Physiology Essay: Micturition Reflex (Urination) and Kidney Stones

Is an autonomic spinal cord reflex that can be overridden voluntarily.

1. Filling of urinary bladder stimulates stretch receptors.

2. Impulses are sent via the (sensory afferent) Pelvic nerve to the Sacral Spinal Cord.

3. The Parasympathetic pelvic nerve (S2-S4) sends impulses back to the bladder. This causes:

   a) Contraction of the Detrusor muscle.

   b) Relaxation (opening) of the internal sphincter muscle.

4. The afferent signal also activates the Pontine Micturition Centre (PMC). This:

   a) Activates the parasympathetic activity (pelvic nerve - see above).

   b) Contracts the detrusor muscles initiating micturition (via the pelvic splanchnic nerve).

   c) Inhibits sympathetic activity (by inhibiting the hypogastric nerve (L2) - this nerve usually inhibits the detrusor and contracts the internal sphincter muscle).

5. If it’s not a suitable time for urination, the cortex sends out inhibitory signals to the PMC to inhibit the parasympathetic activity.

   a) Somatic control of the external sphincter muscle (via the pudendal nerve) - contraction delays urination.

6. As the bladder fills, the reflex becomes more frequent and more powerful.

   a) This causes stronger contractions.

   b) It has a “self-regenerating” feedback loop

   c) If no urination occurs, the reflex may fatique and stop temporarily after a few minutes.

7. If the micturition reflex is strong enough, a 2^nd reflex is triggered via the Pudendal nerve. This inhibits the contraction of the external sphincter muscle.

8. If the inhibition from the cortex is overcome, the bladder empties/urination occurs;

   a) Detrusor: Contracts

   b) Internal sphincter: Relaxes

   c) External sphincter: Relaxes

Kidney Stones (Calculi)

• Is an obstructive disease of the kidney.

• Is painful and requires immediate treatment due to failure of renal function (decreased GFR).

• The stones are typically composed of Calcium oxolate or calcium phosphate.

• Common predisposing factors:

  • Drinking too little water (Dehydration).

  • Obesity or weight loss surgery.

  • Eating food with too much salt or sugar.

  • Infections and/or a family history.

• Treatment: All options aim to clear the obstruction.

  • If the stones are smaller than 1cm, they may pass out spontaneously.

  • If the stones are larger than 1cm, intervention is needed. The type depends on the size, position, shape and composition of the stone.

  • → Lithotripsy (Ultrasonic Fragmentation).

  • → Endoluminal extraction (endoscopic).

  • → Open surgical removal (in severe cases).

• Kidney stones can cause dilation of the renal pelvic (Hydronephrosis).