Micturition Flashcards
What is micturition?
Urination
What three stages are involved in micturition?
- urine made in the kidneys
- urine stored in the bladder
- urine released from the bladder
Describe the basic process of micturition. PART 1
- Urine passes from the collecting ducts of the renal tubules into the renal pelvis.
- Contraction of the smooth muscle of the pelvis aids in the movement of the urine into the ureter.
Describe the basic process of micturition. PART 2
- As the ureter gets distended, the smooth muscle which is arranged circularly contracts.
- Contraction closes the junction between the pelvis and the ureter, and pushes urine further into the ureter, causing distension and further contraction.
- Peristaltic wave is initiated and propagated along the length of the ureter until it propels urine into the bladder.
Describe the basic process of micturition. PART 3
- Ureters open at an oblique angle to the wall of the bladder.
- When the pressure within the bladder rises, the ureters are compressed, preventing the reflux of urine back up into the ureters.
- Ureteric peristalsis is myogenic in origin, i.e. originating in the smooth muscle.
Describe the renal pelvis.
Funnel-like dilated proximal part of the ureter
What is not required for micturition? Explain how you know.
Neural control by the CNS is not needed.
- Ureters seem to function normally when their extrinsic nerves are cut
What is required for control of micturition?
- Coordination of peristaltic waves
- Response to changes in volume of urine produced by the kidney.
Describe kidney stones (renal calculi) and how they develop.
- Common disorder of the urinary tract
- Develop from crystals that separate from the urine within the urinary tract.
What does urine contain to prevent kidney stone formation?
Citrate inhibitors
Outline the composition of kidney stones.
- Calcium usually as calcium oxalate as calcium phosphate.
- Uric acid
- Struvite and cystine.
Why are kidney stones more common in men than women?
Difference in levels of testosterone
What can kidney stones be caused by?
- Poor urine output/ obstruction
- Low concentration of inhibitors
- Infection
- Excess dietary intake of stone-forming substances.
What are the symptoms of kidney stones?
- dysuria (painful urination)
- haematuria
- urinary tract obstruction
- if the stone approaches the tip of the urethra, pain can inhibit micturition (strangury)
Describe the mucosal lining of the bladder.
- Transitional epithelium - capable of stretching without damage.
- Consists of ridges that flatten out the bladder as it fills.
- Impermeable to salts and water - no exchange between the urine and the capillaries of the bladder wall.
Describe the muscle coat lining the epithelium of the bladder.
- Made up of bundles of smooth muscle interlacing and running in various directions.
- Considered as a single structure known as the detrusor muscle.
- Mucosal layer is generally loosely attached to the underlying muscle except at the base of the bladder
Describe the trigone.
- Where entrance of the 2 ureters and the exit of the urethra form a triangle.
- Where the mucosa is firmly attached, making it the thickest and least distensible part of the bladder.
What is the outlet of the bladder into the urethra guided by?
Internal and external sphincter
Describe the internal sphincter.
- Formed by a loop of muscle that is an extension of the detrusor
- Not under voluntary control.
How does the internal sphincter open?
- When detrusor contracts, fibres forming loop shorten and open sphincter.
Describe the external sphincter.
- Composed of skeletal muscle and is continuous with the levator ani.
- 2 striated muscles, the compressor urethrae and the bulbocavernosus.
- Muscles surround the urethra - responsible for continence, so under conscious, voluntary control.
What is the levator ani?
Muscle situated on either side of the pelvis
What is the difference between female and male sphincters? Explain why there is a difference.
- Females require both sphincters for effective continence
- Males can manage with either sphincter intact
- REASON: different anatomical arrangement of urethra.
Describe the differences between a male and female urethra.
- Female urethra only carries urine
- Male urethra needed for the ejaculation of semen during reproduction.
Describe the anatomical differences between a male and female bladder. PART 1
IN WOMEN:
- Structures around the neck of the bladder are the end of the system and point of exit of urine from the body.
- External sphincter is poorly developed
- More prone to incontinence, particularly after childbirth
Describe the anatomical differences between a male and female bladder. PART 2
IN MEN:
Urethra continues through the penis - urine remaining in the urethra can be expelled by contractions of the bulbocavernosus muscles.
