1222 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



ml. 



mm Hg 



FIG. 19. Upper record. Impulses in single afferent fiber of the pelvic plexus (lower trace) aroused 

 by exceedingly rapid passive filling of denervated cat bladder. Upper trace shows the intravesical 

 pressure. Time pips at i-sec. intervals. Lower record. Impulses in a pelvic afferent fiber due to a 

 'spontaneous' contraction during emptying and filling. Lower trace, action potentials; middle trace, 

 intravesical pressure; upper trace, bladder volume. Note emptying is downward. [From Iggo (12).] 



a minute. The same afferent neuron discharges to 

 passive stretch and, more vigorously, to the active 

 tension of isometric contraction of tlie bladder 

 muscle; the neuron also discharges to the actual 

 shortening of the fibers but not to their lengthening 

 during relaxation. 



This behavior is consistent with a stretch receptor 

 arranged in series with the muscle fibers and ac- 

 counts for the sustained character of micturition, but 

 it may not be the only mechanism. The complex 

 brain-stem mechanism for bladder controls suggests 

 that afferent impulses from the bladder may be 

 iong-circuited' through the brain where circularly 

 arranged nerve nets would provide the temporal 

 dispersion of impulses returning to the segmental 

 preganglionic neurons. \Vhether this is the case will 

 not be known until input-output studies are made. 



Spinal Afferent and Efferent Pathways 



The spinal afferent pathway of the bladder in 

 animals is difficult to distinguish experimentally 

 from the descending pathways, but in man, bladder 

 sensations provide additional information. While 

 anterolateral cordotomy in man often interferes 

 transiently or even permanently with bladder func- 

 tion, well-executed cordotomies induce little or no 

 bladder disturbance, either sensory or motor. As- 

 cending impulses from the bladder apparently do not 

 follow the same pathway as sensations of the sexual 

 orgasm which are abolished by cordotomy. The 

 lamination of the spinothalamic tract would predi- 



cate a posterolateral locus of ascending fibers. The 

 available experimental evidence (3) suggests that 

 bladder afferent and motor spinal tracts occupy an 

 e\en more posterolateral position, i.e. superficially 

 just ventral to the posterior horn. A dorsal quadrant 

 section of the spinal cord in the cat seems almost 

 equi\alent to a spinal cord section in severity of 

 bladder disturbance. McMichael (21) described a 

 critical case of severe bladder disturbance, unac- 

 companied by somatosensory disturbance, in which 

 the lesion was restricted to the superficial fibers in 

 the extreme posterior portion of the posterolateral 

 column (fig. 19). Nathan & Smith (24) place the 

 ascending fibers somewhat more ventrally where 

 they would almost certainly be injured by cordotomy 

 which often does not occur. White (35) suggested 

 that the posterior columns may conduct sensory 

 impulses from the bladder, and Talaat (30) con- 

 firmed this electrophysiologically. 



In tracing the ascending degenerations in Marchi 

 preparations of cordotomy cases, Kuru (16) de- 

 scribed, under the name tractus sacrobulbares, 

 fibers pursuing the same general pathway as the 

 spinothalamic tract, some terminating in the juxtasoli- 

 tary nucleus and some dorsal to its rostral end. 

 Sacral components in the posterior column were 

 traced to the same general area. Continuation of the 

 system by second- and third-order neurons could 

 reach the pontine bladder-controlling area which 

 acts when rostral connections are severed. In this 

 fashion, a controlling circuit starting and ending in 

 the bladder would be closed. 



