52) Micturition

Question 1

What is the structure of the ureter?

Answer 1

• The ureter has layers of muscle (called detrusor muscle) surrounded by transitional epithelium

Question 2

What happens when urine enters the ureter

Answer 2

• When urine enters the ureter it forms a bulge (distension) and the smooth muscles surrounding the ureter contracts leading to peristalsis to squeeze urine downwards
• This peristalsis is myogenic and not under CNS (voluntary) control
• There is some coordination needed between peristalsis and changing urine volume

Question 3

Describe the passage of the ureter into the bladder.

Answer 3

• The ureter enters from the side (obliquely) and opens further in the bladder
• Due to the spherical nature of the bladder pressure is exerted in all directions
• This pressure will be used to close the opening of the ureter
• This effect is called the passive flap-valve effect and helps prevent reflux of urine back into the ureters

Question 4

What are the different sphincters of the bladder?

Answer 4

• Internal sphincter

- External sphincter

Question 5

What is the internal sphincter?

Answer 5

• It is located below the bladder neck.
• It is an extension of the detrusor muscle and keeps the urine from leaking out of the bladder into the urethra.
• It is not under voluntary control

Question 6

What is the external sphincter?

Answer 6

• It is located below the urethra.
• It comprises of two striated muscles which surround the urethra
• They are under voluntary control and are responsible for continence (micturition).
• These are under conscious voluntary control

Question 7

What is the difference between male and female bladder and urethras?

Answer 7

• Females have shorter urethras compared to males
• Women have a smaller and more poorly developed external sphincter. This makes women more prone to incontinence especially during child birth
• Males have a prostate gland through which the urethra flows through
• Male urethras are responsible for carrying urine and semen
• Men have greater musculature around the external sphincter to aid in the expulsion of semen and urine.

Question 8

Describe the composition of urine in the bladder?

Answer 8

• Urine in the bladder will have the same composition as urine in the ureters (and the kidneys)
• The bladder is impermeable to salt and water
• However it is permeable to lipophilic molecules

Question 9

What are kidney stones (renal calculi)?

Answer 9

• Crystals that precipitate from the urine within the urinary tract
• Normally urine contains inhibitors (such as citrate) to prevent this
• Calcium is present in majority of stones usually as calcium oxalate or less commonly calcium phosphate
• Kidney stones are the most common disorder of the urinary tract
• Kidney stones, which differ from gall stones, are more common in men than in women as testosterone increases the chances of developing kidney stones

Question 10

What is kidney stones caused by?

Answer 10

• Excess intake of stone-forming substances within the diet
• Poor urine output or an obstruction in the bladder
• Altered urinary pH
• Low concentration of inhibitors
• Urinary Tract Infections (UTIs)

Question 11

What is kidney stone disease (ureterolithiasis)?

Answer 11

• When kidney stones form anywhere within the urinary tract (i.e. kidney, ureter or bladder)

Question 12

What are the symptoms of kidney stone disease?

Answer 12

• Dysuria (painful urination)
• Haematuria
• Loin/back pain
• Reduced urine flow
• Urinary tract obstruction which causes high pressure build up and cause a lot of pain (called renal colic)
• If stone approaches tip of urethra intense pain can inhibit micturition (called strangury)

Question 13

What is the different between afferent nerves and efferent nerves?

Answer 13

• Afferent nerves: Sensory nerves which gives sensation of fullness of bladder and when left for too long sensation of pain
• Efferent nerves: Motor nerves which controls muscles and causes contraction and relaxation of the muscles involved in micturition (detrusor muscles and muscles of the external sphincter)

Question 14

How do nerves affect micturition?

Answer 14

• Neural circuits within the brain and spinal chord co-ordinate activity of bladder and sphincters
• Circuits act as an on-off switch to alternate the bladder between urine storage and elimination

Question 15

What are the main nerves involved in micturition?

Answer 15

• Hypogastric nerve
• Pelvic nerve
• Pudenal nerve

(Each nerve contains an afferent and an efferent component)

Question 16

What is the hypogastric nerve?

Answer 16

• Nerve that branches off the spinal chord at T11-L2
• Afferents connect to stretch receptors which monitors the fullness of the bladder
• Efferent connect to Detrusor muscle (muscle surrounding the bladder and top of the urethra)
• Part of the sympathetic nervous system. Hence (hypogastric) ganglia are located closer to spinal chord.
• There is some interaction with the parasympathetic (pelvic) nerve

Question 17

What is the pelvic nerve?

