Session 7: Neuronal Control of Micturition

Question 2

Functions of nervous system in relation to lower urinary tract.

Answer 2

Provide sensations of bladder filling and pain.

Allow the bladder to relax and accommodate to increasing volumes of urine.

To initiate and maintain voiding so that the bladder empties completely with minimal residual volume.

Provide an integrated regulation of the smooth muscle and skeletal muscle sphincters of the urethra.

Question 3

What is the centre called and where it is found that is involved in the supra spinal regulation of micturition?

Answer 3

Pontine micturition centre (PMC). It is a collection of neuronal cell bodies that can be found in the rostral pons in the brainstem.

Question 4

What nervous systems are involved in micturition?

Answer 4

Sympathetic, Parasympathetic and voluntary somatic.

Question 5

There are two regions in the PMC that controls micturition. Which?

Answer 5

The L region and the M region.

Question 6

What fibres can be found in the L region?

Answer 6

Sympathetic fibres

Question 7

What fibres can be found in the M region?

Answer 7

Parasympathetic fibres

Question 8

Effect of sympathetic system on bladder.

Answer 8

Inhibition of detrusor muscles to prevent them from contracting. Also contraction of the IUS to prevent retrograde ejaculation in men.

Question 9

What other brain centres does the PMC coordinate with?

Answer 9

Medial frontal cortex, insular cortex, hypothalamus, periaqueductal gray.

Question 10

Explain the function of periaqueductal gray in control of micturition.

Answer 10

Relay station for ascending bladder information from the spinal cord and incoming signal from higher brain areas.

Question 11

Explain the function of the M region.

Answer 11

Sends descending excitatory projections to spinally located parasympathetic neurones in (S2-S4).

Question 12

Explain how the M region receives information to activate it.

Answer 12

Mechanoreceptors sensing stretch in the detrusor of the bladder goes to the S2-S4 region and then ascends the spinal cord to feed information to the M region. This switches the M region on an enhances its activity. This leads to parasympathetic activity and relaxation of external urethral sphincter + contraction of the bladder.

Question 13

What are the two phases of the functional activity of the lower urinary tract?

Answer 13

Storage phase and Voiding phase

Question 14

Main innervation of the storage phase.

Answer 14

Sympathetic control

Question 15

Main innervation of the voiding phase.

Answer 15

Parasympathetic control

Question 16

Explain the storage phase.

Answer 16

Filling of the bladder to accommodate increasing volume of urine. This is done by relaxation of the bladder and urinary continence is maintained by increasing the tone of the urethral sphincters.

Question 17

Explain the voiding phase.

Answer 17

Urethral sphincters relax and the bladder contracts. This is done by parasympathetic but also extra help from somatic innervation of skeletal muscles of the sphincters as well as the pelvic floor.

Question 18

What is the bladder lined with?

Answer 18

Transitional epithelium

Question 19

What type of muscle does the IUS mainly consist of?

Answer 19

Smooth muscle

Question 20

What type of muscle does the EUS mainly consist of?

Answer 20

Striated slow twitch muscle

Question 21

What does the sympathetic nervous system innervate in the lower urinary tract?

Answer 21

The bladder (detrusor muscles) as well as the IUS

Question 22

Where does the sympathetic innervation to the lower urinary tract arise?

Answer 22

T10-L2 region.

Question 23

Explain the route of the sympathetic fibres to the lower urinary tract.

Answer 23

Arise in the T10-L2 spinal cord. Preganglionic fibres pass through lumbar splanchnic nerves to the superior hypogastric plexus. Here they give rise to left and right hypogastric nerves. These nerves lead to the inferior hypogastric/pelvic plexus. The preganglionic sympathetic fibres continue to the bladder wall through the distal portion of the hypogastric nerve.

Question 24

What does the distal portion of the hypogastric nerve also contain?

Answer 24

Preganglionic parasympathetic axons.

Question 25

Explain the route of the parasympathetic fibres to the lower urinary tract.

Answer 25

Arise from the S2-S4 spinal cord. Carried via the pelvic splanchnic nerve as preganglionic. Then synapse with postganglionic neurones in the body and neck of the urinary bladder.

Question 26

Explain the route of the somatic innervation to the lower urinary tract.

