Neurological control of micturition

Question 1

Outline the anatomy of the bladder

Answer 1

• Detrusor muscle - smooth muscle fibres passing in all directions
• Internal urethral sphincter - composed of smooth muscle and prevents retrograde ejaculation in males
• External urethral sphincter - composed of skeletal muscle and found within pelvic diaphragm

Question 2

What is the storage phase?

Answer 2

• Ability of bladder to increase in volume with no appreciable change in pressure

Question 3

What is the voiding phase?

Answer 3

• Contraction of detrusor to enable voiding (spikes in pressure)

Question 4

Why doesn’t pressure in the bladder change initially with filling?

Answer 4

• Elastic fibres of rugae allow bladder to stretch

Question 5

Outline the neural control of the storage phase

Answer 5

• Sensory neurone innervates stretch receptor
• Stretch receptor detects stretch of bladder wall
• Sensory neurone also synapses onto a sympathetic neurone between L1 and T10
• Sympathetic neuron keeps bladder walls relaxed so it fills (inhibitory effect)
• Also have stimulatory effect on IUS and cause it to stay shut

Question 6

Which receptors and neurotransmitters allow the bladder to fill?

Answer 6

• Sympathetic neurone acts on IUS via alpha 1 receptor
• Noradrenaline is neurotransmitter
• Sympathetic neurone also acts on bladder wall via beta 3 receptor to relax muscles
• Noradrenaline is neurotransmitter

Question 7

Outline the cortical control of the storage phase

Answer 7

• Stretch receptors report to cerebral cortex
• EUS is controlled by somatic neurones (because it is made of skeletal muscle)
• Somatic motor neurones from S2-S4 stimulate EUS
• Act on a nicotinic receptor via ACh
• L centre within brain stem sends axons to somatic motor neurones
• Keep EUS closed and maintain continence

Question 8

What is the name given to the somatic motor neurones that control the EUS?

Answer 8

• Pudendal nerve

Question 9

Outline the voiding reflex

Answer 9

• Stretch receptor innervated by sensory neurone (enters spinal cord between S2-S4)
• Synapses on parasympathetic neurons
• Parasympathetic neurons stimulate contraction of detrusor muscle
• Results in micturition

Question 10

Which receptors and neurotransmitters enable parasympathetics to stimulate micturition?

Answer 10

• M3 receptor
• ACh

Question 11

Outline the cortical control of voiding

Answer 11

• High levels of bladder stretch cause sensory neurones to activate M centre (micturition centre)
• M centre allows cognitive control over micturition
• Checks in with cerebral cortex to see if it’s appropriate to void - paracentral lobules send excitatory and inhibitory influences onto M centre
• Then stimulates parasympathetics and creates a positive feedback loop

Question 12

Where is the M centre located in relation to the L centre?

Answer 12

• Medially to L centre

Question 13

Why do the M and L centres communicate?

Answer 13

• M centre sends inhibitory projections to L centre
• When we strongly need to void, M centre is stimulated
• Parasympathetics are activated, causing detrusor contraction
• Inhibits L centre
• Switches off pudendal nerve
• EUS can relax and voiding can occur

Question 14

What causes automatic voiding in babies?

Answer 14

• If pressure of bladder overcomes IUS
• Babies haven’t learnt to control EUS via pudendal nerve
• This is learnt as a child potty trains