Physiology of Micturition and Bladder Dysfunctions

Introduction

Micturition is a complex physiological process involving coordinated interaction between the urinary bladder, spinal cord, brainstem, and cerebral cortex. Understanding the mechanisms controlling urine storage and voiding provides the basis for recognizing common bladder dysfunctions, including neurogenic bladder disorders associated with neurological injury.

• The urinary bladder functions as a reservoir that stores urine at low pressure and empties it voluntarily during micturition.
• Urine is transported from the kidneys to the bladder through the ureters by coordinated peristaltic contractions.
• Gravity may assist urine flow, but ureteric peristalsis is the primary mechanism of transport.
• Peristaltic activity originates from pacemaker cells located near the renal calyces and renal pelvis.
• The frequency of ureteric contractions varies with urine production and physiological conditions.
• The oblique passage of the ureters through the bladder wall acts as a functional valve.
• This mechanism prevents vesicoureteral reflux during bladder filling and voiding.
• Ureteric obstruction or urinary calculi may cause ureteric distension and severe colicky pain.

Urinary Bladder

Functional Aspects

Bladder Wall

• The urinary bladder is a distensible muscular organ located within the pelvic cavity.
• Its wall contains the detrusor muscle, which is composed of interlacing smooth muscle fibers arranged in longitudinal, circular, and oblique layers.
• Relaxation of the detrusor facilitates urine storage, whereas contraction produces bladder emptying.
• The bladder is lined by transitional epithelium (urothelium), a specialized epithelium that allows marked stretching without loss of its protective barrier function.
• This structure enables efficient storage and controlled expulsion of urine.

Innervation

• The urinary bladder receives autonomic and somatic innervation that coordinates urine storage and voiding.
• Parasympathetic fibers arise from the second, third, and fourth sacral spinal segments and travel through the pelvic nerves.
• Parasympathetic stimulation causes contraction of the detrusor muscle and promotes micturition.
• Sympathetic fibers originate mainly from the eleventh thoracic to second lumbar spinal segments and reach the bladder through the hypogastric nerves.
• Sympathetic activity facilitates urine storage by relaxing the detrusor muscle and increasing outlet resistance.
• Somatic motor fibers from the second, third, and fourth sacral segments travel through the pudendal nerve to supply the external urethral sphincter.
• Sensory fibers convey information regarding bladder filling, urgency, and painful distension to the central nervous system.

Urethral Sphincters

• Two urethral sphincters regulate urinary continence.
• The internal urethral sphincter consists of smooth muscle located at the bladder neck and is controlled by the autonomic nervous system.
• The external urethral sphincter is composed of skeletal muscle within the urogenital diaphragm.
• It is supplied by the pudendal nerve and remains under voluntary control.
• Coordinated relaxation of both sphincters and contraction of the detrusor muscle are essential for normal micturition.

Functions of Urinary Bladder

• The urinary bladder performs two essential functions: storage of urine and its periodic expulsion during micturition.
• During the storage phase, the detrusor muscle remains relaxed while the external urethral sphincter stays contracted.
• The first sensation of bladder filling is usually perceived at a volume of approximately 150–200 mL.
• The desire to void becomes more noticeable as bladder volume increases to about 250–300 mL.
• At volumes near 400–500 mL, the urge to urinate becomes strong and difficult to ignore.
• Excessive bladder distension may cause pain and can eventually result in involuntary urine leakage.

Micturition Reflex

• Micturition is the process of emptying the urinary bladder and is primarily mediated by a spinal reflex under higher neural control.
• Reflex activity is coordinated in the sacral spinal cord and modulated by centers in the brainstem and cerebral cortex.
• During the filling phase, urine accumulates in the bladder with only a small increase in intravesical pressure.
• This property is due to the high compliance and stress-relaxation characteristics of the detrusor smooth muscle.
• As the bladder expands, muscle fibers adapt to stretching, allowing storage of increasing volumes of urine without marked pressure elevation.
• Cystometry is a urodynamic procedure used to evaluate the relationship between bladder volume and intravesical pressure.
• The bladder is emptied through a catheter and then gradually filled while pressure changes are recorded.
• The resulting pressure–volume graph is known as a cystometrogram.
• Cystometry helps assess bladder compliance, sensation, capacity, and detrusor function.
• A slight rise in intravesical pressure occurs during the initial phase of bladder filling.
• Thereafter, pressure remains relatively constant despite progressive filling, usually up to a volume of approximately 300–400 mL.
• This plateau phase reflects the ability of the bladder wall to accommodate increasing volumes without substantial increases in pressure.
• The relationship is explained partly by Laplace’s law, which states that pressure within a hollow viscus depends on wall tension and radius.
• As bladder volume increases, both wall tension and bladder radius increase, minimizing changes in intravesical pressure over a considerable range.
• When bladder volume exceeds approximately 400 mL, intravesical pressure rises more rapidly.
• Stretch receptors in the bladder wall become increasingly activated and generate afferent signals to the spinal cord.
• These impulses initiate the micturition reflex, producing detrusor contraction and relaxation of the urethral outlet.
• Voluntary control from higher centers can either facilitate or suppress this reflex until an appropriate time for voiding is reached.
• Understanding bladder pressure–volume relationships is essential for evaluating disorders of urine storage and bladder emptying.