What is the effect of men and women having anatomical differences?
Develop different pathologies and diseases
Describe bladder innervation.
- Neural circuits of the brain and spinal cord coordinate smooth muscle activity in the bladder and urethra.
- Circuits act as switches to alternate the urinary tract between the two modes of operation: storage and elimination.
What is the effect of neurological damage on micturition?
- Disrupt the voluntary control of micturition
- Re-emergence of reflex micturition
- Bladder hyperactivity and urge incontinence
What three types of peripheral nerves is the lower urinary tract innervated by?
- Somatic nerves
- Sympathetic nerves
- Parasympathetic nerves
What are the two types of bladder innervation?
SENSORY
MOTOR
What is the effect of sensory innervation?
Causes sensation (awareness) of fullness, and also pain from disease
What is the effect of motor innervation?
- Contraction and relaxation of the detrusor muscle and the external sphincter
Describe the nature of parasympathetic fibres on the bladder.
- Arises in the ventral horn at the sacral region (S2-S4) of the spinal cord
- Preganglionic fibres synapse onto postganglionic fibres on the wall of the bladder and internal sphincter.
Describe the nature of sympathetic fibres on the bladder.
- Arises in the lateral horn at the lumbar region (T11-L2) of the spinal cord.
- Preganglionic fibres synapse onto the postganglionic neurones in the hypogastric ganglia.
- Few neurons end in the detrusor muscle; more are found in the trigone area.
What effects do some of the postganglionic sympathetic fibres have on the bladder?
- Some supply the internal sphincter
- Others innervate with the parasympathetic ganglia in the bladder wall (inhibiting parasympathetic transmission).
Describe the nature of somatic fibres on the bladder.
- Arises from the sacral region of the spinal cord
- Supply the striated muscle of the external sphincter.
Describe the parasympathetic innervation of the detrusor muscle.
- Main transmitter is acetylcholine, but also releases ATP.
What is atropine and what effect does it have?
- Muscarinic agonist
- Inhibits detrusor contraction.
Describe sympathetic innervation of the detrusor muscle.
- Transmitter is noradrenaline - acts on α-receptors to inhibit transmission at the parasympathetic ganglion.
- Relaxation of the bladder body is caused by direct release of noradrenaline acting on β-receptors (usually in the trigone).
Describe the parasympathetic innervation of the sphincters.
- Release nitric oxide to act on muscarinic receptors
- Relaxes the internal sphincter.
Describe the sympathetic innervation of the sphincters.
- Release noradrenaline to act on the α1-receptors
- Contracts the internal sphincter.
Describe somatic innervation of the sphincters.
- Release acetylcholine, which acts on nicotinic receptors
- Holds the external sphincter closed with tonic (continual) activity,
Describe the sensory innervation of the bladder.
- Pelvic, hypogastric and pudendal nerves contain afferent axons that transmit information from the lower urinary tract to the lumbosacral spinal cord.
Describe the afferent pathway through the pelvic nerve.
- Small myelinated Aδ–fibres, which cause a micturition reflex
- Stretch receptors to signal wall tension
- Volume receptors to signal bladder filling
- Pathway is parasympathetic
What are the effect of the unmyelinated C fibres?
Have nociceptors for pain (eg. during infection of the bladder lining, such as cystitis, excessive distension) at epithelium
Compare and contrast the hypogastric and pudendal nerves.
- Hypogastric is sympathetic but pudendal is somatic pathways.
- Both involve nociceptors and flow receptors (on the external sphincter).
With afferent (sensory) nerves, what is the difference between A and C fibres?
A FIBRES: sense tension in the detrusor:
- filling of the bladder
- detrusor contraction
- bladder fullness, discomfort
C FIBRES: respond to damage and inflammatory mediators.
- PAIN as an urgent desire to micturate
Describe the changes in the bladder nerve stimulation when it’s being filled. PART 1
- Initially, the bladder is empty. Bladder pressure is low.
- Sphincters are closed (due to the tonic activity from the sympathetic and somatic nerves).
