micturition Flashcards
what is micturition?
the act of urination (emptying the bladder)
broken down into:
- Urine made in kidneys (formed continuously at a rate of 1ml.min-1 in normally hydrated subjects)
- Urine stored in bladder
- Urine released from bladder
what is the renal pelvis?
funnel-like dilated proximal part of the ureter in the kidney
kidney to bladder - movement
- urine passes from collecting ducts of renal tubules into the renal pelvis
- contraction of the smooth muscle of the pelvis aids movement of urine into the ureter
- urine enters the ureter, ureter becomes distended - circular smooth muscle then contracts
- this contraction closes the junction between the pelvis and the ureter, and pushes urine further into the ureter, causing distention and further contraction
- peristaltic waves are initiated and propagated along the length of ureter until it propels urine into the bladder
- ureters open at an oblique angle to the wall of the bladder
- ureteric peristalsis is myogenic in origin, NOT under CNS control
- coordination required between peristalsis and changing urine volume
- ureter opens up into bladder, flap which closes when the bladder is full, preventing urine from going back up into the ureter
ureters open at an oblique angle to the wall of the bladder - why?
helps ensure that when pressure within bladder rises, the ureters are compressed so preventing reflux of urine back up into the ureters
ureteric peristalsis is myogenic in origin - what does this mean?
- not under CNS control
- originating in smooth muscle, -ureters function normally when their extrinsic nerves are cut, so neural control by CNS not important
Kidney Stones
- most common disorder of urinary tract & develop from crystals that separate from urine within urinary tract
- normal urine contains inhibitors (citrate) to prevent this
- calcium is present in 80% of stones, usually as calcium oxalate or calcium phosphate. others made up of uric acid/struvite/cystine
- more common in men than women due to testosterone
- NOT the same as gall stones
what are kidney stones caused by?
- poor urine output/obstruction
- altered urinary pH
- low concentration of inhibitors
- infection
- excess dietary intake of stone-forming substances
Ureterolithiasis
disease where kidney stones form and become lodged in the ureters
symptoms of ureterolithiasis
- Dysuria (painful urination)
- Haematuria
- Loin pain/back pain
- Reduced urine flow
- Urinary tract obstruction: pressure rises because of continuing peristaltic contractions, causing considerable pain “renal colic”
- if stone approaches tip of urethra – intense pain can inhibit micturition – “strangury”
how can the bladder be almost empty or contain up to 400ml without much of an increase in pressure?
because of its spherical structure - even though tension in the wall may increase as bladder fills, so does the radius, which means tension is spread out over a greater area
structure of the bladder
- mucosal lining
- transitional epithelium - muscle coat
- made up of bundles of smooth muscle interlacing and running in various directions, considered as a single structure known as the detrusor muscle, great ability to expand. - outlet of bladder into urethra -guarded by 2 sphincters: internal and external
transitional epithelium
capable of stretching without damage and consists of (plaque) ridges that flatten out as bladder fills, increaseing the SA)
very impermeable to salts and water, so no exchange between urine and capillaries of bladder wall
trigone
the mucosal layer is generally loosely attached to underlying muscle except at the base of the bladder – entrance of 2 ureters and exit of urethra form a triangle – the trigone – where mucosa is firmly attached – thickest and least distensible part of bladder
internal sphincter
- smooth muscle
- NOT under voluntary control
- formed by a loop of muscle that is extension of the detrusor, such that when the detrusor contracts, the fibres forming this loop shortens and opens the sphincter
external sphincter
- skeletal muscle
- 2 striated muscles (compressor urethrae & bulbocavernosus) surrounding urethra
- these muscles are responsible for continence (control urine output for whenever it is socially convenient)
- under conscious, voluntary control
females vs males (functioning with sphincters)
females require both sphincters for effective continence, males manage with either sphincter intact because of different anatomical arrangement concerning the urethra
female and male bladder
females: shorter urethra, simpler structure, only carries urine
males: carries urine and semen
females: need both internal and external sphincters to be closed properly for continence. external sphincter muscle is poorly developed and women are more prone to incontinence particularly after childbirth
males: don’t need both the sphincters to work well. the urethra continues through the penis – urine remaining in urethra can be expelled by contractions of the bulbocavernosus muscles
Lower urinary tract innervated by…..
