Micturition Flashcards
What is micturition?
→ Act of urination
What are the basic processes of micturition?
→ Made in the kidneys
→ Urine stored in bladder
→ Urine released from bladder
Where does the urine pass from and to?
→ From collecting duct of renal tubules into renal pelvis
What aids the movement of the urine into the ureter?
→ Contraction of the smooth muscle of the pelvis
What pressure do the ureters squeeze urine to?
→ 10-20 mmHg
How do the ureters open into the bladder and why?
→obliquely
→ Prevents reflux of urine back into ureters by passive flap-wave
What type of peristalsis is the ureter under?
→ Myogenic in origin and not under CNS control
What do kidney stones form from?
→ Crystals that separate from urine within urinary tract
What does urine contain to prevent kidney stones?
→ citrate
What do kidney stones usually contain?
→ Calcium
Why are kidney stones more common in men?
→ Due to testosterone
What are kidney stones caused by?
→ Poor urine output → Obstruction → Altered urinary pH → Low concentration of inhibitors → Infection → Dietary intake of stone forming substances
Where can kidney stones form?
→ In the kidney
→ Ureter
→ Bladder
What are symptoms of kidney stones?
→ Dysuria
→ Haematuria
→ Pain
→ Reduced urine flow
→ Urinary tract obstruction - pressure reaches 50 mmHg
→ If stone is at tip of urethra- pain inhibits micturition
How much urine can the bladder hold without pressure?
→ 400ml without much pressure
→ Due to the spherical structure
What can the transitional epithelium do?
→ stretches without damage
→ has ridges that flatten out as the bladder fills
What allows separation of urine and blood in the bladder?
→ Impermeable to salts and water
→ No exchange between urine and capillaries of bladder wall
What is the muscle coat around the epithelium called and made from?
→ detrusor muscle
→ made from bundles of smooth muscle interlacing
What does the trigone region contain?
→ Opening of ureter
→ neck of urethra
What are the sphincters in the bladder?
→ Internal - extension of detrusor - NOT VOLUNTARY
→ External - 2 striated muscles
What is the external sphincter composed of?
→ 2 striated muscles
→ (compressor urethrae and bulbocavernosus surrounding urethra)
→ Responsible for continence - conscious, voluntary control
What is the internal sphincter formed from?
→ Loop of muscle that is an extension of the detrusor
→ When detrusor contracts the fibers forming the loop shorten and open the sphincter
What is the arrangement of the female bladder?
→ Short urethra - only carries urine
→ External sphincter poorly developed - prone to incontinence after childbirth
What is the arrangement of the male bladder?
→ Urethra carries urine and semen
→ urine elimination is aided by the contraction of bulbocavernosus muscle
What is the lining of the bladder?
→ transitional epithelium
What is the bladder muscle?
→ Detrusor
What is the bladder permeable and impermeable to?
→ Impermeable to salt and water
→ Permeable to lipophilic molecules
What do the neural circuits of the bladder act like?
→ an on and off switch to alternate between storage and elimination
What is the lower urinary tract innervated by?
→ 3 sets of peripheral nerves
→ Parasympathetic (pelvic)
→ Sympathetic ( hypogastric)
→ Somatic nervous system ( pudendal)
What are the two bladder innervations?
→ Sensory
→ Motor
What does sensory innervation do?
→ Gives a sensation of fullness
What does motor innervation do?
→ Causes contraction and relaxation of detrusor muscle and external sphincter to control micturition
Where does the parasympathetic motor innervation go from and to?
→ Sacral region of spinal cord S2- S4
→ Pre synaptic neurons synapse to post ganglionic neurons in the bladder wall
→ Synapse in the internal sphincters
Where does the sympathetic motor neurons go from and to?
→ lumbar region T-11 - L2
→ innervate onto post synaptic neurons in the hypogastric ganglia
→ Innervate directly to the internal sphincter
→ inhibiting the parasympathetic system
Where do the somatic neurons go from and to?
→ S2 - S4
→ innervate the external sphincter
What is the innervation of the detrusor muscle like?
→ Parasympathetic pre ganglion synapse onto post ganglion neurons in the bladder wall
→ release AcH
→ acts on nicotinic receptors
→ the post ganglionic neurons release AcH and ATP
What do ACh and ATP do to the detrusor?
→ cause it to contract
What receptor does ATP act on?
→ Purinergic
What inhibits detrusor contraction?
→ Atropine
What causes the detrusor to relax?
