7. Neural control of micturition

Question 1

describe the 3 structures responsible for micturition control

Answer 1

1. detrusor muscle: specialised smooth muscle in bladder wall, with fibres orientated in 3 directions - retains structural integrity when stretched
2. internal urethral sphincter (IUS):
   - in men: well developed circular smooth muscle (prevents retrograde ejaculation)
   - in women: functional sphincter formed by anatomy of bladder neck and proximal urethra
3. external urethral sphincter (EUS): skeletal muscle that is integral component of pelvic floor

Question 2

describe the innervation of the bladder

Answer 2

i. Sensory afferent nerves from stretch Rs in detrusor muscle to brain
ii. PNS: pelvic splanchnic nn. (S2-S4) to detrusor (causes contraction)
iii. SNS: hypogastric n. (T10-L2) to detrusor (causes relaxation) and IUS (causes contraction)
iv. Somatic motor efferent: pudendal n. (S2-S4) to EUS (contraction of relaxation) - voluntary control over micturition

Question 3

describe the 2 phases of micturition

Answer 3

1. FILLING:
   - bladder relaxes and accomodates increasing volumes of urine (compliance and receptive relaxation - transitional epithelium)
   - urethral sphincters contract to maintain continence
2. VOIDING (when max. capacity, 500mL is reached):
   - urethral sphincters relax
   - bladder detrusor contracts (rhythmic contractions)

Question 4

describe the neural control (involuntary and voluntary) of the storage phase

Answer 4

Involuntary control (basic storage reflex - SYMPATHETIC):

i. stretch Rs send low level impulses via afferent sensory fibres to spinal cord - enter at S2-S4 and ascend to T10-L2 to synapse with sympathetic neurones…
ii. sympathetic efferents act at:
- B3 Rs of detrusor SM - inhibit contraction
- a1 Rs of IUS SM - stimulate contraction

Voluntary control:
iii. L centre (pontine storage centre) sends impulses via interneuron… synapse at S2-S4… impulses along pudendal n. (somatic motor fibres) to nAChRs of EUS - stimulate contraction

Question 5

describe the neural control (involuntary and voluntary) of the voiding phase

Answer 5

Involuntary control (basic micturition reflex - PARASYMPATHETIC):

i. stretch Rs send impulses via afferent sensory fibres to spinal cord - enter at S2-S4 and synapse with parasympathetic neurones…
ii. PNS efferents act at M3 Rs on detrusor SM - stimulate contraction

Voluntary control:

iii. M centre (pontine micturition centre) stimulated by high impulses in sensory afferents from bladder stretch Rs:
a) sends impulses via interneurons to PNS efferents at S2-4 to stimulate their action (+ve feedback), i.e. detrusor contraction
b) inhibits L centre, allowing EUS relaxation
c) inhibits sympathetics at T10-L2 allowinf detrusor contraction and IUS relaxation

Question 6

why is micturition mainly under voluntary control in adults but not in infants

Answer 6

• cerebral cortex can hugely activate M centre - voiding mainly consciously controlled
• lack of myelination in infants means basic micturition reflex is only pathway - involuntary voiding

Question 7

suggest 2 classes of agent used to treat overactive bladder

Answer 7

1. B3 R agonists - mimic sympathetic inhibition of detrusor

2. M2 R antagonists - prevent parasympathetic activation of detrusor

Question 8

why does an upper motor neurone lesion (eg lumbar region) cause urinary frequency

Answer 8

• Majority of fibres running down spinal cord inhibit parasympathetic neurones at S2-S4 so act to prevent voiding.
• Lesion of these inhibitory fibres… loss of PNS fibre inhibition… overactive micturition reflex… frequency

Question 9

why does a lower motor neuron lesion (sacral region, e.g. CES) cause overflow incontinence

Answer 9

1. lesion of sensory fibres from bladder stretch Rs… cannot detect how full bladder is
2. lesion of PNS fibres that normally stimulate voiding… cannot stimulate detrusor contraction to initiate voiding…

Bladder fills to max and urine dribbles out - overflow incontience