micturition Flashcards
what is micturition ?
the act of urination
what happens to the ureter when urine enters ?
- distends it and smooth muscles around contract
- Peristaltic waves in ureter occur
why does urine enter obliquely into the bladder ?
- Prevents reflux of urine back into ureters by passive flap-valve effect
- Ureteric peristalsis is myogenic and NOT under CNS control
- Coordination required between peristalsis and changing urine volume
what are the two sphincters of the bladder ?
Internal Sphincter
* Extension of detrusor muscle NOT under voluntary control
External Sphincter:
* Two striated muscles (compressor urethrae & bulbocavernosus) surrounding urethra
* these muscles are responsible for continence
under conscious, voluntary control
describe the bladder
- Lining – transitional epithelium
- Bladder muscle - detrusor
- Impermeable to salt & water
- Permeable to lipophilic molecules
what are kidney stones ?
- Most common disorder of urinary tract
- Develop from crystals that precipitate from urine within urinary tract
- Normal urine contains inhibitors (citrate) to prevent this
- Calcium is present in nearly all stones (80%), usually as calcium oxalate or less often as calcium phosphate.
- Others made up of uric acid (<10%), struvite (<10%), cysteine (<5%).
- not the same as gall stones
what are kidney stones caused by ?
- excess dietary intake of stone-forming substances
- poor urine output/obstruction
- altered urinary pH
- low concentration of inhibitors
- infection
what are the symptoms of ureterolithiasis ( Kidney stone disease) ?
- Dysuria (painful urination)
- Haematuria - blood in urine
- Loin pain/back pain
- Reduced urine flow
- Urinary tract obstruction: pressure reaches 50mmHg - causes considerable pain “renal colic”
- If stone approaches tip of urethra – intense pain can inhibit micturition – “strangury”
describe the 3 types of bladder innervation
describe the efferent innervation of the detrusor muscle
sympathetic pathway (green lines):
* NA inhibits parasympathetic ganglia via α-receptors, indirectly causing detrusor relaxation.
* NA also acts on β-receptors (β-Rs) directly, further promoting detrusor relaxation (also affects the trigone area).
Parasympathetic pathway (red lines):
* ACh acts on nicotinic receptors at the ganglion.
* Parasympathetic post-ganglionic neurons release ACh and ATP:
* ACh acts on muscarinic receptors to contract the detrusor.
* ATP acts on purinergic receptors to also contract the detrusor.
* Atropine blocks muscarinic receptors, inhibiting detrusor contraction.
Final effect:
* Sympathetic activation → detrusor relaxation (prevents urination).
* Parasympathetic activation → detrusor contraction (promotes urination).
describe the efferent innervation of the sphincters
Internal sphincter control:
Sympathetic (green line):
* Noradrenaline (NA) acts on α1 receptors → contracts internal sphincter (prevents urination).
Parasympathetic (red line):
* Nitric Oxide (NO) and Acetylcholine (ACh) cause relaxation of the internal sphincter (allows urination).
External sphincter control:
* Somatic control (blue line):
* ACh acts on nicotinic receptors to keep the external sphincter closed.
* continuous ACh activity maintains contraction of the external sphincter.
Final effect:
* Sympathetic activation → internal sphincter contraction (urine retention).
* Parasympathetic activation → internal sphincter relaxation (urine release).
* Somatic control keeps the external sphincter closed voluntarily until urination is desired.
what are the types of afferent (sensory) nerve fibres ?
A fibres: sense tension in detrusor:
i. Filling of bladder
ii. Detrusor contraction
- bladder fullness, discomfort
C fibres: respond to damage & inflammatory mediators
- PAIN (urgent desire to micturate)
describe the afferent sensory innervation of the bladder
Main afferent pathway is via pelvic nerve (parasympathetic):
* Small myelinated Aδ–fibres micturition reflex
* Stretch receptors - signal wall tension
* Volume receptors - signal bladder filling
* Unmyelinated C fibres - endings in/near epithelium
* Nociceptors - pain (e.g. during infection of bladder lining – cystitis; excessive distension)
Hypogastric (sympathetic) & Pudendal (somatic) pathways:
* Nociceptors
* Flow receptors (external sphincter)
describe the filling of the bladder
Initially – bladder empty
* Sphincters closed
(continuous) activity sympathetic & somatic nerves)
* Bladder pressure low
Arrival of urine
* Detrusor relaxes progressively
(sympathetic activity inhibiting parasympathetic transmission)
* Little increase in pressure
* Sphincters still closed
describe emptying of the bladder
- Micturition is an autonomic reflex
e.g. in babies (<18months), adults with spinal cord transected above sacral region - Reflex is modified by voluntary control
Inhibited or initiated by higher centres in the brain
Maturation of bladder complete by >6 years - Basic circuits act as on/off switches to alternate between 2 modes of operation: storage and elimination
- Disease/injury/ageing to nervous system in adults disrupts voluntary control of micturition
- bladder hyperactivity & urge incontinence
- stress incontinence
describe the micturition reflex
Bladder filling:
* As the bladder fills, receptors in the bladder wall detect tension and begin to “fire off” signals.
