The urinary bladder is a visceral smooth-muscle organ but is under voluntary control from the cerebral cortex. Normal bladder function requires coordinated interaction of afferent and efferent components of both the somatic and autonomic nervous systems. Because many levels of the nervous system are involved in the regulation of voiding function, neurologic diseases often cause changes in bladder function. Examples are multiple sclerosis, spinal cord injury, cerebrovascular disease, Parkinson disease, diabetes mellitus, meningomyelocele, and amyotrophic lateral sclerosis. Injury to the sacral roots or pelvic plexus from spinal surgery, herniation of an intervertebral disk, or pelvic surgery (hysterectomy, abdominoperineal resection) can also cause neuropathic bladder.
Significant bladder dysfunction may occur as a result of poor voiding habits in childhood or of degenerative changes in bladder muscle and nerve endings caused by aging, inflammation, or anxiety disorders. All the above conditions can disrupt efficient reflex coordination between sphincter and bladder, and with time, this leads to symptomatic dysfunction.
The bladder wall is composed of a syncytium of smooth-muscle fibers that run in various directions; however, near the internal meatus, three layers are distinguishable: a middle circular layer and inner and outer longitudinal layers. In females, the outer layer extends down the entire length of the urethra, while in males, it ends at the apex of the prostate. The muscle fibers become circular and spirally oriented around the bladder–urethra junction. The middle circular layer ends at the internal meatus of the bladder and is most developed anteriorly. The inner layer remains longitudinally oriented and reaches the distal end of the urethra in females and the apex of the prostate in males. The convergence of these muscle fibers forms a thickened bladder neck, which functions as the internal smooth-muscle sphincter.
The normal bladder is able to distend gradually to a capacity of 400–500 mL without appreciable increase in intravesical pressure. When the sensation of fullness is transmitted to the sacral cord, the motor arc of the reflex causes a powerful and sustained detrusor contraction and urination if voluntary control is lacking (as in infants). As myelinization of the central nervous system progresses, the young child is able to suppress the sacral reflex so that he or she can urinate when it is appropriate.
The functional features of the bladder include (1) a normal capacity of 400–500 mL, (2) a sensation of fullness, (3) the ability to accommodate various volumes without a change in intraluminal pressure, (4) the ability to initiate and sustain a contraction until the bladder is empty, and (5) voluntary initiation or inhibition of voiding despite the involuntary nature of the organ.
In both males and females, there are two sphincteric elements: (1) an internal involuntary smooth-muscle sphincter at the bladder neck and (2) an external voluntary striated muscle sphincter ...