Micturition Chapter 26

Study Notes

The detrusor muscle is a smooth muscle in the bladder that extends in all directions, increasing pressure in the bladder to 40-60 mm Hg when contracted.
The trigone is a small triangular area on the posterior wall of the bladder where the two ureters enter the bladder.
The internal sphincter is a smooth muscle that normally keeps the bladder neck and posterior urethra empty of urine, preventing bladder emptying until pressure rises above a critical threshold.
The external sphincter is a skeletal muscle under voluntary control, located in the urogenital diaphragm, and can consciously prevent urination.

The ureter is innervated by both parasympathetic and sympathetic nervous systems, enhancing and inhibiting peristaltic contractions, respectively.
The urinary bladder is innervated by:
Autonomic nervous system:
- Parasympathetic fibers (S2 and S3) detect bladder wall stretch and stimulate detrusor muscle contraction.
- Sympathetic fibers (L2) mainly stimulate blood vessels and have little effect on bladder contraction.
Somatic nervous system:
- Skeletal motor fibers through pudendal nerve control the external sphincter.

Vesicoureteral reflux occurs when bladder contraction during micturition does not completely occlude the ureter, leading to urine reflux into the ureter.
This can cause ureter enlargement, increased pressure in the renal calyces, and kidney damage.
The micturition reflex is an autonomic spinal cord reflex that empties the bladder when the tension in its walls rises above a threshold level.

Blockage of a ureter (e.g., by a stone) causes intense reflex constriction, severe pain, and sympathetic reflex back to the kidney, constricting renal arterioles and decreasing urine output.

Voluntary urination involves contraction of abdominal muscles, increasing pressure in the bladder.
Sensory signals from bladder stretch receptors are conducted to sacral segments, reflexively causing contraction of the bladder and simultaneous inhibition of the external urethral sphincter.