Definition
- Micturition is the process of passing urine, primarily a spinal reflex integrated in the sacral S2, S3, S4 segments of the cord, modulated by higher (brainstem and cortical) centres.
- The reflex is normally triggered at a bladder volume of about 300 to 400 mL.
Innervation and Urethral Sphincters
Nerve supply of the urinary bladder
Fibre type
Parasympathetic
- Origin
- Sacral S2, S3, S4
- Nerve
- Pelvic nerve
- Action
- Efferent: detrusor contraction. Afferent: carries filling, desire and pain sensation
Sympathetic
- Origin
- Lumbar L1, L2, L3
- Nerve
- Hypogastric nerve
- Action
- Normally no role in micturition; contracts bladder to prevent semen entering it during ejaculation
Somatic
- Origin
- Sacral S2, S3, S4
- Nerve
- Pudendal nerve
- Action
- Innervates external sphincter; voluntary control