Normal anatomy 

  • To understand the pathophysiology of the neurogenic bladder, the medical professionals must have a thorough understanding of the relevant anatomy and physiology.
  • Normal storage and emptying function of the bladder and urethra requires anatomically and functionally intact structures.
  • These include normal innervation (nervous system), normal smooth and striated musculature (detrusor and urethral muscles), and intact supporting structures (pelvic floor muscle).

Nervous System

  • Nervous system can be classified as the central nervous system (CNS) and peripheral nervous systems (PNS). Both CNS and PNS control the urinary bladder and urinary sphincter.
    • Central nervous system (CNS)
      • consists of the spinal cord and the brain.
      • controls most functions of the body.
    • Peripheral nervous system (PNS)
      • is the nerves projecting to the extremities, heart, lung, skin, and other organs outside the brain including the urinary bladder and urethral sphincter.
      • controls the somatic nervous system, which regulates muscle movements in response to the external stimuli like pain or touch.
      • also controls the autonomic nervous system which provides nerve input to the internal organs and generates automatic reflex responses.
  • PNS can be further classified as an autonomic nervous system and somatic nervous system.
    • Autonomic nervous system
      • Sympathetic
        • Sympathetic nervous system mobilizes organs during times of stress and arousal
      • Parasympathetic
        • Parasympathetic nervous system conserves energy and resources during times of rest and relaxation
    • Somatic nervous system
  • The spinal cord is the primary information pathway between the brain and all the other nervous systems.
    • It receives sensory input from the body and then relays upward to the brain.
    • It also carries signals downward from the brain to the PNS.
    • Any kind of injury to the spinal cord, therefore, can cause significant problems throughout the body.
  • The spinal cord is organized into segments:
    • cervical spinal nerves (C1 to C8),
    • thoracic spinal nerves (T1 to T12),
    • lumbar spinal nerves (L1 to L5) and
    • sacral spinal nerves (S1 to S5).
  • Each segment marks where spinal nerves emerge from the cord to connect to specific regions.
  • Names of spinal cord segments do not correspond exactly to vertebral levels
    • The sacral spinal cord begins at approximately spinal column level T12 to L1.
    • The spinal cord terminates in the cauda equina at the spinal column level of about L1 or L2.
  • Nervous Systems Related to the Lower Urinary Tract Organs

Urinary bladder

  • The urinary bladder is a storage organ as well as an emptying organ.
  • The urinary bladder is divided into two parts, a body and a base consisting of the trigone and bladder neck different in terms of the embryologic origin and innervation.
  • The muscle of the bladder is composed of smooth muscle fibers
  • There are several universal characteristics of smooth muscle:
    • The bladder smooth muscle maintains a steady level of contracture and tone.
    • Tone is important in maintaining the capacity of the bladder.
    • The contraction of smooth muscle is slow, sustained, and resistant to fatigue with very little energy cost. Smooth muscles of the bladder are interwoven.
  • The accommodation of the bladder to increasing volumes of urine is primarily a passive phenomenon of the bladder smooth muscle.
  • The bladder performs the following important functions:
    • It must store a socially adequate volume of urine. The bladder wall must be able to stretch and rearrange itself to allow an increase in bladder volume without significant rise in pressure. In other words, the bladder wall must be extremely compliant.
    • It empties stored urine by synchronous activation of all the detrusor smooth muscle.

Urethral sphincter

  • The structure, origin, the course and nature of the nerves destined for the human urinary sphincter remain the subject of controversy.
  • In the male, there are four segments of urethra are identified: 1) bladder neck or preprostatic portion 2) prostatic portion 3) membranous portion 4) bulbous and 5) penile urethra (fourth segment).
  • In women, the urethra extends throughout the distal third of the anterior vaginal wall from the bladder neck to the meatus
  • The urethral sphincter is a functioning structure in the urethra that blocks urinary incontinence during bladder filling state. Sphincteric structure is not single discrete and also is not visible.
  • Normal urinary sphincter mechanism may be divided into two functionally separate units: There is controversy about the relative roles of the smooth and striated urethral sphincters but it seems likely that both contribute to urinary continence
  Proximal urethral sphincter Distal urethral sphincter
  • Smooth Sphincter,
  • Lissosphincter
  • (Internal sphincter)
  • Striated sphincter, (External sphincter),
  • Covers 80% of the total anatomical urethral length [DeLancey 1990]
  • Rhabdosphincter,
  • Intramural striated sphincter,
  • Paraurethral sphincter
  • Pelvic floor musculature,
  • Periurethral striated sphincter
  • pubourethralis portion of the levator
  • Bladder neck (in male and female)