* Aδ-fiber afferents send sensory information to the spinal cord.
Reflex activation:
* The parasympathetic efferent pathway is activated.
* Acetylcholine (ACh) and Nitric Oxide (NO) cause detrusor contraction and internal sphincter relaxation.
External sphincter control:
* Tonic contraction of the external sphincter is inhibited, allowing urine to pass.
* This is due to the removal of somatic (voluntary) input.
Urine flow reinforcement:
* Flow receptors in the urethra are activated by urine movement, exciting pudendal afferents.
* This reinforces the micturition reflex to ensure the bladder empties completely.
Sacral reflex importance:
* The sacral reflex is crucial in sustaining micturition until the bladder is empty.
Final Effect:
* Detrusor muscle contracts.
* Internal sphincter relaxes.
* External sphincter relaxes, allowing urine flow.
* Bladder empties completely due to reflex reinforcement.
describe the voluntary modification of this reflex
Conscious control:
* External sphincter & levator ani muscle can be contracted voluntarily to delay urination.
* Sympathetic firing to the bladder & internal sphincter can be increased, enhancing retention.
* Parasympathetic transmission can be inhibited, disrupting the positive feedback loop that facilitates bladder emptying.
* Internal sphincter tightens, preventing involuntary leakage.
Factors that can halt urination:
* Strangury (urethral pain due to urethritis or renal calculi) can involuntarily stop the urine stream.
* Pinching the glans penis can reflexively inhibit micturition.
Night-time bladder control:
* If the bladder reaches full capacity at night, the pontine micturition center (PMC) and arousal center detect it, waking you up to urinate.
describe voluntary control of micturition
- The bladder is contained in the floor of the abdominal cavity
By contracting abdominal muscles:
* The increased intra-abdominal pressure is transmitted to the bladder and urethra.
* Reflex contraction of peri-urethral striated muscles also helps compress the urethra ⇒ micturition reflex aided
what is the importance of emptying the bladder ?
Urine
* Normally sterile
* Occasional bacterial entry
* Complete emptying restores sterility
* Bacteria in retained urine seeds fresh urine
* Retained urine - clinical infection (UTI)
- Repeated infections can destroy renal function if ascend to kidney
what is a UTI ?
- Can happen anywhere along the urinary tract
- UTIs have different names, depending on area of infection:
- Bladder – an infection in the bladder is called cystitis or a bladder infection
- Kidneys – an infection of one/both kidneys is called pyelonephritis
- Ureters – rarely the site of infection
- Urethra – an infection of the urethra is called urethritis
what are the risk factors of UTIs ?
- More common in women because of short urethra
- Common in men over 40 due to prostatic disease, causing bladder outflow obstruction
- Diabetes mellitus
- long-term catheterisation
- pregnancy
- enlarged prostate
- prolonged immobility
- kidney stones
- bowel incontinence
- advanced age
what are the problems of an aging bladder ?
Slow urine stream:
* Prostate enlargement (BPH -benign prostatic hyperplasia)
* most common cause of lower urinary tract symptoms in men (25% of men > 40yrs)
* Slow urine stream → incomplete emptying → infection
Incontinence:
Causes
* Weakening of sphincters (e.g. stress incontinence)
Common in women after child-birth, weakened pelvic floor muscles
* Failure of nervous control
* Overactive bladder (OAB) – detrusor contracts spastically – results in sustained high bladder pressure – urge incontinence
Consequence:
* Socially embarrassing
* Diminishes self-esteem
* Reduces quality of life
what are the treatments for bladder problems ?
- Anti-muscarinics relax smooth muscle & ↓ detrusor contraction
* (eg non-specific muscrarinic receptor antagonist Oxybutynin – wide ranging side effects) - Bladder retraining (used for stress & urge incontinence)
* Timetable & Kegel exercises - Surgery
* Bladder neck suspension
* botulinum toxin/collagen injections into muscles around urethra → relaxes bladder (OAB) - Sacral Nerve Stimulation (SNS)
* implanted neurostimulation system
* electrical impulses to sacral nerve - Stem cell therapy
* Frauscher et al (2004) cultured stem cells into bladder wall ⇒ 90% no leakage
* Limited by supply of stem cells (bone marrow) - Tissue engineered bladder
* Synthetic and natural scaffolds to form 3D structure using human tissue.
* Currently in phase II trials.
