S7.1 Control Of Micturition Flashcards
Describe the bladder components involved in micturition
Bladder wall - detrusor muscle, holds urine inside
Internal urethral sphincter - smooth muscle, prevents retrograde ejaculation. In women no IUS and shorter urethra.
External urethral sphincter - voluntary skeletal muscle, use if bladder full but holding urine in
How does the bladder sense what it is holding?
Stretch receptors in the wall detect pressure; communicate with sensory neurone (S2-S4) in dorsal root ganglion, sends info to spinal cord where it synapses with sympathetic fibres projecting to detrusor relaxing it, and the internal sphincter contracting it causing storage.
Describe the storage reflex
Reflex switched on when low stretch detected by sensory nerve.
Sympathetic hypogastric causes relaxation of detrusor (beta 3) and contraction of IUS (alpha 1) (flat line of storage phase on graph).
Pudendal nerve causes contraction of EUS via S2-4 nerve roots which are controlled by the L centre in the brain.
Describe the voiding reflex
M centre stimulated by high frequency activity in the sensory neurone (full bladder)
Activates parasympathetic fibres which contract the detrusor (M3)
M centre sends inhibitory signals to L centre to inhibit the storage phase when voiding, relaxing the EUS and switching off sympathetics (detrusor and IUS)
Describe the graph of micturition
As ureters deliver urine to bladder, pressure initially remains quite constant as the detrusor relaxes to accommodate more urine.
Pressure increases very rapidly near fullness, as the bladder cannot further distend (500ml).
During voiding, rhythmic contractions of the detrusor muscle expel urine until baseline is reached.
What is the role of the pelvic nerve?
parasympathetic - from sacral S2-4.
Releases ACh, binds to M3 receptor in detrusor causing contraction (urine out)
What is the role of the sensory pelvic nerve?
From detrusor, stimulated when bladder stretched S2-4
What is the role of the pudendal nerve?
Somatic, under our control.
Releases ACh, binds to nAChR on EUS, causing contraction (to hold urine in) S2-4
What is the role of the hypogastric nerve?
Storage
NA released, binds to B3 receptor on detrusor causing relaxation
NA released, binds to A1 receptor on IUS causing contraction
What is the role of sympa and parasympathetics in micturition?
Sympathetic for holding in urine (L centre), parasympathetic for voiding (M centre), somatic for holding in.
Describe what nerves are involved when the bladder is empty
Empty bladder - sensory pelvic nerve sends slow impulses towards spinal cord which stimulates the sympathetic hypogastric nerve, which causes contraction of IUS and relaxation of detrusor.
Pudendal nerve stimulated so can contract EUS and hold urine in. Sympathetic stimulated, parasympathetic inhibited.
Describe what nerves are involved when the bladder is full
Stretch in detrusor, sensory pelvic nerve fires signals to sacral SC. New nerve goes straight to M centre, stimulating neurones which inhibit hypogastric nerve (detrusor contracts, IUS relaxes).
Also stimulates pelvic nerve to contract detrusor, and inhibits pudendal nerve so relaxation of EUS.
What is the effect of a lower motor neurone lesion on micturition?
Disconnects bladder from higher centres, bladder and sphincter in relaxed mode as no supply, no sensation so don’t feel bladder filling
Get large residual urine +/- overflow incontinence.
What is the effect of an upper motor neurone lesion on micturition?
Dilated ureters, thickened detrusor, high pressure detrusor contractions, poor coordination with sphincters
Lose control in bladder, constantly in active mode
