively. T ^ to 13, 

 intercostal nerves; L r (0 j, lumbar nerves. Acute spinal cat, 

 cord transected at Ti. [From Downman (47).] 



motor neurons. There was considerable irradiation 

 from the splanchnic input into all intercostal and 

 lumbar nerves and e\en into leg ner\es. The recorded 

 motor volleys were largest in the lower thoracic and 

 upper lumbar nerves, a situation seemingly adapted 

 to the abdominal splinting which occurs with irritation 

 of the abdominal viscera (fig. i). It is of interest that 

 excitation from splanchnic stimulation spreads much 

 faster than does that from intercostal stimulation, the 

 velocity of spread being three to five times greater 

 in the former (20 to 50 m per sec. compared with 7 to 

 II m per sec). Two routes for splanchnic irradiation 

 are involved — a fast extraspinal path in the sympa- 

 thetic chain of the same side and a slower intraspinal 

 route of limited extent. Intercostal reflexes can use 

 only the latter type of pathway. Contralateral spread 

 is entirely intraspinal. This visceromotor reflex dis- 

 charge may be so extensive that in animals with high 

 cervical spinal transection, strong splanchnic stimu- 

 lation evoked diaphragmatic contraction. The effer- 

 ent splanchnic fibers involved are apparently in the 

 small A group, the A gamma-delta size. 



Interrelations with Higher Levels 



Viscerosomatic reflex discharges are greater and 

 more widely spread in the spinal than in the de- 

 cerebrate preparation. Severing the spinal cord seems 



to release the spinal arcs from some inhibiting influ- 

 ence which affects the viscerosomatic reflex pathways 

 more than the somatic ones. Another difl'erence be- 

 tween decerebrate and spinal preparations is the fact 

 that, following an initial conditioning reflex discharge, 

 a later testing discharge into the same nerve of outflow 

 shows facilitation in the former and inhibition in the 

 latter. 



Downman also measured in spinal cats the central 

 delays for reflex discharges evoked by stimulation of 

 the splanchnic and of the intercostal ner\e. The 

 average delays ranged from 5 to 8. i msec, in the case 

 of the viscerosomatic and from 2.4 to 2.8 msec, for 

 the somatic arcs, which indicates that a greater 

 number of neurons is involved in the splanchnic arcs. 

 The extent of this internuncial acti\ ity apparently is 

 less in decereljrate than in spinal animals which may 

 account for the difierences in effect of conditioning 

 afferent stimulation. 



Downman also points out that there are at least 

 three ascending paths available for impulses of 

 splanchnic origin — an extraspinal one by way of the 

 sympathetic chain which eventually enters the spinal 

 cord, an ipsilateral route in the fasciculus gracilis and 

 slower conducting bilateral paths in the anterolateral 

 region of the white matter. Besides setting up visceral 

 reflex arcs at \arious levels of the cord, such pathways 

 also extend to the brain where suprasegmental con- 

 trol mechanisms may be brought into play with 

 feedback to the reflex center. Some of these paths 

 reach the cerebral cortex, and Amassian (4) found 

 that such impulses are conveyed by A beta fibers 

 which lie in the posterior funiculus between the 

 afferent fibers from the upper and lower extreinities. 

 There are also A gamma-delta groups of fibers of 

 splanchnic origin which can affect cortical activity. 

 Amassian believes that such afferent paths may take 

 origin in the mesenteries and visceral Pacinian cor- 

 puscles, accounting for sensory awareness of as well as 

 responses to visceral distention. Aidar et al. (2) found 

 that afferent impulses of splanchnic origin ascend 

 at least as high as the thalamus, the faster impulses 

 passing via fibers in the ipsilateral posterior funiculus 

 and the opposite medial lemniscus (possibly mediating 

 pressure and tension), while the slower traverse the 

 lateral spinothalamic tracts bilaterally (po.ssibly me- 

 diating pain). It is of special interest that some slow 

 impulses reach the posterior part of the hypothal- 

 amus. 



The importance of suprasegmental connections for 

 visceral mechanisms in the spinal cord is attested by 

 the release phenomena exhibited in the spinal animal 



