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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



pressor response disappears. Selective retraction of 

 *he nictitating membrane may also be elicited. Such 

 selective effects may also be obtained from autonomic 

 areas in the brain where these differences are perhaps 

 due to the differences in neuronal thresholds and dis- 

 tribution. In the cord, however, production of specific 

 autonomic discharges depends upon segmental ar- 

 rangement. When a mass discharge takes place, as in 

 emotion, it is probably associated with activation of 

 large areas of higher autonomic mechanisms. An ex- 

 cellent tabulation of the autonomic innervation of the 

 most important structures together with their central 

 nervous representation is to be found in Mitchell (121). 



It seems obvious that reflex mechanisms in the 

 spinal cord are so organized that a variety of reflex 

 patterns may be set up. Thus there are pure visceral 

 reflexes, for example, changes in arterial pressure after 

 mechanical (or other) stimulation of viscera such as 

 the intestine, pancreas, etc. Such reflexes are best 

 studied in spinal animals; otherwise investigators have 

 found themsehes involved in semiphilosophical con- 

 siderations of the specificity of visceral pain, as did 

 Lewis & Kelgren (102) and von Euler & Sjostrand 

 (163). There are also reflex patterns which serve both 

 visceral and somatic functions. Thus we have somato- 

 visceral reflexes, such as the vasomotor response to 

 stimulation of a 'somatic' nerve. There are viscero- 

 somatic ('visceromotor') reflexes, in which contrac- 

 tion of somatic musculature occurs in response to 

 stimulation of visceral afferent fibers, for instance 

 contraction of abdominal muscles upon irritation of 

 abdominal viscera or peritoneum. 



The mass of work done on the vasomotor activities 

 of sympathetic nerves after the original observations 

 of Bernard (23-25) and Brown-Sequard (36) cul- 

 minated in Bayliss' summary of the vasomotor 

 system (19), which e\-ery student of neurophysiology 

 and neuroanatomy should read as an exercise in 

 classical physiology. Segmentally arranged spinal 

 vasomotor reflex arcs are present in the cord and are 

 capable of activity even wlien cut off from the brain, 

 as shown by stimulation of peripheral somatic and 

 splanchnic nerves with resulting increase in arterial 

 pressure and peripheral vasoconstriction. The.se re- 

 sponses are diminished during spinal shock but are 

 very easily obtained after recovery from this state. 

 These spinal reflex mechanisms have been held to be 

 excited by asphyxia (154) or perhaps by chemical 

 changes iirought about ijy anoxia. They are less 

 sensitive to increases in carbon dioxide than are the 

 medullary centers. Bayliss points out that it is difffcult 

 to determine whether the chemical changes them- 



selves stimulate efferent \asoconstrictor impulses or 

 whether reflex stimulation is also necessary. The 

 existence of vasodilator reflex mechanisms at the 

 spinal le\'el is not as certain, but Bayliss felt that they 

 do exist in cases where local vasodilator responses can 

 be induced. The extent to which spinal vasomotor 

 reflexes function in cither the intact or spinal animal 

 is uncertain. 



Other spinal reflex mechanisms may also be aflected 

 by chemical changes in the body fluids. Thus, in 

 spinal cats in which hypoglycemia was induced by 

 insulin in doses equivalent to those which produce 

 strong sympathicoadrenal activation in normal ani- 

 mals, there was also activation of this system and 

 therewith resistance to the hypoglycemia (35). Since 

 this occurred when all nervous connections between 

 the brain and the thoracicolumbar outflow were 

 severed, spinal mechanisms which are capable of ac- 

 tivating the sympathicoadrenal system exist. It is also 

 possible that changes in blood temperature may affect 

 thermoregulatory reflexes, such as those involved in 

 sweating and vasomotor changes. Thus, cooling of 

 one extremity may bring about vasoconstriction in 

 the opposite one according to Sahs & Fulton (144). 

 Certain local sudomotor responses may occur in re- 

 sponse to visceral irritation, and local sweating has 

 been described especially in areas of skin contact in 

 spinal and sympathectomized persons. Such local 

 responses may well be due to axon reflexes (174). 



Viscerosomatic reflexes were studied in amphibians 

 and reptiles by Carlson & Luckhardt (38) in 1921; 

 limb movements in response to visceral stimulation 

 were observed. Miller and his co-workers (116- 119) 

 extended such observations to decapitate mammals, 

 noting contractions of abdominal and hind limb 

 muscles upon mechanical stimulation of abdominal 

 viscera and electrical stimulation of the splanchnic 

 nerves. Downman & MacSwiney (48) found similar 

 responses as well as increases in arterial pressure upon 

 pinching the intestine and mesentery in cats with 

 spinal cord transection in the upper thoracic region. 



Subsequently Downman (46, 47) carried out a 

 more detailed analysis of the central organization of 

 viscerosomatic reflex arcs, using decerebrate and 

 spinal cats and comparing such arcs with those in- 

 volved in somatic reflexes. In the first instance the 

 greater splanchnic nerve was stimulated with single 

 shocks; in the second a nearby intercostal nerve was 

 used. The efferent discharges were recorded from 

 intercostal and lumbar ner\'es. The reflex discharge 

 elicited by splanchnic stimulation in spinal cats was 

 of relativeh' long duration due to repetitive firing of 



