when the person is addressrg a lecture to the public) the cortical center sends down an impulse by which the mieturitor reflex is inhibited,

intermediate horns of 1st and 2nd lumbar segments ofthespnal cord. These fibers eventually end In the hypogastric ganglia Hypogastric ganglia are therefore the relay station of both tie sympathetic and parasympathetic nerves The postganglicnic fibers of the sympathetic end in the detrusor and the internal sphincter Stimulation of the sympathetic efferent leads to relaxation ofthedetrusor and contraction of the internal sphincter and, as a result, the urine is retained in the bladder The afferent fibres belonging to the sympathetic system arise from the bladderproceed to the spinal cord lying side by side with the sympathetic efferents enter the dorsal horns or L1 and L.2 segments of the spinal cords. The sympathetic afferent fibres carry the pain sensation Sensation of the bladder 1 Mucous membrane of the bladder is normally insensitive to the light touch When inflamed (as n cystitis) it becomes sensitive. 2 The sense of stretching, that is. the degree of distension of the bladder (witch Is caused by accumulation of urine), arses from tie detrusor muscles and carried up by the afferent nerves. When distenson is only mild or moderate a 'desire to pass urine' develops, but when the distension cs severe, actual plam t felt and a tremendous desire to pass the urine arises In short, sense of distension is carried in parasympathetic. whereas pain sensation from bladder is earned by the sympathetic Cortical control The S2, S3 and L1 , L2 are spinal centers for bladder. A higher center, which may be called 'cortical center', also axists, one on either side, on the medial aspect of the cerebral hemisphere, in the motor homcnculus, close to the perinea! area (fig. 10C 1.1). Sense of distension from the spinal center (that Is, 52 & S3) reach the cortical center via the dorsal column, whereas pain sensation (from L1 & L2) reach the corlicaE center via the spinothalamlc tract Efferent fibers from this higher center descend down, remaining lateral to the pyramidal fibers and ultimatety reach the spinal centers Through these paths the higher center controls the spiral (or Iower) center, in transection of the spinal cord, these (i e. cortical) controls are. therefore, lost The internal sphincter. therefore, s purely involuntary, as it Is under complete control of the automatic But in addition, there is an external sphincter of the urethra, which surrounds Ihe membranous urethra. It is, his to logically made up of striated muscles and supplied by somatic nerves (via pudenda! nerve), that is, II should be purely voluntary One can voluntarily cause contraction of this (extemaF) sphincter, but voluntary relaxation of it cannot be made Thus,in the middle of an act of micturition one, if he desires, can stop the micturition temporarily by causing the external sphincter to contract voluntarily. But- unless there is some accumulation of urine in the bladder with a consequent rise of hydrostatic pressure within the bladder, relaxation of external sphincter tiy pure voluntary effort is not possible Mechanism of micturition The micturition reflexes Although, to some extent, this rs still a bil speculative, the essentials are as follows 1 When the bladder is sufficiently tilled, the hydrostatic pressure wtthin the bladder nses the rise Of pressure causes simulation of afferent nerves (parasympatheHc) spinal center is stmulated -* efferent parasympathetic fibers (from the spinal center) are now simulated contraction of the detrusor and relaxation of the menial sphincter evacuation This is the fundamental micturition reflex. 2 Durring the abovementioned reflex action, sympathetic efferent are inhibited the contraction of the detrusor and relaxation of tie internal sphincter are facilitated 3 However, filling of the bladder to a certan degree by itself is not enough From the spinal tenter, afferent fibers go to the cortex and a desire to micturate develops If it is inconvenient To pass urine, (eg, when the person is addressrg a lecture to the public) the cortical center sends down an impulse by which the mieturitor reflex is inhibited, so that the detrusor relaxes, the sphincter contracts and the desire disappears But a stage arrives (recall, the two ureters are pouring unne Into Ihe bladder remorselessfy), when the amount of urine to the bladder is greal, and the afferent discharges due to the sttekhing of the bladder arevery strong, at this stage the desire becomes irresistible 4 In the newborn and the nfants. the link between the cortical center and the spriai center Is not established As a result, whenever the bladder has the requisite volume of unrie, the evacuation occurs This is an example of automate bladder 5. During the act of micturition, as the urine passes through the urethra, further set of afferent impulses are produced and they reinforce the micturition reflex (this is one of the Barrington's reflexes, detailed discussion of Barringtons reflexes, however, are no longer popular in standard text books) 6. If the bladder is somewhat filled but the desire to micturate is yet to appear, but the subject decides to pass urine, he can do so This occurs, because, now, the cortex influences tie spinal centers in such a way, that micturition reflex s excited At this stage, some facts aboulthe interrelationship betwen the filling ofthe bladder and aflerent discharges and the desire to micfurate will be given 1 In the beginning, the bladder Is empty and the hydrostatic pressure (i e , the pressure within the bladder) is o( = one atmospheric pressure). 2 In the initial phases of filling, up to about 200 ml of urine accumulation, the rise of pressure Is very smart This Is because, at this stage, as Ihe urine accumulates, the detrusor relaxes to some extent and the rtseof hvdrostatic pressure in the bladder is small* Thereafter, it thses rather markedly After 500 ml of unnary accumulation, rise of pressure is sharp with even every slight degree of futher rise in urinary volume Fig. 8 4.1. in the initial phases of filing, the pressure within the bladder does not rise steeply Only in the later phase, the pressure begins to rise sharply with further rise Of bladder volume 3 Another factor e also important If sudden rise of urinary accumulation occurs, the pressure rises ratier sharply but if the urine is not passed the detrusor muscle again relaxes, and the pressure agar falls All these facts can be show expeimentaliy Maintenance of continence When the bladder is empty or oniymildhy filled, the detrusor remains relaxed and both the tnlemal and external sphincters are contracted This phenomenon, that is. the holding up of urine, is called continence and rs due to a reflex The refiex is initialed from the bladder, and at least in the species cat, the afferent impulses are conveyed by the parasympathetic afferent which terminates in the spiral efferent