MICTURITION Dr Mangala Gunatilake Dept. of Physiology

Body Trigone (on the posterior wall) Neck Smooth muscle of the bladder (detrusor muscle) is arranged in spiral, longitudinal and circular bundles. The muscle bundles pass on either side of the urethra are called the internal urethral spincter. External urethral spincter

Filling of the bladder When urine collects in the renal pelvis, the pressure in the pelvis increases. This increase in the pressure initiates a peristaltic contraction beginning in the pelvis and spreading downward along the ureter to force urine toward the bladder. Peristaltic waves occur 1-5 times/minute The walls of ureters contain smooth muscle arranged in spiral, longitudinal and circular bundles.

In some people, the distance that the ureter courses through the bladder mucosa is less than normal, so that contraction of the bladder during micturition does not always lead to complete occulusion of the ureter. As a result some of the urine in the bladder is propelled backward into the ureter. This is called ‘Vesicoureteral reflux’. Ureterorenal reflex The ureters are well supplied with pain nerve fibers. When a ureter is blocked eg. by a ureteral stone, there will be intense reflex constriction which is associated with very severe pain. These pain impulses cause a sympathetic reflex back to the kidney to constrict the renal arterioles, thereby decreasing urinary output from that kidney. This effect is known as ‘Ureterorenal reflex’. The ureters pass obliquely through the detrusor muscle and it passes little further underneath the bladder mucosa. This oblique passage tends to keep the ureters closed except during peristaltic waves, preventing reflux of urine from the bladder.

L1L2L3L1L2L3 Sympathetic nerve supply Sympathetic chain Hypogastric ganglion Hypogastric nerve Urethra External sphincter Parasympathetic nerve supply S2S3S4S2S3S4 S2S3S4S2S3S4 Pelvic nerve Pudendal nerve Innervation of the bladder Somatic nerve supply

Sympathetic nerve supply and Internal urethral sphincter apparently play no role in micturition. They prevent reflux of semen into the bladder during ejaculation. Parasympathetic nerve supply Sensory fibers in the pelvic nerve carry impulses from stretch receptors present on the wall of the urinary bladder to the spinal centre of micturition. Stimulation of parasympathetic efferent fibers causes contraction of detrusor muscle leading to emptying of urinary bladder. Somatic nerve supply This maintains the tonic contractions of the skeletal muscle fibers of the external sphincter, so that this sphincter is contracted always. During micturition this nerve is inhibited, causing relaxation of the external sphincter and voiding of urine.

What is micturition? Spinal cord reflex activity. * facilitated or inhibited by higher centers * voluntary facilitation or inhibition

The relationship between the volume of urine and pressure in the urinary bladder (intravesical pressure) can be studied by inserting a double lumen catheter and emptying the bladder. Then the pressure is recorded by connecting one lumen of the catheter to a suitable recording instrument while introducing water or air through the other lumen. The graphical recording of the pressure changes in the urinary bladder in relation to rise in the volume of urine collected in it is called cystometrogram. Cystometry is the technique used to demonstrate this relationship.

Cystometrogram Intravesical volume (mL) Intravesical pressure (cm of Water) Ia Ib

When the urinary bladder is empty, the intravesical pressure is zero. When about 50 mL of fluid is collected, the pressure rises sharply (Ia)to about 10 cm H 2 O (Ia in the cystometrogram). The pressure in the bladder remains more or less constant with further addition of about 350 mL of urine (Ib) in an adult. This is in accordance with law of Laplace. In the bladder tension increases as the urine is filled. At the sametime, the radius also increases due to relaxation of the detrusor muscle. Because of this, the pressure rise is almost nil. When bladder wall stretches during filling it will initiate a reflex contraction which has lower threshold. That does not trigger micturition reflex. When bladder is filled about 300 – 400 mL of urine, there will be sharp rise in the intravesical pressure as the micturition reflex is triggered. When, urine of about 400 mL is collected, the contraction of detrusor muscle becomes intense, increasing the consciousness and the urge for micturition. At this point also voluntary control is possible. Beyond 600 – 700 mL of urine voluntary control starts failing.

Micturition contractions Basal cystometrogram Volume (milliliters) Intravesical pressure (centimeters of water) la lb

Filling of the bladder – partially filled Reflex contractions Acute increase in pressure Contractions relax spontaneously Pressure falls back to baseline Bladder continues to fill Reflex contractions – more frequently and powerful Filling of the bladder ………..

Spinal centres of micturition which are present in scral and lumbar segments are regulated by higher centres in the brain stem (Facilitatory area in the pontine region and inhibitory area in the mid brain). The threshold for the voiding reflex is adjusted by the activity of facilitatory and inhibitory centres. When the micturition is facilitated, perineal muscles and external urethral sphincter are relaxed, the detrusor muscle contracts and urine passes out through the urethra. During micturition, the flow of urine is facilitated by increase in the abdominal pressure due to voluntary contractions of abdominal muscles. After urination, the female urethra empties by gravity. Urine remaining in the urethra of the male is expelled by several contractions of the bulbocavernosus muscle.

Stimulates contraction of detrusor muscle Nerve endings sensitive to stretch Spinal cord Simple reflex control of micturition seen in infants. The ability of voluntary control (inhibition) develops at the age of 2 – 3 years.

Spinal cord Nervous control of micturition Brain stem & Cerebral cortex

Abnormalities of micturition 1.Atonic bladder This is due to destruction of sensory nerve fibers from urinary from the bladder. When the dorsal sacral roots are interrupted by diseases of the dorsal roots such as tabes dorsalis or when there is crush injury to sacral segments of spinal cord, person looses bladder control (abolition of reflex contractions of the bladder). Bladder muscle looses the tone (hypotonic) and becomes flaccid). Bladder fills to the capacity and overflows few drops at a time through the urethra (overflow incontinence or overflow dribbling).

2. Automatic bladder (Spastic neurogenic bladder) During spinal shock after complete transection of spinal cord above sacral centres of micturition, the urinary bladder looses its tone and becomes flaccid and unresponsive. So, the bladder is completely filled, and later urine overflows by dribbling. After the spinal shock has passed, the voiding reflex returns although there is no voluntary and higher centre control. Whenever, the bladder is filled with some amount of urine, there is automatic evacuation of the bladder.

3. Uninhibited neurogenic bladder Due to a lesion in some parts of brain stem (interrupting most of the inhibitory signals), there is continuous excitation of spinal micturition centres by the higher centres. There is uncontrollable micturition. Even a small quantity of urine collected in bladder will elicit the micturition reflex increasing the frequency of micturition.

Nocturnal micturition (Bed wetting) This is normal in infants and children below 3 years. It occurs due to incomplete myelination of motor nerve fibers of the bladder resulting loss of voluntary control of micturition.