Physiology of female urinary bladder, urethra and pelvic floor

Unknown April 05, 2012


Introduction 
Mechanisms of urinary continence
  1- Urinary bladder wall
  2- Bladder neck
  3- Urethra
  4- Integral theory for normal function

 

Physiology of pelvic floor

Neural control of the lower urinary tract

 Introduction
The urinary bladder is a complex organ that has a relatively simple function to store urine effortlessly, painlessly and without leakage and discharge urine voluntarily, effortlessly, completely and painlessly. To meet these demands, the bladder must have normal anatomic support as well as neurophysiologic function 
The urethra is composed of striated and smooth muscles. Contraction of the longitudinal smooth muscle could play a role in stabilizing the urethra and allowing force generated by the circular muscular elements to occlude the lumen or in aiding in the opening of the BN during micturition  

Normal lower urinary tract function
          The two-phase concept of function: filling/storage and emptying/voiding. Micturition cycle involves two relatively discrete processes:

1-    Bladder filling and urine storage requires
a-     Accommodation of increasing volumes of urine at a low intravesical pressure with appropriate sensation.
b-    A bladder outlet that is closed at rest and remains so during increases in intra abdominal pressure.
c-     Absence of involuntary bladder contraction.

2-    Bladder emptying/voiding requires
a-     A coordinated contraction of the bladder smooth musculature of adequate magnitude and duration.
b-    A concomitant lowering of resistance at the level of the smooth and striated sphincter.
c-     Absence of anatomic (as opposed to functional) obstruction  

 

Mechanisms of urinary continence

The mechanisms that control urinary continence and voiding are complex. Normal function of the central and peripheral nervous system, bladder wall, detrusor muscle, urethra, correct positioning of the BN and pelvic floor musculature are required. Dysfunction can occur at any of these levels, resulting in various types of LUT dysfunction  

1-Urinary bladder wall
During physiologic bladder filling, little or no increase in the intravesical pressure is observed, despite large increases in urine volume. This process, called accommodation, is caused primarily by the passive elastic and visco elastic properties of the smooth muscle and CT of the bladder wall. As filling increases to a critical intravesical pressure, detrusor muscle contractility is probably inhibited by activation of a spinal sympathetic reflex, which results in inhibition of parasympathetic ganglion transmission and stimulation of beta-adrenergic receptors in the bladder body  

2- Bladder neck (BN)
Early in filling, the BN is in the closed position by neurally modulated activity of the smooth and skeletal muscle combined with non-muscular effects of collagen and elastin tissue as well as urethral supporting structures. The BN only opens as capacity volume is reached and the arrangement of muscle fibers of the proximal urethra and BN is altered allowing for initiation of voiding. By assessing the bladder at 200ml, it eliminates the confounding factor of mechanical effacement of the BN at maximum capacity  

3- Urethra
A- Resting intra urethral pressure
For a patient to remain continent, intra urethral pressure must be greater than intravesical pressure under both resting and stress conditions.  Multiple clinical factors, such as age and obstetric history, can affect the function of these urethral components 

B- Urethral support and pressure transmission concept
It appears that intact support of the BN and proximal urethra in a retro pubic position is important for maintenance of urinary continence under stress. The proximal urethra and BN are supported in a sling like fashion by the anterior vaginal wall which is attached bilaterally to the pelvic diaphragm. The vagina thus provides a stable base onto which the urethra and BN rest. With increase in intra abdominal pressure, as with coughing, pressure increases are transmitted equally to the bladder and urethra, maintaining urethral closure and thus continence 

The stable suburethral layer of vaginal wall and endopelvic fascia prevents urethral and BN descent and causes urethral compression with straining (Hammock hypotheses for urinary continence)  

4- Integral theory for normal function
International Continence Society (ICS) classification for LUT dysfunction is predicated on bladder and urethra interacting as a functional unit   The normal bladder has only 2 stable modes open and closed. The integral theory added an extrinsic mechanism to this concept activated by specific pelvic floor muscles  

