S2: Micruition

Question 1

What is Micrurition?

Answer 1

It is the act of urination (emptying into the bladder)

Question 2

Explain the urine process from kidney to bladder

Answer 2

Urine passes from the collecting ducts of renal tubules into renal pelvis

Contraction of the smooth muscle of the pelvis aids movement of urine into the ureter and force urine down the ureter and close off entry to the kidney so there is no backflow

• Peristaltic waves occur at frequency of 1-6 contractions per minute

Ureters open obliquely into the bladder and this prevents reflux of urine into ureters when pressure in bladder rises. It acts as a flap valve and gets compressed

Cooridination is required between peristalsis and changing urine volume

Question 3

Is ureteric peristalsis under nervous control?

Answer 3

Ureteric peristalsis is of myogenic origin (originating in the smooth muscle) and not under CNS control (as if you denervate you still see peristalsis)

Question 4

What are kidney stones (renal calculi)?

Describe them

Answer 4

Kidney stones are the most common disorder of the urinary tract.
They 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 as calcium oxalate or less often calcium phosphate

-Kidney stones are more common in men than women possible due to testosterone

Question 5

What does normal urine contain to prevent kidney stones?

Answer 5

Citrate

Question 6

What can cause kidney stones?

Answer 6

• Poor urine output/obstruction
• Altered urinary pH
• Low concentration of inhibitors
• Infection of excess dietary intake of stone forming substances

Question 7

What is ureterolithiasis?

Answer 7

When stones form in ureters

Question 8

Where are kidney stones found?

Answer 8

Anywhere within the urinary tract

- In kidney, ureter or bladder

Question 9

Symptoms of kidney stones

Answer 9

• Dysuria (painful urination)- urine can build up and be blocked
  
  • 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’

Question 10

Explain the how the bladder may expand without much increase in pressure

Answer 10

Bladder can be empty or can expand without much increase in pressure - this is due to its spherical structure so even though tension in wall may in increase as bladder fills, so does the ratio

Question 11

Describe the linings of the bladder

Answer 11

The mucosal lining of the bladder is transitional epithelium which is capable of stretching without damage. It is 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 generally loosely attached to the underlying muscle (crosslinked) which helps in it expanding except at the base of the bladder wherethe entrance of the two ureters and exit of urethra form trigone where mucosa is firmly attached.

Question 12

Describe the sphincters of the bladder

Answer 12

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 sphincters
• Males can depend on either

Question 13

Compare male and female bladder

Answer 13

FEMALE

• Urethra shorter and simpler because it only carries urine
• Muscles at external urethral sphincter also poorly developed - childbirth can lead to stress incontinence
• To maintain continence, internal and external sphincter need to work properly.

MALE

• Male urethra is much longer as it continues through the penis and also carries both urine and semen
• Muscles in the penis can control urination by controlling external sphincter

Question 14

What is the micrturition reflex?

When does it come into play?

Answer 14

The micturition is a reflex arc between the bladder and spinal cord - we are born with this and as we grow older, we able to exert some control on it from higher control centres to pass urine during social convenient situations.

It comes into play when there is some neural circuits - there is fine control between storage and elimination. Control of micturition is very elaborate and controls both bladder and sphincter.

Question 15

List the three set of nerves innervating the lower urinary tract

Answer 15

1. The pelvic nerve which has parasympathetic input
2. The hypogastric nerve which has sympathetic nerve
3. The pudendal nerve which is part of the somatic nervous system

Question 16

What properties does bladder innervation need to be?

Answer 16

Sensory to give sensation (awareness) of bladder fullness and also 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)

Question 17

Where does innervation from the bladder arise?

Answer 17

The fibres are coming from the spinal cord to the bladder

Preganglionic parasympathetic fibres will arise from ventral/anterior horn at sacral region of the spinal cord. They will synapse with the post-ganglionic fibres at ganglia which innervate the detrusor muscles and internal sphincter.

Sympathetic fibres arise from the lateral/posterior of lumbar region which will synapse at hypogastric ganglia. We will have post ganglionic fibres innervating internal sphincter and the parasympathetic post ganglion. There is also direct sympathetic innervation to the bladder wall itself, a lot in the trigone region.

The somatic system fibres arise from sacral region and is responsible for innervation of the external sphincter.

Question 18

Describe innervation of the detrusor muscle

Answer 18

PARASYMPATHETIC

• Preganglionic synapse with postganglionic and Ach is released binding to the nicotinic (LG) receptors
• Postganglionic release Ach that act at muscarinic receptors (GPCR) and ATP acting at purinergic receptors at the detrusor muscle fibre.
• This causes the detrusor muscle to contract

SYMPATHETIC

• Postganglionic synapse onto the parasympathetic postganglion by releasing NA acting at a-adrenoreceptors (GPCR). This inhibits 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 causes the bladder to relax.

