structure and function of lower urinary tract

• The ureteral openings and the internal urethral meatus forms a triangular association between these regions (trigone)
• Male bladder has larger muscle layer, to try and combat the resistance provided by the prostate gland
• bladder wall is made of 4 layers:

  • transitional epithelium (uroepithelium)
  • lamina propria
  • submucosa
  • serosa
• The uroepithelium of the bladder:

  • multi-layered epithelium; apical (umbrella cells)
  • functions include: barrier, afferent signalling
• The lamina propria of the bladder:

  • “functional centre” coordinating uroepithelium and detrusor
  • blood vessels, nerve fibres, myofibroblasts
• The submucosa of the bladder:

  • smooth muscle arranged in bundles
  • functional syncytium
  • each detrusor cell - 600 x 5 microns
  • stroma: collagen and elastin
  • innervation of muscle: post ganglionic parasympathetic
• Histology of the bladder:

  • apical membrane, tight junctions ,adhering junction
  • desmosomes, and gap junctions
• The bladder serves as a compliant reservoir for storing urine
• Despite changes in volume, the pressure within the bladder remains constant
• The bladder exhibits high compliance due to its visco-elastic properties (attributed to elastin and collagen). This allows detrusor muscle relaxation without altering tension
• Sensors in the bladder detect increased wall tension during filling (via tight junction stretching)
• Afferent neurons relay real-time sensory data about the bladder state to the brainstem and higher centers
• Barrier Function of the Bladder:

  • The bladder’s barrier function involves glycosaminoglycan (GAG) layers and tight junctions.
  • It allows passive passage of urea, sodium, and potassium.
  • While it resists water passage, it is not entirely waterproof.
  • Damage to the uroepithelium plays a role in certain diseases.
• The Volitional/Micturition Voiding process involves:

  • The spino-bulbar reflex.
  • Modulation of the pontine micturition center (Barrington’s nucleus).
  • Onuf’s nucleus in the internediolateral segments S2, S3, and S4.
  • Feeling uncomfortable when the bladder is full at 250 ml and experiencing detrusor contractions at 500 ml.
  • Coordinating detrusor muscle contraction and urethral relaxation.
  • Relaxing the external urethral sphincter to allow urine to enter the posterior urethra.
• The brainstem plays a central role in controlling micturition
• Micturition as a Positive Feedback Loop (Inhibitory Controls):

  • Detrusor muscle contractions lead to increased wall tension.
  • Afferent signals from the bladder reach the pontine micturition center (PMC).
  • Efferent signals increase detrusor contraction.
• • The urethra is responsible for transporting urine from the bladder to the exterior.
• volitional voiding/micturition, is a muscular function
• micturition is coordinated by detrusor muscle contraction, and urethral relaxation
• Bladder fullness is felt at 250 ml and becomes uncomfortable at 500 ml
• micturition is controlled by:

  • spino-bulbar reflex
  • modulation of pontine micturition centre (Barrington’s nucleus)
  • Onuf’s nucleus (internediolateral of s2,3,4)
• relaxation of external urethral sphincter leads to urine entering the posterior urethra
• micturition is a positive feedback loop (inhibitory controls)
• micturition process is:

  detrusor contacts → wall tension rises → afferent signals to PMC → efferent signals - increases detrusor contraction
• central control of micturition


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• urethra in males is split into parts:

  • anterior urethra forming the penile urethra and
  • posterior urethra forming the prostatic urethra
• the bladder is protected by facia layers and the pubic rami anteriorly, and the iliac wings posteriorly
• normal neurophysiology of bladder is under the control of the highest centres the pons and the cerebrum, and the PMC being the foremost amongst
• Neurophysiology of bladder filling

  • afferent signals sent by the bladder muscle or gland or muscle stretch to the sacral spinal cord ‘relaying centre’
  • the signals are sent up the spinal cord to the higher centres (pons, cerebrum)
  • in the higher centres the afferent signals are processed and efferent signals are sent down
  • once the bladder is full, volitional voiding can begin (when socially convenient)
• neurophysiology of bladder voiding is:

  • coordinated detrusor muscle contraction and sphincter relaxation, via the pelvic nerves, parasympathetic pelvic nerves and the pudendal nerves
• nitric oxide has a role in the relaxation of bladder neck/EUS
• Ach is the excitatory neurotransmitter; GABA and glycine inhibitor neurons for bladder functions
• bladder activity subject higher centres and local reflexes

  • facilitation: contraction of detrusor and relaxation of sphincter when bladder less than full (e.g., anxiety states)
  • inhibition: allows postponement of voiding (e.g., when socially unacceptable)
• spinal cord injury can lead to loss of central inhibition, and the typically reflex voiding, lesions can be grouped into:

  • suprapontine
  • spinal
  • sacral/infrasacral
• suprapontine lesion would present as:

  • history: predominantly storage symptoms
  • ultrasound: insignificant PVR urine volume
  • urodynamics: detrusor overactivity
• spinal (infrapontine - suprasacral) lesions would present as:

  • history: both storage and voiding symptoms
  • ultrasound: PVR urine volume usually raised
  • urodynamics: detrusor overactivity, detrusor-sphincter dyssynergia
• sacral/infresacral lesion would present as:

  • history: predominantly voiding symptoms
  • ultrasound: PVR urine volume raised
  • urodynamics: hypocontractile or acontractile detrusor
• bladder is responsible for the storage of urine
• when bladder contains 300ml (and it is socially convenient) voiding is initiated
• normal voiding pattern is 300-400 ml per void 4-5 per day (<7)- depending on input