  • Prostate, and prostatic urethra to the level of the verumontanum (in male)
  • Male: Below the verumontanum through the membranous urethra; Deficient posteriorly (horseshoe shaped)

  • Female: Attenuated distal sphincter (female) and additional structures (female) [DeLancey 1988]
  • compressor urethrae
  • urethrovaginal sphincter
 
  • Smooth muscle
  • Skeletal muscle and smooth muscle [Karam 2005]
  • Slow-twitch fibers predominant [Elbadawi 1984]
  • Fast: slow = 35%: 65% (male); = 13%: 87% (female) [Padykula 1970]
  • Fast and slow-twitch fibers
  • Sympathetic nerves
  • Somatic and possibly mixed innervation with sympathetic (alpha-adrenergic) [Kakizaki 1991; Torrens 1987] and parasympathetic: * Triple innervation theory
  • Somatic
  • Hypogastric nerve (predominantly sympathetic) [Gosling 1977]
  • Pudendal nerve (predominantly somatic), possibly hypogastric (predominantly sympathetic) [Kakizaki 1991; Torrens 1987] and pelvic nerve (parasympathetic)
  • Levator ani nerve (somatic)
  • Active during urine storage (Involuntary maintenance of urinary continence)
  • Important role in reproduction
  • Primary role in the control of continence
  • Guarding reflex

* Triple innervation theory [Elbadawi 1996]

Ureter

  • Ureter
    • 3 muscular layers (inner longitudinal, middle circular, and outer longitudinal)
    • peristaltic movement triggered by a stretch stimulus provided by the bolus of urine
    • The normal ureter propels boluses of urine into the bladder in an antegrade fashion.
      • Pressure of the peristaltic ureter:
        • 20-35mmHg
      • pressure in the recipient bladder:
        • must be low enough to allow for the free egress of urine low resting pressures
        • 8-12 mmHg
    • Terminal portion of the ureter passes through bladder wall oblique fashion to form the ‘intravesical ureter (tunnel)’
  • Ureterovesical junction (UVJ)
    • As the ureter penetrates the bladder, its muscular layers disperse.
      • Ureteral outer circular layer and adventitia:
        • merge into the detrusor to form Waldeyer’s sheath
        • deep trigone
      • Ureteral inner longitudinal muscles
        • form the borders of the superficial trigone.
    • Gradual bladder filling or detrusor contraction occlude UVJ by the pressure increase on the ureter lumen (“flap-valve” effect), thereby VUR does not occur.

More Information on the Anatomy

  • What has become of the so-called internal sphincter and external sphincter?: These terminology is old and is blamed to be too simplified and not relevant. Recent studies show that the function and structure of the urinary sphincter is very complicated and, therefore, it is considered to be more suitable to describe this as two functional units, proximal and distal sphincters.
  • What role does the proximal sphincter do in the reproduction and how?: The maintenance of continence in men and some women with destruction or opening of the bladder neck argues that the bladder neck may not be the primary site of urinary continence [Chapple 1989]. Rather, the bladder neck is considered as an important for the reproduction. In men, closure of the bladder neck facilitates anterograde ejaculation.
  • What is so-called rhabdosphincter?: Distal urethral sphincter includes the rhabdosphincter and extrinsic paraurethral skeletal muscle. Rhabdosphincter is separate from the periurethral skeletal muscle of the pelvic floor.
  • Is the distal urethral sphincter composed of solely skeletal muscle?: Probably not. Rhabdosphincter is composed of intrinsic skeletal and smooth muscle. Rhabdosphincter consists of longitudinal smooth muscle and slow-twitch (type I) skeletal muscle fibers. These fibers intermingle with smooth muscle fibers of the proximal urethra, suggesting a dynamic or coordination interaction [Karam 2005].
  • Then, what is the periurethral skeletal muscle of the pelvic floor?: It is also called as striated fibers of the extrinsic paraurethral muscle or levator ani complex.
  • Is the periurethral skeletal muscle of the pelvic floor shares the same type of skeletal muscle of the rhabdosphincter?: No. pelvic floor muscle is composed mostly of fast twitch fibers capable of producing voluntary compression of the urethra. [Nanninga 1994; Gosling 1981; DeLancey 1988; Bazeed 1982]
  • What is the role of the periurethral skeletal muscle of the pelvic floor?: During sudden increases in abdominal pressure, these fibers can contract rapidly and forcefully to provide continence.
  • Is the role of the distal sphincter much bigger than that of proximal sphincter? Continence has been shown to be maintained after inducing paralysis of the striated sphincter [Krahn 1965]. This indicates that distal sphincter is not solely responsible for continence. Both proximal and distal sphincter seems to contribute to urinary continence.
  • Does a specific nerve always contain a single population of innervation? No. A single nerve can contain multiple different mixed innervations. For example, the pudendal nerve is generally considered somatic. However, the efferent trunk appears to contain approximately 25% sympathetic fibers. The other 75% are parasympathetic or somatic motor fibers. Only 12% of the efferent fibers are suggested to be somatic motor fibers [Hulsebosch 1982].
Key Points of This Section
  • Both CNS and PNS control the urinary bladder and urinary sphincter.
  • The urinary bladder is a storage organ as well as an emptying organ.
  • The urethral sphincter is a functioning structure in the urethra that blocks urinary incontinence during bladder filling state.
  • Normal urinary sphincter mechanism may be divided into two functionally separate units: the smooth and striated urethral sphincters.
  • Normally, once the urine is excreted from the kidney to the bladder through the ureter, reflux into the upper trat from the bladder is prevented by flap-valve antireflux mechanism.