Describe the changes in the bladder nerve stimulation when its being filled. PART 2
- Detrusor muscle begins to relax progressively (as the sympathetic activity is inhibiting the parasympathetic transmission).
- Little increase in pressure
- Sphincters are still closed.
What is receptive relaxation?
Phenomenon of a hollow organ relaxing as it fills
How do we feel the bladder filling and its pressure?
- Initially become conscious of the bladder filling when there’s 100-150 ml of urine in the bladder.
- Above this volume, the sensation becomes more distinct till discomfort begins between 300-400 ml.
- Further distension beyond this causes mounting distress.
Describe the changes in the bladder nerve activity during the micturition (emptying) reflex. PART 1
- Receptors detect tension in the bladder wall
- Afferent activity is excited
- Triggers activity in the parasympathetic efferents which supply the detrusor and internal sphincter by the spinal reflex arc.
- Detrusor muscle contracts due to acetylcholine, and it is reinforced by ATP.
Describe the changes in the bladder nerve activity during the micturition (emptying) reflex. PART 2
- Other parasympathetic fibres relax the internal sphincter (with acetylcholine and nitric oxide) and urine flows out.
- Urine entry in first part of the urethra triggers the afferent nerves.
- Reinforce micturition by inhibiting the somatic nerves which tonically contract the external sphincter.
Describe the changes in the bladder nerve activity during the micturition (emptying) reflex. PART 3
- Detrusor muscle is contracting, both the internal and external sphincters are relaxed and urine flow is established.
- Positive feedback from the tension receptors in the bladder walls and urine in the urethra reinforce micturition until the bladder is empty.
How can the micturition reflex be modified by higher centres?
- Widespread involvement of the cortical and subcortical areas by viewing activation pattern linked to micturition using MRI
- Neuron populations are involved, including the specific pontine micturition centre (PMC).
Describe the different ways of voluntary modification of the micturition reflex. PART 1
- Contract the external sphincter and levator muscle consciously
- Increase the sympathetic firing to the bladder and the internal sphincter (due to the contraction of the levator and external sphincter muscles, causes the urethra to compress)
Describe the different ways of voluntary modification of the micturition reflex. PART 2
- Interferes with the positive feedback to the bladder emptying by the inhibition of parasympathetic transmission and tightens the internal sphincter.
- Urine stream can be halted by ‘strangury’ (urethral pain) due to urethritis
- Pinching the glans penis can inhibit micturition
Describe the different ways of voluntary modification of the micturition reflex. PART 3
- At night, if the bladder fills to capacity, recognised by the PMC and the arousal centre wakes you up
What is the purpose of increasing sympathetic activity in control of micturition?
- Stop the micturition message from reaching the micturition inhibitory centre in the frontal lobe and the micturition centre in the pons.
What is the result of increasing sympathetic activity during control of micturition?
- Blocks parasympathetic activity
- Keeps the bladder relaxed and the internal sphincter closed
What can be done during control of micturition to allow voluntary release of urine?
- Pons and cerebrum quieten the sympathetic input into the bladder and allow the parasympathetic system to take over
How is urine released voluntarily during control of micturition?
- Brought about by contracting abdominal muscles which increases bladder pressure.
- Urine enters the bladder neck and urethra under pressure
- This stimulates stretch receptors in the wall, which excite the micturition reflex.
What is the importance of the bladder emptying?
- Release the urine within in the bladder.
- Complete emptying restores sterility since urine can contain bacteria.
- Ordinarily, all the urine is emptied, with rarely more than 5-10 ml left in the bladder.
What is the effect of retained urine?
- UTIs
- If repeated, renal function can be impaired
Describe UTIs (urinary tract infections).
- Infection that can happen anywhere along the urinary tract.
Why are UTIs more common in women?
Shorter urethra
Why are UTIs more common in men over 40?
- Prostatic disease - causes bladder outflow obstruction
What are the risk factors for UTIs?
- diabetes mellitus
- long-term catheterisation
- pregnancy
- kidney stones
- bowel incontinence
- advanced age
UTIs have different names depending on what part of the urinary tract is infected.