→ Noradrenaline
→ Inhibits transmission at parasympathetic ganglia indirectly
→ Directly via beta receptors
What relaxes the internal sphincter?
→ NO and ACh
→ Noradrenaline
What holds the external sphincter closed?
→ ACh (tonic and continual)
What effects do the parasympathetic neurons have?
→ Contract detrusor
- ACh (muscarinic) and ATP (purigenic)
→ Relax internal sphincter
- via NO (cGMP) & ACh
What effects do sympathetic neurons have?
→ Relax detrusor
- indirectly via NA & directly via Beta receptors
→ Contract the internal sphincter
- NA (alpha receptors)
What effects do the somatic neurons have?
→ Contract the external sphincter
- ACh
What is the main afferent pathway?
→ Via the pelvic nerve
→ Small myelinated A delta fibres → micturition reflex
What are A delta fibers linked with?
→ Stretch receptors - wall tension
→ Volume receptors - bladder filling
Where are Unmyelinated C fibres found?
→ near the epithelium
What are unmyelinated C fibres linked with?
→ Nociceptors - pain
What are hypogastric and pudendal pathways linked with?
→ Nociceptors
→ Flow receptors
What do A fibres sense?
→ Tension in detrusor
→ Detrusor contraction
→ bladder fullness - discomfort
What do C fibres sense?
→ Respond to damage
→ Inflammatory mediators
How does the bladder fill (flowchart)?
Initally bladder is empty - sphincters closed ↓ bladder pressure low ↓ arrival of urine ↓ detrusor relaxes progressively ↓ little increase in pressure ↓ sphincters still closed
When do we become conscious of bladder filling?
→ 100-150ml
When does discomfort begin?
→ 300-400
What kind of a reflex is micturition?
→ Autonomic
How is the micturition reflex modified?
→ By voluntary control
→ Inhibited by higher centres in the brain
What disrupts voluntary control of micturition?
→ Disease/injury/ageing
→ Bladder hyperactivity + urge incontinence
→ Stress incontinence
Flow chart of micturition
As receptors detect tension
↓
transmit information via afferents ( A delta system) to the spinal cord
↓
synapse to parasympathetic efferents to override storage
↓
detrusor contracts and internal sphincter relaxes
↓
bladder constricts
↓
urine forced out and activates flow receptors in the urethra
↓
activates flow receptors in urethra
↓
pudendal afferents are excited and switch off the tonic system
↓
Tonic contraction removed by inhibition of somatic input
What is the PMC?
→ Pontine micturition centre
How can higher centres modify micturition reflexes?
→ Contract external sphincter and levator muscle
What can the urine stream be stopped by?
→ Strangury ‘urethral pain’ due to urethritis
→ Pinching glans penis
→ at night if bladder fills to capacity PMC wakes you up
Flow chart of innervation to bladder
Cerebral cortex (frontal lobe) ↓ Brain stem ↓ Spinal tract ↓ Spinal efferents ↓ Ganglion ↓ Bladder
What sends signals from the bladder to the spinal efferents?
→ C unmyelinated afferent
What sends signals from the bladder to the spinal tract?
→ A delta myelinated afferent
Where is the bladder?
→ Floor of abdominal cavity
What helps to compress the urethra?
→ Reflex contraction of periurethral striated muscles
What is the result of contracting abdominal muscles?
→ Increased intra-abdominal pressure transmitted to the bladder and urethra
Why do you have to completely empty the bladder?
→ Restore sterility
→ Bacteria in retained urine seeds fresh urine
→ Retained urine - UTI
What can repeated infections do to kidney function?
→ destroy renal function
What is a UTI?
→ Urinary tract infection that can happen anywhere along the urinary tract
What is an infection in the bladder called?
→ Cystitis
What is an infection of the kidneys called?
→ Pyelonephritis
What is a urethra infection called?
→ Urethritis
What are risk factors for a UTI?
→ pregnancy → Enlarged prostate → Kidney stones → bowel incontinence → old age
What are the problems of an ageing bladder?
→ Prostate enlargement
Incontinence - weakening of sphincters
→ Overactive bladder
What is a consequence of slow urine stream?
→ Incomplete emptying
→ Infection
What are medications for incontinence?
→ anti muscarinics
→ detrusor relaxes
→ relax smooth muscle
What is bladder retraining?
→ Kegel exercises
What surgery can be carried out for the bladder?
→ Bladder neck suspension
→ collagen injections
What is SNS?
→ sacral nerve stimulation
→ electrical impulses to sacral nerve