Answer 26

Motor neurons from the S2-S4 spinal cord. Carried via the pudendal nerve where the motor neurons innervate the voluntary skeletal muscles of the EUS.

Question 27

Define bladder tone.

Answer 27

Relationship between bladder volume and the internal pressure of the bladder.

Question 28

How can bladder tone be measured?

Answer 28

Inserting a catheter into the urethra to drain the bladder completely. After this the bladder is filled with water whilst recording the change in pressure.

Question 29

What is this type of measurement called?

Answer 29

Cystometrogram.

Question 30

What happens to the pressure of the bladder from 0-50 ml?

Answer 30

There is a moderately steep increase in pressure

Question 31

What happens to the pressure of the bladder from 50 to around 400 ml?

Answer 31

There is next to no increase in pressure.

Question 32

What happens to the pressure of the bladder beyond 400 ml?

Answer 32

There is a steep rise in pressure.

Question 33

Why is there barely any rise in pressure from 50-400 ml?

Answer 33

Because the sympathetic innervation allows relaxation of the detrusor muscles. Although beyond 400 ml the detrusor muscles can’t relax any further and there is a build up of pressure.

Question 34

What is the micturition reflex also called the reflex arc?

Answer 34

Afferent sensory fibres in the wall of the bladder feeding back to the parasympathetic centre in the S2-S4 stimulating it. This stimulate the efferent parasympathetic fibres to contract the bladder.

Question 35

What centres in the brain normally inhibits the micturition reflex?

Answer 35

Cortical and suprapontine centres under coordination of the PMC.

Question 36

Explain the role of PMC in the storage phase.

Answer 36

Stretch receptors in the bladder send afferent signals to the brain via the pelvic splanchnic nerve. There is a sense of urge for voiding at around 150ml and then a sense of fullness at around 400-500 ml.

Question 37

What makes the parasympathetic system not activate automatically at the sense of fullness to allow voiding, even if not socially acceptable?

Answer 37

A learnt reflex in the brain sending efferent impulses inhibiting the presynaptic parasympathetic neurons in the S2-S4 spinal cord. This leads to a net inhibition of the parasympathetics. Also voluntary contraction of the EUS contributes to continence and storage.

Question 38

Explain the sequence of the voiding phase.

Answer 38

Voluntary relaxation of EUS followed by relaxation of IUS. When a small amount of urine has reached the proximal urethra there are afferents signalling the cortex that voiding is imminent. PMC will no long inhibit parasympathetics and this leads to contraction of the bladder. The cortical centres also inhibit the EUS muscles. Micturition is assisted by voluntary contraction of the abdominal muscles as well as pelvic muscles.

Question 39

Explain the contraction of the detrusor muscles.

Answer 39

They contract rhythmically where the initial contractions lead to further trains of sensory impulse from stretch receptors making it self-regenerating.

Question 40

What receptors does the sympathetic fibres act on in the bladder wall?

Answer 40

B3 receptors

Question 41

What receptors does the sympathetic fibres act on in the IUS?

Answer 41

Alpha1 receptors

Question 42

What receptors does the parasympathetic fibres act on in the bladder wall?

Answer 42

M3 receptors

Question 43

What receptors does the somatic fibres act on in the EUS?

Answer 43

nACh receptors

Question 44

Explain the storage reflex.

Answer 44

Sensory afferent fibres from bladder wall sense distention and feeds back to the sympathetic neurons in the T10-L2 spinal cord. This activates the sympathetic fibres and the IUS contracts and bladder relaxes to allow more storage.

Question 45

Why might a lesion above S2-S4 lead to incontinence?

Answer 45

Because the lesion can obstruct the signals from the efferent impulses from the brain from inhibiting the presynaptic parasympathetic neurons. This leads to no net inhibition of the parasympathetic and a micturition reflex which is overactive.

Question 46

What would a lesion in the S2-S4 region lead to?

Answer 46

No feeling of the bladder being full. No contraction of the bladder. Overactive storage reflex like in cauda equina syndrome e.g.

Question 47

Give example of drugs used to treat an overactive bladder.

Answer 47

B3 agonists to allow relaxation of the bladder.

Question 48

Give examples of drugs used to treat urinary retention.

Answer 48

M3 agonist to start voiding.