Mechanism of Micturition

• Micturition is initiated when bladder filling stretches the bladder wall and activates mechanosensitive stretch receptors.
• These receptors generate sensory impulses that travel to the spinal cord through afferent fibers in the pelvic nerves.

Stimulus and Reflex Arc

• The reflex center is located in the second, third, and fourth sacral spinal cord segments.
• Integration of these signals triggers the micturition reflex when bladder volume generally reaches about 300–400 mL.
• Parasympathetic efferent fibers arising from the sacral segments travel through the pelvic nerves to the bladder.
• Activation of these fibers causes contraction of the detrusor muscle and promotes bladder emptying.
• Simultaneously, urethral outlet resistance decreases, facilitating urine flow.
• Higher centers in the brain can either suppress or facilitate the reflex, allowing voluntary control of voiding.
• Sympathetic nerves primarily contribute to urine storage rather than micturition.
• During ejaculation, sympathetic stimulation causes contraction of the internal urethral sphincter at the bladder neck, preventing retrograde passage of semen into the urinary bladder.

Mechanism

• Micturition is predominantly a parasympathetic reflex that coordinates bladder emptying.
• Parasympathetic stimulation causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter.
• These actions increase intravesical pressure and facilitate the passage of urine into the urethra.
• Relaxation of the pelvic floor muscles and external urethral sphincter further reduces outlet resistance during voiding.
• Voluntary contraction of the external urethral sphincter can temporarily suppress urine flow despite activation of the micturition reflex.
• Contraction of the abdominal muscles and descent of the diaphragm increase intra-abdominal pressure and assist bladder emptying.
• In males, contraction of the bulbospongiosus muscle helps expel the final portion of urine from the urethra.
• Effective voiding requires coordinated activity of the bladder, urethral sphincters, pelvic floor, and abdominal musculature.

Control of Micturition

• Control of micturition involves the spinal cord, brainstem, hypothalamus, and cerebral cortex.
• The pontine micturition center in the brainstem coordinates detrusor contraction with sphincter relaxation and facilitates bladder emptying.
• Brainstem and higher centers regulate the timing and appropriateness of micturition.
• The hypothalamus contributes to autonomic regulation of bladder function.
• The cerebral cortex exerts voluntary inhibitory control over the micturition reflex, allowing continence until a suitable time for voiding.
• Voluntary urinary control develops gradually during early childhood as neural pathways mature.
• Most children acquire daytime bladder control between two and four years of age, although individual variation is common.
• Damage to supraspinal pathways may impair voluntary control and lead to abnormal bladder emptying patterns.
• Proper coordination between autonomic and somatic pathways is essential for normal urinary continence and micturition.

Bladder Dysfunctions

Abnormalities of Micturition

• Bladder dysfunctions occur when neural pathways involved in micturition are damaged at different levels of the nervous system.
• These disorders impair bladder sensation, storage, emptying, or voluntary control of urination.
• Major neurological mechanisms include interruption of afferent pathways, interruption of both afferent and efferent pathways, and loss of supraspinal control following spinal cord injury.
• In many neurogenic bladder disorders, bladder contraction is insufficient for complete emptying, resulting in residual urine and an increased risk of urinary tract infection.

Deafferentation

• Deafferentation occurs when sensory fibers from the bladder to the sacral spinal cord are damaged.

• Loss of afferent input abolishes the normal micturition reflex.
• The bladder becomes enlarged, hypotonic, and poorly responsive to filling.
• Some bladder contractions may persist because smooth muscle can respond directly to stretch.
• This pattern has been described in conditions affecting sensory pathways, such as tabes dorsalis.

Denervation

• Denervation results from interruption of both sensory and motor innervation of the bladder.
• Initially, the bladder becomes flaccid, distended, and unable to empty effectively.
• Urinary retention is common during the early phase.
• Over time, intrinsic activity of the bladder wall may produce intermittent contractions and urinary dribbling.
• Chronic changes may lead to bladder wall hypertrophy and reduced bladder capacity.
• Tumors involving the cauda equina or lesions affecting sacral nerve roots may produce this pattern.
• Increased responsiveness of denervated smooth muscle may contribute to later bladder hyperactivity.

Spinal Cord Transection

• Spinal cord transection commonly produces three stages: spinal shock, reflex recovery, and late deterioration.
• During spinal shock, the bladder is areflexic, resulting in urinary retention and overflow incontinence.
• As spinal reflexes recover, the micturition reflex returns, but voluntary control remains absent.
• The bladder develops involuntary contractions, reduced capacity, and detrusor hypertrophy.
• This condition is known as a spastic neurogenic bladder or reflex bladder.
• Patients may learn techniques that trigger reflex voiding.
• Recurrent urinary tract infections and chronic urinary retention can worsen bladder dysfunction and impair long-term bladder emptying.