Neural Control of Micturition

Question 1

Describe the sensory innervations to the bladder to sympathetic fibres

Answer 1

• Along pelvic nerve
• Stretch receptors send signal which enters spinal cord at S2-S4 and then projects upwards and synapses at T10-L2
• Synapses into postganglionic sympathetic fibres

Question 2

Describe the sympathetic input to the bladder in the storage phase

Answer 2

• Sympathetic fibres to detrusor muscle and internal urethral sphincter
  
  • Inhibit detrusor muscle through ß3 receptor
    
    • ß3 agonist can be used to treat overreactive bladder
    • Stimulate internal urethral sphincter through α1 receptor

Question 3

Describe the somatic innervation to the bladder in the storage phase

Answer 3

• Somatic innervation along pudendal nerve to external urethral sphincter
  - nACh receptor

Question 4

Describe the L center in the storage phase

Answer 4

• L centre of the pontine micturition centre stimulates contraction of external urethral sphincter to prevent incontinence
• Located within pons of brainstem

Question 5

Describe the sensory innervation of the bladder to parasympathetic fibres

Answer 5

• Stretch receptors divided into 2 pathways
• Synapses with parasympathetic fibres at S2-S4
• At high bladder loads, also sends signals directly to the M centre in the pons

Question 6

Describe the parasympathetic innervation to the bladder in voiding phase

Answer 6

• Parasympathetic fibres to detrusor muscle in bladder and causes contraction through M3 ACh receptor
• Anti-muscarinic drugs can cause urinary retention problem

Question 7

Describe what happens when the M center is activated

Answer 7

• When M centre in brain is activated, it inhibits L centre to decrease sympathetic and somatic innervation to the bladder
• Causes relaxation of sphincters and decreases inhibition to detrusor muscle
• M centre also has innervation from brain to precisely control bladder function

Question 8

Describe the normal voiding reflex

Answer 8

• Stretch receptors in the bladder are stimulated and send inputs to the parasymapthetic fibre as well as the M center
• Parasymapthetic fibres cause contraction of detrusor muscle
• M center inhibits L center and thus inhibits sympathetic input to detrusor muscle and decrease somatic contraction of external urethral sphincter

Question 9

Describe the role of the internal and external urethral sphincter

Answer 9

• Internal urethral sphincter prevents retrograde ejaculation
  
  • Well developed in males
    Assists continence
• External urethral sphincter - integral component of pelvic floor
  
  • Skeletal muscle - involuntary

Question 10

What are the effects of high and low spinal lesion to bladder

Answer 10

• Majority of nerves down spinal cords are inhibitory
• A high spinal lesion takes away inhibition input - overactive micturition
• A low spinal lesion (sacral lesion) causes overactive storage - incontinence
  
  • Parasympathetic input to the bladder prevented
    
    • Same effect as cauda equina syndrome