12 - Neuronal Control of Micturition Flashcards
In simple terms, how does urination occur?
- Filling where bladder relaxes and urethral sphincters increase their tone
- Detrusor smooth muscle contracts (more in men)
- Sphincters relax (IUS smooth, EUS slow twitch skeletal)
What is the function of the IUS?
- Primarily stop retrograde ejaculation
- Assist continence
What functions do the nervous system have on the lower urinary tract?
Where in the CNS is the process of micturition controlled?
- Pontine Micturition Centre (Barrington’s nucleus) in the rostral pons
- L region: sympathetic, storage, relaxation of the bladder detrusor and contraction of EUS
- M region: parasympathetic, voiding, contraction of bladder and relaxation of sphincters
What other areas of the brain does the PMC coordinate with?
- Medial frontal cortex
- Insular cortex
- Hypothalamus
- Periaqueductal Grey (PAG - relay for ascending bladder information from the spinal cord and higher brain areas)
What is the role of the M region in the PMC?
- Sends descending excitatory projections to P.S neurones controlling detrusor muscle of bladder
- Sends inhibitory interneurones to Onuf’s nucleus
- During filling M region is turned off but when mechanoreceptors detect too much stretch it is switched on
What are the muscular layers of the bladder?
- Posterior urethra has smooth muscle fibres from detrusor and along with elastic fibres forms the IUS
What is the storage reflex of micturition?
- Under sympathetic control
- Sensory receptors synapse T10-L2 and preganglionics go through lumbar splanchnic nevrves to superior hypogastric plexus where they form left and right hypogastric nerves
- Inferior hypogastric plexus pre-ganglionics synapses and post gangionics continue through hypogastric to detrusor (inhibitory) and IUS (excitatory)
- S2-S4 somatic pudendal nerve, under control of L centre in pons, acts on EUS to cause it to contract
What is the voiding reflex of micturition?
- Under parasympathetic control
- S2-S4 origin via pelvic splanchnic nerve
- Synapse in wall of bladder
- High levels of stretch M centre can deactivate L centre
What is the somatic innervation to the bladder?
- S2-S4 pudendal nerve which controls EUS
- Pelvic splanchnic nerves are autonomic
What control is the storage and voiding reflex under?
Storage = Sympathetic
Voiding/Peeing = Parasympathetic
What happens to the pressure of the bladder during voiding and storage?
- Stretch receptors in storage phase cause bladder to relax so pressure doesn’t change much
How do we measure the pressure in the bladder at different volumes?
- Catheter through urethra and empty bladder
- Add 50ml increments of water and record pressure
- Make cystometrogram, 0-50ml steep increase in pressure, up to 300ml almost no change, higher than 400ml increase
When does the bladder sense the need to urinate and how does it combat this if it is not socially acceptable to void?
- 150ml and full at 400-500ml
- Cortical and suprapontine centres have learned reflexes that inhibit presynaptic parasympathetic neurones that would otherwise stimulate the detrusor muscle
How does voluntary voiding occur?
- Voluntary relaxation of EUS followed by IUS
- Pontine centres stop inhibiting parasympathetic and rhymic contractions
- Abdominal muscles also contract to assisting voiding and complete bladder emptying
Why may happen to micturition if you have a spinal cord lesion at the sacral level (below S2)?
- Will lose your parasympathetic innervation so will have urinary retention
- Therefore will have overfow incontinence
- Overactive storage reflex
What may happen to micturition if you have a spinal cord lesion above S2?
- Lose descending inhibition (majority of fibres from M centre in parasympathetics are inhibitory neurones unlike the two excitatory in micturition)
- P.S not inhibited so overactive micturition reflex so incontinence
What are the different receptors involved in the voiding and storage reflex?
- B3: sympathetic detrusor muscle
- M3: parasympathetic detrusor muscle
- nAChR: EUS
- a1: sympathetic IUS
Why does this patient have a low HR and blood pressure, and why may she have issues with incontinence? (she is in a C-spine collar)
- She may have a lesion affecting the sympathetic system (T1 to L2) so unopposed parasympathetics lowering HR
- Loss of sympathetics holding tone of bladder so incontinent
What are the nerves involved in continence?
- Pudendal: somatic motor to external urethral sphincter
- Hypogastric: motor for sympathetic
- Pelvic: sensory for S and PS and motor for PS
