Micturition Flashcards
Micturition
where is the urine stored and released from? How is this possible?
What does the urine pass through to get to its end location?
What helps the movement of urine?
What stops blackflow of urine?
What pressure is reached and how frequent is peristaltic waves?
How is reflux of urine prevented when pressure rises? what does this act as?
What controls the ureteric peristalsis?
Urine made in kidneys is formed continuously at a rate of 1ml/min
Urine is stored in the bladder and released from the bladder
The last two steps involve function of bladder and associated sphincters
Kidney to Bladder
Urine passes from the collecting ducts of renal tubules into the renal pelvis.
The renal pelvis is the funnel like dilated proximal part of the ureter in the kidney.
Contraction of the smooth muscle of the pelvis aids movement of urine into the ureter.
Urine enters into the ureter and smooth muscle around lumen of the ureter contract and force urine down the ureter and close off the entry to the kidney, so you don’t get backflow.
Peristaltic waves in the ureter occur at a frequency of 1-6 contractions/minute. Ureters squeeze urine to pressure of 10-20mmHg.
Ureters open obliquely into the bladder and this prevents reflux of urine back into the ureters when pressure in bladder rises, it acts as a flap valve and gets compressed.
Ureteric peristalsis is of myogenic origin, originating in the smooth muscle, and not under CNS control.
- Coordination is required between peristalsis and changing urine volume.
Kidney Stones
what are kidney stones?
What prevents the formation of kidney stones?
What is usually present in most stones?
Are they similar to gallstones?
Who are kidney stones more common in and why?
why else can you get kidney stones?
Where can kidney stones form?
What are the symptoms of kidney stones?
what is renal colic?
what is strangury?
What is the term used when stones from in ureters?
- Kidney stones (renal calculi) develop from crystals that separate from urine within the urinary tract.
- Normal urine contains inhibitors (citrate) to prevent this occurring.
- Calcium is present in nearly all stones (80%), usually as calcium oxalate or less often calcium phosphate.
- Kidney stones are not the same as gallstones.
• Kidney stones are more common in men than women, possibly due to testosterone. But you also can get it caused by poor urine output/obstruction, altered urinary pH, low concentration of inhibitors, infection or excess dietary intake of stone-forming substances.
Kidney stones can form anywhere within the urinary tract: In the kidney, ureter or bladder (and cause blockages)
Symptoms:
– Dysuria (painful urination)
– Haematuria
– Referred pain - Loin pain/back pain
– Reduced urine flow
– Urinary tract obstruction: pressure reaches 50mmHg, causes considerable pain as peristalsis continues, called renal colic
– If stone approaches tip of urethra, intense pain can inhibit micturition, this is called ‘strangury’
[Ureterolithiasis when stones form in ureters]
Bladder
How much volume can the bladder handle without an increase in pressure? What is this due to? How does it’s lining allow stretch?
Why is there no exchange between urine and capillaries of the bladder?
What is the trigone? and how is the mucosal layer different elsewhere?
what guards the outlet of the bladder? name the 2 structures and describe their musculature and if they are under voluntary control
The bladder can be almost empty or can expand and contain up to 400ml without much increase in pressure. This is due to its structure of being spherical so even though tension in wall may increase as bladder fills so does the ratio.
As well as this the mucosal lining of the bladder is transitional epithelium, which can stretch without damage.
It is also impermeable to salts and water so there is no exchange between urine and the capillaries of the bladder (composition of urine doesn’t change in the bladder).
The mucosal layer is also generally loosely attached to the underlying muscle which helps in it expanding, except at the base of the bladder where the entrance of the two ureters and exit of ureter form a triangle the trigone where the mucosa is firmly attached.
The outlet of the bladder into the urethra is guarded by two sphincters, the internal sphincter and external sphincter.
The internal sphincter is just an extension of the detrusor muscle, the smooth muscle just continues down to form the internal sphincter. This is not under voluntary control.
The external sphincter is composed of 2 striated muscles surrounding the urethra. These muscles are responsible of continence, so they are under conscious, voluntary control. (women require both, males can depend on either).
Female and male bladder
Which urethra is shorter? why?
What is stress incontinence and why does it happen during child birth?
The female urethra is far shorter and much simpler because it only carries urine, as well as this the muscles of the external urethral sphincter are poorly developed.
For this reason, in childbirth it can get stretched leading to stress incontinence.