          Perineal sonography (PU) can visualize micturition in a physiologic setting position. Bladder base, BN and midurethra, the important structures of micturition, are visible consistently throughout the voiding phase  

Factors contributing to the female continence mechanism under resting and stress conditions  

Pressure transmission concept for SUI and incontinence: A, in continent women, rises in intra abdominal pressure are transmitted equally to the bladder and urethra. B, in women with anterior vaginal support defects, the bladder base descends and the urethra rotates during increases in intra abdominal pressure. This can lead to decreased pressure transmission to the urethra relative to the bladder, which then results in SUI  


Lateral view of pelvic floor with urethra, vagina and fascial tissues at level of BN indicating compression of urethra by downward force “arrow” against supportive tissues, indicating influence of abdominal pressure on urethra (Quoted from DeLancey, 1994).


Schematic 3 dimensional view of bladder supported by vagina. The arrows represent directional striated muscle forces. 'N' = stretch receptors at bladder base  

Physiology of pelvic floor
      Pelvic floor muscles play an important role in maintaining urinary continence 
   The muscles of the pelvic floor form a complexly built sustaining structure, which bears the whole weight of the visceral column. 

Improved vesical neck stabilization has showed that a volitional contraction of the pelvic floor muscles just before and throughout a cough, a preemptive maneuver called the knack because skill is involved, can be used to reduce stress related urine leakage significantly  
         
Neural control of the lower urinary tract
Urine storage in the bladder occurs via autonomic processes. In contrast, bladder emptying is usually a voluntary process in older children and adults. Both the central and peripheral nervous systems are involved in micturition and continence 
The precise neurological pathways and neurophysiologic mechanisms that control micturition are complex and not completely understood. The storage and expulsion of urine are part of a complex neurophysiologic function that involves autonomic and somatic nervous systems 

Peripheral innervation of the female LUT  





Actions of the autonomic and somatic nervous systems during bladder filling/storage and voiding  

Autonomic nervous system
        It controls the LUT by its actions on the ganglia, detrusor muscle, and smooth 
muscles of the trigone and urethra  
Peripheral somatic motor system
          Somatic efferent branches of the pelvic nerves variably innervate proximal intramural component of the striated urogenital sphincter (sphincter urethrae, rhabdosphincter), a component of the pelvic plexus. The more distal periurethral striated muscles (compressor urethrae and urethrovaginal sphincter) are innervated by the pudendal nerve, as is the skeletal muscle of the external anal sphincter and perineal muscles. The neuronal cell bodies for the sphincter urethrae and for the distal periurethral striated muscles and pelvic floor muscles are located in Onuf’s somatic nucleus in the lateral aspect of the anterior horn of the gray matter of the sacral spinal cord from S2 to S4. The pudendal nerve passes between the coccygeus and piriformis muscles, leaves the pelvis through the greater sciatic foramen, crosses the ischial spine, and reenters the pelvis thorough the lesser sciatic foramen.  

Course and branches of the pudendal nerve in the female pelvis. Shaded area represents section of the nerve that is sometimes damaged with childbirth


Sensory innervation
          Detrusor proprioceptive endings exist as nerve endings in collagen bundles. They are stimulated by stretch or contraction and are responsible for the feeling of bladder fullness. Pain and temperature nerve endings are free in bladder mucosa and submucosa. The sensory endings in the detrusor probably contain acetylcholine and substance P. Two types of bladder sensors have been postulated, the first sensor perhaps are being at the trigone, the second being stretch receptors in the bladder body. Loss of the first sensor may lead to urge incontinence because the bladder is ready to contract before sensation is noted. Sensory innervation can follow both the sympathetic and parasympathetic nerves. Urgency is transmitted along parasympathetic pathways. Principally the pudendal nerve carries urethral sensation, although the pelvic nerve also contributes. Urethral smooth muscle sensory innervation, like that of the detrusor, has both a contralateral and an ipsilateral supply