The bladder
Hollow muscular reservoir which functions to:
- Store adequate volumes of urine
- At low pressure (neither exceeding the outlet or the safe drainage of upper tracts)
- With continence
- Allowing complete efficient expulsion of urine and voiding at appropriate times
- Also at low pressure
Trigone and rest of the bladder derive from different embryological origins and are functionally distinct.
3 layers:
- Urothelium and lamina propria
- Barrier and impermeable to urine
- Continuous with urethra and ureters
- May have afferent signalling roles regarding stretch and noxious stimuli
- Detrusor
- Smooth muscle fibres allowing for relaxation and contraction
- Inner longitudinal fibres continuous with urethra
- Middle circular fibres condensed at bladder neck giving rise to internal sphincter
- Additional longitudinal fibres at trigone continuous with ureters
- Adventitia
- Connective tissue and fasciae holding bladder in place
Storage and voiding
The whole micturition cycle should be thought of in two discrete phases and processes.
- Storage (filling)
Accommodation of increasing volumes of urine at low pressure with normal compliance.
Appropriate sensation of filling.
Closed bladder outlet at rest which stays closed during rises in intra-abdominal pressure.
Absence of involuntary bladder contractions.
- Voiding (emptying)
Co-ordinated contraction of bladder smooth musculature of appropriate duration and strength.
Lack of resistance at bladder outlet and relaxation of sphincters (no functional obstruction).
Lack of anatomical obstruction.
Adequate bladder function in both phases requires functioning and co-ordination of multiple pathways (in conjunction with normal anatomy and anatomical supports)
- Autonomic pathways – sympathetic (storage) and parasympathetic (voiding)
- Afferent sensory pathways
- Somatic pathways – pudendal nerve and sphincteric control
- Higher brain centres – modulation and control
Anatomical supports and sphincters (outlet)
- External (voluntary) sphincter (rhabdosphincter)
Striated muscle fibres under voluntary control.
Innervated by somatic nerves originating from Onuf’s nucleus in S2 – S4, travelling via pudendal nerve.
Combination of slow and fast twitch fibres – fast twitch fibres more useful for rapid increases in abdominal pressure, slow twitch more tonic contraction during normal slow filling.
Males:
- Between membranous urethra and prostate apex
- Circular fibres
Females:
- Horseshoe shaped, running ventrally and intermixed with connective tissue of anterior vaginal wall
- Thickest mid urethra
- Predominantly slow twitch fibres of urethral sphincter
- Periurethral striated muscle of the pelvic floor – can be strengthened with pelvic floor exercises
- Internal (involuntary) sphincter (bladder neck)
Concentric circular smooth muscle.
Remains closed during filling, not under voluntary control.
Generally less well developed in women cf. men.
Supplied by alpha-adrenergic nerves from T11 – L2.
The viscoelastic properties of the bladder also contribute to continence – as a compliant structure without significant increase in pressure and the bladder fills.
Other mechanisms of female continence
Continence in women is largely a result of proximal and mid urethral forces due to absence of a functional internal or bladder neck sphincter.
External sphincter
As above – thickest mid urethrally, with tonic pudendal stimulation
Distally – striated muscle from perineal membrane and from pelvic floor (urethra compressae)
Urethral attributes
Spongy vascular submucosal layer (oestrogen sensitive) enhances mucosal apposition
Mucosa itself tends to adhere to itself and aid in further compression
Surrounding supports
Strong posterior support and compression from anterior vaginal wall
Pubourethral ligaments fix mid urethra anteriorly and avoid excess force transmission from abdomen
Extra support from fascial attachments anterolaterally to ATFP / white line
Neural control of voiding
- Autonomic pathways – sympathetic and parasympathetic
Sympathetic (T11 – L2)
Originating from sympathetic chain
Alpha-1 a receptors at bladder neck, prostatic and urethral smooth muscle – contraction and promotion of storage
Beta-3 receptors in bladder body – relaxation of detrusor and promotion of storage
Parasympathetic (S2 – S4)
Via pelvic splanchnic nerves (nervi erigentes) and pelvic plexus
Muscarinic receptors (predominantly M3) triggered by acetylcholine cause detrusor contraction and voiding
Additional parasympathetic mediated urethral smooth muscle relaxation (?mediated by NO)
- Afferent sensory pathways
Information from LUT conveyed to CNS via sympathetic, parasympathetic and pudendal pathways.
Respond to both distension (detrusor receptors) and various chemical stimuli (detrusor and urothelium). Both myelinated Aδ and unmyelinated C fibres.
- Somatic pathways
From S2 – S4 ventral horn – Onuf’s nucleus – travelling in pudendal nerve.
Innervation of the striated voluntary rhabdosphincter – causes contraction of the sphincteric muscle and closure of the outlet.
- Higher brain centres and brain stem reflexes
There are reflex pathways in action during both storage and voiding – once toilet trained, reflex voiding can be ‘overridden’ by higher centres.
Storage reflexes
Low level afferent signalling during filling causes sympathetic firing promoting ongoing relaxation of detrusor (sympathetic storage reflex)
Guarding reflex – with increasing volume, pudendal nerve somatic firing to sphincter increases
Voiding reflexes
Periaqueductal grey matter (PAG) in midbrain integrates afferent signals
When threshold of bladder filling detected by PAG, the pontine micturition centre (PMC, or Barrington’s nucleus) is activated
PMC co-ordinates switch from storage phase to voiding phase – parasympathetic stimulation, sympathetic inhibition and inhibition of Onuf’s nucleus
*The above is all ‘supraspinal’ – hence lost in SCI – hence atonic bladder initially before slow development of spinal reflexes after spinal shock passes
Higher centre control
Multiple areas receive afferent signals from PAG giving conscious awareness of bladder filling
Inhibitory signals to PAG and PMC can be voluntarily sent until socially appropriate time, when inhibition is lifted, and voiding phase activated.