Elaborate.
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 a site of infection
URETHRA: an infection of the urethra is called URETHRITIS
What are some problems associated with an aging bladder? PART 1
- Slow urine stream - commonly due to prostate enlargement (BPH - benign prostatic hyperplasia)
- Incomplete emptying of the bladder, which could lead to an infection.
What are some problems associated with an aging bladder? PART 2
- Sphincters will also weaken with age.
- Normal bodily functions such as coughing or laughing raises bladder pressure sufficiently enough to squeeze urine past them.
- Socially very embarrassing and distressing.
What are some problems associated with an aging bladder? PART 3
Overactive bladder (OAB)
- Detrusor muscle contracts spastically, sometimes without a known cause, which results in sustained, high bladder pressure and the urgent need to urinate.
What are some problems associated with an aging bladder? PART 4
- Experience urgency at inconvenient and unpredictable times and sometimes lose control before reaching the toilet (urge incontinence).
- Interferes with work, daily routine and diminishes self-esteem and quality of life.
What normally maintains continence?
- Increased intra-abdominal pressure forces the urethra against the intact pubocervical fascia
- This closes the urethra
Where is the ureterovesical junction (UV junction) located?
Located where the ureter (the tube that drains urine from the kidney) meets the bladder.
What occurs during stress urinary incontinence?
- Defective fascial support allows posterior rotation of the UV junction due to the increased pressure
- Urethra opens and urine loss occurs.
How would we treat incontinence with medications? PART 1
- Antimuscarinics, which affect the central nervous system and muscarinic receptors in smooth muscle.
- Relax the smooth muscle of the bladder, reducing detrusor contraction and subsequent wetting accidents within two weeks.
How would we treat incontinence with medications? PART 2
- Side effects such as headache, blurred vision, hypertension, drowsiness and urinary retention.
- Used with caution in patients with narrow-angle glaucoma / certain kidney, liver and urinary problems.
How can exercise be used to treat incontinence?
- Kegel exercises can also be used for bladder retraining (used for stress and urge incontinence).
- Squeeze pelvic flow muscles (as if trying to stop urine flow) for 10 seconds, then rest for 10 seconds. Done 10 to 20 times a day for several weeks can make a huge difference.
How can surgery be used to treat incontinence?
- Examples - bladder neck suspension or sling, collagen injections around the urethra, implantation of an artificial urinary sphincter, tension-free vaginal tape (TVT).
- Botox is injected into the sides of the bladder to treat urge continence and OAB
How can Botox treat incontinence?
- Relaxes the bladder and lasts for several months before repeat injections are needed.
How can sacral nerve stimulation be used to treat incontinence?
- Electrical current is applied to the sacral nerve, supplying the lower urinary tract via an implanted pulse generator.
- Provides complete continence in 50% of patients and improvements in 80% of patients - Works better in women than in men.
How can stem cell therapy be used to treat incontinence?
- Cultured stem cells are inserted into the bladder wall.
- Limited by the supply of stem cells (bone marrow).
How can synthetic organs be used to treat incontinence?
- Synthetic and natural scaffolds are used to form a 3D structure using human tissues.
- Currently in Phase II trials.
What is the innervation of the detrusor muscle like?
→ Parasympathetic pre-ganglion synapse onto post-ganglion neurons in the bladder wall
→ release ACh (and ATP)
→ acts on nicotinic receptors - cause contraction
What receptor does ATP act on?
Purinergic
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/aging
→ Bladder hyperactivity + urge incontinence
→ Stress incontinence
Using what you know, outline the steps for micturition. PART 1
- 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
Using what you know, outline the steps for micturition. PART 2
- Urine forced out and activates flow receptors in the urethra
- Pudendal afferents are excited and switch off the tonic system
- Tonic contraction removed by inhibition of somatic input
Using what you know, outline the path of innervation to the bladder.
Cerebral cortex (frontal lobe)
Brain stem
Spinal efferents
Ganglion
Bladder
What sends signals from the bladder to the spinal efferents?
Unmyelinated afferents
Where is the bladder?
→ Floor of abdominal cavity