The male urethra is much longer as it continues through the penis and carries both urine and semen. Because it continues through the penis it means muscles in the penis can control urination.
Summary of Bladder
- Lining is transitional epithelium
- The bladder muscle is the detrusor muscle
- The epithelium is impermeable to salts and water
- It is permeable to lipophilic molecules
- The outlet of bladder is guarded by sphincters
- Internal sphincter is composed of smooth muscle and under involuntary control
- External sphincter is composed of striated muscle and under voluntary control
Bladder Innervation
What does the control of micturition control? When does the micturition reflex come into play?
What 3 nerves innervate the lower urinary tract? and what is the nature of their input?
What does the bladder innervation need to be? and why?
Control of micturition is very elaborate and controls both bladder and sphincter. There is a very fine control between storage and elimination. If there is a problem with these neural circuits (e.g. with spinal cord) what happens is the micturition reflex comes into play.
The lower urinary tract is innervated by three sets of nerves:
o The pelvic nerve which has parasympathetic input
o The hypogastric nerve which has sympathetic nerve
o The pudendal nerve which is part of the somatic nervous system
Bladder innervation needs to be:
Sensory, to give sensation (awareness) of bladder fullness and pain due to infection/disease.
Also needs to have motor, to cause contraction and relaxation of the detrusor muscle and external sphincter to control micturition (fill and excrete).
Motor Innervation of Bladder
Where do the fibres come from?
Where do the parasympathetic fibres arise from and synapse at? What else do these fibres innervate too?
Where do the sympathetic fribres arise from and where do they synapse and what do they innervate? Where else do the fibres innervate?
Where do the somatic fibres arise from and what do they innervate?
The fibres are coming from the spinal cord (SC -> Bladder)
The parasympathetic fibres will arise from the sacral region of the spinal cord, these will synapse with post-ganglionic fibres at ganglia which innervate the detrusor muscle (which are on the wall of the muscle). The parasympathetic fibres also innervate the internal sphincter.
The sympathetic fibres arise from the lumbar region, these synapse at the hypogastric ganglia and we have the post-ganglionic fibres innervating the internal sphincter and the parasympathetic post-ganglionic fibres (at parasympathetic ganglia) at bladder wall.
We also have some direct sympathetic innervation to the bladder wall itself, a lot in trigone region.
The somatic system fibres arise from sacral region and is responsible for innervation of the external sphincter.
Innervation of Detrusor
(This is important for knowing to prescribe drugs!)
what happens when preganglionic fibres synapse with postganglionic fibres? (receptor and molecules involved)
What do parasymoathetic fibres release and at what receptors at the detrusor muscle fibres? What is the effect? Which is the more important molecule and why? (drug name)
Where do the sympathetic fibres synapse? What do they release and what receptors do they act on? What is the effect?
Why is sympathetic innervation important in ejaculation?
The preganglionic fibres synapse with the postganglionic fibres and Ach is released at the ganglia binding to nicotinic (LGIC) receptors on the postganglionic fibre.
The parasympathetic postganglionic fibres release Ach (acts at muscarinic receptors, GPCR) and ATP (acts at purinergic receptors) at the detrusor muscle fibres, but the primary signalling molecule is Ach. These both cause the detrusor muscle to contract.
We know Ach is more important because if we give atropine which blocks muscarinic receptors we inhibit contraction of the detrusor muscle.
The sympathetic postganglionic fibres synapse onto the parasympathetic postganglionic fibres in the ganglia located on the detrusor muscle wall.
The sympathetic fibres release NA which acts at alpha receptors which inhibit the parasympathetic postganglionic action, therefore they are preventing detrusor contraction so indirectly cause it to relax.
Sympathetic fibres also directly innervate the detrusor muscle which acts on beta adrenoceptors causing relaxation, particularly in the trigone region.
This is thought to be important in men in ejaculation, as sympathetic closes internal sphincters which stops semen moving back into the bladder.
Innervation of Sphincters
What do the parasympathetic fibres innervate? What do they release? effect?
What do the sympathetic fibres release? what receptors? effect?
What do the somatic fibres release and act on? effect?
Parasympathetic fibres innervate the internal sphincter, the postganglionic fibres release NO (more important) and Ach to relax the internal sphincter.
Sympathetic system does the opposite, it contracts the internal sphinter by NA acting at alpha receptors.
The somatic nerves release Ach tonically at the external sphincter keeping it closed.
