Micturition Flashcards
Define Micturition
Micturition is the act of urination (emptying the bladder).
The basic process of Micturition is broken down into 3 stages, what are these stages?
Urine made in kidneys (1ml.min-1)
Urine stored in bladder
Urine released from bladder
The last 2: Function of bladder & associated sphincters
Describe the basic 3 point process of how urine is transported from the kidney to the bladder
- Urine from all collecting ducts of all nephrons
- Empty into Renal Pelvis
- Urine enters Ureter
What happens when the ureter gets distended and the smooth muscle contracts?
This contraction closes junction between the pelvis & ureter and pushes urine further into ureter, causing distension and further contraction – peristaltic wave initiated and propagated along length of ureter until it propels urine into the bladder. Peristaltic waves initiated about 5x per minute from renal pelvis.
Peristaltic waves in ureter occur @ freq ~1-6 contractions/minute
Ureters squeeze urine to pressure of 10-20mmHg
How does the ureter open up into the bladder and what does this prevent?
Ureters open obliquely into the bladder
Prevents reflux of urine back into ureters by the passive flap-valve effect
What controls uretic peristalsis?
Myogenic in origin, not under the control of CNS
Coordination required between peristalsis and changing urine volume
How do kidney stones form?
most common disorder of urinary tract & develop from crystals that separate from urine within urinary tract
What prevents the formation of kidney stones?
Normal urine contains inhibitors (citrate) to prevent this
Describe the composition of kidney stones
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%), cystine (<5%).
What causes kidney stones?
Caused by poor urine output/obstruction, altered urinary pH, low concentration of inhibitors, infection, excess dietary intake of stone-forming substances
Where can kidney stones form and what are the symptoms?
Kidney stones can form anywhere within urinary tract: kidney, ureter or in the bladder
Symptoms:
Dysuria (painful urination)
Haematuria
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 structure of the bladder
On image
What are the 2 sphincters of the bladder?
What control them?
Locate them on the diagram
Internal Sphincter: extension of detrusor muscle NOT under voluntary control
External Sphincter:
2 striated muscles (compressor urethrae & bulbocavernosus) surrounding urethra = these muscles responsible of continence = under conscious, voluntary control
Compare and contrast the female and male bladder
Female:
Short urethra - only carries urine
External sphincter poorly developed » more prone to incontinence particularly after childbirth.
Male:
Carries urine and semen
Urine elimination aided by contraction of bulbocavernosus muscles in penis
What is the lower urinary tract innervated by (3 sets of peripheral nerves)
parasympathetic (pelvic nerve)
sympathetic (hypogastric nerve)
somatic nervous system (pudendal nerve)
What does sensory and motor innervation of the bladder cause?
Sensory: gives sensation (awareness) of fullness and also pain from disease
Motor: causes contraction and relaxation of detrusor muscle and external sphincter to control micturition
Where does the parasympathetic, sympathetic and somatic run to and from
Motor innervation:
Parasympathetic:
Arises from the sacral region of the spinal cord, delivering pre-synaptic neurones to the synapse in the ballder wall, this gives off-post-ganglionic neurones.
We also have more pre-synaptic neurones from the sacral region, forming post-synaptic neurones in the internal sphincters
Sympathetic nervous system:
Arises in the lumbar region: innervate post-synaptic neurones in the Hypogastric ganglia and innervate the internal sphincter, they act on parasympathetic post-ganglionic neurones - inhibiting the PS NS and direct innervation to the ballder wall
Somatic: this innervates the external sphincter
These also convey signals to the brain
What causes the detrusor to contract?
What inhibits contraction - what drug?
What causes the detrusor to relax?
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
The parasympathetic nervous system releases Ach acting at nicotinic receptors causing the release of Ach and ATP that act on muscarinic receptors and purinergic receptors causing attraction of detrusor
The sympathetic pre-ganglion releases NA that acts on alpha receptors inhibit the parasympathetic nervous system - causing relaxation. It also releases NA at the bladder wall acting on Beta recptors allowing semen to pass, preventing urine from entering
What cause the sphincters to contract and relax?
Nitric Oxide (NO) & ACh - relaxes internal sphincter Noradrenaline (NA) – contracts internal sphincter ACh – tonic (continual) activity holds external sphincter closed
Describe the 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
- Unmeylinated 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)
What do A fibres sense (sensory fibres)?
A fibres: sense tension in detrusor:
i. Filling of bladder ii. Detrusor contraction - > bladder fullness, discomfort
What do C fibres sense?
C fibres: respond to damage & inflammatory mediators
PAIN (urgent desire to micturate)
Describe the state of the ballder when empty and when it is filling
Initially – bladder empty
Sphincters closed
(tonic 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
How does the bladder develop?
- Micturition is an autonomic reflex - when ballder is full it empties
- 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 signals initiated during Micturition (Emptying)
This occurs in babies
- Stretch and volume receptors fire, activating the afferent system - alpha delta fibres - to the spinal cord
- These synapse with the parasympathetic referents causing detrusor contraction and internal sphincter to relax by the release of NO acting on MR.
- As urine empties it activates flow receptors in the urethra -> pudendal afferents excited
- Tonic contraction of external sphincter removed by inhibition of somatic input
What are the voluntary controls that prevent random urination?
What disease process inhibit urination?
Higher centres can modify micturition reflex for a while:
Contract external sphincter & levator muscle consciously
Increase sympathetic firing to bladder and internal sphincter (voluntary??)
Interferes with positive feedback to bladder emptying by inhibition of parasympathetic transmission
Tightens 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
Summarise the voluntary control of Micturition
On slides
What happens when we contract the abdominal muscles?
Reflex contraction of peri-urethral striated muscles - what does this cause?
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
Is the bladder sterile?
What restores the sterility of the bladder?
What can a bacterial infection caused in the retained urine?
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 ascends to kidney
What is a urinary tract infection?
A urinary tract infection, or UTI, is an infection that can happen anywhere along the urinary tract.
Urinary tract infections have different names depending on which part is infected, what are these names?
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
Why is UTI more common in women?
What is prostatic disease?
More common in women because of short urethra
Common in men over 40 due to prostatic disease, causing bladder outflow obstruction
Some risk factors:
What are the issues with an ageing bladder?
What are the consequences of a slow urine stream?
What causes these issues?
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)
Consequence of slow urine stream → incomplete emptying → infection
Incontinence
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
Socially embarrassing
Diminishes self-esteem
Reduces quality of life
What treatment is available for UTI?
Medication: 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) – inserted 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.
