Physiology of Micturition

Overview

• Micturition is the process of urination, involving multiple organs and structures.
• Key structures: kidneys, ureters, bladder, urethra, and in males, the prostate.
• Urine produced in the kidneys travels via ureters to the bladder, which stores urine for eventual release through the urethra.

Key Organs and Structures

• Kidneys: Site of urine production.
• Ureters: Muscular tubes carrying urine from kidneys to bladder.
• Bladder: Stores urine; comprised of the detrusor muscle.
• Urethra: Tube through which urine exits the body.
  • Male: Includes spongy tissue and is longer than female urethra.
  • Female: Shorter urethra, no internal sphincter or prostate.

Sphincters and Prostate

• Internal Sphincter: Involuntary control; absent in females.
• External Sphincter: Voluntary control; part of the urogenital diaphragm.
• Prostate: Found only in males; involved in sperm activation.

Receptors Involved in Micturition

• Bladder Receptors: Muscarinic (M3) and beta-adrenergic (B3).
• Internal Sphincter: Alpha-adrenergic receptor.
• External Sphincter: Nicotinic receptor.

Nervous System Control

• Nervous system regulates micturition via various nerves.

Key Nerves

• Pelvic Nerve:
  • Efferent: Acetylcholine binds to M3, causing detrusor contraction.
  • Afferent: Sensory, activated when bladder stretches.
• Pudendal Nerve: Controls external sphincter via nicotinic receptors; voluntary.
• Hypogastric Nerve: Part of sympathetic system.
  • Noradrenaline binds to B3 (relax detrusor), alpha-1 (contract internal sphincter).

Sympathetic vs. Parasympathetic Systems

• Sympathetic Nervous System: Responsible for urinary retention.
• Parasympathetic Nervous System: Responsible for voiding (peeing).

Micturition Process

• Empty Bladder:
  • Minimal stretching, slow sensory signals.
  • Stimulates hypogastric nerve, causing internal sphincter contraction and detrusor relaxation.
  • Pudendal nerve is stimulated for voluntary retention.
• Full Bladder:
  • Significant stretching, fast sensory signals to the spinal cord.
  • Signals bypass thoracic-lumbar area, stimulating pontine micturition center.
  • Inhibits hypogastric nerve, causing relaxation of internal sphincter.
  • Stimulates pelvic efferent nerves, causing detrusor contraction.
  • Inhibits pudendal nerve, relaxing external sphincter.

Voiding Reflex

• Continues peeing involves repeated feedback loop.
• Sensory signals keep stimulating parasympathetic efferent nerves.

Conclusion

• Understanding the physiology of micturition involves recognizing the roles of various organs, muscles, receptors, and nerves.
• The interaction between sympathetic and parasympathetic systems is crucial in controlling urination.

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End of lecture notes.