Summary of Motor Innervation:
- Parasympathetic neurones:
• Contract detrusor via Ach (muscarinic R) and ATP (purigenic R)
• Relax internal sphincter via NO (cGMP) and Ach (Nicotinic (probs Cl-))
This encourages micturition
- Sympathetic neurones
• Relax detrusor indirectly via NA (alpha R) and directly via NA (beta R)
• Contract the internal sphincter via NA (alpha R)
This inhibits micturition
- Somatic neurones
• Contract external sphincter via tonic release of Ach (on nicotinic R)
This inhibits micturition
Parasympathetic = Pee Sympathetic = Storage
Sensory Innervation
What is is the main afferent pathway? (nerve and nature of nerve) What do these fibres consist of and what are they involved in? What are these fibres linked to?
What fibres detect pain and what are they linked to?
What are the sympathetic firbes linked to?
what are the somatic fibres linked to?
We also need sensory innervation, and this is done by afferent fibres which are also connected to receptors to relay the info back.
The main afferent pathway is via the pelvic nerve (which is parasympathetic).
These consist of small myelinated A-delta fibres which are involved in the micturition reflex.
They link to:
• Stretch receptors which signal wall tension
• Volume receptors which signal bladder filling
We also have un-myelinated C-fibres which link to nociceptors which detect pain, important during infection of bladder lining, cystitis, or excessive distension.
The hypogastric (sympathetic) and pudendal (somatic) pathways are mainly linked to nociceptors, the latter are also linked to flow receptors important in the micturition reflex.
Afferent (sensory) nerves:
A-fibres sense distension in the detrusor
- Filling of the bladder (because connected to stretch receptors)
- Signal if we need to contract detrusor (fullness, discomfort)
C-fibres respond to damage and inflammatory mediators Signal pain (urgent desire to micturate)
Bladder Filling
What is happening when the bladder is empty in relation to nerves and pressure?
what happens when urine enters? why is the bladder able to fill?
What is receptive relaxtion and where else is it seen?
describe the bladder filling and pressure curve
Initially the bladder is empty, the sphincters are closed due to tonic activity of the sympathetic and somatic nerves (both internal and external closed). The bladder pressure is low due to being deflated like.
Arrival of urine into the bladder from the urine will result in relaxation of the detrusor muscle to allow the bladder to fill, the reason we have this is because the sympathetic activity inhibits the parasympathetic transmission to contract the detrusor.
There will be a little increase in pressure and sphincters will remain closed
This concept is known as receptive relaxation (also seen in stomach).
Bladder Filling and Pressure
On X-axis we have bladder volume and Y-axis the pressure in bladder.
As urine enters the bladder we are not really aware of it until it reaches about 150mls, by the time we get to 300mls it is usually uncomfortable, but this threshold volume does vary from person to person.
Once you get to around 400mls there will start being a real urgency to pee.
Bladder Emptying (Micturition)
Why kind of reflex is this? When do you lose this ability in adults?
How is this reflex modified by voluntary control?
What can disrupt this voluntary control?
when emptying takes place
What happens when bladder fills with urine?
What receptors are activated when urine reaches external sphincter?
why is the sacral reflex important for proper emptying?
Micturition is an autonomic reflex that you’re born with, it is seen in babies (<18months, they just pee and you need nappies) it and also in adults when spinal cord is transacted above the sacral region.
We know that the reflex is modified by voluntary control, it can be inhibited or initiated by higher centres in the brain, we learn how to control bladder voiding.
Basic circuits act as on/off switches to alternate between the two modes of operation: storage and elimination.
Disease/injury/aging to the nervous system in adults disrupts voluntary control of micturition. For instance, bladder hyperactivity and urge incontinence, also stress incontinence.
Micturition (Emptying) Reflex
As the bladder fills with urine, the stretch/volume receptors will start firing as the tension in bladder increases.
These join onto the A- delta fibres which then go on to synapse with preganglionic parasympathetic efferent fibre.
This synapses to the postganglionic fibres to cause contraction of the detrusor muscle. There is also synapsing with postganglionic neurones that innervate the internal sphincter to relax it.
So, you’re contracting the bladder and opening the sphincter to allow urine to flow through.
As urine flows into the urethra, flow receptors are activated at the external sphincter which send back signals via somatic afferents to inhibit the tonic contraction of external sphincter.
This allows urine to pass through.