Answer 17

• Branches off from the spinal chord at S2-S4
• Afferents connect to stretch receptors which monitors the fullness of the bladder
• Efferent connect to Detrusor muscle (muscle surrounding the bladder and top of the urethra)
• Part of the parasympathetic nervous system. Hence ganglia are located closer to bladder
• There is some interaction with the sympathetic (hypogastric) nerve

Question 18

What is the Pudendal nerve?

Answer 18

• Branches off from the spinal chord at S2-S4
• Afferents connect to flow receptors further down in the urethra which detect the flow of urine
• Efferents connect to musculature of the pelvic floor allowing it to control the external sphincter
• Part of the somatic nervous system (under voluntary control)

Question 19

Explain how the efferent innervation of the detrusor muscles causes contraction

Answer 19

• Impulses are sent down a parasympathetic preganglionic neurone.
• When these impulses reach the synapse ACh (a neurotransmitter) is released which act on nicotinic receptors.
• This triggers an impulse on the parasympathetic postganglionic neurone which reaches the detrusor muscle
• At the neuromuscular junction ACh is released (which act on muscarinic receptors) and ATP is also released (which act on purinergic receptors) to cause contraction of the detrusor muscle
• Atropine can be used to inhibit ACh action on muscarinic receptors at the detrusor muscle and hence can inhibit detrusor contraction affecting our ability to urinate

Question 20

Explain how the efferent innervation of the detrusor muscles causes relaxes

Answer 20

• Impulses are sent down sympathetic post ganglionic neurones which synapses with the parasympathetic post ganglion
• At the ganglion it releases noradrenaline which acts on α-adrenergic receptors and inhibits transmission of implses down the parasympathetic post-ganglionic neurone
• This indirectly causes the detrusor to relax due to the inhibition of ACh and ATP activity
• Furthermore another sympathethic post ganglionic neurone can innervate the detrusor directly
• In this case it will secrete noradrenaline which acts on β-adrenergic receptors and directly causes the detrusor to relax

Question 21

Explain how efferent innervation of the sphincters causes relaxation

Answer 21

• Impulses are sent down preganglionic parasympathetic neurones which release ACh, at the ganglion, that act on nicotinic receptors
• This causes an impulse to be generated and is passed down the postganglionic parasympathetic neurones where it reaches the sphincter
• At the sphincter nitric oxide (NO) and ACh is released which causes the internal sphincter to relax

Question 22

Explain how efferent innervation of the sphincters causes contraction

Answer 22

• Impulses are sent down sympathetic post ganglionic neurones which synapses with the internal sphincter
• At the synapse they release noradrenaline which causes contraction of the internal sphincter
• Somatic nerves directly innervate the external sphincters and release ACh at their synapse which interact with nicotinic receptors
• The effect is continual so the ACh activity is tonic which holds the external sphincter closed and so in order to urinate we consciously switch off the tonic activity

Question 23

What are the different types of afferent (sensory) nerve fibres?

Answer 23

• Aδ fibres

- Cδ fibres

Question 24

What is the role of the Aδ fibres?

Answer 24

• Sense tension in detrusor muscle which means they can detect filling of the bladder and contraction of the detrusor.
• Hence they are responsible for detecting bladder fullness and discomfort when we need to go to the toilet

Question 25

What is the role of the Cδ fibres?

Answer 25

• They respond to damage and inflammatory mediators.

- They cause pain due to the urgent desire to micturate

Question 26

Describe the afferent innervation via the pelvic pathway

Answer 26

• Here there are small myelinated Aδ-fibres control micturition reflexes
• They work via stretch receptors which detects tension within the wall of the bladder and volume receptors which measure the filling of the bladder
• There are also unmyelinated Cδ-fibres which have endings in/near the epithelium of the bladder
• They work via nociceptors which detect pain which is typically following an infection (e.g. cystitis) or during extensive distension

Question 27

Describe the afferent innervation via the Hypogastric and Pudenal pathways?

Answer 27

• There are nociceptors
• There are also flow receptors (in the external sphincter) which detect the flow of urine through the external sphincter

Question 28

Explain the process of bladder filling

Answer 28

• Initially the bladder is empty which means the bladder pressure will be low
• The sphincters remain closed due to the tonic activity of sympathetic and somatic nerves
• As time passes urine enters via the ureter which causes the detrusor to relax
• In this case sympathetic activity will inhibit parasympathetic activity and so there will be a little increase in pressure while the sphincters still remain closed
• This is called receptive relaxation

Question 29

What is micturition?

Answer 29

• Micturition is an autonomic reflex (something we do not control).
• As we grow up this reflex is modified by voluntary control as it is inhibited or initiated by higher centres in the brain
• Furthermore we also experience maturation of the bladder.
• Basic circuits governing micturition act as on/off switches which toggle the function of the bladder between two modes of operation: storage and elimination
• Disease, injury or ageing of the bladder (or nervous system) in adults can disrupt voluntary control of micturition
• This can lead to bladder hyperactivity, urge incontinence and stress incontinence

Question 30

Explain the process of micturition.