3 sets of peripheral nerves
- parasympathetic (pelvic nerve)
- sympathetic (hypogastric nerve)
- somatic nervous system (pudendal nerve)
neural circuits in the brain and spinal cord coordinate what…..
the activity of the smooth muscle in the bladder and urethra
these circuits act as on/off switches to alternate the lower urinary tract between 2 modes of operation: storage and elimination
how can injury/disease of the nervous system affect the kidneys?
disrupt the voluntary control of micturition and cause the re-emergence of reflex micturation, resulting in bladder hyperactivity & urge incontinence
bladder innervation - sensory and motor
Sensory: gives sensation (awareness) of fullness and pain from disease
Motor: causes contraction and relaxation of detrusor muscle and external sphincter to control micturition
parasympathetic route (motor innervation, SC to bladder)
parasympathetic arises in ventral/anterior horn at sacral region of spinal cord and preganglionic fibres synapse onto postganglionic fibres on wall of bladder and internal sphincter
sympathetic route (motor innervation, SC to bladder)
sympathetic arises in lateral/posterior horn at lumbar region of spinal cord and preganglionic fibres synapse onto post-ganglionic neurones in hypogastric ganglia
some post-ganglionic neurones supply the internal sphincter
others innervate with the parasympathetic ganglia in bladder wall (inhibit neurotransmission)
a few sympathetic neurones end in the detrusor muscle, more found in the trigone region nearer to the internal sphincter
somatic route (motor innervation, SC to bladder)
arise from sacral region and supply the striated muscle of the external sphincter
Innervation of Detrusor (look at diagram)
ACh & ATP – cause detrusor to contract
NA – inhibits transmission at parasympathetic ganglia indirectly causes detrusor to relax
NA - also directly via β-Rs (also in trigone area) causes detrusor to relax
Innervation of Sphincters (look at diagram)
Nitric Oxide (NO) & ACh - relaxes internal sphincter
Noradrenaline (NA) – contracts internal sphincter
ACh – tonic (continual) activity holds external sphincter closed (somatic)
afferent (sensory innervation)
The pelvic, hypogastric and pudendal nerves contain afferent axons that transmit information from lower urinary tract to the lumbosacral spinal cord
Main afferent pathway is via pelvic nerve (parasympathetic):
- Small myelinated Aδ–fibres, micturition reflex
- Unmyelinated C fibres, endings in/near epithelium
Hypogastric (sympathetic) & Pudendal (somatic) pathways
- Nociceptors
- Flow receptors (external sphincter)
A fibres and C fibres (pelvic nerve, parasympathetic, afferent)
A fibres: sense tension in detrusor:
i. Filling of bladder ii. Detrusor contraction - -> bladder fullness, discomfort - Stretch receptors signal wall tension - Volume receptors signal bladder filling
C fibres: respond to damage & inflammatory mediators
-Nociceptors = pain, urgent desire to micturate (e.g. during infection of bladder lining – cystitis; excessive distension)
Bladder Filling
Initially – bladder empty
Sphincters closed
(tonic activity sympathetic & somatic nerves)
Bladder pressure low
Arrival of urine
Detrusor relaxes progressively to accommodate
(sympathetic activity inhibiting parasympathetic transmission)
Little increase in pressure
Sphincters still closed
RECEPTIVE RELAXATION
bladder filling and pressure
we first become conscious of bladder filling when there’s 100-150ml of urine in the bladder
above this volume the sensation becomes more distinct until discomfort begins between 300-400ml
further distension beyond this causes mounting distress and and eventually pain
Threshold volume differs from person to person
Bladder Emptying (Micturition)
this is an autonomic reflex, 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)
why do girls develop continence faster?
because girls nerve’s mature faster than boys
stress incontinence
(mostly to women) loss of sphincter control, especially after childbirth. E.g. little urine when you cough
Micturition (Emptying) Reflex
detrusor muscle contracts, both internal and external sphincters relax and urine flow is established - how does this work?
