Urinorganen 5(5) Miktion

Scientific Concepts and Physiological Mechanisms of Urination (Micturition)

• Anatomy of the Urinary Bladder and Sphincters:
  • The urinary bladder is composed of smooth muscle called the detrusor muscle.
  • There are two sphincters controlling urine flow:
    • internal sphincter: smooth muscle, involuntary control.
    • external sphincter: striated muscle, under voluntary control.
  • The ureters bring urine from the kidneys to the bladder, opening at the back of the bladder near the urethra.
• reflex arc in Urination:
  • Urination in infants is primarily a reflex involving:
    1. Receptors: Stretch receptors in the bladder wall detect bladder filling.
    2. Afferent sensory neurons: Carry signals from stretch receptors to the spinal cord.
    3. Integration center: Spinal cord processes the signals.
    4. Efferent motor neurons: Parasympathetic neurons send signals to contract the detrusor muscle.
    5. Effectors: detrusor muscle contracts and internal/external sphincters relax, allowing urine to flow.
  • This reflex arc allows automatic urination without brain involvement in infants (autonomous bladder).
• Role of the Brain in Adult Micturition:
  • In adults, the brain modulates the reflex by sending inhibitory signals to prevent urination until socially appropriate.
  • As the bladder fills, stretch receptors send signals to the spinal cord and brain.
  • The brain suppresses the reflex until voluntary relaxation of the external sphincter and contraction of the detrusor muscle occur.
  • This control allows voluntary initiation and inhibition of urination.
• Voluntary Control and Reflex Interaction:
  • Adults can initiate urination voluntarily by relaxing the external sphincter and contracting abdominal muscles.
  • Once urination starts, a reflex is triggered that promotes detrusor contraction and sphincter relaxation.
  • Urination can also be voluntarily stopped midstream by contracting the external sphincter.
• Clinical Relevance:
  • In people with spinal cord injuries, brain signals may not reach the bladder, leading to an autonomous bladder where urination occurs reflexively at a certain bladder volume without voluntary control.

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Summary of Micturition Mechanism:

• Bladder fills → stretch receptors activated → sensory signals to spinal cord.
• In infants: spinal reflex triggers detrusor contraction and sphincter relaxation → urination.
• In adults: brain sends inhibitory signals to suppress reflex until voluntary urination.
• Voluntary relaxation of external sphincter + detrusor contraction → urination.
• Voluntary contraction of external sphincter can stop urination midstream.
• spinal cord injury can lead to loss of voluntary control → autonomous bladder reflex.

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Researchers or Sources Featured

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