Control of Micturition

appearance of the urothelium when it is relaxed

The top layer of cells are dome shaped/cuboidal when the bladder is relaxed, and the epithelium is thicker with more visible layers. This allows the epithelium to maintain a protective barrier

appearance of the urothelium when contracted

When the bladder is full and therefore stretched, the top layer appears flattened, squamous and therefore the whole epithelium appears thinner as the layers have stretched, so the cell surface is less rounded and bumpy, but is more smooth due to the SA increase to allow for distension and storage

3 methods of control for micturition

-          Urine release – voiding phase – parasympathetic with the pelvic nerve from the sacral vertebra

-          Urine retention – storage phase – sympathetic with the hypogastric nerve from the lumbar vertebra

-          Somatic – this is the conscious control of the release of urine, so is involved in both phases through the pudendal nerve

vesicoureteral reflex

-          the ureters enter the bladder obliquely which creates a valve-like entry when the bladder fills up as detrusor contraction compresses the intramural ureter which prevents retrograde flow of urine towards the kidneys which reduces urinary reflux and damage to the kidneys, and maintains the GFR.

Note that this is to prevent backflow to the kidney, not prevent urine from entering the kidney, as the peristatic effect of the ureters overcomes this valve-like mechanism, and the bladder is made to stretch

what is it called when there is intense pain due to contraction of the ureter which causes the renal arterioles to contract which reduces GFR

ureterorenal reflex

what controls the ANS

pons micturition centre

what controls the voluntary control

the cerebral cortex by inhibiting or activating the pudendal nerve

upper motor neurone lesion at pons

– this causes increased sympathetic tone so the detrusor muscle and internal sphincter retain tone, and the sacral spinal reflex remains functional.

The external sphincter will increase in tone as there is no voluntary relaxation due to a loss of descending brain control

This means that there is retention of urine, making the bladder full, firm and difficult to express. It also causes incomplete emptying which risks infection and an increased bladder pressure

Incomplete emptying leaves behind residual urine which can lead to infection, stone formation and increased upstream pressure compromising GFR

LMN lesion at sacral segments S1-S3

this means that the pelvic and pudendal nerves are both damaged. This means that there is a loss of spinal reflex arc so the detrusor muscle is flaccid and can’t contract and there is reduced tone in the internal sphincter.

The loss of function to the pudendal nerve means the external sphincter is relaxed, so the animal has no control over the release of urine.

This presents as a bladder that is large due to retention, but is easy to express with continuous dribbling of urine