Micturition

Question 1

Define Micturition

Answer 1

Micturition is the act of urination (emptying the bladder).

Question 2

The basic process of Micturition is broken down into 3 stages, what are these stages?

Answer 2

Urine made in kidneys (1ml.min-1)
Urine stored in bladder
Urine released from bladder

The last 2: Function of bladder & associated sphincters

Question 3

Describe the basic 3 point process of how urine is transported from the kidney to the bladder

Answer 3

1. Urine from all collecting ducts of all nephrons
2. Empty into Renal Pelvis
3. Urine enters Ureter

Question 4

What happens when the ureter gets distended and the smooth muscle contracts?

Answer 4

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

Question 5

How does the ureter open up into the bladder and what does this prevent?

Answer 5

Ureters open obliquely into the bladder

Prevents reflux of urine back into ureters by the passive flap-valve effect

Question 6

What controls uretic peristalsis?

Answer 6

Myogenic in origin, not under the control of CNS

Coordination required between peristalsis and changing urine volume

Question 7

How do kidney stones form?

Answer 7

most common disorder of urinary tract & develop from crystals that separate from urine within urinary tract

Question 8

What prevents the formation of kidney stones?

Answer 8

Normal urine contains inhibitors (citrate) to prevent this

Question 9

Describe the composition of kidney stones

Answer 9

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%).

Question 10

What causes kidney stones?

Answer 10

Caused by poor urine output/obstruction, altered urinary pH, low concentration of inhibitors, infection, excess dietary intake of stone-forming substances

Question 11

Where can kidney stones form and what are the symptoms?

Answer 11

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”

Question 12

Describe the structure of the bladder

Answer 12

On image

Question 13

What are the 2 sphincters of the bladder?

What control them?

Locate them on the diagram

Answer 13

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

Question 14

Compare and contrast the female and male bladder

Answer 14

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

Question 15

What is the lower urinary tract innervated by (3 sets of peripheral nerves)

Answer 15

parasympathetic (pelvic nerve)
sympathetic (hypogastric nerve)
somatic nervous system (pudendal nerve)

Question 16

What does sensory and motor innervation of the bladder cause?

Answer 16

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

Question 17

Where does the parasympathetic, sympathetic and somatic run to and from

Answer 17

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

Question 18

What causes the detrusor to contract?

What inhibits contraction - what drug?

What causes the detrusor to relax?

Answer 18

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

Question 19

What cause the sphincters to contract and relax?

Answer 19

Nitric Oxide (NO) & ACh - relaxes internal sphincter 
Noradrenaline (NA) – contracts internal sphincter
ACh – tonic (continual) activity holds external sphincter closed

Question 20

Describe the sensory innervation of the bladder

Answer 20

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)

Question 21

What do A fibres sense (sensory fibres)?

Answer 21

A fibres: sense tension in detrusor:

i. Filling of bladder
ii. Detrusor contraction
- > bladder fullness, discomfort

Question 22

What do C fibres sense?

Answer 22

C fibres: respond to damage & inflammatory mediators

PAIN (urgent desire to micturate)

Question 23

Describe the state of the ballder when empty and when it is filling

Answer 23

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

Question 24

How does the bladder develop?

Answer 24

1. Micturition is an autonomic reflex - when ballder is full it empties
2. Reflex is modified by voluntary control
   Inhibited or initiated by higher centres in the brain
   Maturation of bladder complete by >6 years
3. Basic circuits act as on/off switches to alternate between 2 modes of operation: storage and elimination
4. Disease/injury/ageing to nervous system in adults disrupts voluntary control of micturition
   bladder hyperactivity & urge incontinence
   stress incontinence

Question 25

Describe the signals initiated during Micturition (Emptying)

This occurs in babies

Answer 25

1. Stretch and volume receptors fire, activating the afferent system - alpha delta fibres - to the spinal cord
2. These synapse with the parasympathetic referents causing detrusor contraction and internal sphincter to relax by the release of NO acting on MR.
3. As urine empties it activates flow receptors in the urethra -> pudendal afferents excited
4. Tonic contraction of external sphincter removed by inhibition of somatic input

Question 26

What are the voluntary controls that prevent random urination?

What disease process inhibit urination?

Answer 26

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

Question 27

Summarise the voluntary control of Micturition

Answer 27

On slides

Question 28

What happens when we contract the abdominal muscles?

Reflex contraction of peri-urethral striated muscles - what does this cause?

Answer 28

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

Question 29

Is the bladder sterile?

What restores the sterility of the bladder?

What can a bacterial infection caused in the retained urine?

Answer 29

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

Question 30

What is a urinary tract infection?

Answer 30

A urinary tract infection, or UTI, is an infection that can happen anywhere along the urinary tract.

Question 31

Urinary tract infections have different names depending on which part is infected, what are these names?

Answer 31

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

Question 32

Why is UTI more common in women?

What is prostatic disease?

Answer 32

More common in women because of short urethra

Common in men over 40 due to prostatic disease, causing bladder outflow obstruction
Some risk factors:

Question 33

What are the issues with an ageing bladder?

What are the consequences of a slow urine stream?

What causes these issues?

Answer 33

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

Question 34

What treatment is available for UTI?

Answer 34

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.