Neural Control of Micturition and Urinary Incontinence

Anatomy and Musculature of the Bladder

  • Micturition Definition: Elimination of urine.
  • Bladder Function:
    • Fundus: Urine storage.
    • Bladder neck: Connects bladder to urethra.
  • Major Muscular Units:
    • Detrusor muscle.
    • Internal urethral sphincter.
    • External urethral sphincter.
  • Bladder Wall:
    • Layers: Mucosa, Smooth muscle, Lamina propria.
    • Smooth muscle layers are criss-crossed for strength during stretch.
  • Sphincter Composition:
    • Internal: Smooth muscle, autonomic nervous system.
    • External: Skeletal muscle, somatic nervous system (voluntary control).
  • Urinary Incontinence (Post-Birth):
    • Causes: Intrinsic sphincter deficiency, urethral hypermobility, or both.

Nervous Control

  • Divisions Innervating Bladder:
    • Parasympathetic.
    • Sympathetic.
    • Somatic.
  • ANS Components:
    • Sympathetic: Noradrenaline, Alpha and Beta receptors.
    • Parasympathetic: ACH, M2 or M3 receptors (bladder).
  • Sympathetic Efferents Innervate: Detrusor, trigone, internal sphincter
  • Parasympathetic Efferents Innervate: Detrusor, trigone, internal sphincter
  • Somatic Nerve: Pudendal nerve (S2, S3, S4).
  • Physiological Sphincter with Same Root Values: Anal sphincter.

Storage/Continence Phase

  • Brain Centers:
    • Cortical Storage Centers command Pontine Continence Centers.
    • Cortical centers reduce parasympathetic drive and increase sympathetic output.
  • Spinal Cord Neurons: Sympathetic neurons mediate continence.
  • Neuronal Consequences of Stimulating Spinal Continence Neurons:
    • Silencing of detrusor activity.
    • Increase in urethral sphincter pressure.
  • Adrenoreceptor Agonists:
    • Bladder: Beta adrenoreceptors (relaxation of detrusor muscle).
    • Urethral Sphincter: Alpha adrenoreceptors (bladder neck).
  • Other Effector Sites: External urethral sphincter (closure via somatic motor system).
  • Intravesical Pressure: Slight increase during bladder filling.
  • Stretch-Relaxation Phenomenon: Compensatory accommodation of increased urine volume.

Sensory Function of the Bladder

  • Afferent Fiber Origins: Throughout the wall of the urinary bladder.
  • Sensory Information (Pudendal Nerve):
    • General pain.
    • Degree of fullness.
    • Urine temperature.

Micturition Phase

  • Areas Responsible for Voiding: Micturition or voiding centers.
  • Brain Centers:
    • Cortical Micturition Centers command Pontine Micturition Centers.
    • Cortical centers increase parasympathetic output and reduce sympathetic drive.
  • Highest Brain Center: Cerebral Cortex (conscious decisions).
  • Pons Region Active During Micturition: M region (dorsomedial pontine tegmentum); damage leads to urinary retention.
  • Nervous System Division for Voiding: Parasympathetic (S2-S4).
  • Detrusor and Internal Sphincter: Contraction of detrusor, relaxation of trigone and internal sphincter.
  • Somatic Nervous System: Reduced activity, leading to relaxation of external urethral sphincter (S2-S4).
  • Frequency of Action Potentials:
    • Bladder Stretch: Increase in frequency of action potentials from urinary bladder.
    • Urination: Decreased action potentials cause relaxation of external urinary sphincter.