Physiology of urination


This is initiated by the micturition reflex which include the following events

If the micturition reflex is powerful enough it generates another micturition reflex and which passes through the pudendal nerve to inhibit the external urethral sphincter. Urination occurs when the inhibition is less for the urethral sphincter than for the reflex.  The urinary bladder then empties itself. The micturition reflex is controlled from the micturition center probably in the pons.

The detrusor muscle is innervated by the parasympathetic outflow of the pelvic splanchnic nerve and they cause the detrusor to contract.

Internal urethral sphincter exists only in the male and it prevents retrograde ejaculation  and it contracts when its contained α1 adrenoceptors are stimulated. Urine is stored in the bladder when the detrusor relexases and the sphincters close.

Micturition reflex is an autonomic spinal cord reflex. But it is also under the control of the brain at the following centers

These centers maintain a hold on micturition as follows

  1. Micturition reflex is abrogated by higher centers unless micturition is desired.
  2. Cortical centers allow the initiation of micturition reflex simultaneous with inhibition of the external urethral sphincter to allow urination
  3. The higher centers can prevent micturition even in the presence of micturition reflex.

Urination animation: fast with detrusssor contracting

















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Slow with urethral sphincter contracting



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