Micturition Reflex - QS Study
QS Study

Micturition Reflex

Micturition is fundamentally a spinal cord reflex. It is facilitated and inhibited by higher centers and subject to voluntary facilitation and inhibition. When urination is desired, signals from the brain stimulate the micturition reflex. The bladder normally accommodates up to 300-400 ml in adults. When the bladder is distended it sends signals to the brain, which is perceived as the ‘full bladder’ sensation.

Components

  • An inherent contractile activity of bladder smooth muscle
  • Stretch receptors in the bladder wall
  • Spinal voiding reflex.
  • Facilitator and an inhibitory area in the brain.

Mechanism

The bladder smooth muscle has some inherent contractile activity; however, when its nerve supply is intact, stretch receptors in the bladder wall initiate a reflex contraction that has a lower threshold than the inherent contractile response of the muscle.

Fig: reflex by Dr. Irum

  • Stretch reflex initiated by sensory stretch receptors in the bladder wall (esp by the receptors in the posterior urethra)
  • Signals conducted to the sacral segments of the spinal cord through pelvic nerves.
  • Signals back to bladder wall through the parasympathetic nerve.
  • Contraction of detrusor muscles.
  • This contraction ceases spontaneously and bladder wall relaxes.
  • As the bladder fills more, micturition contraction becomes stronger.
  • The cycle repeats again and again until bladder has reached a strong degree of contraction.
  • Once the micturition reflex becomes powerful.
  • Once the micturition reflex becomes powerful enough
  • It causes another reflex
  • Through pudendal nerves
  • To external sphincter to inhibit it.

It this inhibition is more potent in the brain than the voluntary constrictor signals to external sphincter → urination occurs.

If not, urination will not occur until the bladder fills further and the micturition reflex becomes more powerful.