What is micturition?
The act of urination (emptying the bladder)
Basic process
Urine made in kidneys (1 ml.min-1)
Urine stored in the bladder
Urine released from bladder
Kidney to bladder
Urine from al collecting ducts of al nephrons
Emptying into the renal pelvis
Urine enters ureter
Peristaltic waves
~1-6 contractions/min
What is the pressure in the ureters
10-20 mmHg
What control are ureters under?
Myogenic in origin not under CNS
Kidney stones
Crystals that separate from the urine within the urinary tract
Normal urine contains citrate to prevent this
What is present in most kidney stones?
Calcium (80%) as calcium oxalate or less often as calcium phosphate.
Then uric acid (<10%)
Struvite (<10%)
Cystine (<5%)
What causes kidney stones?
Poor urine output/ obstruction, altered urinary pH, infections, excess dietary intake, low concentration of inhibitors
Whos is more likely to have kidney stones
Men due to testosterone
Symptoms of kidney stones
Dysuria (painful urination)
How much can the bladder store?
400 ml w/o much increase in pressure - spherical structure
Female bladder
Short urethra - only carries urine
External sphincter poorly developed - more prone to incontinence particularly after childbirth
Male bladder
Carries urine and semen
Urine elimination aided by contraction of bulbocavernosus muscle in the penis
Summary of bladder
Lining - transitional epithelium
Bladder muscle - detrusor
Impermeable to salt & water
Permeable to lipophilic molecules
Internal sphincter
Smooth muscle, involuntary control
External sphincter
Striated muscle, voluntary control
Bladder innervation 1
Neural circuits in brain & SC coordinate activity of bladder & sphincters circuits act as an on-off switch to alternate between storage & elimination
Bladder innervation 2
3 peripheral nerves
- Parasympathetic (pelvic nerve)
- Sympathetic (hypogastric nerves)
- The somatic nervous system (pudendal nerve)
Bladder innervation 3
- Sensory: gives sensation (awareness) of fullness and also pain from disease
- Motor: causes contraction and relaxation of the detrusor muscle and external sphincter to control micturition
What do ACh and ATP do the Detrusor
Causes detrusor to contract
What does NA do the detrusor
Inhibits transmission at parasympathetic ganglia - indirectly causes detrusor to relax
Also directly via B-Rs (also in trigone area) causes detrusor to relax
What does NO and ACh do the internal sphincter?
Relaxes
What does Noradrenaline (NA) do to the internal sphincter?
Contracts
What does the ACh to the internal sphincter?
Tonic activity holds external sphincter closed
Parasympathetic neurons
Contract detrusor - via ACh (muscarinic) + ATP (Purigenic R)
-Relax internal sphincter - via NO (cGMP) & ACh (nictotinic)
Encourage micturition
Sympathetic neurons
RELAX detrusor- Indirectly via NA (a-R) & directly via NA (beta receptors)
CONTRACTS internal sphincter - NA (alpha R)
Inhibits Micturition
Somatic neuron
Contract external sphincter - ACh (nicotinic R)
Inhibit micturition
Afferent innervation
The main afferent pathway is via pelvic nerve (parasympathetic):
Small myelinated A/delta fibers - micturition reflex
Stretch receptor - signal wall tension
Volume receptors - signal bladder filling
A fibres
Sense tension in detrusor
Filling of bladder
Detrusor contraction
BLADDER FULLNESS, discomfort
C fibres
Respond to damage & inflammatory mediators
Unmeylinated C fibres - endings in/near epithelium
What happens when the bladder is empty?
Sphincters closed
Bladder pressure is low
What happens when urine arrives?
Detrusor relaxes progressively
Little increase in pressure
Sphincters still closed
How is micturition modified?
Voluntary controlled
The act is an autonomic reflex
However, is inhibited by higher centers in the brain
What do disease and injury do to the nervous system?
Disrupts voluntary control of micturition
- Bladder hyperactivity & urge incontinence
- Stress incontinence
Retained urine
Clinical infection UTI
Bladder infection
Called Cystitis or a bladder infection
Kidney infection
One or both kidney - called pyelonephritis
Ureter infection
Rarely the site of infection
Urethra infection
Urethritis
How is micturition aided?
Reflex contraction of peri-urethral striated muscles also helps compress the urethra
How can higher centers modify micturition?
Contract external sphincter & levator muscle consiously
What are some risk factors of UTIs
Diabetes Long term catheter Pregnancy Prolonged immobility Kidney stones Bowel incontinence Older age
Problems with an ageing bladder
Slow urine stream - lead to incomplete emptying and an infection
Incontinence
Treatment for ageing bladder
Surgery - bladder neck suspension
Bo