Lecture 12 Flashcards
(17 cards)
What is the tip of the renal pyramid called?
renal papilla
Describe the exit of urine into the renal pelvis and ureter
Minor and major calyces lead to renal pelvis
Fluid deposition into renal pelvis stretches smooth muscle
Distension triggers peristaltic contractions at hilus
Fluid moves down ureter in pulses towards bladder for storage and controlled release
Describe the structure and function of the ureters
Structure
Mucosal layer: transitional epithelium
– 3-8 cells thick, impermeable to urine
Supported by layers of smooth muscle:
- inner: longitudinal muscle (L)
- outer: circular/spiral muscle (C)
- extra outer layer of longitudinal muscle
Function
Dilation of renal pelvis generates action potential from pacemaker cells in hilum
Peristaltic waves generated – between 1 to 6 per minute…
…the number of contractions can be modulated by nervous system:
- parasympathetic NS: enhanced
- sympathetic NS: inhibited
Describe the structure and function of the urinary bladder
Structure
A hollow muscular organ, consisting of fundus (body) and neck
Outer “Detrusor” Muscle layer:
- consists of longitudinal, circular/spiral
muscles
Inner Mucosal layer:
- transitional epithelium - folded into “rugae” when bladder empty - highly elastic – expands as bladder fills
The Trigone
Triangular area bounded by openings of ureters and entrance to urethra…
…acts as a funnel to channel urine towards neck of bladder
Function
Temporary storage of urine
Up to 1 L capacity
Stimulated to contract by parasympathetic NS
Describe the exit of urine from the urinary bladder
The urinary bladder is guarded by two sphincters:
Internal urethral sphincter
Loop of smooth muscle
Convergence of detrusor muscle
Under involuntary control
Normal tone keeps neck of bladder
and urethra free of urine
External uretheral sphincter
Circular band of skeletal muscle where urethra passes through urogenital diaphragm
Acts as a valve with resting muscle tone
Under voluntary control
Voluntary relaxation permits micturition
Describe the female and male elimination of urine
Females:
Opens via external urethral
orifice located between clitoris and vagina
Shorter urethra in females
- more susceptible to UTIs
External sphincter not as well developed
- incontinence following childbirth due to injury
Males:
Urethra passes through prostate gland and through urogenital diaphragm and penis
Longer urethra compared to females provide some protection against UTIs
Prostate gland enlarges in
50% of males >60 yrs (along with hypertrophy of detrusor muscle)
- may require surgical or hormone treatment
-Prostate cancer – one of the commonest cancers affecting older men
Describe micturition
Bladder progressively fills until pressure within bladder reaches a threshold level…
…this elicits the “micturition reflex” which produces a conscious desire to urinate or eventual emptying of the bladder
The micturition reflex is an autonomic reflex which is
(i) inhibited by higher centres in the brain and
(ii) facilitated by cortical centres in the brain:
Higher centres keep the micturition reflex under inhibition…
…prevents micturition by stimulating continual tonic contraction of the external sphincter
Cortical centres facilitate urination by initiating the micturition reflex and relaxing the external sphincter
Internal sphincter relaxes at the same time and urination occurs
Describe filling the urinary bladder
Bladder fills at 1 mL/min at normal levels of hydration
As bladder fills, (intravesical) pressure increases
– these are periodic reflex contractions of short duration which occur above approximately 200 mL urine volume
- Partially full bladder: contractions relax spontaneously after a few seconds
- Increasingly full bladder: contractions more frequent, intense and last longer
Describe the bladder and sphincter innervation
Hypogastric nerve-Sympathetic and involuntary control
Pelvic nerve- parasympathetic and involunatry cotrol
Pudendal nerve- Somatic ad volutary cotrol
Micturition is inhibited by activity in the hypogastric and pudendal nerves
It is facilitated by activity in the pelvic nerves
Describe how the guarding reflex promotes cotinece
During bladder filling:
Progressive bladder distension stimulates the pelvic nerve via activation of stretch receptors in bladder wall and internal sphincter (1)
Activation of the pelvic nerve leads to stimulation of the hypogastric nerve
Hypogastric nerve stimulation causes:
(i) relaxation and reduced excitability of the bladder detrusor muscle (2)
(ii) constriction of the internal sphincter (3)
Also, the external sphincter is held closed by pudendal nerve (4)
These responses are known as the “guarding reflexes” which promote continence
Describe the micturition reflex
Micturition is facilitated
by activity in the pelvic nerve
Stretch receptors in bladder continue to stimulate the pelvic nerve
Stimulation of the pelvic nerve also causes:
