Pocket ObGyn – Physiology and Mechanisms of Micturation
See Abbreviations
Innervation of Bladder and Urethra (Nature Rev Neurosci 2008;9:453)
| Control of micturition | |||||
| Target | Effect | Nerve | Type | Transmitter | Receptor |
| Bladder
(detrusor) |
Contraction/ voiding | Pelvic plexus efferents (S2–4) | Parasymp | ACh | M3 muscarinic |
| Relaxation/ filling | Hypogastric (T11–L2) | Symp | NE | b3-adrenergic | |
| Urethral sphincter | Contraction/ filling | Hypogastric | Symp | NE | a1-adrenergic |
| External urethral sphincter | Contraction/ voluntary retention | Pudendal (S2–4) | Somatic | ACh | Nicotinic
cholinergic |
- CNS involvement (pontine micturition center) – afferent signal through spinothalamic tracts & dorsal columns ® intensity of signal reaches threshold of consciousness triggering void when socially acceptable ® efferent signal through reticulospinal & corticospinal tracts
Anatomy
- EAS – striated muscle innervated by hemorrhoidal branch of pudendal nerve, voluntary squeeze
- IAS – continuation of smooth circular muscle of rectum under autonomic control, constant contraction contributes 70–80% of resting anal tone
- Levator ani complex – defines prox border of anal canal – PR muscle – striated musc sling originating from pubic bone supporting the rectum, innervated via direct branches from S3, S4, & pudendal nerve, constant tone at rest creates the anorectal angle (~90∞)
Mechanism of Normal Defecation
- Rectum acts as reservoir ® receptors in PR sense distention ® IAS reflexively relaxes to sample contents & then contracts RAIR ® voluntary relaxation of pelvic floor (PR) & EAS straightens anorectal angle by >15° & allows passage of contents