Micturition

• What is micturition?

  The process of urine excretion, including its transport from the kidney to the bladder and eventual elimination from the body.
• What structures are involved in the transport of urine from the kidney to the bladder?

  • Ureters
  • Renal pelvis
  • Peristaltic waves
  • Bladder
• What is the renal pelvis?

  A funnel-shaped structure in the kidney that collects urine from the major calyces before it enters the ureter.

  
•  What is the function of the ureter?

  A muscular tube that transports urine from the renal pelvis to the bladder via peristalsis.
• How long is the ureter?

  Approximately 25–30 cm.
• What are the three anatomical constrictions of the ureter?

  1. Ureteropelvic junction – where the renal pelvis meets the ureter.
  2. Crossing over the iliac vessels – as the ureter descends into the pelvis.
  3. Ureterovesical junction – where the ureter enters the bladder.
• What mechanism drives urine transport in the ureters?

  Peristalsis, a wave-like contraction of smooth muscle in the ureter walls.
• What controls peristalsis in the ureters?

  Pacemaker cells in the renal pelvis that generate rhythmic contractions.
• How does the autonomic nervous system regulate ureteral peristalsis

  • Parasympathetic stimulation (via vagus nerve) enhances peristalsis
  • Sympathetic stimulation (via hypogastric nerve) decreases peristalsis
•  How does urine move despite gravity?

  Peristalsis, hydrostatic pressure, and the oblique entry of the ureters into the bladder prevent backflow.
• What prevents urine reflux from the bladder into the ureters?

  The oblique entry of the ureters into the bladder wall, forming a physiological valve that closes when the bladder fills.
• What is the vesicoureteral reflux (VUR)?

  A condition where urine flows backward from the bladder into the ureters, often due to a defective ureterovesical junction.
• What is the function of the urinary bladder?

  A hollow, muscular organ that stores urine before it is excreted via the urethra.
• Where is the bladder located in males and females?

  • Males: Located anterior to the rectum, superior to the prostate gland.
  • Females: Located anterior to the vagina and uterus.

  
• What type of epithelium lines the bladder?

  Transitional epithelium (urothelium), which allows stretching and recoiling.
• What are the four layers of the bladder wall?

  1. Mucosa: Transitional epithelium + lamina propria
  2. Submucosa: Connective tissue with blood vessels & nerves
  3. Detrusor muscle: Smooth muscle layer (3 layers: inner longitudinal, middle circular, outer longitudinal)
  4. Adventitia/Serosa: Connective tissue layer

  
• What is the function of the detrusor muscle?

  The detrusor muscle contracts during micturition to expel urine from the bladder.
• What are the two sphincters that control urine flow?

  1. Internal urethral sphincter (IUS) – involuntary
  2. External urethral sphincter (EUS) – voluntary
• What type of muscle makes up each sphincter?

  • Internal urethral sphincter (IUS): Smooth muscle
  • External urethral sphincter (EUS): Skeletal muscle 
• Where is the internal urethral sphincter located?

  It is located at the junction of the bladder and urethra (bladder neck)
•  Where is the external urethral sphincter located?

  • Males: Surrounds the membranous urethra, just below the prostate.
  • Females: Located in the deep perineal pouch, surrounding the urethra.
• What nervous system controls the internal urethral sphincter?

   Autonomic nervous system (ANS):

  • Sympathetic (Hypogastric nerve: T11–L2): Contracts IUS (prevents involuntary urination)
  • Parasympathetic (Pelvic nerve: S2–S4): Relaxes IUS (allows urination)
• What nervous system controls the external urethral sphincter?

  Somatic nervous system (via the pudendal nerve, S2–S4) – allows voluntary control.
• What is the role of the pudendal nerve in micturition?

  It contracts the external urethral sphincter to prevent urination until consciously relaxed.
• What is the trigone of the bladder?

  A triangular region at the base of the bladder, formed by:

  • Two ureteric orifices (where ureters enter)
  • The internal urethral orifice (where urine exits into the urethra)

  
• Why is the trigone clinically significant?

  Most common site of bladder infections due to its smooth mucosal lining and close proximity to ureters.
• What is the function of parasympathetic innervation in micturition?

  • Stimulates detrusor muscle contraction → bladder empties.
  • Inhibits the internal urethral sphincter (in males).
  • Mediates the voiding phase of micturition.
• What neurotransmitter is released by parasympathetic nerves to the bladder?

  Acetylcholine (ACh), which acts on muscarinic (M3) receptors in the detrusor muscle.
• What is the function of sympathetic innervation in micturition?

  • Relaxes the detrusor muscle (β3 receptors) → allows bladder filling.
  • Contracts the internal urethral sphincter (α1 receptors in males) → prevents leakage.
  • Mediates the storage phase of micturition.
• What neurotransmitters are released by sympathetic nerves to the bladder?

  Noradrenaline (norepinephrine) acts on:

  • β3 adrenergic receptors → relaxes detrusor muscle.
  • α1 adrenergic receptors → contracts internal urethral sphincter.
• Which nerve provides somatic innervation to the bladder?

  The pudendal nerve (S2-S4).
• What neurotransmitter is released by the pudendal nerve?

  Acetylcholine (ACh), which acts on nicotinic receptors in the external urethral sphincter.
• What are the two main types of sensory innervation to the bladder?

  1. Afferents via the pelvic nerve (S2-S4) → detect bladder distension.
  2. Afferents via the hypogastric nerve (T11-L2) → detect pain and fullness.
• What sensory information is carried by the pelvic nerve?

  • Detects bladder stretch (filling) → activates the micturition reflex.
  • Sends signals to the pontine micturition center (PMC) to initiate urination.
• What sensory information is carried by the hypogastric nerve?

  • Detects bladder distension and pain from overfilling.
  • Provides feedback to the brain to maintain the storage phase.
• How does the brainstem regulate bladder function?

  • The pontine micturition center (PMC) in the pons coordinates urination.
  • The prefrontal cortex provides voluntary control over micturition.
• What are the two main phases of micturition?

  • Bladder Filling (Storage Phase) – The bladder stores urine while preventing leakage.
  • Bladder Emptying (Voiding Phase) – The bladder contracts, and urine is expelled through the urethra.
• Which nervous system division is dominant during bladder filling?

  The sympathetic nervous system dominates, inhibiting detrusor contraction and promoting storage.
• What are the key neural pathways involved in bladder filling?

  • Sympathetic nerves (hypogastric nerve, T11-L2):
  • Releases noradrenaline, which binds to β3-adrenergic receptors in the detrusor muscle → relaxation (bladder expands).
  • Activates α1-adrenergic receptors in the internal urethral sphincter → contraction (prevents leakage).
  • Somatic nerves (pudendal nerve, S2-S4):
  • Releases acetylcholine (ACh) to stimulate nicotinic receptors → external urethral sphincter contraction (voluntary control).
• What happens to the detrusor muscle during bladder filling?

  The detrusor muscle relaxes to allow urine storage due to sympathetic inhibition via β3-adrenergic receptors.