(i) contraction of the detrusor muscle (1)
(ii) relaxation of the internal sphincter (2)
Periodic reflex micturition contractions are also stimulated above 200 mL. These relax spontaneously after a few seconds
These micturition contractions continue to be stimulated and relax but at > 300 mL bladder contractions begin to predominate
Full bladder sensation conveyed to thalamus and then to cerebral cortex. Desire to urinate starts to increase…
At appropriate time, voluntary relaxation of external sphincter occurs via pudendal nerve. Micturition occurs, aided by lowering of diaphragm, contraction of abdominal muscles and opening of internal sphincter
Describe the neural disruption of micturition
Paraplegia: complete severing of nerve inputs from cerebral cortex (1)
Mictuiation reflexes return, but without cortical control
Periodic but unannounced bladder emptying – “Automatic bladder”
Partial spinal cord damage with loss of inhibitory descending signals (2)
Frequent urination as excitatory impulses from cerebral cortex remain unopposed
Known as “Uninhibited bladder”
Crush injury of dorsal roots (3)
Afferent nerve destruction - micturition reflexes lost despite complete efferent system
Bladder fills to capacity and overflows dropwise - “overflow incontinence”
Known as “Atonic bladder”
Infants lack voluntary control over urination until corticospinal connections are established
Explain the problems with the micturition reflex
Control of micturition can be lost due to:
- stroke injury, Alzheimer’s disease, problems affecting cerebral cortex or the hypothalamus (e.g. a brain tumour)
(2) Bladder sphincter muscles can lose tone (e.g. after pregnancy):
- leading to urinary incontinence
Urinary retention may develop in males if enlarged prostate gland compresses the urethra and restricts urine flow
Functional classifications
Failure to store urine = incontinence
Failure to empty bladder = retention
- Due to bladder dysfunction
- Due to problem with outlet of urine
Describe the three main types of urinary incontinence
Loss of sensory nerves – due to injury
Bladder fills to capacity
No signals from stretch receptors in bladder
Overflow incontinence occurs (Atonic bladder)
Involuntary bladder contractions – due to injury
Urge incontinence or increased frequency
Heightened urge incontinence – sensitive bladder Spicy food (capsaicin) Caffeine/chocolate (xanthines) Citrus fruits (citric acid) Carbonated beverages (sugar, sweeteners)
Describe the drugs used in urinary incontinence
Anticholinergics (muscarinic ACh receptor antagonists, i.e. Antimuscarinic drugs)
Actions:- inhibit bladder contractions
- facilitates involuntary contraction of internal bladder sphincter
Examples:OXYBUTININ, Tolterodine, Flavoxate (less side effects but less effective)
Unwanted effects: Dry mouth, blurred vision, palpitations, drowsiness, facial flushing (Atropine-like)
Tricyclic antidepressants, e.g. IMIPRAMINE can be used at low doses for short-term treatment for nocturnal enuresis in children > 10 years but can have side effects, e.g. behaviour problems on withdrawal
Mechanisms: - anticholinergic and direct muscle relaxant effects on the bladder but also inhibits re-uptake of
noradrenaline and serotonin increasing their levels
DULOXETINE also increases serotonin and noradrenaline levels – for moderate to severe stress urinary incontinence
DESMOPRESSIN (ADH analogue) – also useful in children for nocturnal bed-wetting
MIRABEGRON – selective beta-3 receptor agonist – useful for overactive bladder syndrome
Purified bovine collagen implants – useful for urinary incontinence
Describe the alpha-adrenergic blocking drugs
Actions:- Antagonist action at 1A adrenoceptors in bladder neck
- Relaxes smooth muscle at bladder neck and increases urine flow rate
Examples: Alfuzosin, Doxazosin, Indoramin, Prazosin, Tamsulosin, Terazosin
Cautions: - Can reduce blood pressure so careful dosing required in patients already receiving antihypertensive treatments
- Should be avoided in patients with postural hypotension
Unwanted effects: Hypotension, drowsiness, depression, headache, dry mouth, GI disturbances
Describe the Parasympathomimetics (Choline Esters)
Actions: - Agonist action at muscarinic ACh receptors
- Increases contraction of the bladder detrusor muscle - Limited role in relief of urinary retention – now superseded by catheterisation
Examples: BETHANECOL
Cautions: - Use with care or avoid using in patients with cardiac disorders (e.g. arrhythmias)
- Avoid in cases involving GI ulceration, asthma, hypotension, epilepsy, Parkinsonism, pregnancy
Unwanted effects: Nausea, vomiting, intestinal colic, bradycardia, blurred vision, sweating
Neostigmine, Pyridostigmine – inhibit breakdown of acetylcholine – promotes incontinence
Finasteride, Dutasteride are inhibitors of androgen synthesis (5α–reductase inhibitors) useful for
relieving urinary retention caused by prostate hyperplasia