receptors detect tension in bladder wall – afferent activity excited, which triggers activity in parasympathetic efferents (supply detrusor and internal sphincter by spinal reflex arc)
Act causes detrusor muscle to contract, reinforced by ATP
other parasympathetic fibres relax internal sphincter (Ach & NO) and urine flows out
entry of urine into the first part of the urethra triggers afferent nerves, which reinforce micturition by inhibiting somatic nerves which tonically contract external sphincter
Continued positive feedback from tension receptors in bladder wall and urine in urethra all reinforce micturition until the bladder has emptied
Modification of Micturition Reflex by Higher Centres
Evidence for involvement of higher centres in micturition reflex because get incontincence-provoking lesions in motor cortex
-pontine micturition centre (PMC) in the pons
Voluntary Modification of Reflex
Emptying of bladder in older children and adults same as in reflex emptying BUT higher centres can inhibit reflex for a time until socially convenient
Higher centres can modify micturition reflex for a while:
- Contract external sphincter consciously
- Increase sympathetic firing to bladder and internal sphincter
- Urine stream can be halted by “strangury” (urethral pain) due to urethritis (inflammation of urethra from STI or renal calculi)
- Pinching glans penis can inhibit micturition
- At night, if bladder fills to capacity, recognised by PMC and arousal centre wakes you up
Normal Control of Micturition
to stop micturition, messages from micturition inhibitory centre in the frontal lobe and the micturition centre in the pons increase sympathetic activity to block parasympathetic activity and hence keep bladder relaxed and internal sphincter closed
to allow voluntary release of urine the pons and cerebrum quietens the sympathetic input into the bladder and allows the parasympathetic system to take over (as in the reflex)
usually brought about by contracting abdominal muscles which increase pressure in the bladder
hence urine enters into bladder neck and urethra under pressure, stimulating stretch receptors in the wall which excite the micturition reflex as described
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 to the normally supported urethra.
Reflex contraction of peri-urethral striated muscles also helps compress the urethra ⇒micturition reflex aided
Importance of Bladder Emptying
Urine
- Normally sterile
- Occasional bacterial entry
Complete emptying restores sterility
Bacteria in retained urine seeds fresh urine
Retained urine clinical infection (UTI)
Urinary Tract Infections (UTI)
- can happen anywhere along the urinary tract
- have different names, depending on what part of the urinary tract is infected:
Bladder – an infection in the bladder is also 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
are UTI’s more common in women or men?
women because of short urethra
risk factors for UTI’s:
Diabetes mellitus; long-term catheterisation; pregnancy; enlarged prostate; prolonged immobility; kidney stones; bowel incontinence; advanced age
how is diabetes mellitus a risk factor for UTI’s?
glucose in the urine - glucose is a breeding ground for bacteria
how is advancing age a risk factor for UTI’s?
as men age, the prostate enlarges and this can press down against the urethra and bock the flow or urine
Problems of Ageing Bladder
Slow urine stream
- Prostate enlargement (BPH -benign prostatic hyperplasia). Most common cause of lower urinary tract symptoms in men (occurring in 25% of men > 40yrs)
- Slow urine stream means there is incomplete emptying → infection
Incontinence
- weakening of sphincters (stress incontinence)
- failure of nervous control
- overactive bladder - detrusor contracts spastically, resulting in sustained high bladder pressure – urge incontinence (urgency at inconvenient and unpredictable times and sometimes lose control before reaching a toilet)
- Socially embarrassing
- Diminishes self-esteem
- Reduces quality of life
treatments
- Medication: anti-muscarinics relax smooth muscle & decrease detrusor contraction (Oxybutynin, a non-specific muscarinic receptor antagonist)
- Bladder retraining - used for stress & urge incontinence (Kegel exercises)
- Surgery
- Sacral Nerve Stimulation (SNS)
- implanted neurostimulation system
- electrical impulses to sacral nerve - Stem cell therapy
- inserted cultured stem cells into bladder wall - Tissue engineered bladder
- synthetic and natural scaffolds to form 3D structure using human tissue
how can surgery help incontinence?
- bladder neck suspension
- botulinum toxin/collagen injections into muscles around urethra → relaxes bladder (OAB). lasts for several months before repeat injections needed.
