Micturition Physiology

Bladder Basics - Anatomy Unveiled

• Location: Pelvic cavity, retroperitoneal.
• Structure: Hollow, muscular organ.
  • Apex: Anterior.
  • Body: Main storage area.
  • Fundus (Base): Posterior; ureters enter.
  • Neck: Connects to urethra.
• Trigone:
  • Smooth, triangular region at base.
  • Defined by ureteric orifices & internal urethral orifice.
  • Stretch-sensitive.
• Wall Layers:
  • Urothelium (Transitional Epithelium): Lines bladder; highly distensible.
  • Lamina Propria
  • Detrusor Muscle: Smooth muscle (3 layers); contracts for voiding.
  • Adventitia/Serosa (superiorly).

⭐ Urothelium's unique ability to stretch allows the bladder to accommodate varying urine volumes, typically 300-500 mL, without a significant rise in intravesical pressure (Laplace's Law application).

Nerve Networks - Wiring the Flow

• Autonomic Control (Involuntary):
  • Sympathetic (Storage): Origin T11-L2 (Hypogastric N.)
    • Detrusor: Relaxation (β3-receptors) → ↑ capacity.
    • Internal Sphincter: Contraction (α1-receptors) → continence.
  • Parasympathetic (Voiding): Origin S2-S4 (Pelvic N.)
    • Detrusor: Contraction (M3-receptors) → emptying.
    • Internal Sphincter: Relaxation (NO).
• Somatic Control (Voluntary): Origin S2-S4 (Pudendal N.)
  • External Sphincter (EUS): Voluntary contraction → continence.
  • 📌 S2, S3, S4 keep the pee off the floor.
• Sensory Pathways:
  • Bladder stretch receptors → pelvic nerves → spinal cord & PMC.
• Higher CNS Control:
  • Pontine Micturition Center (PMC): Key coordinator; switches storage/voiding.

    ⭐ The pontine micturition center (PMC) acts as the master switch for voiding.

  • Cerebral Cortex (Frontal Lobe): Conscious awareness, voluntary control (initiation/inhibition).
  • Others: Basal ganglia, thalamus, cerebellum influence.

Filling & Emptying - The Cycle Unpacked

Micturition cycle: urine storage and voiding, neurally orchestrated. Sensations: first urge 150-200ml, fullness 300-400ml. (📌 Sympathetic Stores, Parasympathetic Pees).

Laplace's Law: $P = 2T/r$.

Micturition Mishaps - When Flow Goes Wrong

• Urinary Incontinence:

  Type	Key Cause(s)	Key Symptom(s)
  Stress	Weak pelvic floor, urethral hypermobility	Leak with ↑intra-abdominal pressure (cough, sneeze)
  Urge	Detrusor overactivity	Sudden, intense urge; frequency, nocturia
  Overflow	Bladder outlet obstruction (BPH), Atonic bladder	Dribbling, poor stream, incomplete emptying

• Other Neurogenic Bladder Types:
  • Atonic Bladder (Sensory/Motor Paralytic): Damage to S2-S4 (e.g., DM, tabes dorsalis, cauda equina). Large, flaccid bladder; overflow incontinence; high PVR.
  • Automatic Bladder (Reflex/Spastic): SCI above sacral micturition center. Intact reflex arc; involuntary, frequent, small voids.

⭐ Detrusor overactivity is the hallmark of urge incontinence.

High‑Yield Points - ⚡ Biggest Takeaways

• Micturition reflex: A spinal reflex coordinated by the pontine micturition center (PMC) and cerebral cortex.
• Detrusor contraction: Parasympathetic (pelvic n., S2-S4; M3 receptors).
• Internal sphincter: Sympathetic (hypogastric n., T11-L2; α1 receptors) for contraction (storage); Parasympathetic for relaxation (voiding).
• External sphincter: Somatic (pudendal n., S2-S4) for voluntary control.
• Storage phase: Sympathetic dominant - detrusor relaxes, internal sphincter contracts.
• Voiding phase: Parasympathetic dominant - detrusor contracts, internal sphincter relaxes.
• Bladder stretch receptors initiate the reflex when bladder volume reaches threshold (around 150-300 mL for first urge).