Question 19

How do we know Ach is important for contraction of detrusor muscle?

Answer 19

We know Ach is more important because if we give atropine which blocks muscarinic receptors we inhibit contraction of the detrusor muscle.

Question 20

What receptor does ATP act on for detrusor muscle contraction?

Answer 20

Purinergic receptors

Question 21

What NS innervating the detrusor muscle is though to be important in ejaculation?

Answer 21

Sympathetic
This is thought to be important in men in ejaculation, as sympathetic closes internal sphincters which stops semen moving back into the bladder.

Question 22

Describe innervation of sphincters

Answer 22

• Parasympathetic fibres innervate the internal sphincter - postganglionic releases NO 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. It also aids parasympathetic by releasing Ach at internal sphincter.

Question 23

Explain sensory innervation to bladder

Answer 23

We also need sensory innervation and this is done by afferent fibres (from lower urinary tract to SC) 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 noiciceptors 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.

Question 24

What is the main role of afferent sensory nerves (A-fibres and C-fibres)?

Answer 24

A-fibres sense distension in the detrusor
i. Filling of the bladder (because connected to stretch + volume receptors)
ii. Signal if we need to contract detrusor (fullness, discomfort)

C-fibres respond to damage and inflammatory mediators
Signal pain (urgent desire to micturate)

Question 25

Describe bladder filling

Answer 25

Initially the bladder is empty and sphincters are closed due to tonic acitivity of sympathetic and somatic nerves. The bladder pressure is low due to being deflated.

Arrival of urine into the bladder results in relaxation of the detrusor muscle to allow the bladder to fill -this is because sympathetic activity inhibits the parasympathetic transmission to contract the detrusor.
There is little increase in pressure and sphincters remain closed –> RECEPTIVE RELAXATION

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.

Question 26

Summary of bladder emptying

Answer 26

• Micturition is an autonomic reflex that we are born in (seen in small babies and in adults when the spinal cord is transected above sacral region)
• Reflex is modified by voluntary control and can be inhibited or initiated by higher centres in the brain
• 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.

Question 27

Describe the Micturition (Emptying) reflex

Answer 27

As bladder fills with urine, the stretch/volume receptors start firing as the tension in the 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 (in bladder) 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 afferent nerves (pudendal) to inhibit the tonic contraction of external sphincter by inhibition of somatic input.

Question 28

What reflex is important to reinforce micturition until whole bladder is empty?

Answer 28

Sacral reflex

Question 29

Explain voluntary modification of the micruition reflex

Answer 29

The higher centres can modify the mictruition reflex for a while
- This is done by contracting the external sphincter and levator muscle around it consciously

This:
• Interferes with positive feedback to bladder emptying by inhibition of parasympathetic transmission
• Tightens internal sphincter

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

Question 30

What can halt urine stream?

Answer 30

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.

Question 31

Explain voluntary control of mictruition (different from m.reflex)

Answer 31

The bladder is contained in the floor of the abdominal cavity
By contracting abdominal muscles:
- increased intra-abdominal pressure is transmitted to bladder and normally supported urethra
- reflex contraction of peri-urethral striated muscles help compress urethra –> micruition reflex added

Question 32

Describe the path of micruition inhibitory centres

Answer 32

Cerebral cortex (frontal lobe) –> Pons of brain stem –> Spinal tract –> Spinal efferents –> Symp(block) Ganglion Para –> Bladder

Bladder –> C ubmyelinated afferents -> spinal efferents

Bladder –> A myeinated afferents –> spinal tracts –> brain stem (if socially acceptable you pee)

Question 33

What is the importance of bladder emptying?

Answer 33

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.

Question 34

List the places UTI can happen

Answer 34

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 (usually site of blockages)
Urethra - An infection of urethra is called urethritis

Question 35

Risk factors for UTI

Answer 35

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

Question 36

Problems of ageing bladder

Answer 36

Slow urine stream: more common in men due to benign prostatic hyperplasia which compresses the urethra.
- Most commonly cause lower urinary tract symptoms in men - can lead to incomplete emptying and infection

Inconinence: 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.

Question 37

Explain how damage of nervous control leads to problems with micruition

Answer 37

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.

Question 38

Treatments of incontinence

Answer 38

• Medicationee.g. anti-muscarinincs which relax detrusor muscle (reduce spasiticity)
• Bladder retraining - Kegel excercises
• Botox for spastic bladder
• Surgery can be used for bladder neck suspension
• Sacral nerve stimulation - restablish the micturition reflex
• Stem cell therapy
• Tissue engineered bladder