As this occurs it reinforces the whole positive feedback loop and reflex arc.
The sacral reflex is important to reinforce micturition until the bladder is empty.
Voluntary Modification of the Reflex
how do we get volunatry control? where does it act? what is the feedback loop?
how can a urine stream be halted? (3 ways)
The higher centres can modify the micturition reflex for a while.
We do this by contracting the external sphincter and levator muscle (around it) consciously.
There is also a feedback loop to increase sympathetic firing to the 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 also inhibit micturition.
We know we consciously can control our micturition reflex because of MRI scanning. Also, in people with lesions in their brain can end up with incontinence.
Normal Control of Micturition
the pathway from the brain?
where is the bladder contained and how does this help start the reflex?
how does the peri-urethral muscle help too?
There are micturition inhibitory centres in the brain (frontal lobe and pons) which can go both ways, to brain or to brain stem.
Fibres will travel down the spinal tract to the spinal efferents and sympathetic fibres will block parasympathetic activity.
However, as the bladder fills signals are sent back to the spinal tract via the A-delta myelinated fibres. These signals can travel up the brain stem and if it’s socially acceptable you can pee.
Sympathetic activity is stopped, enabling the parasympathetic activity to kick in and allow bladder voiding.
What actually controls this is that the bladder is contained in the floor of the abdominal cavity and contraction of the abdominal cavity can increase intra-abdominal pressure to start/aid the micturition reflex through parasympathetic system.
Reflex contraction of peri-urethral striated muscles also helps compress the urethra to aid micturition.
Importance of Bladder Emptying
is urine sterile? how do you get a uti? why is repeated infections bad?
Where can uti’s take place?
What are the different name of UTI?
who are they more common in and why?
What are risk factors for utis?
Urine is normally sterile but there can be occasional bacteria entry. If you don’t empty your bladder properly the bacteria can result in UTI if there is retained urine.
Repeated infections can destroy renal function if it ascends to the kidney.
Urinary Tract Infections (UTI)
A UTI can happen anywhere along the urinary tract. UTIs have different names, depending on what part of the urinary tract is infected.
Bladder -> An infection of the bladder is also called cystitis or bladder infection
Kidneys -> An infection of one/both kidneys is called pyelonephritis
Ureters -> Rarely site of infection
Urethra -> An infection of urethra is called urethritis
They are more common in women because of their short urethra. Also common in men over 40s due to prostatic disease-causing bladder outflow obstruction.
Some risk factors include diabetes mellitus, long term catheritisation, pregnancy, enlarged prostate, immobility, kidney stones, bowel incontinence, advanced age
Problems of Ageing Bladder
What happens to urine stream as you get older? who is it more common in and why?
what is BPH?
3 reasons for incontinence
3 reasons for failure of nervous control
(atonic, automatic and neurogenic bladder)
Slow urine stream:
Often as people age they get a slow urine stream, this is common in men due to the prostate gland enlarging with age (BPH -> Benign prostatic hyperplasia) which compresses the urethra.
- Most common cause of lower urinary tract symptoms in men.
Consequence of slow urine stream is incomplete emptying which can lead to infection.
Incontinence:
- There can be weakening of the sphincters (e.g. stress incontinence) this is common in women after child-birth due to weakened pelvic floor muscles. (normal body actions like coughing or laughing raises bladder pressure enough to push urine past sphincters)
- There could be a failure of nervous control
- Or an overactive bladder (OAB), where the detrusor contracts spastically causing a sustained high bladder pressure leading to urge incontinence.
Incontinence is socially embarrassing, diminishes self-esteem and reduces quality of life.
Failure of Nervous Control
Destruction of sensory nerve fibres from bladder to spinal cord (Atonic bladder)
- Means the micturition reflex contraction cannot occur, because stretch signals are not transmitted, and it leads to the bladder overfilling a few drops at a time into the urethra = overflow incontinence
Destruction of spinal cord above sacral region (Automatic bladder)
- The micturition reflex can occur but is not controlled by the brain, this is common after initial trauma causing spinal shock resulting in a suppression of this reflex.
- Catheritisation to empty the bladder can prevent bladder injury and you may regain the reflex, however sometimes unannounced bladder emptying can occur.
Partial damage to spinal cord/brainstem (neurogenic bladder)
- Inhibitory signals get interrupted, meaning there is continuous excitable impulses travel down the cord. Resulting in frequent and uncontrolled micturition.
