Urethral sphincter, contracting

The urethral sphincter, a combination of smooth and striated muscles within and external to the urethra, maintains adequate resistance to the flow of urine from the bladder until voluntary voiding is initiated. Normal bladder emptying occurs with opening of the urethra concomitant with a volitional bladder contraction. 

The effect of catecholamines on the human uterus, which can be mediated by a- and /3-adrenoceptors, depends on its functional state. During pregnancy /32-adrenoceptor stimulation decrease the uteral tonus, an effect that can be used therapeutically. /32-Adrenoceptor agonists are in use as tocolytics. In the bladder base and the urethral sphincter a-adrenoceptors are present, mediating a contraction, whereas the /32-adrenoceptors of the bladder wall induce a relaxation of the particular smooth muscles present at these structures. Ejaculation is regulated by a-adrenoceptors. 

Einally, external urethral sphincter (voluntary sphincter, or rhabdosphincter) is a striated circumscribing structure emanating from the bladder neck and bladder base detrusor through the mid-urethra in the female and intermediate prostatic urethra in the male. While also surrounding Cowper s glands in the male, these rhabdosphincter subunits contract, most likely, only with ejaculation (Hutch, 1972 Elbadawi, 1980), along with simultaneous anal rhabdosphincter, bulbo-cavernosus muscle, and cremaster muscle contractions. 

In genitourinary organs, the bladder base, urethral sphincter, and prostate contain a receptors that mediate contraction and therefore promote urinary continence. The specific subtype of 04 receptor involved in mediating constriction of the bladder base and prostate is uncertain, but cxia receptors probably play an important role. This effect explains why urinary retention is a potential adverse effect of administration of the 04 agonist midodrine. 

The human uterus contains and B2 receptors. The fact that the Breceptors mediate relaxation may be clinically useful in pregnancy (see Clinical Pharmacology). The bladder base, urethral sphincter, and prostate contain receptors that mediate contraction and therefore promote urinary continence. The specific subtype of ai receptor involved in mediating constriction of the bladder base and prostate is uncertain, but uia receptors probably play an important role. The B2 receptors of the bladder wall mediate relaxation. Ejaculation depends upon normal a-receptor (and possibly purinergic receptor) activation in the ductus deferens, seminal vesicles, and prostate. The detumescence of erectile tissue that normally follows ejaculation is also brought about by norepinephrine (and possibly neuropeptide Y) released from sympathetic nerves. Alpha activation appears to have a similar detumescent effect on erectile tissue in female animals. 

The bladder functions as a low-pressure reservoir, filling at the rate of 2 mL/min until approximately 360-400 mL is reached, and the intravesical pressure increases. This pressure activates proprioceptive receptors in the bladder wall to signal the sacral spinal cord, thus triggering detrusor contraction. Sensory stimulation occurs at the micturation center in the brainstem that coordinates urethral sphincter relaxation as the detrusor muscle contracts. Higher controls in the frontal lobe can block this sensory message until conscious direction permits a voluntary void. Medical insults to the spinal column, peripheral sensory nerves, and cerebral cortex will cause malfunction in the voiding pattern (7). 

Fig. 6.7. a VCU in a 7-year-old boy with dysuria, showing extrinsic compression of the urethra due to abnormal sphincter contraction during micturition. This functional anomaly should not be mistaken for posterior urethral valves. Urody-namic studies favor dysfunctional voiding with bladder-sphincter dyscoordination during voiding. Biofeedback physiotherapy was carried out. Clinical outcome was favorable, b Follow-up VCU shows normalization of urethral anatomy during... 

By the age of 4-5 years many children have been toilet-trained successfully and have adopted an adult pattern of urinary control. This is also characterized by the absence of involuntary or uninhibited detrusor contractions during bladder fiUing. Even if the bladder is full and there is a strong desire to void, no bladder contractions will occur. With micturition, coordinated relaxation of the external urethral sphincter takes place. Therefore, bladder emptying is under low intravesical pressure in children and adults. 

The child, attempting to maintain continence during such contractions, must voluntarily and tightly constrict the external urethral sphincter to stay dry. This results in simultaneous and unphysi-ological contraction of both the bladder and external urethral sphincter. During this event functional urinary obstruction and high intravesical pressure... 

In about 70% of cases this dysfunction leads to (urge) incontinence, which is clinically manifested as wetting (mostly daytime, but nighttime as well). But even in severe cases the obligatory voluntary contraction of the striated urethral sphincter against the contracting detrusor can prevent leakage in up to 30% of cases. 

Fig. 14.2a-c. Male, 2 months old moderate bilateral fetal hydronephrosis, VCU. Reduced bladder filling volume (20 ml) residual urine early uninhibited detrusor contractions transformed into premature micturition, a Minor bladder trabecu-lation, short phase of normal micturition. b,c Dyscoordinated voiding, contraction of external urethral sphincter dilated posterior urethra, male spinning top urethra... 

The combination of a transient opening of the bladder neck with a flow of contrast material into the posterior urethra up to the voluntarily contracted striated urethral sphincter (Potter et al. 1986 Passerini-Glazel et al. 1992) together with cessation and/or back-up of contrast material drip flow suggests the presence of an uninhibited detrusor contraction (Fig. 14.3). These findings are valid they can stand alone without urodynamic results. Modified VCU allows detection of the majority of these dysfunctions in neonates, infants and small children with the same reliability and in the same way as in older age groups. 

The detrusor, whose smooth muscle fibres comprise the body of the bladder, is innervated mainly by parasympathetic nerves which are excitatory and cause the muscle to contract. The internal sphincter, a concentration of smooth muscle at the bladder neck, is well developed only in the male and its principal hmction is to prevent retrograde flow of semen during ejaculation. It is rich in aj-adrenoceptors, activation of which causes contraction. There is an abvmdant supply of oestrogen receptors in the distal two-thirds of the female urethral epithelium which degenerates after the menopause causing loss of urinary control. 

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