CENTRAL CONTROL OF THE BLADDER I 22 I 



the internal urethral orifice. The internal sphincter 

 does not act like a gate at the command of the Ijrain. 

 Much of the information on \esical action was pro- 

 vided by Denny-Brown & Robertson (7, 8j who 

 recorded the pressure in the bladder and distal to 

 the internal sphincter in man. The internal sphincter 

 cannot be voluntarily opened or closed independently 

 of detrusor contraction and relaxation, respectiveh'. 

 The external sphincter cannot be \oluntarily opened, 

 but it can be closed vigorously in the face of detrusor 

 activity. The internal sphincter cannot be forced by 

 straining (increased intra-abdominal pressure) nor 

 does it open before or immediately when detrusor 

 activity begins. It opens later after a variable rise in 

 pressure (18 to 43 cm), i.e. after an initial period 

 which might be termed the 'initial isometric phase of 

 vesical contraction.' 



The internal sphincter is therefore not a primary 

 agent, but it opens sequentially to detrusor contrac- 

 tion and closes sequentially to detrusor relaxation. 

 (Although a constrictor action is exerted by the 

 hypogastric nerve, it is believed to be a part of the act 

 of ejaculation, preventing reflux into the bladder.) 

 Thus, there is little evidence that the internal 

 sphincter is relaxed by either the sympathetic or 

 parasympathetic innervation as a part of micturi- 

 tion, but see Evans (10). The mechanism of the in- 

 ternal sphincter remains speculative. The contrac- 

 tion of the detrusor musculature in the bladder and 

 sphincter may open the sphincter orifice in a me- 

 chanical fashion, although this is difficult to \isualize 

 from the anatomical arrangements. Or, as the de- 

 trusor is activated, impulses may spread through the 

 intramural plexus, relaxing the sphincter. Or, a 

 spinal reflex may sequentially activate tiic two 

 muscles, a mechanism analogous to that of the 

 cardiac sphincter of the stomach. The latter ex- 

 planations assume that the sphincter, unlike the 

 mural muscles, possesses neurogenic tonus when, in 

 fact, it restrains fluid under pressure during normal 

 filling when the bladder wall is acting passively or 

 when extrinsic neural activity is in abeyance, as in 

 the acute phase of spinal shock or after sacral nerve 

 section. The mechanical hypothesis is perhaps the 

 most economical. 



During micturition, the external sphincter ap- 

 parently opens as a result of central inhibition of the 

 motoneurons of the pudendal nerve. Evans (10) 

 noted a cessation of nerve impulses in the pudendal 

 nerve when the external sphincter opened as a part 

 of micturition. The tone of this sphincter, like that of 

 other striate muscle, is inaintained by a continuous 



stream of impulses which are inhibited centrally as 

 a part of reflex micturition. 



.\FFERENT B.'^SIS OF MICTLRITION 



Vesical afTerents and their pathways complete the 

 segmental and supraspinal control of the bladder. 

 When Iggo (12) introduced fluid into the bladder at 

 such excessively high rates that it was filled to its 

 capacity in less than half a minute, single afferent 

 neurons in the pelvic nerve discharged occasionally, 

 i.e. irregularly, beginning at about 20 ml of volume. 

 The discharges accelerated briefly to a maximum of 

 20 to 30 per sec. (at about 40 ml of fluid) and then 

 decelerated to i per sec. despite further filling. 

 During sustained, but not increasing, filling ap- 

 proaching micturition threshold volume (30 to 60 

 ml), the frequency was very low (4 to 6 per min.). 

 This suggests either that the end organs adapt 

 rapidly or that the internal viscous flows end the 

 stimulus. Although the evidence is fragmentary, there 

 presumably is only a slight discharge during the 

 initial phases of the cystometrogram, not sufficient in 

 itself to induce reflex micturition in the normal ani- 

 mal but sufficient when the preganglionic fibers are 

 heavily facilitated from above as in the intercollicular 

 or transhypothalamic decerebrate preparation when 

 the micturition threshold is low and the resistance to 

 passive stretch is like that in the normal animal. 



The inicturition reflex is perhaps the longest sus- 

 tained phasic spinal reflex. If this long duration is 

 gained peripherally, a certain arrangement of stretch 

 receptors can be predicted. If the stretch receptors 

 are arranged in parallel with the muscle fibers, their 

 shortening would slacken them, ending the stimulus 

 to the receptor and to the continuation of micturi- 

 tion. On the other hand, if the receptors are ar- 

 ranged in series, isometric and even isotonic contrac- 

 tion should increase the rate of firing during detrusor 

 activity. In a chloralosed cat, the bladder distended 

 with 5 to 15 ml of water characteristically contracts 

 rhythmically, although not strongly. A few impulses 

 at low frequency occurred in a single fiber as fluid 

 returned to the bladder (fig. 19, lower record). As 

 the bladder contracted, there was a sudden increase 

 in the rate of firing which stopped completely when 

 the bladder started to relax. When flow of fluid 

 from the bladder was restrained, firing coincident 

 with contraction was sharph' increased (from 5 per 

 sec. to 30 or 40 per sec), and the discharge was sus- 

 tained at a high rate, 20 per sec, for as long as half 



