Micturition, or the expulsion of the urine, does not normally depend on elastic forces alone, as in the case mentioned of paralytic incontinence, when the urine commences to dribble away as soon as a certain pressure is attained within the bladder, but is accomplished by the- detrusor urinoz muscle which lies in the wall of the bladder.
Under ordinary circumstances there is a relationship between the expelling and retaining powers (neither the muscle in the wall of the bladder nor voluntary effort, however, coming into action), in which the retaining power of the sphincter is just able to resist the elastic pressure. If the urine be retained for a considerable time, the reflex stimulation of the sphincter no longer suffices to keep back the fluid, and the voluntary effort has to be called to the aid of the reflex action of the sphincter. If a drop of urine happen now to make its way into the sensitive urethra, matters are altered. Even voluntary effort does not suffice to keep back the stream, and an irresistible call to empty the bladder is made upon the spinal nerve mechanisms. This is accomplished by the contraction of the muscular coat of the bladder, which is excited reflexly by the stimulus starting from the mucous membrane lining the urethra.
When the urine once commences to flow, it continues until the bladder is quite empty, the last drops of urine being expelled from the urethra by rhythmical spasms of the muscles around the bulbous portion of that canal. The sequence of events will then be - (i) stimulation of the mucous membrane of the urethra by a drop of urine; (2) contraction of the detrusor urinae; (3) relaxation of the sphincter; (4) rhythmical contraction of the ejaculator urinae, and, finally, a twitch of the levator ani and neighboring muscles.
The evacuation of the bladder is, under these circumstances, accomplished independently of the will, by a reflex act, of which we may even be unconscious.
This reflex micturition may occur during the sleep, as the result of slight local excitations. In infants this is the normal mode of emptying the bladder, and the gradual education of the centres controlling the retention mechanisms is watched with interest in young children.
At an early age, generally, we learn to control the acts of these centres by our will. We feel a desire to empty the bladder before it becomes so distended that the reflex contraction of the sphincter is insufficient to retain the urine. But the volition serves to call into activity the reflex mechanism just described. Almost at any time we can call forth the reflex act by increasing the pressure on the bladder by voluntary contraction of the abdominal muscles; the diaphragm being depressed and fixed, the muscles of expiration are put into action so as to press upon the pelvic viscera. At the same time the contraction of the sphincter muscle is probably checked by the will, and thus the power of retention is overcome.
The moment the balance of power is thus turned in favor of the expelling agencies, a drop of urine reaches the beginning of the urethra and excites reflexly the spinal centres, and thus brings about the complete evacuation of the bladder without further voluntary effort. The nervous mechanism that controls the act of micturition consists essentially of ganglionic centres which are situated in the lumbar enlargement of the spinal cord, and of two sets of nerve channels passing to and from these centres. The centres may be said to be composed of functionally distinct parts - a retaining and evacuating part. The retaining centre causes the sphincter muscle to contract. The evacuating centre can excite the detrusor to action. One set of nerve channels (3, 4, r, t) communicates between these centres and the urinary organs (b), and the other (1, 2) between the cord centres and the cerebral hemispheres (c). That which connects the special lumbar centres with the bladder, contains efferent fibres of two kinds, going to the antagonistic muscles, the sphincter vesicae (t), and the detrusor urinae (4) respectively, and afferent fibres of different kinds; those (r) going from the bladder to the nerve cells in the cord which stimulate them and cause the sphincter to remain tonically contracted; those passing from the mucous membrane of the urinary passages to the ganglionic cells in the cord have two functions; one (4) excites the contractions of the detrusor urinae and the other (3) inhibits the tonic action of the retaining centre.
Fig. 176. Diagram of the Nervous Mechanism of Micturition. B. Bladder. M. Abdominal muscles.
C. Cerebral centres.
R. Represents impulses which pass from the bladder to the centre in the spinal cord, whence tonic impulses are reflected and pass along T to sphincter, which retains the urine.
When the bladder is distended, impulses pass to the brain by 1, and when we will, the tonus of the spinal centre stimulating the sphincter is checked, and the abdominal muscles are made by 2 to force some urine into the neck of the bladder, whence impulses pass by 3 to inhibit the sphincter centre and excite the detrusor through 4.
The action of the ganglionic cells that stimulate the sphincter muscle can to a certain extent be either aided or checked by means of voluntary or other cerebral influences, so that two kinds of fibres - a stimulating and inhibitory one - must pass from the hemispheres to the micturating centre in the cord.
Those cells which govern the motions of the detrusor seem to be least under voluntary control, and are probably only stimulated to action by the impulses arising from the urinary passages, and hence are simply reflex, centres.
The effect of certain emotions on the act of micturition seems to show that those ganglion cells in the cord which cause the bladder to contract are connected with the higher centres. Thus, extreme terror (in a dog at least) often causes a forcible expulsion of urine, and great anxiety or impatience seems in man often to have a checking influence, causing great delay in initiating micturition.