 

References

  • Chapple CR, Helm CW, Blease S, Milroy EJ, Rickards D, Osborne JL. Asymptomatic bladder neck incompetence in nulliparous females. Br J Urol. 1989 Oct;64(4):357-9.
  • DeLancey JO. Structural aspects of the extrinsic continence mechanism. Obstet Gynecol. 1988 Sep;72(3 Pt 1):296-301.
  • Bazeed MA, Thuroff JW, Schmidt RA, Tanagho EA. Histochemical study of urethral striated musculature in the dog. J Urol. 1982 Aug;128(2):406-10.
  • DeLancey JO. Anatomy and physiology of urinary continence. Clin Obstet Gynecol. 1990 Jun;33(2):298-307.
  • Elbadawi A. Functional anatomy of the organs of micturition. Urol Clin North Am. 1996 May;23(2):177-210.
  • Elbadawi A. Ultrastructure of vesicourethral innervation. II. Postganglionic axoaxonal synapses in intrinsic innervation of the vesicourethral lissosphincter: a new structural and functional concept in micturition. J Urol. 1984 Apr;131(4):781-90.
  • Gosling JA, Dixon JS, Lendon RG. The autonomic innervation of the human male and female bladder neck and proximal urethra. J Urol. 1977 Aug;118(2):302-5.
  • Hulsebosch CE, Coggeshall RE. An analysis of the axon populations in the nerves to the pelvic viscera in the rat. J Comp Neurol. 1982 Oct 10;211(1):1-10.
  • Kakizaki H, Koyanagi T, Kato M. Sympathetic innervation of the male feline urethral rhabdosphincter. Neurosci Lett. 1991 Aug 19;129(2):165-7.
  • Karam I, Droupy S, Abd-Alsamad I, Uhl JF, Benoit G, Delmas V. Innervation of the female human urethral sphincter: 3D reconstruction of immunohistochemical studies in the fetus. Eur Urol. 2005 May;47(5):627-33
  • Krahn HP, Morales PA. The effect of pudendal nerve anesthesia on urinary continence after prostatectomy. J Urol. 1965 Sep;94(3):282-5.
  • Nanninga JB. Some aspects of striated urethral sphincter structure and function. J Urol. 1994 Dec;152(6 Pt 2):2321-3.
  • Gosling JA, Dixon JS, Critchley HO, Thompson SA. A comparative study of the human external sphincter and periurethral levator ani muscles. Br J Urol. 1981 Feb;53(1):35-41.
  • Padykula HA, Gauthier GF. The ultrastructure of the neuromuscular junctions of mammalian red, white, and intermediate skeletal muscle fibers. J Cell Biol. 1970 Jul;46(1):27-41.
  • Sebe P, Schwentner C, Oswald J, Radmayr C, Bartsch G, Fritsch H. Fetal development of striated and smooth muscle sphincters of the male urethra from a common primordium and modifications due to the development of the prostate: an anatomic and histologic study. Prostate. 2005 Mar 1;62(4):388-93
  • Torrens MJ: Human physiology. In: Torrens MJ, Morrison JFB (eds). The physiology of the lower urinary tract. Springer-Verlag, Berlin, 1987;333.

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