Answer 30

• Receptors in the bladder wall detect filling of the bladder with urine
• This generates an impulse that is sent down the Aδ-fibres efferent neurones which travel to the spinal chord which sends it down a parasympathetic efferent
• The parasympathetic efferent will synapse at the detrusor muscles causing them to contract
• It will also synapse at the internal sphincter where it releases ACh and NO causing relaxation of the internal sphincter
• This means urine will begin to flow out which will be detected by flow receptors within the urethra
• This causes the excitation of pudendal nerves which will in turn cause inhibition/removal of the tonic contraction of the external sphincter (called the sacral reflex)
• This will allow for the complete emptying of the bladder

Question 31

What is the name of the area of the brain that is activated during micturition?

Answer 31

• Pontine Micturition Centre (PMC)

Question 32

How is the micturition reflex modified?

Answer 32

• Contraction of external sphincter and levator muscle consciously
• Increased sympathetic firing to bladder and internal sphincter which interferes with positive feedback to bladder emptying by inhibiting parasympathetic activity and tightening internal sphincter
• Urine stream can be halted by strangury (urethral pain) due to urethritis (e.g. inflammation of ureter from STI or kidney stone)
• Pinching glans in the penis can inhibit micturition
• At night if bladder fills to capacity it is recognised by the PMC and arousal centres wakes us up

Question 33

What is the voluntary control of micturition?

Answer 33

• The bladder is contained in the floor of the abdominal cavity and so its activity can be controlled by the abdominal muscles
• An increase in intra-abdominal pressure is transmitted to the bladder and ureter
• The reflex contraction of striated muscles within the urethra help to compress the urethra which aids micturition

Question 34

What is the importance of bladder emptying?

Answer 34

• Urine is normally sterile with occasional bacterial entry
• Complete emptying restores sterility
• Bacteria in retained urine can cause new bacteria to grow as new urine is added
• Retained urine in the bladder can cause UTIs
• Repeated infections can cause destruction of renal function if they ascend to the kidney

Question 35

What are Urinary Tract Infections (UTIs)?

Answer 35

• Infections that occur within the urinary tract

- UTIs have different names depending on the area of their infection

Question 36

What are the different names for UTIs?

Answer 36

• Bladder: An infection in the bladder is called cystitis or a bladder infection
• Kidneys: An infection in one/both kidneys is called pyelonephritis
• Ureters: Rarely the site of infection
• Urethra: An infection in the urethra is called urethritis

Question 37

What are the risk factors for UTIs?

Answer 37

• More common in women than in men due to length of the urethra
• More common in men over 40 due to prostatic disease causing bladder outflow obstruction
• Diabetes
• Long term catheterisation
• Pregnancy
• Enlarged prostate
• Prolonged immobility
• Kidney stones
• Bowel incontinence
• Advanced age

Question 38

What are problems associated with ageing bladders?

Answer 38

• Slow urine stream due to prostate enlargement which results from the benign prostatic hyperplasia (BPH)
• The slow urine stream leads to incomplete emptying that causes infection
• BPH is the most common cause of UTIs in men
• It also leads to incontinence

Question 39

What are causes of incontinence?

Answer 39

• Weakening of the sphincters (e.g. stress incontinence) which is common in women after child-birth which causes weakened pelvic-floor muscles
• Failure of nervous control due to ageing
• Overactive Bladder (OAB) where the detrusor muscles contract spastically resulting in a sustained high blood pressure leading to urge incontinence (feeling of going to the toilet all the time)

Question 40

What are the consequences of incontinence?

Answer 40

• Socially embarrassing
• Destroys self-esteem
• Reduces quality of life

Question 41

What are the treatments for bladder problems?

Answer 41

• Anti-muscarinic drugs: They relax smooth muscles which decreases contraction of the detrusor muscles (e.g. Oxybutynin) and have a range of side effects.
• Bladder retraining: Used to treat stress and urge incontinence (e.g. Kegel exercise)
• Surgery: In particular if there is an issue with the suspension of the bladder neck (i.e. the ligaments that hold the bladder in place). It can be done via Botox in the muscles around the urethra which relaxes the bladder
• Sacral Nerve Stimulation (SNS): An electrical device is implanted which stimulates the sacral nerves which is controlled externally to improve continence
• Stem cell therapy: However limited supply of stem cells
• Tissue engineered baldder: Synthetic and natural scaffold used to form 3d structures using human tissue. However it is currently in clinical trials