Urinary Sphincters: How Continence Is Maintained

Two sphincters control urine flow:

1. Internal urethral sphincter (IUS) – smooth muscle at bladder neck; involuntary (autonomic: sympathetic T11–L2 causes contraction). Maintains baseline continence.
2. External urethral sphincter (EUS) – skeletal muscle in urogenital diaphragm; voluntary (somatic: pudendal nerve S2–S4). Provides conscious control.

Storage Phase

• Bladder fills → IUS contracts (sympathetic), EUS maintains tonic contraction.

Voiding Phase

• Voluntary decision → EUS relaxes (somatic inhibition).
• Parasympathetic (pelvic nerve S2–S4) triggers detrusor contraction.
• IUS relaxes (sympathetic inhibition).
• Urine flows.

Dysfunction of either sphincter or their neural control causes incontinence or retention.