Lecture on Micturition Reflex

Anatomy of the Urinary System

• Ureters: Tubes carrying urine from kidneys to bladder (left and right).
• Bladder: Muscular organ, primary muscle is the detrusor muscle (red layer), responsible for contraction.
• Urethra: Tube through which urine exits the body.
• Internal Urethral Sphincter: Muscle surrounding the bladder exit, controls urine release.
• External Urethral Sphincter: Part of the urogenital diaphragm, under voluntary control.
• Deep Transverse Perineal Muscle: Part of the pelvic floor muscles.

Control of Micturition

• Central Nervous System: Controls the bladder through reflexes.
• Stretch Receptors: Located in the detrusor muscle, respond to bladder filling.

Spinal Cord Involvement

• Sacral Region (S2-S4): Important for bladder control.
• Thoracolumbar Region (T11-L2): Controls sympathetic outflow.
• Pons: Contains pontine storage center and pontine micturition center.
• Cerebral Cortex: Higher brain center for conscious awareness of bladder filling.

Mechanism of Empty Bladder

• Sensory Input: Stretch receptors send slow impulses to spinal cord.
• Sympathetic Pathway:
  • Preganglionic neurons in thoracolumbar region synapse on inferior mesenteric ganglion.
  • Postganglionic neurons (hypogastric nerve) release norepinephrine.
  • Beta-3 Adrenergic Receptors: Located on detrusor muscle, cause relaxation.
  • Alpha-1 Adrenergic Receptors: Located on internal urethral sphincter, cause contraction.
• Parasympathetic Pathway: Inhibits bladder contraction by preventing acetylcholine release.
• Somatic Pathway: Pudendal nerve keeps external sphincter contracted using acetylcholine.

Process of Full Bladder

• Increased Stretch: Around 200-500 mL, triggers significant sensory neuron activity.
• Signal to CNS: Impulses sent to the pons and cerebral cortex.
• Pontine Centers:
  • Pontine Micturition Center: Activated to initiate urination.
  • Pontine Storage Center: Inhibited during urination.
• Inhibition of Sympathetic Nerves: Reduces norepinephrine, causing relaxation of sphincters and contraction of detrusor muscle.
• Activation of Parasympathetic Nerves: Stimulates detrusor muscle to contract via m3 receptors (acetylcholine).
• Somatic Nerve Inhibition: External sphincter relaxes, allowing urine to flow out.

Urine Composition

• Normal Components: Majority water, some electrolytes, urea, uric acid.
• Abnormal Components:
  • Glucose: Suggests diabetes mellitus.
  • Proteins: Possible sign of renal disease or excessive exercise.
  • Ketones: May indicate diabetes or low-carb diet.
  • Bilirubin: Suggests liver disease.
  • Hemoglobin: Indicates hemolytic anemia or burns.
  • RBCs: May imply trauma, tumors, infections, or stones.
  • WBCs: Sign of urinary tract infection.

Summary

• The micturition reflex involves complex interactions between the nervous system components and the bladder muscles.
• Emptying and filling of the bladder are managed by balancing sympathetic and parasympathetic inputs.
• Understanding urine composition is essential for diagnosing various pathological conditions.